Chapter 3 Climate of the Nile Basin

59

Chapter 3

CLIMATE OF THE NILE BASIN 3.1

HISTORICAL INTRODUCTION

T h e r e i s e v i d e n c e o f some c l i m a t i c c h a n g e s i n t h e N i l e B a s i n , e s p e c i a l l y i n Egypt and t h e S u d a n , i n t h e l a s t 25 000 y e a r s . I n Egypt t h e w e t p h a s e t h a t t e r m i n a t e d some 2 5 000 y e a r s BP ( b e f o r e p r e s e n t ) was f o l l o w e d by a d r y p h a s e t h a t l a s t e d a h o u t 7 000 y e a r s . The s u b s e q u e n t p e r i o d from 18 000 t o , s a y , 5 000 y e a r s BP w a s c h a r a c t e r i z e d by i t s h e a v y w i n t e r r a i n and by i n c r e a s e d f l o w i n t h e N i l e coming f r o m t h e E t h i o p i a n P l a t e a u . The g r a d u a l a r i d i t y which s w e p t o v e r E g y p t s i n c e t h e n w a s i n t e r r u p t e d by some w e t , t h o u g h s h o r t , i n t e r v a l s . T h e s e w e r e f r o m a b o u t 10 000 t o 8 000 y e a r s BP and f r o m a b o u t 6 000 t o 4 500 BP. The m o i s t i n t e r v a l s h a v e b e e n t e r m i n a t e d s i n c e a b o u t 2 500

years B.C.

(Butzer, K.W.,

1966 and 1 9 7 1 ) .

I n t h e S u d a n t h e p e r i o d f r o m 20 000 t o 1 5 000 y e a r s BP w a s v e r y a r i d . T h i s

was f o l l o w e d by a w e t p e r i o d t h a t l a s t e d f r o m 1 2 000 t o 7 000 y e a r s BP and by somewhat f l u c t u a t i n g i n t e r v a l s f r o m 7 000 t o 6 000 y e a r s BP. The c l i m a t e i n t h e i n t e r v a l from 6 000 t o 3 000 y e a r s BP c a n b e d e s c r i b e d as f a i r l y w e t . From 3 000 y e a r s a g o up t o t h e p r e s e n t ,

t h e climate i n t h e Sudan, l i k e t h a t i n Egypt, has

g r a d u a l l y b e e n becoming a r i d (Wickens, G . ,

1975).

A d e t a i l e d d e s c r i p t i o n o f t h e s e changes are beyond t h e s c o p e o f t h i s book.

Our i n t e r e s t h e r e i s f o c u s e d on t h e c l i m a t e as i t h a s b e e n i n t h e p a s t 100 y e a r s

or s o .

3.2

CLIMATIC REGIONS

A s h o r t d e s c r i p t i o n o f t h e c l i m a t e i n t h e a r e a o c c u p i e d by t h e N i l e B a s i n i s given i n V o l .

I of t h e N i l e Basin ( H u r s t , H . E .

and P h i l i p s , P . , 1 9 3 1 ) . I n t h i s

d e s c r i p t i o n t h e c l i m a t e h a s b e e n d i v i d e d i n t o t h r e e main t y p e s . T h e s e a r e : Type 1

- M e d i t e r r a n e a n c l i m a t e c o v e r i n g t h e a r e a from t h e sea coast t o a

little

s o u t h o f C a i r o . The a n n u a l r a i n f a l l d e c r e a s e s f r o m 150 t o 200 mm/yr o n t h e c o a s t t o a b o u t 2 5 t o 30 mm/yr a t C a i r o . Type 2

-

Desert or S a h a r a n c l i m a t e c o v e r i n g t h e a r e a f r o m a l i t t l e s o u t h of C a i r o t o A t h a r a . T h e r e i s p r a c t i c a l l y no r a i n f a l l i n t h i s a r e a .

Type 3

-

T r o p i c a l climate c o v e r i n g t h e a r e a s o u t h of Atbara. This type has f u r t h e r been sub-divided i n t o :

3a

- The S u d a n P l a i n s - T h e r e

is a steady increase i n r a i n f a l l south

o f t h e r a i n l e s s r e g i o n ( t y p e 2 ) . An a n n u a l d e p t h o f , s a y , 1 0 0 0 mm is reached i n t h e s o u t h .

60

3b

- The H i g h l a n d s of A b y s s i n i a - T h i s c o u l d be a r e g i o n o f r e l a t i v e l y heavy r a i n f a l l , s i n c e an a n n u a l d e p t h of 1 600 mm i s r e a c h e d i n some p l a c e s .

3c

- The H i g h l a n d s of t h e Lake P l a t e a t i - The a v e r a g e a n n u a l r a i n f a l l c o u l d b e i n t h e o r d e r o f 1 250 m m .

The c l i m a t e of t h e Sudan was d e s c r i b e d by I r e l a n d i n " A g r i c u l t u r e i n t h e Sudan" ( e d i t e d by T o t h i l l , J . D . ,

1 9 4 8 ) . The Sudan, h e m e n t i o n e d , l a y w h o l l y

w i t h i n t h e t r o p i c s between l a t i t u d e s 22'

and 3 O N .

I t i s almost e n t i r e l y lana-

l o c k e d and h a s a p r e d o m i n a n t l y c o n t i n e n t a l c l i m a t e . The e f f e c t o f t h e Red S e a i s q u i t e l i m i t e d and t h e r e a r e no l a k e s or i n l a n d w a t e r s u r f a c e s l a r g e enough t o p r o d u c e e v e n l o c a l c l i m a t i c e f f e c t s . Broadly s p e a k i n g ,

t h e r e f o r e , t h e Sudan i s

one v a s t p l a i n , i n t e r r u p t e d o n l y by t h e Marra Mountains o f D a r f u r and t h e Nuba Mountains o f s o u t h e r n K o r d o f a n . The cli.mate of t h e Sudan may be d i v i d e d i n t o 3 r e g i o n s . Kegion 1 i s s i t u a t e d n o r t h o f a b o u t l a t i t u d e 19'N.

In t h i s d e s e r t i c region the

d r y n o r t h e r l i e s p r e v a i l t h r o u g h o u t t h e y e a r and r a i n i s i n f r e q u e n t . I t e x p e r i e n c e s l a r g e d i u r n a l and a n n u a l v a r i a t i o n s i n t e m p e r a t u r e ,

c h a r a c t e r i s t i c s of a

desert climate. Region 2 i s s i t u a t e d s o u t h o f l a t i t u d e 19'N is t y p i c a l of a t r o p i c a l c o n t i n e n t ,

t o l a t i t u d e 3 O N . Here t h e c l i m a t e

though t h e n o r t h e r n p a r t is s e m i - a r i d .

Region 3 c o m p r i s e s t h e a r e a s a l o n g t h e Red S e a c o a s t and t h e e a s t e r n s l o p e s of t h e Red S e a h i l l s . L i k e r e g i o n 1, t h e n o r t h e r l i e s p r e v a i l t h r o u g h o u t t h e y e a r , e x c e p t t h a t t h e c l i m a t e i s p r o f o u n d l y a f f e c t e d by t h e m a r i t i m e i n f l u e n c e o f t h e Red S e a . The r a i n i s p a r t l y o r o g r a p h i c and p a r t l y c o n v e c t i o n a l . The c l i m a t e of A f r i c a w a s c l a s s i f i e d ( T r e w a r t h a , G . T . ,

1 9 6 2 ) , b a s e d on a

s i m p l i f i e d form o f t h e c l a s s i f i c a t i o n t h a t had o r i g i n a l l y been d e v e l o p e d by W.

Koppen. The r e s u l t s o f t h a t work have been used t o d e r i v e t h e map of t h e

c l i m a t e of A f r i c a (Stamp, D. and Morgan, W . ,

1 9 7 2 ) . The p a r t of t h e map c o v e r -

i n g t h e N i l e B a s i n i s shown i n F i g . 3 . 1 . The c a t e g o r i e s i n c l u d e d i n i t a r e : 1) The e q u a t o r i a l or t r o p i c a l r a i n f o r e s t c l i m a t e

- This

i s c h a r a c t e r i z e d by

a l m o s t c o n s t a n t h e a t , c o n s t a n t h u m i d i t y and c o n s t a n t r a i n f a l l . P l a n t growth t a k e s p l a c e throughout t h e y e a r , s o t h a t l u x u r i a n t v e g e t a t i o n i s t h e r u l e . The s u n h e a t c a u s e s e v a p o r a t i o n from l a k e s and m o i s t l a n d s u r f a c e s . The h e a t e d , s a t u r a t e d , a i r r i s e s and i s c o o l e d by c o n v e c t i o n s o t h a t r a i n f a l l s i n a l m o s t t h e same a r e a from which t h e m o i s t u r e o r i g i n a t e s ( c o n v e c t i v e rains). 2) The t r o p i c a l savannah

-

T h i s r e g i o n e x t e n d s from t h e t r o p i c a l r a i n f o r e s t on

one s i d e t o t h e d e s e r t m a r g i n s on t h e o t h e r . A c c o r d i n g l y , t h e savannah c l i m a t e v a r i e s g r e a t l y between t h e s e two l i m i t s . The v a r i a t i o n i s p r i m a r i l y i n t h e a n n u a l d e p t h of p r e c i p i t a t i o n , commonly 1 500 mm or more i n t h e

61

e q u a t o r i a l m a r g i n s t o 400 mm i n t h e s e m i - a r i d p a r t . The s e a s o n from November t o F e b r u a r y i s r e l a t i v e l y cool a n d d r y . T h i s i s f o l l o w e d by a h o t , d r y , s e a s o n w h e r e t h e h o t t e s t month i s A p r i l o r May. The coming of r a i n s from J u n e t o O c t o b e r , c a u s e s t h e l o w e r i n g o f t h e t e m p e r a t u r e . The amount o f t h e l o w e r i n g d e p e n d s on t h e amount o f r a i n f a l l . T h e s e a s o n a l d e p t h o f r a i n h a s a w i d e r e l a t i v e r a n g e b u t t h a t o f t h e m o n t h l y d e p t h i s much w i d e r . 3 , 4 ) The s e m i - a r i d

and a r i d c l i m a t e o r t h e s t e p p e and d e s e r t

l i m i t may b e t a k e n as t h e 400 mm/yr

isohyet.

-

The s a v a n n a h

I f less t h a n t h i s d e p t h , t h e

c l i m a t e may b e d e s c r i b e d as s t e p p e . I t i s r a t h e r d i f f i c u l t t o s a y where t h e s t e p p e c h a n g e s i n t o d e s e r t . On t h e p o l e w a r d m a r g i n s ,

3

low r a i n f a l l l i m i t

may b e t a k e n t o i n d i c a t e w h e r e t h e s t e p p e l a n d s f a d e i n t o t h e M e d i t e r r a n e a n Along t h e s o u t h e r n m a r g i n s , t h e r a i n y s e a s o n i s t h e h o t s e a s o n , w h e r e a s along t h e Mediterranean margins i t is t h e w i n t e r .

5 ) The H i g h l a n d s

-

T h e r e a r e two a r e a s whose c l i m a t e b e l o n g s t o t h i s c a t e g o r y ,

a r o u n d t h e E q u a t o r i a l L a k e s a n d a good p a r t o f t h e E t h i o p i a n P l a t e a u ( s e e map, F i g . 3.1.). T h e c l i m a t e h e r e i s v e r y much m o d i f i e d by t h e e l e v a t i o n . Some p a r t s r i s e t o t h e snow l i n e . The a n n u a l p r e c i p l t a t l o n e a s i l y r e a c h e s

1 500 m m . The m o i s t u r e i n d e x method h a s b e e n u s e d i n t h e h y d r o m e t e o r o l o g i c a l s u r v e y o f t h e catchments of Lakes V i c t o r i a ,

Kyoga a n d A l b e r t f o r t h e p u r p o s e o f c l a s s i f y -

i n g t h e c l i m a t e o f t h e L a k e P l a t e a u a r e a . The i n d e x h a s b e e n computed f o r s e v e r a l p o i n t s w i t h i n t h e a r e a from t h e e x p r e s s i o n I

m

100 ( S =____.__-.

-

0.6D)

(3.1)

PET

where Im

= moisture index,

S

= cumulated monthly s u r p l u s ,

(R

D

= cumulated monthly d e f i c i t ,

(PET

R

=

m

= s u b s c r i p t r e f e r r i n g t o month,

-

PET)

-

m'

R)m,

rainfall, and

PET = a n n u a l p o t e n t i a l e v a p o t r a n s p i r a t i o n The d i f f e r e n t t y p e s o f c l i m a t e c o r r e s p o n d t o t h e f o l l o w i n g I Type

Im value

__

100 and a b o v e

Perhumid Humid

Moist sub-humid

20

to

100 20

0

to

Dry sub-humid

-

20

to

0

Semi - a r i d

-

40

to

-20

Arid

-100

to

-40

values

62

Fig. 3.1.

C l i m a t o l o g i c a l r e g i o n s o f the N i l e B a s i n

63

The c o n t o u r l i n e s of e q u a l m o i s t u r e i n d e x v a l u e s i n t h e E q u a t o r i a l Lake P l a t e a u a r e as shown i n F i g . 3.2. From t h i s map one c a n e a s i l y see t h a t t h e g r e a t e r p a r t o f t h e p l a t e a u a r e a f a l l s i n t h e c l a s s of t h e d r y sub-humid c l i m a t e and o n l y a few s m a l l p a r t s f a l l i n t h e m o i s t sub-humid c l a s s . The i n f o r m a t i o n p r e s e n t e d on t h e map i n F i g . 3.2. d o e s n o t f u l l y a g r e e w i t h t h e c o r r e s p o n d i n g p a r t of t h e map i n F i g . 3 . 1 . They d i f f e r mainly i n t h a t p a r t s i t u a t e d w e s t of Lake V i c t o r i a ,

29'

E

30'

i n t h e b a s i n o f t h e Kagera and n o r t h o f i t .

31 '

32

33'

3 4'

35'

36'

F i g . 3.2. Contour l i n e s of e q u a l m o i s t u r e i n d e x v a l u e s f o r t h e E q u a t o r i a l Lake P l a t e a u a r e a (WMO, 1974)

64

3.3

NETWORK OF METEOROLOGICAL STATIONS

A d e s c r i p t i o n o f some o f t h e c l i m a t i c f e a t u r e s of t h e N i l e B a s i n a r e a i s

included i n t h e f o l l o w i n g s e c t i o n s of t h i s c h a p t e r . These f e a t u r e s i n c l u d e t h e t e m p e r a t u r e , h u m i d i t y , r a d i a t i o n , s u n s h i n e , c l o u d i n e s s , wind and g e n e r a l c i r c u l a t i o n of t h e a i r m a s s e s . T h i s d e s c r i p t i o n i s b a s e d on t h e d a t a o b s e r v e d a t some, or a l l , of t h e m e t e o r o l o g i c a l s t a t i o n s l i s t e d i n T a b l e 3.1.

TABLE 3.1

D a t a o f some o f t h e m e t e o r o l o g i c a l s t a t i o n s i n t h e N i l e B a s i n ( I r e l a n d , 1948; M i n i s t r y of War and M a r i n e , E g y p t , 1950; WMO, 1974)

Station Sidi Barrani S a 1 um ( Obs e r va t o r y )

Damietta Rosetta Mersa Matruh Edfina Port Said (Airport) Sirw Alexandria Atf Arish Sakha Mansura Damanhur Qurashiya Gemmeiza Tanta Zagazig Benha Delta Barrage C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa Beni Suef Minya ( A i r p o r t ) Hurghada Assiut Qena Qusseir Nag-Hammadi Luxor Dakhla Kharga Esna Deadalus KOQ Ombo Aswan

Country Egypt

Latitude N

31’ 31 31 31 31 31 31 31 31 31 31 31 31 31 30 30 30 30 30 30 30 30 29 29 29 29 29 28 27 27 26 26 26 25 25 25 25 24 24 24

38’ 33 25 24 22 18 17 14 12 11 07 07 03 02 51 48 47 35 28 11 03 02 56 52 18 12 04 05 14

11 10 08 03 39 29 26 18 55 29 02

Longitude E

25O 25 31 30 27 30 32 31 29 30 33 30 31 30 31 31 31 31 31 31 31 31 32 31 30 25 31 30 33 31 32 34 32 32 29 30 32 35 32 32

58’ 11 49 25 14 31 15 39 53 31 46 57 23 28 07 07 00 30 11 08 15 13 33 20 51 29 06 44 51 13 43 18 15 39 00 34 34 52 56 53

Altitude

metres 22 4 3 2 7 3 1 2 32 10 10 6 7 6 8 9 14 13 14 20 20 21 10 116 30 - 15 28 39 3 55 75 7 70 78 122 72 82 4 10 2 111

65

TABLE 3 . 1

(continued)

Station Wadi H a l f a P o r t Sudan Dongola Karima Tokar Atbara Khartoum Kassala J e b e l Aulia Wad-Medani El-Dueim E l Fasher Sennar Geneina E 1-0be i d S i nga Gallabat E l - Nahud El-Roseires Renk Ma1ak a 1 Addis Ababa Raga Gambe i 1a Akobo Wau Jimma Juba Tori t Loka Gulu Moroto Lira But i ab a Soroti Masindi Fort Portal Mubende Namulonge Eldoret K amp a 1a Entebbe Kitale Equator Kisumu Kericho Mbarara Kabale Bukoba Musoma Mwanza

Country

Latitude

__ Sudan

Ethiopia Sudan

Ethiopia Sudan

Uganda

Kenya Uganda Kenya Kenya Uganda Tanzania

21° 19 19 18 18 17 15 15 15 14 14 13 13 13 13 13 12 12 11 11 09 09 08 08 07 07 07 04 04 04 02 02 02 01 01 01 00 00 00 00 00 00 00 00 00 00 00 01 01 01 02

55 37 10 33 26 42 37 28 14 24 00 38 33 29 11 09 58 42 51 45 33 02 28 15 47 42 39 51 25 22 45 33 18 50 43 41 40 35 32 31 20 03 01 S 01 06 21 37 15 20 30 28

Longitude E

Altitude metres

31° 37 30 31 37 33 32 36 32 33 32 25 33 22 30 33 36 28 34 32 31 38 25 34 33 28 36 31 32 30 32 34 32 31 33 31 30 31 32 35 32 32 35

20 13 29 51 44 58 32 24 30 29 30 21 37 27 14 57 10 26 23 47 39 45 41 35 01 01 51 37 33 57 20 36 56 20 37 43 17 22 37 17 36 27 00

125 5 225 250 20 345 380 500 380 405 380 740 4 20 805 565 4 30 760 565 465 380 390 2450 460 450 400 435 1750 460 6 25 965 9 26 1524 1095 621 1127 1146 1539 1542 1148 2084 1230 1146 1896

35 34 35 30 29 31 33 32

33 35 20 39 59 49 48 55

2762 1146 2070 1443 1868 1137 1147 1140

66

The d a t a u s e d , p a r t o f which is i n c l u d e d i n t h e c l i m a t i c t a b l e s a v a i l a b l e i n t h i s book, are e x t r a c t e d from a number o f r e f e r e n c e s . Examples of t h e s e r e f e r e n c e s a r e : t h e p u b l i c a t i o n s o f t h e E a s t A f r i c a n M e t e o r o l o g i c a l Department (E.A.M.D.),

t h e C l i m a t o l o g i c a l Normals f o r E g y p t , t h e C l i m a t e s of A f r i c a and

some volumes o f t h e N i l e B a s i n . Each o f t h e c o u n t r i e s s h a r i n g t h e b a s i n o f t h e N i l e h a s i t s own n a t i o n a l network o f m e t e o r o l o g i c a l s t a t i o n s . However, t h e h y d r o m e t e o r o l o g i c a l p r o j e c t of t h e E q u a t o r i a l Lake a r e a had among i t s o b j e c t i v e s t h e s t r e n g t h e n i n g o f t h e p r e v i o u s l y e x i s t i n g networks i n Uganda, Kenya and T a n z a n i a , and l a t e r i n Rwanda and B u r u n d i . To f u l f i l l t h i s o b j e c t i v e , t w e n t y - f i v e m e t e o r o l o g i c a l s t a t i o n s have been e s t a b l i s h e d and t h i r t y e x i s t i n g s t a t i o n s have a l r e a d y been up-graded by t h e p r o v i s i o n of a d d i t i o n a l i n s t r u m e n t s . Moreover, 200 o r d i n a r y r a i n gauges have been i n s t a l l e d and 2 3 t o t a l i z e r s p l a c e d i n remote p l a c e s and i s l a n d s . F o r t h e e s t i m a t i o n of e v a p o r a t i o n from t h e E q u a t o r i a l L a k e s , a network of s i x s o l a r i m e t r e s f o r r a d i a t i o n measurement, s e v e n s t a t i o n s w i t h wind m a s t s f o r measurement of wind s p e e d , and e i g h t s t a t i o n s f o r t h e measurement of l a k e s u r f a c e water t e m p e r a t u r e , have been i n s t a l l e d . F o r t h e e s t i m a t i o n of e v a p o t r a n s p i r a t i o n ,

a network o f T h o r n t h w a i t e t a n k s

and some s p e c i a l l y s i m e t r e s have b e e n i n s t a l l e d a t a number o f l o c a t i o n s . An a u t o m a t i c w e a t h e r s t a t i o n h a d b e e n e s t a b l i s h e d on t h e Nabiyongo I s l a n d i n Lake V i c t o r i a , w i t h an a u x i l i a r y s t a t i o n a t E n t e b b e . The p r i n c i p a l s t a t i o n a t t h e l a t t e r h a s among i t s equipment t h e R u s s i a n 20 m 2 and GGI 3000 e v a p o r a t i o n pans (WMO, 1 9 7 4 ) . The m a j o r i t y o f t h e m e t e o r o l o g i c a l s t a t i o n s i n Egypt and t h e Sudan a r e s t a t i o n s of t h e s e c o n d o r d e r , where o b s e r v a t i o n s a r e t a k e n e v e r y day a t 0 8 . 0 0 , 1 4 . 0 0 and 20.00 h o u r s s t a n d a r d l o c a l t i m e . A t f i r s t - o r d e r s t a t i o n s , t h e o b s e r v a t i o n s a r e u s u a l l y t a k e n e i g h t t i m e s a day i n t h e s y n o p t i c h o u r s of o b s e r v a t i o n s , and a t t h i r d - o r d e r

s t a t i o n s , o b s e r v a t i o n s a r e taken a t 08.00 hours l o c a l t i m e

only. S c r e e n o b s e r v a t i o n s u s u a l l y c o m p r i s e t h e a i r t e m p e r a t u r e , maximum and minimum t e m p e r a t u r e s , a l l i n d e g r e e s c e n t i g r a d e , t h e b a r o m e t r i c p r e s s u r e i n m i l l i b a r s , t h e h u m i d i t y a s o b t a i n e d from t h e w e t and d r y b u l b t h e r m o m e t e r s , and t h e e v a p o r a t i v e c a p a c i t y of t h e a i r a s measured w i t h a P i c h e e v a p o r i m e t e r .

The

d u r a t i o n o f t h e b r i g h t s u n s h i n e i s measured m o s t l y by a Campbell-Stokes r e c o r d e r . The wind i s e x p r e s s e d by a number on t h e B e a u f o r t S c a l e when t h e wind f o r c e i s e s t i m a t e d , or i n k i l o m e t r e s p e r h o u r when i t s s p e e d i s measured by means o f an anemometer. The s o i l t e m p e r a t u r e i s measured a t a few l o c a t i o n s o n l y , and a t a d e p t h v a r y i n g from 0.60 m t o 2 . 1 0 m , whereas t h e g r a s s minimum t e m p e r a t u r e i s measured a t a h e i g h t o f 0 . 1 0 m above t h e ground l e v e l . The r i v e r and s e a t e m p e r a t u r e s a r e measured a t some s e l e c t e d s i t e s i n Egypt and t h e Sudan

67

The r a i n f a l l i s measured n o t o n l y a t t h e m e t e o r o l o g i c a l s t a t i o n s , b u t a l s o a t many o t h e r l o c a t i o n s . Most of t h e r a i n gauges i n s t a l l e d are c y l i n d e r i c a l i n form w i t h a c a t c h o f 200 c m 2 i n s u r f a c e a r e a and a r i m o f a b o u t 1 . 0 m h e i g h t from t h e ground s u r f a c e . The a n a l y s i s of t h e r a i n f a l l d a t a i s p r e s e n t e d i n Chapter 4 .

3.4

TEMPERATURE The mean d a i l y t e m p e r a t u r e f o r t h e months of t h e y e a r a t a number of s t a t i o n s

i n t h e N i l e B a s i n i s l i s t e d i n T a b l e 3 . 2 . These d a t a a r e b a s e d on t h e d a i l y mean t e m p e r a t u r e which is c a l c u l a t e d on t h e b a s i s of t h e number of o b s e r v a t i o n s taken e v e r y d a y . S o , f o r a l l s t a t i o n s i n Egypt and Sudan o b s e r v i n g t h r i c e d a i l y , t h e mean t e m p e r a t u r e i s o n e - f o u r t h

t h e sum o f t h e t e m p e r a t u r e s a t 0 8 . 0 0 , 1 4 . 0 0 ,

and 20.00 h o u r s p l u s t h e minimum t e m p e r a t u r e . F o r s t a t i o n s o b s e r v i n g t w i c e d a i l y , t h e t e m p e r a t u r e is t h e mean of t h e 0 8 . 0 0 h o u r and 20.00 h o u r o b s e r v a t i o n s , and for s t a t i o n s o b s e r v i n g once d a i l y t h e t e m p e r a t u r e is s i m p l y t h e mean of t h e maximum and minimum t e m p e r a t u r e s . The m e t e o r o l o g i c a l s t a t i o n s i n s t a l l e d by t h e h y d r o m e t e o r o l o g i c a l p r o j e c t i n t h e E q u a t o r i a l Lake P l a t e a u as w e l l a s t h e up-graded are spot-read

s t a t i o n s i n East Africa

v i s u a l l y d u r i n g t h e s y n o p t i c h o u r s : 06.00 Z*,

09.00 Z a n d 1 2 . 0 0 Z .

The mean d a i l y t e m p e r a t u r e shows a d i s t i n c t p a t t e r n c h a r a c t e r i s t i c of each p a r t i n t h e N i l e B a s i n . G e n e r a l l y s p e a k i n g , t h e c o o l e s t month i n Egypt i s January and t h e warmest i s J u l y , e x c e p t a l o n g t h e c o a s t s of t h e M e d i t e r r a n e a n and t h e Red S e a s , where August i s t h e warmest month. F o r t h e g r e a t e r p a r t of t h e Sudan, J a n u a r y i s t h e c o o l e s t month. The month w i t h t h e h i g h e s t mean d a i l y t e m p e r a t u r e changes r a p i d l y w i t h l a t i t u d e from J u l y i n Wadi H a l f a , s i m i l a r t o Egypt, t o May i n Wad Medani, A p r i l i n Malakal and F e b r u a r y i n J u b a down s o u t h . This main c y c l e i s f o l l o w e d , i n many p l a c e s , by a less pronounced c y c l e where t h e second minimum f a l l s i n August and t h e second maximum i n September or O c t o b e r . I n t h e E q u a t o r i a l Lake P l a t e a u t h e wave o f t h e mean d a i l y t e m p e r a t u r e

is q u i t e s i m i l a r t o t h a t i n t h e s o u t h e r n p a r t o f t h e Sudan. The primary maximum o c c u r s i n F e b r u a r y and t h e l o w e s t t e m p e r a t u r e i n J u l y . The s e c o n d a r y maximum t a k e s p l a c e i n O c t o b e r and i s f o l l o w e d by a s e c o n d a r y minimum i n November. The r a t i o of t h e mean d a i l y t e m p e r a t u r e i n t h e warmest month t o t h e mean d a i l y t e m p e r a t u r e i n t h e c o o l e s t month,

(Tmx /? mn ) ,

h a s been computed f o r a l l s t a t i o n s

given i n T a b l e 3 . 1 and p l o t t e d v e r s u s t h e l a t i t u d e ,

4.

Three curves a r e

o b t a i n e d a s shown i n F i g . 3 . 3 . I n t h e v e r y n o r t h e r n l a t i t u d e , a b o u t N 3 Z 0 ,

the

grouping of t h e s t a t i o n s i s n o t c l e a r . South of t h i s l a t i t u d e one c a n e a s i l y = Greenwich M e r i d i a n Time = mean d a i l y t e m p e r a t u r e i n t h e warmest month, and TmX= mean d a i l y t e m p e r a t u r e i n t h e c o o l e s t month mn

*Z

T

= G.M.T.

68

d i s t i n g u i s h o n e c u r v e f o r t h e s t a t i o n s l o c a t e d w e s t of t h e N i l e , a n o t h e r c u r v e

for t h e s t a t i o n s on t h e N i l e and i t s t r i b u t a r i e s and between t h e b r a n c h e s and a t h i r d c u r v e f o r t h o s e s t a t i o n s on t h e Red S e a c o a s t and e a s t o f t h e N i l e .

In

each c a s e t h e c u r v e c o n s i s t s o f a v e r y s h o r t r i s i n g limb f o l l o w e d by a l o n g f a l l i n g t a i l . The peak o c c u r s a t a p p r o x i m a t e l a t i t u d e s o f 28O, 27O and 30° n o r t h f o r t h e t h r e e c u r v e s i n t h e o r d e r d e s c r i b e d a b o v e . The c o r r e s p o n d i n g

(Tmx /? mn )

r a t i o is about 2.65,

2 . 5 and 2 . 0 5 r e s p e c t i v e l y .

The a n n u a l mean d a i l y t e m p e r a t u r e a t t h o s e s t a t i o n s l i s t e d i n T a b l e 3 . 2 have

been used f o r p l o t t i n g t h e mean a n n u a l i s o t h e r m o v e r t h e N i l e B a s i n . The map i n F i g . 3 . 4 . shows t h a t t h e mean of t h e a n n u a l mean d a i l y t e m p e r a t u r e i n c r e a s e s a l m o s t s t e a d i l y from a b o u t 19OC on t h e M e d i t e r r a n e a n S e a c o a s t i n t h e n o r t h t o a l m o s t 29OC i n A t b a r a down s o u t h . A mean t e m p e r a t u r e of 29 t o 29.5OC c o v e r s t h e b e l t from A t b a r a t o Khartoum. S o u t h o f Khartoum t h e t e m p e r a t u r e f a l l s , b u t s l o w l y , t o r e a c h 26OC a l o n g t h e s o u t h e r n f r o n t i e r of t h e Sudan. North-west

of

Lake V i c t o r i a t h e t e m p e r a t u r e d r o p s r a t h e r r a p i d l y t o r e a c h a b o u t 2l0C i n Entebbe and 20°C

i n F o r t P o r t a l . The topography of t h e h i g h l a n d s i n t h e e a s t e r n

p a r t o f t h e N i l e B a s i n c a u s e s t h e c o o l i n g of t h e mean t e m p e r a t u r e

t o about

17OC a s shown i n F i g . 3.4. The mean a n n u a l t e m p e r a t u r e h a s a s m a l l r a n g e of v a r i a t i o n . T h i s r a n g e v a r i e s from a b o u t 3OC i n t h e major p a r t o f t h e E q u a t o r i a l Lake P l a t e a u t o a maximum o f less t h a n 6 O C i n t h e c e n t r a l p l a i n s of t h e Sudan. T h i s narrow r a n g e i s produced by t h e r e l a t i v e l y s m a l l a n n u a l v a r i a t i o n o f radiation.

I n c o n t r a s t , t h e d i u r n a l r a n g e of t e m p e r a t u r e is q u i t e l a r g e . The

f i g u r e s p r e s e n t e d i n T a b l e 3 . 3 show t h e r a n g e t o b e l a r g e s t i n t h e n o r t h e r n p a r t o f t h e Sudan and t h e s o u t h e r n p a r t o f Egypt,

and s m a l l e s t i n t h e

M e d i t e r r a n e a n and Red S e a a r e a s and t h e Lake P l a t e a u a r e a .

TABLE 3 . 2

Mean d a i l y t e m p e r a t u r e a t c e r t a i n s t a t i o n s i n t h e N i l e B a s i n ( d a t a a r e from G r i f f i t h s , 1972; I r e l a n d , 1948; M i n i s t r y o f War and Marine, Egypt, 1950; WMO, 1974)

Mean d a i l y temperature i n

Station Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

OC

for Sep.

Oct.

Nov .

Dee.

Year

~

S i d i Barrani Salum ( O b s e r v a t o r y ) D ami e t t a Rosetta Mersa Matruh Port Said Alexandria Mansura Damanhur Tanta Zagazig D e l t a Barrage C a i r o (Ezbek i y a ) Giza S u e z ( P o r t Tewfik) Helwan Fay urn Siwa Beni Suef Minya Hurghada Assiut Qena Quss e i r Nag Hammadi Luxor Dakhla Kharga

12.3 11 3 13.2 15.2 12 4 13.7 13.7 13.4 13 6 11.6 11.5 13.0 12.3 11.2 13.8 12.3 11.6 10.7 12.4 12.2 15.8 11.7 13.2 17.8 1 2 .o 13 .O 12.3 13.1

13.2 12.3 14 . O 15.2 12.9 14 . 3 14.1 14 . O 14.2 12.3 12.6 14.0 13.5 12.5 14.6 13.5 13.2 12.6 14.1 14.1 16.2 13.3 15 .O 18.4 12.7 15.4 14.1 14.9

14.2 14.1 15.3 16 , 7 14.5 16.2 15.8 16.2 16.2 14.9 15.2 16 . O 16.3 15.4 17.1 16.4 16 1 15.8 16.9 17.1 18 . 7 17.1 19.4 20.7 16.6 19.4 18.1 19.1

16.6 16.8 18.3 19 . o 17.2 18.7 18.1 19.8 19.4 18.7 18.9 19.8 20 . 2 19.2 20.5 20.4 20.4 20.3 20.2 21.4 22.2 22.2 24.6 23.4 21.5 25 . o 23.4 23.9

19.3 19.4 22.2 22.0 19.9 21.8 2 1 .o 23.8 22.9 22.9 23.0 23.7 24.2 23.3 24.4 24.3 25.1 25.3 23.7 26.1 26.2 26.6 29.8 26.8 27.3 30.2 28.2 29 . o

22.0 22.3 24.2 24 . 5 22 9 24.6 23.6 2fi 3 25 2 25.4 25.7 26.2 26.8 26 . O 26.9 26.6 27.2 27.9 26.6 28 . O 28.5 28.8 31.7 28.9 28.5 31.4 30.4

23.8 24.3 26 . O 26.3 24 7 26.4 25.4 27 . 8 26.4 26.5 26.8 27.7 27.7 26.9 28.4 27.5 28.1 28.9 27.4 29 . O 29.5 29.4 32 .O 29.8 29.1 32.3 30.8

24.5 24.3 26.2 27.2 25.5 26.9 26.2 27 8 26.6 26.4 26.6 27.4 27.6 26.7 28.5 27.4 28.0 28.5 27.6 28.7 30 .O 29.1 32.1 30.3 29 4 32.1 30.5

23.5 23.0 24.6 26.3 24.4 25 . 8 25.3 26.1 25.2 24.4 24.5 25.2 25.3 24.3 26.3 25.4 25.6 26 . O 24.6 26.1 28.0 26.5 29.1 28.7 26.9 29.7 28.1

20.9 21.2 23.2 24.5 22.2 23.9 23.3 24 .o 23.5 22.1 22.4 23.2 22.7 22.0 24 . O 23.3 23.2 22.4 22.6 23.8 25.2 23.8 26 . O 26.7 24.6 26.8 24.9

17.4 17.8 19.8 21.2 18.7 20.4 19.9 2F. 3 19.8 18.3 18.4 19.2 18.7 18 . O 20.0 19 18.7 17.4 18.5 19.2 21.2 18.6 20.3 23.4 19.6 20.5 19.2

13.6 13.4 15.4 17.2 14.4 15.6 15,7 15.4 15.5 13.6 13.4 14.8 14 . O 13.2 15.4 14.1 13.5 12.3 13.5 14 . O 17.7 13.6 15 . O 19.6 14.6 15.1 13.8

18.4 18.4 20.2 21.3 19.1 20.7 2c.2 21.2 20.7 19 . 8 19.9 20.8 20.8 19.9 21.7 20.8 20.9 20.7 20.7 21.6 23.3 21.7 24.0 24.5 21.9 24 , 2 22.8

31.0

31.4

31.1

28.6

26 - 0

20.2

15 . O

23.6

.o

TABLE 3.2

4 0

(continued)

Mean d a i l y temperature i n OC f o r Station

Esna D e a d a l us Kom Ombo Aswan Wadi H a l f a P o r t Sudan Tokar Atbara Khar tourn Kassala W ad-Medani E l -Dueim Fasher Sennar Geneina El-Obeid Singa Gal l a b a t El-Nahud E l Roseires Renk Malakal Addis Ababa Raga Gambei l a Akobo Wau Jimma Juba Tori t

Jan.

Feb.

Mar.

Apr

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

13.1 21.8 14.9 15.5 15.9 23.5 24.4 22.2 23.6 25 .O 24.2 23.7 20.6 25 .O 22.4 21.0 25.7 26.0 23.4 26.2 24.7 27 .O 15.6 24.2 27.5 28.4 26.7 19.5 28.8 28.3

14.4 21.5 16.3 17.2 17.5 23.2 24.3 23.4 25 .O 26.1 25.2 25 . O 22.2 26.1 24.5 22.6 26.8 27.5 24.2 27.5 26.5 28.4 16.9 25.8 28.6 29.8 28 . O 20 .o 29.6 28.9

18.5 22.7 20.4 21.3 21.9 24.2 26 . O 26.6 28.2 28.8 28.2 27.8 25.3 29.1 25.8 25.8 29.6 29.4 25.6 29.8 28.1 30.4 18.2 27.4 29.9 30.9 29.5 20.0 29.5 28.8

23.3 24.6 25 . O 26.2 26.7 26.6 28 . O 30.4 31.4 31.6 31.0 30.9 28.4 32.0 29.1 29.3 32.0 30.6 30.4 31.6 31.9 31.0 18.0 28.8 29.4 30.2 29.8 20 .o 28.8 28.1

27.6 27.2 29.6 30.5 30.5 29.4 30.9 33.4 33.6 33.2 32.5 32.1 30.1 32.4 29.6 30.7 32.1 29.7 30.4 31.0 31.2 29.4 18.7 28.0 27.5 28 .O 28.4 19.5 27.4 26.8

29.7 28.3 31.2 32.9 32.2 32.3 33.4 34.8 33.6 32.1 31.9 31.8 30.5 31.2 29.4 30.2 30.3 26.6 30 . O 28.6 29.3 27.4 17.5 26.6 26.2 27.1 27.2 19 .o 26.5 25.9

30.1 29.7 31.4 33.2 32.2 34.5 35 . O 33.6 31.7 29.1 29.1 29.6 28.7 28.4 27.1 28.0 27.8 24.0 27.4 26.8 27.1 26.3 16.5 25.5 25.6 26.1 26.2 18 . O 25.5 24.8

30.1 30.3 31.4 33 . O 32.2 34.8 35.0 32.7 30.6 28.0 27.8 28.4 27 .O 27.4 25.4 27.0 26.8 23.8 26.6 26.4 26.3 26.2 16.1 25 , 2 25.6 26.1 26 . O 18.0 25.6 24.8

27.9 29.1 29.4 30.9 30.5 32.2 33.6 33.6 32.2 29.6 28.9 29.5 28.1 28.2 26.1 27.9 27.6 24.6 27.6 26.9 26.9 27 . O 16.3 25.8 25.9 26.9 26.6 18.5 26.4 25.8

24.2 28 .O 27.2 28.3 28.2 29.4 30.6 31.6 32.1 31.2 30.2 30.8 27.8 29.9 26.1 28.7 29.4 26 . O 28.4 28.0 28.5 27.8 16.5 26.6 26.5 27.4 27.4 18.0 27 .2 26.4

19.1 26.1 21.6 22.6 22.6 27.4 28.0 27.4 28.4 29.4 27.7 28.2 23.8 28.4 24.2 25.3 28.5 26 . O 24.8 27.8 27.3 27.6 15.1 25.3 26.8 28.2 27.5 18 . O 27.7 27 . O

14 .G 23.3 16.8 17.4 17.6 25 .O 25.6 28.2 25 .O 26.2 25.0 24.8 21.1 26 .O 23.2 22.0 26.4 25.6 23.7 26.5 26.5 27 . O 14.8 24.2 27.0 27.8 26.8 18.0 28.1 26.9

22.7 26 .O 24.6 25.8 25.7 28.6 29.6 29.5 29.6 29.2 28.5 28.6 26.1 28.7 26.1 26.6 28.6 26.6 26.9 28.1 27.9 28 . O 16.7 26.1 27.2 28.1 27.5 19 .o 27.6 26.9

TABLE 3.2

(continued)

S t a t ion

Gulu Lira Butiaba Soroti Masindi Fort Portal Entebbe Kisumu Bukoba Mwanza

Mean d a i l y t e m p e r a t u r e i n

OC

for

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

24.2 24.5 26.1 25.5 23.8 19.9 21.7 25.1 20.2 22.6

24.8 25.1 26.5 25.9 24.1 20.2 21.7 24.3 20.3 22.5

24.5 24.9 26.5 25.7 24.0 20.1 21.8 24.0 20.4 22.2

23.6 23.8 25.9 24.4 23.3 20.0 21.3 23.6 20.3 22.0

22.5 22.9 25.7 23.5 22.9 19.6 21.0 23.2 20.2 22.2

22.2 22.9 25.3 23.1 22.3 19.2 20.8 22.7 20.3 22.0

21.6 21.7 24.8 22.5 21.6 19.0 20.4 22.2 20.2 21.8

21.8 21.9 24.5 22.6 21.5 19.2 20.4 22.6 20.4 22.2

22.3 22.5 25.1 23.3 21.9 19.4 20.8 23.3 20.5 23.0

22.8 22.9 25.5 24.1 22.5 19.0 21.2 23.8 20.6 23.4

23.6 23.3 25.6 24.5 22.9 19.2 21.2 24.6 20.4 23.1

23.4 23.5 25.7 24.7 22.9 19.6 21.3 24.9 20.2 22.8

23.2 23.3 25.6 24.1 22.8 19.5 21.1 23.5 20.4 22.5

72 0

c

a,

.r(

1

o m

a

a,

k

5

Y

fi

;d

C

x

.d

rl

a,

a

o 0

9

9 P Y

a,

9

Y

73

Fig. 3 . 4 .

Map showing t h e mean annual isotherms i n t h e N i l e B a s i n area

4

TABLE 3.3

Mean d a i l y r a n g e of t e m p e r a t u r e a t a number of s t a t i o n s i n t h e N i l e

B a s i n ( I r e l a n d , 1948; M i n i s t r y of War

and M a r i n e , E g y p t , 1950; WMO 1974)

Station

Mean d a i l y r a n g e , i n OC, for Feb

Jan.

.

~

S i d i Barrani S a 1um ( Ob s e r va t o ry ) Damiet t a Rosetta Mersa Matruh Port Said A l e x a n d r ia Mansura Damanhur Tanta Zagazig D e l t a Barrage C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa Beni Suef Minya Hurghada Assiut Qena Qusseir Nag Hammadi Luxor Dakhla Kharga

10.6 9.8 9.8 5.8 9.2 8.2 7.9 12.6 12.1 13.6 14 .O 13.3 12.1 13.8 10.5 10.5 14.6 16.1 14.3 13.8 11.5 14 . O 15.8 8.8 15.0 1S.O 16.7 16.1

10.5 9.9 9.6 6.5 9.1 8.6 8.1 13.1 12.5 14.3 14.3 14.3 13.0 14.9 11 .o

11.5 14.9 16.4 15.4 14.5 12.4 15.3 17.5 8.7 16.8 19.4 17.7 17.1

Mar.

Apr.

__ 10.7 10.2 8.8 6.2 9.0 8.0 8.4 13.7 13.2 15.5 15.2 15.3 13.9 16.1 11.9 12.8 15.6 17.1 16.8 15.8 12.4 16.5 19.1 8.3 17.4 19.7 18.3 17.3

11.0 10.9 8.8 6.5 9.4 7.9 8.5 15.3 14.7 17.2 16.6 17.3 15.2 17.8 13.5 14.4 17.0 17.9 18.1 17.1 12.5 17.0 19.2 7.9 18.5 20.0 19.1 17.9

Jun .

May ~

11.6 9.8 8.8 5.7 8.8 7.9 8.2 15.7 14.8 17.4 16.8 17.6 15.7 17.9 13.9 15 .O 17 .O 18.0 18.2 17.0 12.1 16.6 18.0 7.6 17.5 19.5 18.3 16.8

Jul

.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

8.5 9.7 8.7 4.5 G .6 8.0 6.8 14.0 12.4 15.2 14.8 14.6 13.3 15 .O 13.1 13.5 14.8 17.7 15.1 14.4 10.0 13.8 16.5 7.1 17.7 18.4 15.9 16.6

9.9 9.5 8.6 5.0 7.1 7.8 7.3 13.6 12.6 15.1 14.5 13.5 12.7 14.2 12.2 12.4 14.1 17.3 15.1 13.0 10 .o 12.8 16.1 6.9 14.6 17.8 15.5 15.4

10.4 10.6 8.3 5.8 8.4 7.6 8.2 12.9 13.0 14.7 14.4 13.7 13.0 14.4 11.8 11.7 14.3 17.6 14.7 12.7 11.0 12.1 16.3 7.3 15.3 18.2 16 .O 15.6

10.7 9.4 7.9 5.7 8.4 7.5 8.1 11.8 11.9 13.5 13.4 13.1 12.4 13.7 11.0 10.7 13.8 16.7 14.6 12.8 11.1 13.7 16.5 7.9 16.6 18.3 16.1 15.8

10.5 9.8 9.3 6.2 9.1 8.1 8.0 12.2 11.8 13.1 13.4 13.1 11.8 13.3 10.9 10.2 13.9 16.1 13.8 12.9 11.4 13.7 15.5 8.4 15.8 18.0 16.2 15.8

10.3 10.0 8.8 5.5 8.2 7.9 7.7 13.7 13.0 15.1 14.9 14.9 13.5 15.4 12.3 12.7 15.1 17.2 15.8 14.6 11.3 14.7 17.1 7.7 16.5 18.8 16.9 16.4

~

10.0 10.4 8.3 4.6 7.6 7.8 7.4 15.2 13.9 16.8 16.4 16.9 15.4 17.3 14 .O 14.9 16.3 18.5 17.7 16.3 11 .o 16.7 17.8 7.1 17.1 19.7 16.9 15.9

8.4 10.1 9.0 4.2 6.4 8.1 6.6 14.7 12.7 15.5 15.3 16.2 14.2 16.1 13.6 14.4 15.5 17.6 16.3 15.2 10.4 14.8 16.9 7.2 16.8 18.9 16.1 16.1

P

TABLE 3.3

(continued)

Mean d a i l y r a n g e , i n OC, f o r Station Jan.

Feb.

Mar.

Apr.

May

Jun. ~

Esna Deadalus I s l a n d K o m Ombo Aswan Wadi H a l f a P o r t Sudan Tokar Atbara Kh a r t o um Kassala Wad Medani El Dueim El Fasher Sennar Geneina El -0be i d Singa Gallabat E l -Nahud E l Roseires Renk Malakal Addis Ababa Raga Gambeila Akobo W au Juba Gulu Moro to

16.4 3.8 16.2 13.4 16.2 7.2 8.7 16.3 16.9 18.1 19.8 18.6 21.5 19.8 22.9 18.8 18.8 20 . o 16.9 20.6 19.1 17.5 17.0 22.8 18.6 16.4 18.2 17.2 15.5 15.0

17.1 3.8 17.4 14.3 17.6 8.0 9.2 17.2 17.6 19 .o 20.4 19.6 21.8 20.6 22.6 19.5 19.4 19.1 18.7 20.6 19.0 17.5 17.0 22.4 17.9 15.7 17.2 16.2 15.1 14.8

18.1 4.1 18.9 15.9 18.9 9 .o 10.2 18.1 19.0 19.1 20.8 20.3 22.2 21.2 21.2 19.9 20 .o 18.7 18.3 20 .o 19.5 17.4 16 .O 21.2 17.2 15.2 16.8 15.1 13.6 13.4

18.4 4.4 19.7 16.3 19.5 10.1 11.5 18.7 18.7 18.1 20.3 20.0 21.3 19.9 20.4 18.8 18.8 17.7 17.7 18.4 16.7 15.3 15 . O 16.9 14.8 13.7 14.7 13.6 11.6 12.2

19 .o 4.2 18.7 15.9 18.8 11.3 15.0 16.9 16.7 16 . O 17.4 17.6 18.8 17.3 19.3 16.8 16 .O 15.8 16.2 15.4 14.6 13.1 16 .O 14.4 12.2 11.9 13.2 12.1 10.5 12.1

18.0 4.5 18.7 15.9 18.2 13.2 17.4 16.1 15.0 14.6 15.2 15.4 17.2 15.6 17.3 14.6 14.6 13.7 13.5 13.1 12.3 11.3 13.0 12.9 11.1 11.4 12.1 12.0 10.5 12.3

Jul

.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

16.5 5.1 17.8 14.7 16.8 11.9 13.1 14 .O 12.2 11.3

14.3 4.2 17.5 15.4 16.2 11.4 14.7 14.9 14.2 13.5 13.8 13.6 16 . O 13.7 14.8 13.4 12.9 12.5 12.5 12.6 11.5 10.7 11 .o 12.3 11.6 9.8 12.2 12.8 11.3 14.3

14.5 4.2 17.4 15.2 17.0 9.1 11.3 15.6 16.2 15.4 16.7 16.2 19.1 16.8 20.0 15.8 16.5 16.1 15.1 15.9 15.4 12.3 15 .O 14.0 13.8 11.3 13.5 14.0 12.0 13.6

16.4 4.2 16.3 14.1 16.5 7.4 10.1 15.5 16.3 16.3 18.6 17.3 21.3 18.7 20.5 17.9 18.6 19.5 16.3 17.4 18.3 16.3 18.0 19.6 15.7 13.8 16 .O 15.6 13.2 13.8

15.7 3.7 15.7 13.5 16.3 7 .O 8.9 15.9 16.7 17.3 19.3 17.9 21.8 19.3 22.1 18.8 18.5 10.2 18.2 20.4 18.8 17.7 17 .O 22.3 17.5 15.7 17.9 16.6 14.1 14 .O

16.8 4.3 17.8 15 .O 17.5 9.9 12.1 16.1 16.1 15.9 17.2 16.7 19.0 17.3 18.8 16.5 16.4 16.3 15.4 16.6 15.3 14.1 14.4 16.8 14.2 13.0 14.6 14.0 12.3 13.3

~

17.6 5.1 18.4 14.9 17.9 12.8 15.1 14.1 13.6 12.2 13.0 12.9 14.2 13.1 13 . O 12.1 11.9 11.1 10.8 11.2 10.1 10.8 9 .o 11.2 10.3 10.2 11.0 11.2 9.8 11.6

11.5 11.7 13.1 11.9 11.6 11.3 10.8 11 .o 10.4 11.1 8.9

9.7

9 .o 11.1 10.5 9.8 10 . 9 11.4 10.1 12.1

TABLE 3.3

(continued)

S t a ti o n

Lira Butiaba Mas i n d i F o r t Portal Mubende E l d o r et Entebhe Kitale Kericho Mharara I.:ab a 1e Bukoba Musoma Mwanza

Mean d a i l y r a n g e , i n 'C,

.~

tor

Jan.

Feb.

Mar.

Apr.

lay

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

17.2 7 9 14.2 14.6 10.4 16 0 9 8 10 0 15.2 12.6 14.1 10 7 8.8 8 8

16.5 7.5

14 9 7.2 12 8

12 8 7.3 11.5 11.9 9 .0 13 3 7.8 12.7 1: 8 11 . o 11.5 9 4 9.0 9.2

11.7 7.2 10.7 11.2 8.2 12.9 7.7 12 4 11 8 10.8 11.0 9.1 9.4 9.8

11.5 7.3 11.2 12.1 8.3 13.3 8.3

11.2 7.0 10.6 12.2 8.7 11.7 8.8

12.7

11 4

11.5

10.6 12.1 13.2 10.1 9.9 11.8

11.1 13.1 14.5 10.3 10.2 12.4

11.7 6.5 10.7 11.7 8.9 12.1 8.8 11.2 12.0 12.2 13.6 15.3

13.8 7 3 11.7 12.1 8 8 13 8 9 2 11 1 13 5 11 2 12.9 10.1

14.7 7.4 12.2 11.8 9.1 13.5 8.9 10 7 13 7 11.5 12.5 9.9 8.8 9.2

15.5 7.8 12.9 12.8 9.1 14.0 9 0 10.6 12.5 11.5 13.0 10.3 8.8 8.8

13.7 7.3 12.0 12.6 9.2 13.9 8.9 11 0 12.9 11.8 13.1 10.1 9.3 10 . o

14.1

14.6 10.6

16 8 9 7 10.4 15 4 12 5

13.8 10.5 9.0

9.3

1.3 5 10 0 15 9

8 8

11 5

14 6 1 : 8 13 1 10.3 9.2 9.4

9.5

11.2

7.4 11.3 12.2

a .O

14 .Q 9 4 10 . 7 i2 9 11.6 13 6 10.4 9.4 10.5

__

9.4 9.7

__

3.5

ATMOSPHERIC HUMIDITY The a t m o s p h e r i c h u m i d i t y i n t h e N i l e B a s i n area i s e x p r e s s e d m o s t l y i n terms

of

t h e r e l a t i v e h u m i d i t y . I n E g y p t and t h e S u d a n t h i s m e a s u r e m e n t i s made o n c e

or twice a day, except, of course, a t t h e f i r s t - o r d e r

s t a t i o n s . For s t a t i o n s

o b s e r v i n g t w i c e or more a d a y , t h e mean r e l a t i v e h u m i d i t y is t h e mean o f t h e r e l a t i v e h u m i d i t i e s m e a s u r e d a t 08 00 h o u r s and 20 00 h o u r s , and f o r t h o s e o b s e r v i n g o n c e d a i l y , i t i s s i m p l y t h a t m e a s u r e m e n t made a t 08 00 h o u r s . F o r t h e up-graded s t a t i o n s i n t h e c a t c h m e n t s o f t h e E q u a t o r i a l L a k e s , t h e measurements o f t h e r e l a t i v e h u m i d i t y a r e made a t 0 3 0 0 , 0 6 00 and 1 2 00 Z h o u r s , and t h e o t h e r s a r e a t 0 6 00 a n d 1 2 00 Z h o u r s o n l y . The d e v i a t i o n s from t h e mean d a i l y r e l a t i v e h u m i d i t y a v e r a g e d f o r J a n u a r y , A p r i l , J u l y and O c t o b e r f o r A l . e x a n d r i a , E g y p t , Khartoum a n d t h e S u d a n a r e shown i n F i g s . 3 . 5 ( a ) and 3 . 5 ( b ) r e s p e c t i v e l y . The g r a p h s f o r A l e x a n d r i a , and s i m i l a r l y f o r many s t a t i o n s , show t h a t t h e d a i l y mean is r e a c h e d t w i c e e v e r y d a y ; o n c e b e t w e e n 0 7 30 h o u r s and 10 30 h o u r s ( f o r e noon) and a n o t h e r t i m e b e t w e e n 17 30 h o u r s a n d 22 00 h o u r s ( a f t e r n o o n ) , dependi n g o n t h e l o c a t i o n and month o f t h e y e a r . T h i s i s n e a r l y s o e v e r y w h e r e i n t h e N i l e B a s i n , e x c l u d i n g Khartoum ( F i g . 3 . 5 ( b ) ) . G e n e r a l l y s p e a k i n g , t h e r e f o r e ,

in

t h e a b s e n c e of c o n t i n u o u s , o r f r e q u e n t , m e a s u r e m e n t s o f t h e r e l a t i v e h u m i d i t y , t h e a v e r a g e o f t h e o b s e r v a t i o n s made a t 08 00 h o u r s and 20 00 h o u r s , o r s i m p l y t h e o b s e r v a t i o n a t 08 00 h o u r s w i l l n o t b e t o o f a r f r o m t h e t r u e mean ( O l i v i e r , H.,

1 9 6 1 ) . The mean d a i l y r e l a t i v e h u m i d i t y f o r some s t a t i o n s i n E g y p t ,

the

h u m i d i t y a t 08 00 h o u r s i n t h e S u d a n a n d a t 0 6 00 h o u r s e l s e w h e r e , a r e i n c l u d e d i n Table 3 . 4 , The r e l a t i v e h u m id ity a t noon, H n ,

s e e m s t o bear a c e r t a i n relationship t o

t h e d a i l y mean h u m i d i t y , H m , o r t o t h e h u m i d i t y a t 08 00 h o u r s o r any o t h e r r e f e r e n c e hour'. thru'

T h i s r e l a t i o n s h i p i s shown g r a p h i c a l l y i n F i g s . 3 . 6 ( a ) and

( d ) f o r a number o f c o m b i n a t i o n o f s t a t i o n s f r o m some p a r t s o f t h e N i l e

Basin. Of i n t e r e s t i s F i g . 3 . 6 ( c ) , w h i c h shows two d i s t i n c t r e l a t i . o n s h i p s ; one for s t a t i o n s l o c a t e d a l o n g t h e M e d i t e r r a n e a n S e a c o a s t and t h e S u e z C a n a l , and t h e o t h e r f o r t h o s e s t a t i o n s l o c a t e d a l o n g t h e Red S e a c o a s t . F o r v a l u e s of H t h a n 80%, t h e r e l a t i v e h u m i d i t y a t n o o n , H n ,

less

i s much more f o r t h e s t a t i o n s on

t h e Red S e a t h a n f o r t h e s t a t i o n s o n t h e c o a s t o f t h e M e d i t e r r a n e a n S e a . The p l o t o f t h e HO6 h u m i d i t y v e r s u s t h e H12

r e l a t i v e humidity f o r t h e Equatorial

Lake P l a t e a u a r e a d o e s n o t e x h i b i t a s i n g l e r e l a t i o n s h i p f o r a l l t h e s t a t i o n s i n t h e a r e a . Among t h e f a c t o r s i n f l u e n c i n g t h e r e l a t i o n s h i p

are t h e geographic

l o c a t i o n o f t h e s t a t i o n , t h e a l t i t u d e , a n d t h e d i s t a n c e from t h e s t a t i o n t o t h e n e a r e s t l a k e . The g r a p h i c p l o t f o r t h e s t a t i o n s e a s t and s o u t h - e a s t o f Lake + R e l a t i v e h u m i d i t y a t 06 00 Z h o u r s

78

V i c t o r i a p r e s e n t s a much w i d e r s c a t t e r t h a n d o e s t h e p l o t f o r t h e s t a t i o n s i n and w e s t of t h e same l a k e .

t h e n o r t h , north-west

The t i m e s a r e n o t u n i f o r m a c r o s s t h e N i l e B a s i n a r e a and t h e e a r l y o b s e r v a t i o n o f h u m i d i t y i s t a k e n a t a t i m e when t h e r e l a t i v e h u m i d i t y i s c h a n g i n g r a p i d l y . I n some c a s e s t h i s i s done w i t h o u t h a v i n g a r e a d i l y a v a i l a b l e s l m u l taneous temperature t o s p e c i f y t h e c l i m a t e completely, a s i s t h e c a s e with t h e noon h u m i d i t y . I n t h i s c o n n e c t i o n , G r i f f i t h s ,

F.J.

(1972) recommends t h e u s e

Of

o t h e r p a r a m e t e r s t o d e s c r i b e t h e a t m o s p h e r i c h u m i d i t y a s t h e dew p o i n t or t h e a b s o l u t e h u m i d i t y . T h e s e two p a r a m e t e r s do n o t show t h e l a r g e d i u r n a l f l u c t u a t i o n t y p i c a l of t h e r e l a t i v e humidity c u r v e .

p

10

A

0

0

5

+

0

c

o

5

0

€ - 5

-

E

2

-10

ul

-

C .-0

-15

.5_

>

0

2

4

8

6

Local

b

n

10

14

12

standard

16

18

22

20

24

t i m e (hours )

Fig. 3.5(a) D e v i a t i o n s from mean d a i l y r e l a t i v e h u m i d i t i e s f o r A l e x a n d r i a , Egypt ( O l i v i e r , H . , 1961)

$ 20 2' 15

-<

l

~

:

:

0

._....

10

1

. ....... .,

..-.

/'

'.

~

1

..

~

1

~

1

~

1

~

P e r i o d of r e c o r d :

1

~

1

~

1

II

1908 - 1911 & 1920 -1921

0

F

5

-P E ul

o

5-

.c

5

.-0 > - lo aJ n

- 15 - 20

0

2

4

6

8

10

Local s t a n d a r d

1 2 14 16 18 time (hours)

20

22

24

Fig, 3.5(b) D e v i a t i o n s from mean d a i l y r e l a t i v e h u m i d i t i e s f o r Khartoum, Sudan ( O l i v i e r , H . , 1961)

1

1

1

~

TABLE 3.4

The mean d a i l y r e l a t i v e h u m i d i t y a t a number o f s t a t i o n s i n t h e N i l e B a s i n ( G r i f f i t h s , 1972; I r e l a n d , 1948; M i n i s t r y of War and M a r i n e , E g y p t , 1950; W M O ,

-

Station Jan. Salum ( O b s e r v a t o r y ) Mersa Matruh Edfina Port Said Alexandria Tanta Zagazig C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa M i ny a Assiut Qena Qusseir Nag Hammadi Luxor Dakhla Kharga Esna Deadalus I s l a n d Aswan Wadi H a l f a Merowe Atbara Khartoum

71 76 79 76 69 81 83 74 79 68 61 68 70 64 69 63 56 69 68 51 58 61 68 45 48 31 38 29

Feb

72 73 78 75 68 78 79 68 72 66 56 63 64 58 62 56 54 59 58 47 54 54 70 40 40 26 34 24

1974)

Mean d a i l y r e l a t i v e h u m i d i t y , i n p e r c e n t , f o r

Mar.

Apr.

69 74 74 73 68 76 75 65 67 63 52 58 61 52 54 44 52 48 46 41 46 41 74 32 31 20 27 16

66 72 70 73 69 68 67 58 60 60 45 50 56 43 41 31 52 40 34 35 40 32 70 27 24 14 18 15

May

70 74 68 73 72 61 60 52 53 59 41 42 52 39 36 27 52 38 30 32 38 27 77 27 22 15 19 22

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

67 77 70 75 74 63 62 55 56 61 44 46 53 42 37 28 50 40 32 30 38 25 78 25 21 18 24 34

70 81 73 77 77 70 68 61 62 62 51 51 55 46 42 31 52 44 34 30 38 27 77 26 23 23 32 47

76 80 73 76 75 74 73 65 68 65 54 57 58 51 46 32 51 45 37 33 38 31 76 29 26 22 33 52

74 77 71 73 69 77 78 69 72 67 58 62 60 58 55 44 53 54 46 37 44 42 78 33 30 23 30 46

74 75 74 72 69 79 80 72 73 68 59 64 63 61 62 53 56 58 53 43 48 49 79 35 34 22 28 31

73 75 78 73 70 81 82 74 78 69 62 69 66 65 67 59 58 64 60 47 55 52 73 40 39 24 31 30

70 74 79 76 70 80 83 76 81 68 62 72 70 68 69 63 57 71 66 53 60 59 68 45 42 26 34 29

71 76 74 74 71 74 74 66 68 64 54 58 61 54 53 44 54 52 47 40 46 42 74 34 32 22 29 31

m

0

TABLE 3 . 4

(continued) Mean d a i l y r e l a t i v e h u m i d i t y , i n p e r c e n t , for Station

Gallabatx KassalaX P o r t SudanX E l RoseiresX Wad MedaniX E l ObeidX E l FasherX Malakal Addis AbabaX AkoboX W au Jub a ToritX Mongall a Gulu+ Moroto+ Lira+ Butiaba+ Masindi+ Fort Portal+ Mubende+ Eldoret+ Entebbe K i tale+ Kericho+ Mbarara+ Kab a l e + Bukoba+ Musoma+ Mwanza+ x =

Jan.

Feb.

Mar.

Apr.

45 62 66 41 36 37 35 30 61 43 35 43 39 50 63 57 70 68 73 87 77 62 84 70 66 85 94 85 74 77

43 56 65 34 26 28 28 24 64 43 29 41 45 52 66 55 73 67 73 87 77 58 88 76 70 84 95 86 74 78

36 48 64 27 21 23 24 36 58 45 36 50 53 55 73 62 78 70 75 89 81 62 90 73 78 85 96 86 76 79

38 40 56 31 20 26 21 46 65 63 45 63 67 68 83 72 84 73 80 90 86 73 85 80 85 87 97 88 80

r e ? a t i v e h u m i d i t y a t 0 8 h r 30

-

81

May 53 40 45 48 31 41 31 58 63 75 64 85 73 75 85 75 89 74

81

95

88

76 78 83 83 86 95 85 79 77

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

68 49 37 66 48 56 47 75 76 79 73 77 75

78 66 39 79 67 73 65 84 86 84 78 82 79 84 87 79 90 79 86 90 82 82 91 85 85 79 95 76 70 66

80 72 41 83 77 79 74 87 86 85 80 83 79 83 88 76 89 80 86 90 84 82 92 84 81 80 92 82 69 63

77 65 47 80 70 69 61 85 76 84 76 77 74 81 85 69 86 78 83 90 83 72 88 79 78 82 91 82 70 61

67 48 66 70 50 48 37 80 56 78 71 71 72 78 85 66 82 77 81 89 94 66 85 74 74 84 93 81 69 66

54 52 68 49 36 33 31 63 59 71 55 63 62 73 74 63 79 74 78

49 60 69 42 39 36 34 34 62 58 46 52 53 57 70 62 74 71 77 88 84 67 85 72 75 85 95 83 64 76

57 55 55 54 43 46 41 58 68 67 57 66 64 70 79 68 78 74 79 89 83 71 87 78 78 84 96 83 72 72

81

84 75 87 75 83 88 80 79 87 84 84 82 95 79 73 68

+ = r e l a t i v e h u m i d i t y a t 06 00 2 h o u r s

88 81

67 88 73 72 85 94 82 67 68

81

70

LEGEND

o Sakha 60

50

Qurashiya x Gemrneiza Tanta z Zagazig 3 Cairo I Giza Fayurn +

M Minya r Assiut 1 Qena Y N a g - Harnrnadi A Luxor lsna -0. A s w a n

$ 40 C

I

30

20

10

0

Fig. 3.6(a) The mean d a i l y r e l a t i v e h u m i d i t y , H , v e r s u s t h e r e l a t i v e humid i t y a t n o o n , H n , for t h e i n l a n d s t a t i o n s i n Egyp?.

70

LEGEhD

x Karirna 60

+

0 3

v 50

c

1

A

$40

Y

o

Wadi-Halfa r Atbara A Khatourn 2 Kassala I Wad Medani 3 E l Fasher 0 Geneina M El-Obeid z

Gallabat

E l - Nahud

Renk Malakal Akobo Wau Loca Juba Raga

C

I

30

20

10

0 Fig. 3.6(b) The r e l a t i v e h u m i d i t y a t 08 00 h o u r s , Hg8, v e r s u s t h e r e l a t i v e h u m i d i t y a t noon, H for t h e i n l a n d s t a t i o n s i n t h e u d a n . n'

82

0

co

0

h

0 W

0 ul

0 -3

'10 '

H

0

z

0

-

0

m

o

M

5 m

._1

fi

rl 0

X

.

L >

--..-

s

n L L

n

0 n

n >L<

> >> >< <

0 ID

n

L

0

'10

<

'(

0

0 0

0

N

n

0

OOZ1)u H

NI-

O L

0 In

N

00

L

N L

0

n

0

0

Y.

0

n

0

m

m

o m 0 .d I-'

m

E

L)

k w X

6

83

84

R A D I A T I O N , SUNSHINE AND CLOUDINESS

3.6

A l l a t m o s p h e r i c p r o c e s s e s s u c h as t e m p e r a t u r e , p a t t e r n o f b a r o m e t r i c p r e s s u r e , wind f l o w , r a i n f a l l , h u m i d i t y and e v a p o r a t i o n , a r e i n f l u e n c e d d i r e c t l y o r i n d i r e c t l y by t h e f l u x o f t h e s h o r t wave r a d i a t i o n . U n f o r t u n a t e l y , t h e d e n s i t y of t h e network o f r a d i a t i o n s t a t i o n s i n t h e N i l e B a s i n is s t i l l u n s a t i s f a c t o r y . This is t h e s t a t e of a f f a i r s

i n s p i t e o f t h e network s t r e n g t h e n i n g and t h e

u p d a t i n g o f some o f t h e p r e v i o u s l y e x i s t i n g s t a t i o n s i n t h e c a t c h m e n t s of t h e E q u a t o r i a l L a k e s . Many more s u n s h i n e m e a s u r i n g s t a t i o n s a r e , however, a v a i l a b l e than f o r r a d i a t i o n . The g l o b a l r a d i a t i o n ( s o l a r and s k y ) a t t h e t o p of t h e e a r t h ' s atmosphere depends on t h e g e o g r a p h i c l a t i t u d e and t h e t i m e o f t h e y e a r . T h i s r a d i a t i o n , e x p r e s s e d a s A n g o t ' s v a l u e , c a n b e computed from a s t r o n o m i c a l c o n s i d e r a t i o n s , and i s a v a i l a b l e i n some r e f e r e n c e s i n meteorology i n t a b u l a r or g r a p h i c a l form. The monthly Angots f o r l a t i t u d e s from 30°N up t o 10°S w i t h 10'

latitude inter-

v a l s have b e e n q u o t e d and i n c l u d e d i n T a b l e 3.5 j u s t f o r comparison w i t h t h e a c t u a l g l o b a l r a d i a t i o n a t t h e e a r t h ' s s u r f a c e measured a t some l o c a t i o n s i n t h e N i l e Basin a r e a .

TABLE 3.5

S o l a r r a d i a t i o n a t t h e t o p of t h e atmosphere e x p r e s s e d i n Langleys/day*

Latitude

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

30°N

520 660 780 885 965

630 750 840 915 960

775 850 900 925 915

895 920 925 900 840

975 960 915 850 755

1000 965 900 820 710

990 960 905 830 730

925 935 915 870 795

820 875 905 905 875

685 785 865 910 935

560 685 800 890 955

490 630 760 875 960

20°N

10°N Equator 10's

The a c t u a l r a d i a t i o n r e c e i v e d on t h e e a r t h ' s s u r f a c e depends mainly on t h e t h e o r e t i c a l r a d i a t i o n , t h e c l o u d i n e s s and t h e a t m o s p h e r i c t u r b i d i t y . The e s t i mate of t h e a c t u a l r a d i a t i o n , R ,

from t h e r e l a t i v e d u r a t i o n of s u n s h i n e , n/N,

was f i r s t s u g g e s t e d by Angstrom, A .

(1924). S i n c e t h e n t h e e x p r e s s i o n i n common

use i s

R = R

A

(a + b

'

n/N)

(3.2)

where RA = t o t a l r a d i a t i o n r e c e i v e d i f t h e atmosphere were p e r f e c t l y t r a n s p a r e n t .

-.

*Langley/day

= 1 gm cal/cm2/day

85

The o v e r a l l r e g r e s s i o n e q u a t i o n d e v e l o p e d by B l a c k , J . , Bonython, C . ,

and

P r e s c o t t , J . ( 1 9 5 4 ) , f o r 3 2 s t a t i o n s s c a t t e r e d o v e r a r a n g e of l a t i t u d e s from about 7OS t o a b o u t 65ON is

R = RA ( 0 . 2 3

+ 0 . 4 8 n/N)

(3.3)

The same a u t h o r s upon g r o u p i n g t h e 32 s t a t i o n s i n t o s e v e n groups found t h a t t h e r e g r e s s i o n c o n s t a n t , a , v a r i e s from 0.19 t o 0 . 4 0 and t h e s l o p e of t h e r e g r e s s i o n l i n e , b , from 0 . 2 7 4 t o 0 . 6 1 3 . D a r l o t and L e C a r p e n t i e r found a and b a t 0 . 1 8 and 0 . 6 2 r e s p e c t i v e l y , whereas t h e d a t a a n a l y z e d by Woodhead f o r 1 5 s t a t i o n s i n E a s t A f r i c a gave 0 . 2 3 f o r a and 0 . 5 3 f o r b . The e q u a t i o n d e v e l o p e d by G l o v e r , J . , and McCulloch, J . S . t h e e f f e c t o f t h e g e o g r a p h i c a l l a t i t u d e of t h e s t a t i o n ,

R = RA ( 0 . 2 9 c o s

4 +

4,

(1958), includes

on i t . I t r e a d s

0 . 5 2 n/N)

(3.4)

For E a s t A f r i c a w e may t a k e N = 1 2 . 1 (maximum e r r o r cos $ = 1 (maximum e r r o r

-

+

-

0 . 6 ) and

c$

= 0 , so

0 . 0 1 5 ) s o t h a t e q . 3 . 4 becomes

R = R A ( 0 . 2 9 + 0 . 0 4 3 n)

For a n n u a l v a l u e s R R = 250

+ 37 n.

A

(3.5) = 850 gm cal/cm2/day

(maximum e r r o r 1 p e r c e n t ) , so t h a t

The g l o b a l r a d i a t i o n a t t h e e a r t h ' s s u r f a c e , R , i s u s u a l l y measured by t h e Gunn-Bellani

i n s t r u m e n t and t h e d u r a t i o n o f t h e s u n s h i n e i s r e c o r d e d by t h e

Campbell-Stokes

s u n s h i n e r e c o r d e r . The d a t a a v a i l a b l e f o r t h e s e two meteoro-

l o g i c p a r a m e t e r s i n t h e N i l e B a s i n a r e a a r e summarized and g i v e n i n T a b l e s 3 . 6 and 3 . 7 r e s p e c t i v e l y . From t h e d a t a i n T a b l e s 3 . 5 and 3 . 6 t h e r a t i o R / R A was computed f o r 14 s t a t i o n s s c a t t e r e d o v e r t h e r a n g e from a b o u t 2OS t o a b o u t 30°N l a t i t u d e . The r e l a t i v e d u r a t i o n o f b r i g h t s u n s h i n e was computed from T a b l e 3 . 7 f o r t h o s e s t a t i o n s h a v i n g N e q u a l t o 12 h o u r s . For t h e r e m a i n i n g s t a t i o n s , n/N was t a k e n d i r e c t l y from l i t e r a t u r e by O l i v i e r , H . The p l o t t e d p o i n t s o f n/N v e r s u s R/RA

(1961) and WMO ( 1 9 7 4 ) .

a r e shown i n F i g . 3 . 7 . I t i s c l e a r

t h a t the s c a t t e r of t h e p o i n t s is s o l a r g e t h a t a s i n g l e r e g r e s s i o n r e l a t i o n between n/N and R / R A

a p p l y i n g t o a l l t h e s t a t i o n s used h e r e w i l l not be s t a t i s -

t i c a l l y j u s t i f i e d . T h i s s c a t t e r i s p a r t l y due t o t h e d i f f e r e n c e i n t h e t i m e p e r i o d and t h e number o f y e a r s each s e t o f p o i n t s b e l o n g i n g t o a s t a t i o n r e p r e s e n t s . Another r e a s o n c o u l d b e t h a t n o t a l l t h e s u n s h i n e d a t a were measured by t h e same i n s t r u m e n t . I f t h e r e i s a n e e d , however, t o l i n k t h e two v a r i a b l e s n/N

TABLE 3.6

The monthly and annual mean s o l a r r a d i a t i o n a t t h e e a r t h ' s s u r f a c e f o r some r a d i a t i o n s t a t i o n s i n t h e N i l e Basin

Average g l o b a l r a d i a t i o n i n cal/cm2 day f o r Station

Tahrir Giza Wadi H a l f a Port Sudan Khartoum Wad-Medani El-Fasher Tozi Malakal Jub a Culu Lira Masindi Kabale Tororo Namulonge Jinja Entebbe Kericho Mbarara Kisumu Bukoba Mwanza

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

Oct.

Nov.

Dec.

Year

29 3 290 457 354 483 483 457 422 471 458 474 405 39 3 334 456 403 405 474 446 375 490 339 4 33

40 1 375 524 449 557 433 534 474 521 464 468 49 3 423 4 10 50 1 397 450 459 481 408 542 305 502

489 498 588 539 612 576 579 5 16 527 46 2 397 4 37 421 378 473 415 448 445 483 376 462 386 477

568 576 639 6 12 648 602 594 528 536 454 428 459 4 15 35 3 501 423 457 461 470 386 5 10 381 49 1

659 635 667 625 622 584 592 529 5 10 479 4 13 474 400 316 444 405 423 402 397 377 46 3 338 470

684 667 655 577 587 459 550 499 448 459 408 452 376 383 445 408 435 432 437 420 490 403 494

682 663 638 564 571 474 532 466 448 415 378 409 4 15 380 4 26 366 390 39 2 380 411 443 4 27 487

627 610 609 552 56 3 484 534 470 486 466 409 423 4 16 325 409 379 4 19 421 397 418 4 70 448 486

538 533 582 547 56 1 56 2 56 3 504 484 509 46 1 498 444 399 452 4 16 453 433 4 35 444 500 355 492

413 420 525 486 522 533 529 476 487 480 477 528 490 417 483 431 452 445 482 4 16 527 433 496

325 319 46 1 383 488 489 486 415 487 457 486 512 477 347 503 421 447 460 404 404 521 395 445

282 266 426 340 455 466 456 399 476 445 483 485 511 35 1 477 411 432 421 450 4 19 533 386 4 30

497 488 564 502 556 512 534 475 490 46 2 436 464 432 366 464 403 4 34 438 437 404 499 383 475

TABLE 3 . 7

The monthly and annual means of t h e d a i l y s u n s h i n e hours a t c e r t a i n s t a t i o n s i n t h e N i l e Basin ( G r i f f i t h s , 1972; M i n i s t r y of War and Marine, Egypt, 1950; WMO, 1974)

S t a ti o n

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

Aug.

Sep.

Nov

Oct.

.

~

Sallum Mersa Matruh Edfina Port Said Alexandria Tanta Cairo Giza Helwan Wadi H a l f a Karima Atbara Khartoum Kassala P o r t Sudan Wad Medani E l Obeid E l Fasher Malakal Addis Ababa Jimma W au Juba Gulu Soroti Namulonge

7.0 6.5 6.9 6.9 7 .O 7 .O 7.4 7.3 7.6 10.2 9.9 10.1 10.6 10.1 6.9 10.6 10.3 10.3 9.7 8.7 8.0 9.7 9.3 8.9 7.4 6.1

7.5 7.3 7.7 7.3 7.8 7.5 8.1 8.1 8.5 10.4 10.6 10.6 10.7 10.4 8.2 10.6 10.6 10.7 9.3 8.5 6.4 9.3 8.2 8.7 8.4 6 .O

9.0 7.8 8.7 8.6 9 .o 9.2 9.0 8.9 9.4 10.3 10.6 10.5 9.9 10.2 9.1 10.5 9.9 10 .o 9 .o 8.0 6.7 8.4 7.1 8 .O 8 .O 6.1

9.9 10 .o 10.4 9.8 10.6 10.1 10.2 10.2 10.6 10 .o 11.0 12.0 10.4 10.8 10.4 10.8 10.3 10.1 8.3 7.1 6.3 7.6 6.5 7.4 7.5 5.4

9.9 10.7 10.7 10.9 10.9 11.0 10.7 11.o 11.4 11.3 11.4 11.1 10.3 10.5 10.9 10.2 9.8 9.9 7.7 7.0 6.3 8.2 7.8 7.8 7.9 5.7

12.3 11.9 12.2 12.0 11.9 12.2 12.1 12.4 12.8 11.9 10.7 10.4 9.9 10.2 10.3 9.4 8.5 9.2 5.5 5.2 5.1 7.9 7.5 7.7 7.4 5.6

12.7 12.2 12.2 12.2 12.0 12.1 12 .o 12.1 12.6 11.0

11.1 9.9 8.6 8.4 9.9 7.7 7.3 7.7 5.3 2.2 3.4 5.7 5.9 6.2 6.2 5 .O

12.2

11.9 11.9 11.8 11.9 11.6 11.5 11.6 12.1 11.1 9.6 9.5 8.7 8.4 9.6 7.8 6.8 7.4 5.9 2.7 4 .O 6.2 6.9 6.4 6.9 4.9

10.9 10.4 11.o 11.0 11.1 10.7 10.6 10.6 11.1 10 .o 9.6 9.8 10 .o 9.7 10 .o 8.8 8.3 8.6 6 .O 4.6 5.7 7.0 7.5 8.1 7.9 5.2

9.7 8.8 9.5 10.0 9.9 9.4 9.5 9.3 9.8 10.7 10.2 10.2 10.3 10.3 9.9 10 .o 9.3 9.9 7.3 8.6 6.6 7.4 7.6 8.1 8.5 5.4

7.3 8.3 7.9 8.7 8.2 8.0 8.2 8.2 8.6 10.4 10.7 10.3 10.8 10.1 8.3 10.6 10.6 10.9 8.6 8.9 6.2 8.9 7.9 8.2 8.1 5.7

.

Year

6.8 6.6 6.7 6.6 6.7 6.6 6.8 7.2 7.6 9.8 10.5 10.1 10.6 10.0 7.6 10.5 10.5 10.5 10.1 8.6 7.3 9.8 8.1 8.6 8.3 5.7

9.6 9.4 9.7 9.6 9.8 9.6 9.7 9.7 10.2 10.6 10.5 10.4 10.1 9.9 9.3 9.8 9.3 9.6 7.7 6.4 6 .O 8.0 7.5 7.8 7.7 5.5

Dee

__

4 00

m W

TABLE 3.7

( continued)

Station Jan. Kampala Entebbe Eldoret Equator Kericho Kabale Kisumu Mwanza

7.7 7.5 8.9 8.9 8.0 4.8 8.6 7.4

Feb.

7.9 7.2 9.7 9.3 8.1 5.6 8.8 7.7

Mar.

6.6 6.6 8.6 8.6 7.6 5.0 8.5 7.5

Apr.

6.0 6.0 8.1 7.2 5.7 4.3 7.7 8.0

May

6.2 6.2 7.5 7.7 6.4 3.7 7.8 8.2

Jun.

6.3 6.2 7.6 6.7 6.8 5.7 7.5 9.4

Jul.

5.6 6.4 5.9 5.4 5.8 5.5 6.9 9.6

Aug.

5.8 6.3 5.9 5.4 5.6 4.8 6.9 9.0

Sep.

6.1 6.5 7.5 7.4 6.0 5.2 7.6 8.4

Oct. 6.3 6.5 7.8 8.0 5.7 5.1 7.7 7.8

Nov.

6.3 6.6 7.4 7.5 5.6 4.5 7.3 7.0

Dec.

Year

7.1 6.8 8.8 8.3 6.9 4.4 8.2 7.2

6.5 6.6 7.8 7.5 6.5 4.9 7.8 8.1

0.70

0

z

0

z

LEGEND

o Giza 0.65

0.60

A

z Wadi H a l f a v s Port S u d a n 1 o Khartoum y 3 Wad Medani I El- Fasher r + Malakal Juba

.

t

Gulu Namulonge Entebbe Kisumu Kericho Mwanza

-----'::

I Y

z

Y Y

h

0.55

y h'i

6

E

r

3

CK

0.50

O.LE A

0. LC

, 0.45

0.50

I

I

0.55

0.60

I

0.65

nlN

0.70

I

I

I

I

0.75

0.80

0.85

0.90

0.1

Fig. 3.7. The r e l a t i v e d u r a t i o n of b r i g h t s u n s h i n e , n/N, v e r s u s t h e r a t i o of t h e s h o r t wave r a d i a t i o n a t t h e e a r t h ' s s u r f a c e t o t h e s h o r t wave r a d i a t i o n a t t h e t o p boundary o f t h e e a r t h ' s atmosphere f o r 14 s t a t i o n s i n t h e N i l e B a s i n area

OD

W

90 and R/RA f o r t h e whole b a s i n area by a s i n g l e r e g r e s s i o n r e l a t i o n ,

the line

b e s t f i t t e d by e y e w i l l b e t h a t r e p r e s e n t e d by e q . 3.2 ( s e e F i g . 3.7). I n t h e a b s e n c e o f o b s e r v e d s u n s h i n e d a t a , one may t r y t o estimate t h e r a t i o n/N from t h e amount o f s k y c l o u d i n e s s t h a t c o u l d b e a v a i l a b l e a t t h e p l a c e i n q u e s t i o n . The monthly and a n n u a l mean c l o u d i n e s s a t a number o f s t a t i o n s s p r e a d i n t h e N i l e B a s i n a r e a a r e summarized and p r e s e n t e d i n T a b l e 3.8. The monthly v a l u e s , Sc, a r e p l o t t e d v e r s u s t h e c o r r e s p o n d i n g monthly mean r e l a t i v e d u r a t i o n o f t h e b r i g h t s u n s h i n e , n/N,

as shown i n F i g . 3.8. From t h i s f i g u r e i t i s c l e a r

t h a t t h e p o i n t s are so w i d e l y s c a t t e r e d t h a t a f a i r l y a c c u r a t e r e l a t i o n s h i p and n/N f o r a l l s t a t i o n s combined c a n h a r d l y b e d e v e l o p e d . Among t h e

between S

r e a s o n s b e h i n d t h i s w i d e s c a t t e r is t h e i n c o n s i s t e n c y o f t h e s k y c l o u d i n e s s produced by d i f f e r e n c e s i n t h e l e n g t h o f r e c o r d and f r e q u e n c y and p r e c i s i o n w i t h which measurement i s t a k e n . Doorenbos, J . , and P r u i t t , W.O. (1977), s u g g e s t e d t h e u s e o f T a b l e 3.9 as a rough g u i d e f o r c o n v e r t i n g c l o u d i n e s s i n t o e q u i v a l e n t v a l u e s o f n/N. P . (1965), upon i n v e s t i g a t i n g t h e r e l a t i o n s h i p between S

and n/N,

Palayasoot,

suggested t h e

use of t h e e q u a t i o n

n/N = 100 where S

-

1.6 S

-

0.84 Sc2

i s i n t e n t h s on t h e s c a l e 0

-

10.

From t h e d a t a i n c l u d e d i n T a b l e 3.9 and t h e r e s u l t s o b t a i n e d from e q . 3.6, two l i n e s h a v e been drawn, as shown i n F i g . 3.8. The l i n e g i v e n by Doorenbos and P r u i t t i s t h e a p p r o x i m a t e lower e n v e l o p e f o r n e a r l y a l l p l o t t e d p o i n t s , whereas t h e c u r v e d e s c r i b e d by e q . 3.6 forms t h e upper e n v e l o p e o f t h e n/N r a t i o from c l o u d i n e s s from z e r o up t o s l i g h t l y less t h a n 4 o k t a s (1 o k t a = 1 . 2 5 times 1 t e n t h ) . The t h i c k s o l i d l i n e drawn i n F i g . 3 . 8 . a n d which c a n b e d e s c r i b e d by t h e equation 2

n/N = 88

+ 0.43 S

where S

i s i n o k t a s , i s t h e c u r v e f i t t e d by t h e method of l e a s t s q u a r e s t o t h e

- 1 . 1 9 Sc

(3.7)

a n n u a l mean v a l u e s ( n o t shown i n F i g . 3.8.). T h i s c u r v e as judged by e y e seems t o f i t t h e monthly mean v a l u e s w i t h S

above 1 o k t a much b e t t e r t h a n any of t h e

e n v e l o p i n g l i n e s . I n t h e lower h a l f o f t h e g r a p h i t f a l l s i n s i d e t h e band formed by t h e boundary l i n e s , and t h e r e u p o n i t p a s s e s halfway t h r o u g h t h e p l o t t e d p o i n t s . F o r o k t a s more t h a n 4, t h e n/N r a t i o o b t a i n e d from e q . 3.6 i s c l o s e s t t o t h e measurement t h a n t h o s e e s t i m a t e d from t h e o t h e r methods. The m a j o r drawback of e q . 3.6 i s t h a t i t g i v e s f o r S

S

= 8, n/N = 15% i n s t e a d o f 0 % .

= 0 , n/N = 88% i n s t e a d of 100% and f o r

TABLE 3.8

Monthly and a n n u a l mean s k y c l o u d i n e s s a t c e r t a i n s t a t i o n s i n t h e N i l e B a s i n a r e a ( G r i f f i t h s , 1972; M i n i s t r y o f War and Marine, E g y p t , 1 9 5 0 ; WMO,

1974)

Amount of c l o u d i n e s s , i n o k t a s , f o r Station

S i d i Barrani* Sallum ( O b s e r v a t o r y ) D a m i e t taf Rosetta* Mersa Matruh Port Said Alexandria Mansura' Damanhur* Tanta Zagazig D e l t a Barrage* C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa Beni S u e f * Minya Hurghada* A s siut* Qena* Q u s se i r Nag-Hammadi Luxor Dakhla Kharga Esna

* = observed

Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

3.4 3.8 3.3 4.0 3.8 3.2 4.1 2.7 3.4 3.3 2.4 2.8 3.2 3.4 2.7 3.2 2.8 1.4 1.2 1.9 1.6 1.3 1.9 1.4 1.6 2.3 1.4 1.2 1.1

3.5 3.8 2.7 3.5 3.4 2.9 3.7 2.4 2.9 3.3 2.3 2.3 2.8 3.1 2.4 2.9 2.5 1.5 1.1 1.8 1.4 1.0 1.5 1.1 0.7 1.8 1.0 0.9 0.9

2.6 3.1 2.9 3.4 2.9 2.7 3.2 2.2 2.6 2.7 1.8 2.1 2.4 2.8 2.0 2.5 2.1 1.0 0.7 1.6 1.3 0.8 1.6 0.9 1.5 1.8 0.9 0.7 0.9

2.7 2.5 2.5 2.7 2.0 2.3 2.6 1.6 2.2 2.2 1.4 1.8 1.8 2.2 1.6 2.2 1.8 0.9 0.9 1.2 0.8 0.7 1.4 0.7 0.8 1.8 0.8 0.6 0.6

2.2 3.3 2.1 2.3 2.2 1.9 2.4 1.5 1.9 2.0 1.0 1.5 1.7 2.2 1.5 1.9 1.9 1.2 0.4 1.4 0.9 0.8 1.5 0.7 0.8 1.5 0.9 0.7 0.6

1.4 1.0 1.1 1.6 1.2 1.1 1.3 1.o 0.9 0.6 0.5 0.9 0.6 0.9 0.4 0.5 0.4 0.3 0.2 0.2 0.1 0.2 0.3 0.1 0 .o 0.2 0.2 0.2 0.0

1.8 0.5 1.4 1.7 1.o 1.2 1.4 1.5 1.3 1.o 0.7 1.6 0.9 1.0 0.3 0.5 0.3 0.1 0.5 0.2 0 .o 0.1 0.2 0 .o 0 .o 0.3 0.1 0 .o 0.1

a t 0 8 00 h o u r s l o c a l t i m e

Aug.

Sep.

Oct.

Nov.

Dec.

Year

1.9

1.1 0.9 2.1 1.1 1.2 0.5 0.6 0.5 0.1 0.3 0.2 0.1 0 .o 0.2 0.1 0 .o 0.2 0 .o 0.0

1.5 1.3 1.7 2.2 1.4 1.5 1.9 0.9 1.1 1.3 0.9 1.7 1.1 1.6 0.6 0.6 0.5 0.2 0.4 0.3 0.2 0.1 0.2 0 .o 0.0 0.2 0.0 0 .o

2.2 2.4 2.1 2.3 2.1 2.0 2.4 1.4 1.6 1.8 1.2 1.8 1.7 2.0 1.3 1.5 1.3 0.6 0.6 1.1 0.7 0.4 0.9 0.6 0.4 1.o 0.3 0.3

2.4 3.8 3.0 3.4 3.4 2.6 3.4 2.4 2.7 2.8 2.0 2.3 2.5 3.0 2.0 2.4 2.3 1.2 0.6 1.7 1.1 0.8 1.4 0.8 0.8 1.2 0.6 0.6

2.8 3.5 3.4 3.7 3.5 3.2 4.1 2.7 3.2 3.4 2.3 2.6 3.0 3.4 2.6 3.1 3.0 1.7 1.2 2.3 1.9 1.3 2.0 1.1 1.6 2.1 1.1 1.0

2.4 2.4 2.3 2.7 2.3 2.2 2.7 1.8 2.1 2.1 1.4 2.0 1.9 2.2 1.5 1.8 1.6 0.8 0.7 1.2 0.8 0.6 1.1 0.6 0.7 1.2 0.6 0.5

0.2

0.1

0.4

0.5

0.9

0.5

0 .6 1.7 2.1 1.2 1.4 1.6 1.3 1.4

W

to

TABLE 3 . 8

(continued)

Station Deadalus I s l a n d Kom Ombo Aswan Wadi H a l f a + P o r t Sudan+ Dongola' Khartoum+ Kassala+ E l Roseires+ Ma1akal' Addis Ababa Wau+ Jimma++ Juba+

Gulu

Entebbe Equator Kisumu Kabale

+

Amount of c l o u d i n e s s , i n o k t a s , f o r Jan.

Feb.

Mar.

Apr.

May

Jun.

Jul.

2.3 1.2 1.2 1 3

2.0 0.8 1

1.7 0.7 0.9 1 2

1.6 0.5 0.9

1.6 0.9 0.8

0.6 0.1 0.2

1

<1

1

<1 1

<1 1

1

1 1 1

<1

1

2 3 1 3 5 4 4 5 6 6 6 6 7

2 2 2 4 5 5 5 6 6 6 7 6 7

2 2 3 5 5 5 4 6 6 6 6 5 7

3 3 4 6 6 5 5 5 6 6 6 5 6

1 1 2 3 2 3 3 5 6 6 5 7

1

3

1

1 3 4 3 3 4 5 6 5 5 7

1

= mean of 06 0 0 , 12 00 and 18 00 h o u r s l o c a l t i m e

++ = mean of 08 00 and 14 00 h o u r s l o c a l t i m e

Aug.

Sep.

Oct.

Nov.

Dec.

Year

0.8

1.0

0.1 0.2

0.1

1.1 0.1 0.2

1.6 0.3 0.4

1

1

1

<1

2.1 0.4 0.9 1

2 1 4 5 5 6 7 6 7 6 6 6 6 5 6

2 2 5 5 5 6 7 6 6 5 6 6 6 5 6

2 1 4 3 4 6 6 6 5 5 6 6 5 5 7

2.5 0.9 1.0 1 2

1.6 0.5 0.7 1 2

0.3

2

<1 2 2 3 5 4 5 3 5 6 6 6 5 7

3 <1

1 1 1 3 2 4 2

5 5 6 6

6 7

<1 1 1 1 2 2 3 2 4 5 6 6 5 7

1 2.3 2 .o 3.0 4.3 4.7 4.5 4.1 4.9 5.7 6 .O 5.9 5.2 6.8

L

\ z 0

. -

\

r s

i v A

L

9

\l"a5

1-4.

,

.,':'.

r

+

'\

LEGEND

o Sallum z Port S a i d + Alexandria x Giza 3 E l Kharga I Port S u d a n Y E l Khartoum

s I

\ x

Kassala Malakal Wau Juba Gulu Entebbe Kisumu

..

\.

045

Fig. 3 . 8 . Basin area

0.50

'.

Y Y

5 I

\.

I

I

I

I

I

I

I

0.55

0.60

0.65

0.70

0.75

0.80

0.85

nlN

'.

'.-.

L 2

0.90

0.95

The amount o f s k y c l o u d i n e s s v e r s u s t h e r e l a t i v e d u r a t i o n of b r i g h t sunshine f o r 14 s t a t i o n s i n t h e N i l e

W W

94

TABLE 3 . 9

Conversion of sky c l o u d i n e s s , Sc, i n t o r e l a t i v e d u r a t i o n of b r i g h t s u n s h i n e , n/N,

S c , o k t a s (0-8) n/N r a t i o S c , t e n t h s (0-10) n/N r a t i o 3.7

as s u g g e s t e d by Doorenbos and P r u i t t (1977)

0

1

2

3

4

5

6

7

8

.95

.85

.75

.65

.55

.45

.30

.15

-

0

1

2

3

4

5

6

7

8

9

10

.95

.85

.80

.75

.65

.55

.50

.40

.30

.15

-

WIND

I n t h i s s e c t i o n w e s h a l l t r y t o draw a s k e t c h f o r t h e s u r f a c e winds blowing i n t h e N i l e B a s i n area. Wind i s u s u a l l y measured a t a b o u t 2 metres from t h e e a r t h ’ s s u r f a c e by means o f a t m o m e t e r s . The wind c h a r a c t e r i s t i c s a t a h e i g h t o f 1 0 metres above t h e s u r f a c e a r e a u t o m a t i c a l l y r e c o r d e d a t a l i m i t e d number o f p l a c e s by means of m e c h a n i c a l r e c o r d i n g anemographs. The wind f o r c e i s g i v e n i n t e r m s of numbers on t h e B e a u f o r t S c a l e when t h e f o r c e i s e s t i m a t e d , o r i n u n i t s of v e l o c i t y as m / s e c o r km/hr,

when t h e v e l o c i t y

is measured by an anemometer. The f r e q u e n c y w i t h which t h i s v e l o c i t y i s measured v a r i e s from o n e c o u n t r y t o a n o t h e r , and f o r t h e same c o u n t r y from one g r o u p of s t a t i o n s t o a n o t h e r . I n Egypt, t h e wind f o r c e i s e s t i m a t e d o n c e a t 0 8 00 h o u r s l o c a ? t i m e , whereas t h e v e l o c i t y measurements are t a k e n a t 0 8 00, 14 00 and 20 00 h o u r s l o c a l t i m e and t h e mean g i v e n . I n t h e Sudan, t h e wind s p e e d and d i r e c t i o n are measured a t 0 7 00 and 1 3 00 h o u r s l o c a l t i m e .

I n east A f r i c a ,

wind measurements a r e t a k e n a t 06 00 Z and 1 2 00 Z ( 0 9 00 and 1 5 00 h o u r s l o c a l t i m e ) . A summary o f t h e wind s p e e d , p r e v a i l i n g d i r e c t i o n and p e r c e n t a g e f r e q u e n c y of calm c o n d i t i o n s i n E g y p t , Sudan and E t h i o p i a , Uganda and Kenya, i s g i v e n i n T a b l e s 3 . 1 0 ( a ) , ( b ) and ( c ) r e s p e c t i v e l y . I n E a s t A f r i c a , t h e v e l o c i t y measured a t a h e i g h t o f 10 metres above t h e s u r f a c e does n o t compare t o t h e f a s t b l o w i n g wind o f t e n n o t e d i n t h e t e m p e r a t e z o n e s . The monthly mean v a l u e h a r d l y r e a c h e s 10 m/sec. The maximum g u s t r e c o r d e d i s a b o u t 30 m/sec a t Kisumu, w h i l e v a l u e s above 18 m/sec are rare and seldom l a s t f o r more t h a n a m i n u t e or two ( G r i f f i t h s , J . F . ,

1971).

The s e a s o n a l v a r i a t i o n i n t h e wind s p e e d i s n o t s t r o n g e v e r y w h e r e . I n t h e h i g h l y e l e v a t e d p l a c e s s u c h as Addis Ababa, Jimma and E q u a t o r , t h e r a t i o of t h e maximum t o t h e minimum monthly mean i s between 2 . 7 t o 1 and 2.0 t o 1. Low elev a t e d s t a t i o n s s u c h a s G u l u , E n t e b b e , Kisumu and Kabale show t h e maximum t o minimum r a t i o of between 1 . 5 t o 1 and 1 . 1 5 t o 1. I n g e n e r a l , t h e wind blows f a s t e r i n t h e e a r l y s p r i n g and i n autumn t h a n i n t h e r e m a i n i n g s e a s o n s o f t h e year.

95

F o r a l l s t a t i o n s i n c l u d e d i n T a b l e 3 . 1 0 ( c ) and f o r o t h e r E a s t A f r i c a n s t a t i o n s which are n o t i n c l u d e d i n t h i s t a b l e , t h e wind v e l o c i t y measured a t

12 00 Z ( s e c o n d measurement) i s more t h a n t h a t measured a t 06 00 Z ( f i r s t measurement). The r a t i o

between t h e s e two measurements v a r i e s from 1 . 0 5 : l f o r

t h e a n n u a l mean a t t h e E q u a t o r t o 2.72 a t K a b a l e . The magnitude o f t h i s r a t i o depends, among o t h e r f a c t o r s , on t h e a l t i t u d e o f t h e p o i n t i n q u e s t i o n and i t s d i s t a n c e t o t h e n e a r e s t l a k e s h o r e . The p r e v a i l i n g wind a l o n g t h e s h o r e of Lake V i c t o r i a c h a n g e s i t s main d i r e c t i o n from s o u t h - e a s t

a t t he southern p a r t of t he

w e s t e r n s h o r e t o s o u t h a t t h e n o r t h e r n p a r t o f t h e same s h o r e , and t o s o u t h - w e s t a t t h e n o r t h e r n p a r t o f t h e e a s t e r n s h o r e . North of Lake V i c t o r i a , t h e d i r e c t i o n of t h e p r e v a i l i n g wind s w i n g s between t h e e a s t and t h e n o r t h . A more d e t a i l e d review o f t h e c h a r a c t e r i s t i c s o f t h e s u r f a c e winds i n t h e E q u a t o r i a l Lake P l a t e a u c a n b e found i n t h e r e p o r t on t h e H y d r o m e t e o r o l o g i c a l Survey o f t h e Catchments o f Lakes V i c t o r i a , Kyoga and A l b e r t ((WMO,

1974).

The wind b l o w i n g on t h e E t h i o p i a n h i g h l a n d s i s c h a r a c t e r i z e d by t h e c o n s i d e r a b l e v a r i a t i o n i n t h e p e r c e n t a g e of calm c o n d i t i o n s d u r i n g t h e y e a r w i t h t h e l o c a l c o n d i t i o n s (compare Addis Ababa t o Jimma, T a b l e 3 . 1 0 ( c ) ) . G e n e r a l l y speaki n g , however, t h e p e r c e n t a g e o f calm c o n d i t i o n i s g r e a t e s t a t t h e e a r l y morning r e a d i n g ( 0 6 00 h o u r s ) . A t Addis Ababa i t r e a c h e s t h e maximum i n May and t h e minimum i n November, b o t h a t t h e e a r l y morning r e a d i n g and i n J u l y and O c t o b e r r e s p e c t i v e l y , b o t h a t t h e midday r e a d i n g . A t Jimma i t i s autumn and w i n t e r which e n j o y t h e g r e a t e s t p e r c e n t a g e of t h e calm c o n d i t i o n . Wind d i r e c t i o n s a r e g e n e r a l l y d o m i n a n t l y e a s t t o s o u t h - e a s t e r l y d u r i n g t h e d r y s e a s o n b u t v e e r around t o w e s t e r l y o r n o r t h - w e s t e r l y

i n t h e rainy season.

The a n n u a l mean v e l o c i t y of t h e s u r f a c e wind o v e r t h e Sudan i s g e n e r a l l y n o t high.

I t i n c r e a s e s s l i g h t l y from Wadi H a l f a t o Khartoum and from t h e r e d e c r e a s e s

r a t h e r r a p i d l y i n t h e d i r e c t i o n t o t h e s o u t h . To t h e e a s t , e x c e p t on t h e Red S e a c o a s t , and t o t h e w e s t ,

t h e wind s p e e d i s , on a v e r a g e , less t h a n a t t h o s e

s t a t i o n s o n , o r n e a r , t h e N i l e and i t s main t r i b u t a r i e s . From t h e d a t a l i s t e d i n T a b l e 3 . 1 0 ( b ) i t i s c l e a r t h a t t h e maximum a v e r a g e speed o c c u r s i n t h e w i n t e r t i m e ,

e x c e p t f o r Wadi H a l f a and K a s s s a l a as i t o c c u r s

i n t h e s p r i n g and summer, r e s p e c t i v e l y . The minimum o c c u r s i n t h e summer everywhere, e x c e p t a t K a s s a l a where l a t e autumn is t h e t i m e o f o c c u r r e n c e of t h e minimum o f t h e a v e r a g e wind s p e e d . The s e a s o n a l e f f e c t i s q u i t e s m a l l , e x c e p t f o r Malakal and K a s s a l a where t h e r a t i o of t h e maximum t o t h e minimum, b o t h a t a v e r a g e s p e e d , i s a b o u t 2.7 o r 2.8 t o 1. The p r e v a i l i n g wind c h a n g e s i t s d i r e c t i o n from n o r t h a t H a l f a t o n o r t h - w e s t a t Dongola and t h e n back t o n o r t h a t Khartoum e x c e p t d u r i n g t h e summer s e a s o n where t h e d i r e c t i o n becomes m o s t l y s o u t h and s o u t h - s o u t h - w e s t . t h e "haboob"

( A r a b i c word meaning " b l o w i n g " ) ,

I n t h i s period

which r e f e r s m o s t l y t o d u s t s t o m

96

or s a n d s t o r m , o c c u r s r a t h e r f r e q u e n t l y . T h i s wind a p p l i e s t o a r a p i d l y - m o v i n g and u n s t a b l e t y p e of a i r mass a s s o c i a t e d w i t h t h u n d e r a c t i v i t y and r e l a t e d t o a d v a n c i n g or e a r l y monsoon. The a v e r a g e number o f d u s t s t o r m s r e a c h e s a maximum around Khartoum w i t h a v e r a g e s 4 , 6 , 4 f o r May, J u n e and J u l y , r e s p e c t i v e l y .

At

A t b a r a t h e maximum o f 5 o c c u r s i n August. Along t h e d i a g o n a l j o i n i n g K a s s a l a and Malakal and on b o t h s i d e s of i t s e x t e n s i o n , i . e . J u b a and Wau, t h e n o r t h - n o r t h - e a s t

wind p r e v a i l s i n t h e w i n t e r

t i m e . D u r i n g t h e s p r i n g , summer and p a r t o f t h e autumn, t h e s o u t h e r l i e s and t h e south-westerlies prevail. The u n u s u a l n a t u r e of t h e d i u r n a l wind v a r i a t i o n , i . e . t h e d e c r e a s e o f t h e wind s p e e d from morning t o midday was i n v e s t i g a t e d by F a r q u h a r s o n , J . S . H e concluded t h a t t h i s t y p e

(1939).

o f v a r i a t i o n i s a c h a r a c t e r i s t i c of t r o p i c a l A f r i c a

and n o t o n l y of C e n t r a l Sudan. I n h i s comment on t h a t work, S u t t o n p r e s e n t e d t h e two maps which a r e shown h e r e i n F i g s .

3 . 9 ( a ) and 3 . 9 ( b ) . T h e s e maps show t h a t

t h e u n u s u a l t y p e o f d i u r n a l v a r i a t i o c o c c u r s i n an e x t e n s i v e a r e a between l a t i t u d e s 20°N and S . W i t h i n t h e s e l a t i t u d e s t h e u s u a l t y p e of d i u r n a l v a r i a t i o n c o v e r s , however, l a r g e r e g i o n s ( S u t t o n , L . J . ,

1 9 3 9 ) . S u t t o n added f u r t h e r t h a t

i n some p a r t s o f t h e Sudan t h e r e are l a r g e s e a s o n a l v a r i a t i o n s . For example, i n t h e n o r t h (Merowe) t h e morning wind i s s t r o n g e r t h a n t h a t i n t h e a f t e r n o o n o n l y from August t o O c t o b e r d u r i n g t h e SW monsoon. I n t h e s o u t h ( M a l a k a l , N a s s e r ) , t h e morning wind i s s t r o n g e r t h a n t h e a f t e r n o o n wind o n l y from December t o March d u r i n g t h e p e r i o d o f NE w i n d s . But i n t h e s o u t h - w e s t ,

Bahr e l Ghazal P r o v i n c e

( A w e i l , Raga, Wau) t h e morning wind i s weaker t h a n t h e a f t e r n o o n wind a l l t h e y e a r r o u n d . The c o n t r a s t i n t h e b e h a v i o u r o f t h e wind i n t h e s e s o u t h and s o u t h -

w e s t r e g i o n s , which form a c o n t i n u o u s p l a i n i n t h e same l a t i t u d e s , d o e s n o t seem a t t r i b u t a b l e t o l o c a l v a r i a t i o n s of t o p o g r a p h y . I n t h e w i n t e r season t h e s t a t i o n s s i t u a t e d along t h e c o a s t of t h e M e d i t e r r a n e a n S e a i n Egypt and t h e s t a t i o n s i n t h e N i l e D e l t a a r e s t r o n g l y a f f e c t e d by t h e s o - c a l l e d M e d i t e r r a n e a n d e p r e s s i o n s . A t S a l l u m , which i s l o c a t e d a t t h e r e a r o f t h e t r a c k o f t h e s e d e p r e s s i o n s , t h e p r e v a i l i n g wind d i r e c t i o n i s

w e s t t o north-west.

Those p l a c e s l o c a t e d i n f r o n t of t h e d e p r e s s i o n s , . l i k e

A l e x a n d r i a and Q u r a s h i y a r e c e i v e s o u t h - w e s t e r l y winds ( s e e T a b l e 3 . 1 0 ( a ) ) . Upper Egypt and t h e n o r t h e r n Red S e a a r e a a r e p r a c t i c a l l y u n a f f e c t e d by t h e M e d i t e r r a n e a n d e p r e s s i o n s and a r e , f o r most o f t h e t i m e , u n d e r t h e e a s t e r n f l a n k o f t h e s u b - t r o p i c a l h i g h p r e s s u r e c e l l t h a t c o v e r s t h e w e s t e r n d e s e r t of Egypt (Stamp, D . L .

and Morgan, W.T.W.,

1 9 7 2 ) . The p r e v a i l i n g winds a r e , t h e r e f o r e ,

n o r t h e r l i e s (see d a t a o f Helwan, Minya and Luxor i n T a b l e 3 . 1 0 ( a ) ) . I n t h e summer s e a s o n t h e n o r t h e r l y winds p r e v a i l o v e r most o f E g y p t . The n o r t h e r n p a r t o f t h e N i l e D e l t a a r e a and w e s t o f i t up t o A l e x a n d r i a a r e an e x c e p t i o n t o t h i s r u l e . The n o r t h - w e s t

d i r e c t i o n usually p r e v a i l s .

97

The d i r e c t i o n o f t h e p r e v a i l i n g wind o v e r Lower Egypt changes i n t h e s p r i n g and autumn t o n o r t h - e a s t

b e c a u s e of t h e p a s s a g e of t h e c e n t r e of t h e Khamasin

d e p r e s s i o n s ( d u s t s t o r m s blowing from t h e w e s t e r n q u a d r a n t and l a s t i n g f i f t y days) s o u t h of t h e M e d i t e r r a n e a n c o a s t l i n e . The mean a n n u a l wind s p e e d d e c r e a s e s a l o n g t h e M e d i t e r r a n e a n c o a s t from w e s t to east.

I t a l s o d e c r e a s e s from n o r t h t o s o u t h . The n o r t h e r n p a r t of t h e Red

Sea c o a s t i s a r e l a t i v e l y windy a r e a . The wind c o n d i t i o n s i n l a n d a r e a d j u s t e d , however, by t h e l o c a l t o p o g r a p h y .

Late w i n t e r and e a r l y s p r i n g a r e t h e s e a s o n s i n which g a l e s and s t r o n g winds t a k e p l a c e i n L o w e r E g y p t . The f r e q u e n c y of t h e s e g a l e s d e c r e a s e s c o n s i d e r a b l y along t h e north-south

d i r e c t i o n . I n g e n e r a l , autumn, e s p e c i a l l y O c t o b e r , i s t h e

t i m e o f l o w e s t wind s p e e d .

TABLE 3.10(a)

Month and Year

Wind s p e e d and d i r e c t i o n a t c e r t a i n s t a t i o n s i n Egypt ( M i n i s t r y of War and Marine, E g y p t , 1950)

Avg. speed, km/hr

Prev. direction

Calm, 96

Sallum (Observatory) January February March April May June July August September October Novemh e r December Year Month and Year January February March April May June July August September October November December Year

19.9 20.4 20.3 20.0 17.1 19.8 24.9 22.6 19.3 16.1 16.6 19.6 19.7

W

Nw N

N NE N N N N N

Nw Nw N

5.5 4.8 5.4 4.6 4.2 3.2 1.3 1.7 2.4 6.7 6.1 4.6 4.2

Avg. speed, km/hr

sw NE NE NE NE N N N N NE NE NE NE

8.8 7.3 5.0 2.5 3.7 2.9 6.2 6.9 12.4 15.7 14.4 13.1 8.2

Calm, %

Avg. speed, km/hr

Port Said (Airport)

15.6 14.2 17.9 15.3 14.3 14.7 14.5 11.8 11.3 11.8 13.7 11.7 13.9

C a i r o (Almaza A i r p o r t )

12.3 11.5 14.2 13.5 13.0 11.3 7.7 8.5 8.7 9 .o 9.8 9.4 10.9

Prev. direction

W

W

N N N N N N N N N W N

8.4 7.8 1.7 2.5 3.2 2.4 1.3 3.2 4.6 5.8 5.3 10.1 4.7

9 .o

Nw Nw N N N

N Nw N

N N N N N

Calm, ~

15.8 16.4 16.3 15.5 14 .O 15.3 16.2 14.9 13.1 11.1 12.0 12.8 14.4

sw Nw NE NE NE Nw

Nw Nw N NE NE

sw Nw

13.1 14.4 17.2 19 .o 20 .o 20.2 18.1 18 .O 19 .o 18.4 15.5 12.6 17.1

N N N N N N

Prev. direction

Calm, %

Qurashiya

11.6 9.0 7.1 4.6 6.1 4.6 1.3 3.8 5.2 9.9 11.5 15.6 7.5

Helwan

26.4 22.6 16.9 14.4 15.6 10.5 11 .o 14.1 15.6 19.3 24.4 30.7 18.5

Avg. speed, km/hr

Alexandria

Giza

7.6 8.6 9.8 9.9 10.5 10.5 9.7 9.6 9.4 8.2 7.4 6.8

Prev. direction

4.9 5.7 5.5 5.9 5.4 5.6 5.4 4 .O 2.9 2.3 3.0 4.2 4.6

sw sw SE SE SE SE Nw

sw Nw

SE

sw sw sw

53.0 45.6 49.5 43.1 49.7 42.6 44.7 57.2 71 .O 76.0 71.3 59.4 55.3

Minya ( A i r p o r t )

N N N

2.5 3.5 2.3 1.2 0.3 0.2 0.2 0.5 0.7 2.7

6.4 6.8 9.4 9.6 10.7 12.7 9.5 7.7 8.3 6.9

N N N N N N N N N

N

23.9 20.8 11.0 13.7 11.1 7.1 16.8 23.2 17.1 23.6

N N N

4.6 3.7 1.9

7.1 5.6 8.4

N N N

27.3 21.5 18.1

Nw

TABLE 3.10(a)

Month

and Year

(continued)

Avg. speed, km/hr

Prev. direction

Calm, %

Avg. speed, km/hr

Qusseir January February March April May June July August September October November December Year

18.0 18 .o 18.0 18.0 18 .o 14.4 14.4 18.0 14.4 18.0 14.4 14.4 16.5

w w N N N N N N N N N/W

w

N

Prev. direction

Calm,

Avg. speed, km/hr

Luxor

4.2 5.3 6.1 9.3 9.4 7.2 10.8 13.2 8.9 7.2 2.8 2.1 7.5

6.8 7.2 7.9 7.2 7.6 7.6 7.2 7.6 6.9 5.8 6.1 6.5 7 .O

N N Nw Nw Nw Nw Nw Nw N N N N N/NW

Prev. direction

Calm, %

Avg. speed, km/hr

Dakhla

28.2 27.6 14.3 14.1 13.9 12.7 13.0 13.2 18.6 19 .o 19.3 27.5 18.5

6.3 5.9 6.8 7.2 6.8 7.2 7.7 7.7 8.1 6.8 5.4 5.9 6.8

Nw Nw Nw Nw Nw Nw

Nw Nw Nw Nw Nw

Nw Nw

Prev. direction

Calm,

%

Aswan

23.8 23 .O 15.7 12.8 10.6 9.4 6.4 6.9 5.8 11.1 16.5 19.8 13.5

7.7 7.7 8.6 8.6 8.1 8.5 7.2 7.2 8.1 7.7 6.8 7.2 7.8

N N N N N N N N N N N N N

48 . O 43.0 36.4 35.9 42.8 36.1 52.3 41.0 29.6 37.9 46 . O 45.6 41.2

W

W

TABLE 3.10(b)

Month

and Year

Wind s p e e d and d i r e c t i o n a t c e r t a i n s t a t i o n s i n t h e Sudan ( G r i f f i t h s , 1972)

Avg. speed,

Prev. direc-

km/hr

tion

Calm,

%

Avg. speed, km/hr

Prevailing direction

07 h r

Wadi H a l f a January February March April May June July August September October November December Year

14 15 16 16 16 15 14 13 14 14 13 11 14

N N N N N N N N N N N N N

13 h r

Calm, %

07 h r

13 h r

percentage frequency of avg. wind s p e e d

1-3

4-7

P o r t Sudan

5 4 3 6 5 6 10 9 4 3 4 6 5

18.5 18.5 16.7 16.7 14.8 13.0 14.7 13.0 13.0 13.0 14.9 16.7 15.3

Nw Nw Nw Nw Nw Nw

sw sw Nw Nw Nw Nw Nw

NE NE NE NE NE NE E E NE NE NE NE NE

Prev. direction

Dongola

2 3 9 15 23 33 30 23 33 6 1 4 15

0 0

0

0 0

1 1 0 0 0

0

0 0

97 96 92 93 94 95 94 94 95 96 94 96 95

2 3 8 5 4 4 4 4 5 4 6 4 5

NE NE NE NE NE NE NE NE NE NE NE NE NE

~

Calm, %

TABLE 3.10(b)

Month and Year

(continued) Avg. speed, km/hr

Prev. direction

Calm, ~

a

Avg. speed, km/hr

Khartoum January February March April May June July August September October November December Year

16 16 15 14 16 14 16 15 11 13 14 16 15

Month and Year January February March Apri 1 May June July August September October November December Year

N N

S

N N N N

1 0 0 1

S

ssw

Jub a

5 5 5 5 5 3 3 3 3 3 3 3 4

"E "E S S

S S S S S

S

S "E S

Calm, 111

b

Avg. speed, km/hr

Kassala

<1 <1 1 1 2 2 1 2 3

N N N S

Prev. direction

36 29 21 17 19 30 31 26 27 23 29 32 27

-

5 5 5 5 5 6 8 6 5 3 3 5 5

"E NNE "E N

S S S S S S

"E "E

-

Prev. direction

Calm, (I

B

Avg. speed, km/hr

Malakal

6 6 6 7 12 9 3 5 9 12 8 7 7

14 14 10 10 10 8 8 5 5 5 8 12 9

N NE

"E N S S S S S

V

V N "E

Prev. direction

Calm, 10

W au

2 5 6 8 4 8 10 13 15 14 9 5 8

5 3 5 5 5 3 3 3 3 3 3 3 4

"E "E V S

S

sw sw sw V V

NNE "E

34 29 28 23 23 28 33 35 35 38 43 34 32

TABLE 3.10(c)

Month and Year

Wind speed and d i r e c t i o n a t c e r t a i n s t a t i o n s i n E t h i o p i a , Kenya and Uganda ( G r i f f i t h s , 1972; WMO, 1974) Avg. speed, km/hr

Prev. direction

06 h r

% 12 h r

Avg. speed,

km/hr

Prev. direct ion

11.1 9.3 11.1 13.0 11.1 9.3 7.4 7.4 9.3 13.0 14.8 13.0 10.9

SSE E E E E

S S S S E

SE E

-

56 51 40 41 64 52 53 51 57 33 29 40 47

Avg. s p e e d , km/hr 09 h r 15 h r

Jimma

Addis Ababa January February March April May June July August September October November December Year

Calm, % 06 h r 12 h r

12 10 11 10 16 23 39 13 21 6 7 8 15

7.4 9.3 7.4 5.6 7.4 5.6 7.4 5.6 3.7 5.6 3.7 6.2

E

SE E E SE SSE SE E

E

SSE E

-

Prev. direction

Calm, %

Gulu

97 92 87 87 88 95 87 83 95 97 96 92

53 42 56 47 49 54 63 54 50 64 76 56

13.0 13.0 13.0 14.8 13.0 11.1 11.1 11.1 11.1 13.0 13.0 13.0 12.5

18.5 18.5 18.5 14.8 14.8 13.0 13.0 13.0 14.8 14.8 14.8 16.7 15.4

N/E N E

S

s s

N N N E E

E

-

0 0

0 0 0 0 1 0 0 0 0 0 0

TABLE 3.10(~)

Month and Year

(continued) Avg. s p e e d , km/hr 09 h r 15 h r

Prev. direction

Calm,

’

Avg. s p e e d , km/hr 09 h r 15 h r

Entebbe January February March April May June July August September October November December Year

7.4 7.4 9.3 11.1 11.1 11.1 11.1 9.3 11.1 9.3 7.4 7.4 9.4

18.5 22.2 22.2 18.5 18.5 22.2 18.5 18.5 18.5 18.5 18.5 16.7 19.3

5.6 3.7 3.7 3.7 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.6 5.1

13.0 13.0 13.0 13.0 13.0 14.8 14.8 14.8 14.8 14.8 14.8 13.0 13.9

Month and Year January February March April May June July August September October November December Year

Prev. direction

Calm,

’

Avg. s p e e d , km/hr 09 h r 15 h r

Equator

S S S S S S S S

S S S S

s

0 0 0 0 0 0 1 1 0 1 1 1 <1

Kabale NE/SE NE/SE E/SE SE SE SE SE SE SE SE E/SE E/SE

-

1 0 0 0 0 0 0 0 0 0 0 0 0

31.5 29.6 31.5 25.9 20.4 14.8 13.0 13.0 16.7 25.9 33.3 35.2 24.2

27.8 29.6 31.5 27.8 25.9 22.2 18.5 16.7 20.4 24.1 29.6 29.6 25.3

Prev. direction

Calm, %

Kisumu E E E E E E

W W

E E E E

-

0 0 0

0 1 1 4 2 1 0 0

0 1

5.6 5.6 5.6 5.6 5.6 7.4 7.4 7.4 7.4 7.4 5.6 5.6 6.4

22.2 24.1 20.4 18.5 14.8 14.8 14.8 16.7 18.5 18.5 18.5 18.5 18.4

SW SW SW SW SW SW SW SW S SW SW SW SW

1 0 1 1 1 0 1 1 0 0 1 1 1

104

al m

h

m

m

5

c 0 0

m 4J

0

0

c 0 M

c

4J

a

al

0

m 'CI

a

m m k

0

m 3

c

?

m

105

3.8.

CIRCULATION OF A I R MASSES

The g e n e r a l c i r c u l a t i o n o f t h e atmosphere i n A f r i c a i s dominated by two sub-

t r o p i c a l h i g h - p r e s s u r e b e l t s and t h e e q u a t o r i a l low-pressure b e l t between. These b e l t s move northwards i n t h e n o r t h e r n summer, l a g g i n g behind t h e overhead s u n , and southwards i n t h e s o u t h e r n summer. The c i r c u l a t i o n o f t h e a i r masses i n t h e N i l e B a s i n area i s p a r t o f t h e g e n e r a l c i r c u l a t i o n o v e r t h e c o n t i n e n t , modified

by t h e l o c a l c o n d i t i o n s . The two h e m i s p h e r i c a n t i c y c l o n e s which e x t e n d from l a t i t u d e s 30° n o r t h and s o u t h t o t h e E q u a t o r , c o n t r o l t h e wind systems i n t h e e q u a t o r i a l r e g i o n and t h e t r o p i c s . The n o r t h a n d . s o u t h a n t i c y c l o n e s a r e most i n t e n s e d u r i n g t h e n o r t h e r n w i n t e r and summer r e s p e c t i v e l y . The E q u a t o r i a l Lake P l a t e a u is covered by a p a r t of t h e e q u a t o r i a l trough which e x t e n d s o v e r C e n t r a l A f r i c a and where winds a r e g e n e r a l l y l i g h t and variable.

The e a s t e r l i e s , n o r t h - e a s t

and s o u t h - e a s t monsoons, p r e v a i l o v e r t h e

Lake P l a t e a u a r e a . The r e s u l t i n g w e a t h e r p a t t e r n , however, i s s i g n i f i c a n t l y

modified by t h e w e s t e r l i e s . The l a t t e r a r e known t o b e a r a i n - b r i n g i n g a g e n t . The n o r t h - e a s t monsoon o r i g i n a t e s from t h e n o r t h s u b - t r o p i c a l

anticyclone.

I t flows towards lower l a t i t u d e s when t h e s u n i s s o u t h of t h e E q u a t o r , from October t o March. I t i s t h a t segment which o r i g i n a t e s o v e r Saudi Arabia w h i l e t r a v e l l i n g o v e r t h e Red Sea and p a r t o f t h e I n d i a n Ocean t h a t a c q u i r e s some m o i s t u r e as i t goes down t o lower l a t i t u d e s . T h i s a i r mass p r e v a i l s o v e r t h e Lake P l a t e a u . The s o u t h - e a s t monsoon t a k e s p l a c e from A p r i l t o September and o r i g i n a t e s as a n e a s t e r l y wind from t h e s o u t h - t r o p i c a l

a n t i c y c l o n e over t h e south Indian

Ocean. By t h e t i m e t h i s a i r m a s s e n t e r s t h e e a s t e r n s e a b o a r d o f A f r i c a i t already h a s a maritime t r a j e c t o r y . T h e w e s t e r l i e s c o v e r a narrow b e l t s t r e t c h i n g from t h e e q u a t o r i a l r e g i o n s of

South America t o e q u a t o r i a l A f r i c a . They i n t r u d e i n t o t h e c i r c u l a t i o n e s t a b l i s h e d by t h e e a s t e r l i e s and p l a y a r o l e i n t h e change of t h e w e a t h e r p a t t e r n s . The n o r t h - e a s t monsoon c r o s s e s t h e Equator d u r i n g t h e w i n t e r i n t h e n o r t h e r n hemisphere and under t h e i n f l u e n c e o f t h e change i n t h e d i r e c t i o n o f t h e C r i o l i s f o r c e , t h e stream blowing s o u t h o f t h e Equator a p p e a r s a s i f coming from t h e

w e s t . The w e s t e r l y flow i s s u p p o r t e d by t h e p r e s s u r e g r a d i e n t caused by t h e Lake V i c t o r i a low. A segment o f t h e s o u t h - e a s t monsoon mentioned above and which h a s c o n t i n u e d i t s t r a j e c t o r y t o t h e w e s t s p l i t s b e f o r e l e a v i n g t h e w e s t c o a s t and i n v a d e s a narrow b e l t n o r t h o f t h e E q u a t o r . Here t h e s t r e a m moving t o t h e e a s t i s d e f l e c t e d by t h e C r i o l i s f o r c e and caused t o blow o v e r t h e Z a i r e

Basin as a w e s t e r l y wind which t h e n i n t r u d e s i n t h e e s t a b l i s h e d c i r c u l a t i o n . The zone of s e p a r a t i o n of t h e n o r t h e r n from t h e s o u t h e r n h e m i s p h e r i c a i r masses

i s known as t h e i n t e r t r o p i c a l convergence zone, I T C Z . T h i s zone m i g r a t e s i n t h e c o u r s e of t h e y e a r between t h e two t r o p i c s , i . e . between l a t i t u d e 23°30’N and

106

23O3O.S.

The winds are g e n e r a l l y s o u t h - e a s t t o s o u t h from 5's

and n o r t h t o n o r t h - e a s t

t o t h e Equator

from t h e Equator t o 5 O N . T h i s e x t e n t c o v e r s t h e Lake

P l a t e a u and t h e extreme s o u t h o f t h e Sudan. P a r t o f

t h e r a i n f a l l i n g on t h i s

area comes from t h e ITCZ a s i t f o l l o w s t h e p o l a r m i g r a t i o n o f t h e s u n and t h e thereupon l a g g e d monsoons. S i n c e t h e movement and p o s i t i o n o f ITCZ are i n f l u e n c e d by t h e s t r e n g t h o f t h e monsoons, t h e r a i n f a l l t h u s d e r i v e d is v a r i a b l e i n amount and i n t e n s i t y . A d e t a i l e d summary o f t h e work

o f T r e w a r t h a (1962) and Flohn and S t r u n i n g

(1969) c o n c e r n i n g t h e a t m o s p h e r i c c i r c u l a t i o n i n t h e Lake P l a t e a u area a p p e a r s i n t h e r e p o r t o n t h e h y d r o m e t e o r o l o g i c a l s u r v e y o f t h e catchments o f Lake V i c t o r i a , Kyoga and A l b e r t (WMO, 1 9 7 4 ) . I n t h e n o r t h e r n summer (mainly June t o August) a t r o u g h o f l o w p r e s s u r e c o v e r s t h e n o r t h e r n Sudan. An o s c i l l a t o r y b a r o m e t r i c minimum i s l o c a t e d o v e r t h e n o r t h - e a s t Sudan, w h i l e a b a r o m e t r i c maximum (of t h e I n d i a n Ocean a n t i c y c l o n e ) e x t e n d s o v e r t h e g r e a t e r p a r t o f A b y s s i n i a and s o u t h - e a s t Sudan. T h i s maximum s u f f e r s f l u c t u a t i o n s i n i t s s t r e n g t h and northward e x t e n t . The g e n e r a l wind c i r c u l a t i o n meets, t h e r e f o r e , w i t h some d i s t u r b a n c e s . Four t y p e s o f d i s t u r b a n c e s h a v e been d i s t i n g u i s h e d by El-Fandy

( 1 9 4 9 ) . These are:

Sand storms (haboobs) w i t h a m o d i f i c a t i o n o f t h e g e n e r a l c i r c u l a t i o n by t h e o n s e t o f s t r o n g s o u t h e r n l y winds. These storms a r e common o v e r t h e d e s e r t a r e a s o f t h e c e n t r a l and n o r t h e r n Sudan. Marked s t r e n g t h e n i n g and d e e p e n i n g o f t h e w e s t e r l i e s u s u a l l y accompany t h e northward e x t e n s i o n o f t h e b a r o m e t r i c maximum o f t h e s o u t h - e a s t Sudan t o g e t h e r w i t h a northward o s c i l l a t i o n o f I T C Z . The m o d i f i c a t i o n m a n i f e s t s i t s e l f by a rise o f p r e s s u r e and f a l l o f t e m p e r a t u r e i n t h e s o u t h e r n Sudan-Uganda a r e a and s p r e a d s northwards r a p i d l y . Duststorms (haboobs) w i t h "induced c o l d f r o n t s " o r l i n e s q u a l l s set up by thunderstorms. Low-level

t h u n d e r s t o r m s , which a r e formed under f a v o u r a b l e c o n d i t i o n s o f

t e m p e r a t u r e and h u m i d i t y , by d i r e c t c o n v e c t i o n w i t h i n t h e s o u t h t o s o u t h -

east monsoon. These g e n e r a l l y o c c u r when t h e ITCZ is a t i t s f a r t h e s t n o r t h , t h e m o i s t u r e c o n t e n t showing a marked i n c r e a s e w i t h i n t h e lowest layers. High-level

t h u n d e r s t o r m s f o l l o w e d by o u t b r e a k s o f r a i n c o v e r i n g a w i d e

a r e a and p o s s i b l y p e r s i s t i n g f o r about 24 h o u r s i n some p l a c e s . El-Fandy (1948) a l s o found t h a t thundery c o n d i t i o n s i n Egypt o c c u r when t h e Sudan monsoon l o w i n t e n s i f i e s towards t h e n o r t h and s u p p l i e s t h e e a s t e r n M e d i t e r r a n e a n w i t h a warm s o u t h - e a s t e r l y c u r r e n t . Q u i t e o f t e n s h a l l o w d e p r e s s i o n s form o v e r t h e s o u t h - e a s t e r n M e d i t e r r a n e a n , o r t h e a d j a c e n t l a n d a r e a s , a s a r e s u l t o f t h e m e e t i n g o f t h e above c u r r e n t w i t h t h e r e l a t i v e l y c o l d n o r t h -

107

e a s t e r l y a i r o f a n t i c y c l o n i c d i s t r i b u t i o n i n A s i a Minor. I n a few c a s e s o n l y ,

t h e y form o v e r t h e w e s t e r n d e s e r t . Once f u l l y d e v e l o p e d , t h e s e d e p r e s s i o n s move eastwards o r north-eastwards,

while t h e inflow of t h e c o l d a i r i n t h e r e a r gives

thundery showers. Such t h u n d e r s t o r m s a r e s p e c i a l l y marked o v e r t h e sea, S y r i a and I s r a e l . Over Egypt, however, h i g h - l e v e l

i n s t a b i l i t y frequently exists

between t h e w a r m c u r r e n t and t h e c o o l a i r advancing above. Owing t o t h e s t a b i l i t y of t h e s u r f a c e layers, t h i s high-level

i n s t a b i l i t y cannot pass i n t o t h e

thunderstorm s t a g e w i t h o u t s u f f i c i e n t convergence o f damp a i r . With a h i g h d e g r e e o f l a t e n t i n s t a b i l i t y , s e v e r e t h u n d e r s t o r m s are t h e n produced. The above d i s c u s s i o n o f t h e a t m o s p h e r i c c i r c u l a t i o n might h e l p us form an idea about t h e r a i n y seasons i n t h e N i l e Basin a r e a . Generally speaking, t h e r a i n f a l l o v e r t h e E q u a t o r i a l Lake P l a t e a u , t h e extreme s o u t h o f t h e Sudan up t o Juba and t h e E t h i o p i a n Highlands c a n o c c u r i n e v e r y month o f t h e y e a r . Over t h e Sudan, away from t h e Red S e a c o a s t , r a i n f a l l i s e f f e c t i v e l y c o n f i n e d t o a d e f i n i t e s e a s o n . T h i s s e a s o n l a s t s from March t o November a l o n g t h e l i n e c o n n e c t i n g Wau and M a l a k a l . F u r t h e r n o r t h , t h e s e a s o n becomes p r o g r e s s i v e l y s h o r t e r ; from May t o O c t o b e r on t h e l i n e p a s s i n g through Geneina, E l Nahoud, El-Obeid, Khartoum and Kassala, from J u n e t o September a t A t b a r a , and J u l y and August f o r p l a c e s s i t u a t e d more t o t h e n o r t h up t o Wadi H a l f a . The r a i n f a l l on Lower Egypt and t h e M e d i t e r r a n e a n c o a s t c a n o c c u r i n a l l months o f t h e y e a r e x c e p t J u n e , J u l y , and August. The peak r a i n f a l l o v e r t h e E q u a t o r i a l Lake P l a t e a u o c c u r s i n A p r i l w i t h t h e p o s s i b i l i t y o f a less pronounced second peak around November i n some p l a c e s . The r a i n f a l l o v e r t h e E t h i o p i a n P l a t e a u and t h e Sudan shows t h a t J u l y and August a r e t h e months o f peak r a i n . The month of maximum r a i n f a l l o v e r t h e Mediterranean c o a s t and Lower Egypt i s J a n u a r y , and o v e r Middle Egypt, December. Though t h e r a i n f a l l o v e r Upper Egypt i s e x t r e m e l y low, y e t i t s d i s t r i b u t i o n among t h e months of t h e y e a r c l e a r l y shows two marked maxima, one i n October and t h e o t h e r i n May. The d i u r n a l v a r i a t i o n o f t h e i n c i d e n c e of r a i n f a l l on t h e Lake P l a t e a u was i n v e s t i g a t e d and d i s c u s s e d by Thompson, B.W.

(1957). A few o f h i s many funda-

mental f i n d i n g s i s t h a t t h e maximum o f r a i n f a l l i n a day o v e r w e s t e r n Lake V i c t o r i a d u r i n g November t o May, i . e . i n t h e n o r t h - e a s t e r n monsoon, o c c u r s i n t h e e a r l y morning (00

-

06 h o u r s l o c a l time) and o v e r north-western Lake

V i c t o r i a d u r i n g May t o September, i . e . i n t h e s o u t h - e a s t monsoon, i n t h e morning (06

-

1 2 h o u r s l o c a l t i m e ) . The heavy r a i n f a l l s east o f Lake V i c t o r i a o c c u r b e t -

ween 1 2 and 18 h o u r s and o v e r n o r t h - e a s t e r n V i c t o r i a i n t h e l a t e evening (18

-

24 h o u r s ) . T h e r e i s a l a g i n t i m e between t h e i n c i d e n c e o f heavy r a i n s

i n l a n d and o n t h e l a k e . A d e t a i l e d i n v e s t i g a t i o n and d i s c u s s i o n o f t h e d i u r n a l v a r i a t i o n of t h e i n c i d e n c e of monsoon r a i n s o v e r t h e Sudan and of r a i n and

108

t h u n d e r a t A s m a r a and Addis Ababa, E t h i o p i a were g i v e n by P e d g l e y , D . E . , ( i n 1969 a and b , and 1 9 7 1 r e s p e c t i v e l y ) .

A s i m i l a r i n v e s t i g a t i o n on t h e r a i n f a l l o v e r E g y p t , w i t h emphasis on t h e d i u r n a l v a r i a t i o n o f t h e f a l l o n A l e x a n d r i a , P o r t S a i d and C a i r o (Almaza) w a s c a r r i e d o u t and r e p o r t e d by S o l i m a n , K .

(1953).

A summary o f t h e s e r e s u l t s is p r e s e n t e d i n F i g . 3.10. S i n c e J u l y and August are t h e t w o months w i t h t h e m o s t f r e q u e n t o c c u r r e n c e o f r a i n i n t h e Sudan and t h e E t h i o p i a n P l a t e a u , t h e r e s u l t s o b t a i n e d by P e d g l e y h a v e b e e n a v e r a g e d o v e r J u l y and A u g u s t . The r e s u l t s o b t a i n e d by Soliman f o r Egypt have b e e n l e f t as t h e y a r e f o r t h e whole d u r a t i o n of t h e r a i n y s e a s o n . The p a t t e r n s o f d i u r n a l v a r i a t i o n may b e grouped under f o u r t y p e s w i t h t h e following c h a r a c t e r i s t i c s : R a i n f a l l r e a s o n a b l y u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t t h e d a y , w i t h a weak

(i)

maximum i n t h e a f t e r n o o n o r e a r l y e v e n i n g . T h i s is t h e case a t J u b a , M a l a k a l , E l O b e i d , Addis Ababa, e t c . (ii)

S i m i l a r t o ( i ) e x c e p t t h a t t h e weak maximum t a k e s p l a c e d u r i n g t h e e a r l y morning. T h i s p a t t e r n c a n b e found a t K o s t i and T o z i i n t h e Sudan and a t Alexandria i n Egypt.

(iii)

A w e l l - d e f i n e d peaked d i s t r i b u t i o n where t h e peak t a k e s p l a c e i n t h e l a t e a f t e r n o o n or e a r l y e v e n i n g . Examples of t h i s p a t t e r n c a n b e found a t Wadi H a l f a , Abu Hamed, P o r t Sudan, G e n e i n a , e t c .

(iv)

A w e l l - d e f i n e d peaked d i s t r i b u t i o n where t h e peak t a k e s p l a c e d u r i n g t h e l a t e e v e n i n g or e a r l y morning and w i t h i n f r e q u e n t r a i n s d u r i n g t h e m i d d l e o f t h e d a y . T h i s i s t h e case a t Wad Medani and El-Khartoum.

P e d g l e y c o n c l u d e d t h a t t h e d i s t r i b u t i o n of t h e f o u r t y p e s o f s t a t i o n s forms a d e f i n i t e p a t t e r n o v e r t h e Sudan. Type ( i i i ) i s t o b e found i n t h e m i d d l e of t h e c o u n t r y , between t h e e x t e n s i v e h i g h l a n d m a s s i f o f E t h i o p i a and t h e smaller Marra Mountains which a r e s i t u a t e d w e s t o f E l - F a s h e r .

Except f o r E l - F a s h e r

and Geneinq

t h e r e m a i n i n g s t a t i o n s o f t y p e s ( i ) and ( i v ) l i e w i t h i n 700 km o f t h e E t h i o p i a n H i g h l a n d s . On t h e d i u r n a l v a r i a t i o n o f p r e c i p i t a t i o n i n E g y p t , g e n e r a l l y speaki n g , t h e m a r i t i m e t y p e h a s i t s maximum i n t h e e a r l y morning o r d u r i n g t h e n i g h t , and t h e c o n t i n e n t a l t y p e , which h a s i t s maximum i n t h e a f t e r n o o n .

109

i

.

;

> L L

i

i N

k

r

i

:!

a

:i

W

S

a

i

d

2oBldl h ,r;

c .- 0 -; 0 # #

z

6 12 18 24

0 6

0

12 18 24

6 12 18 24

Time, hr.

E l - Fasher

30

4 0 r ~ b uH a m a d

Halfa

Port Sudan

30

20

10 0

0 6 12 18 24

O

r

0

10

10 0

10

0

6 12 18 2 4

0 6

.-c 10 L

6 12 18 24

0

6 12 18 24

20

10

10

10

0

0 6 12 18 2 4

0

-

0

Kassala

30

20

0 6 12 18 24

0

0 6

0 6 12 18 24

6 12 18 2 4

Addis Ababa

10

m

0 6 12 18 24

0

0

6 12 18 24

10

0 6 12 18 2 4

7zl 0 0 6 12 18 2 4

0

0

12 18 24

D B L2l Kosti

0

Atbara

20

i 10 0

0

Ge nei n a

30

- 'go 0

12 18 24

20b

0 6 12 18 24

0

0

Tozi

6

12 18 2 4

2Ob Malakal

2 0 b Juba

l0 o0 6

l0 o0

12 18 24

6 12 18 2 4

Time, hr

Fig. 3.10. Diurnal variation of rainfall incidence over the Sudan and Ethiopiq average of July and August, and over Egypt average for rainy season

110

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Thompson, B . W . , 1957. The d i u r n a l v a r i a t i o n o f p r e c i p i t a t i o n i n B r i t i s h East A f r i c a . Techn. M e m o r . 8 , E a s t A f r i c a n Meteo. D e p t . , N a i r o b i . T r e w a r t h a , G . T . , 1962. The e a r t h ' s problem c l i m a t e s , Univ. o f Wisconsin P r e s s , Madison, and Methuen 81 Co. L t d . , London, 334 p p . Wickens, G . , 1975. Changes i n t h e c l i m a t e and v e g e t a t i o n of t h e Sudan s i n c e 20 000 BP. P r o c e e d i n g s 8 t h P l e n a r y s e s s i o n AETFAT. Geneva, 1974, B o i s s i e r a 2 4 . WMO, 1974. H y d r o m e t e o r o l o g i c a l s u r v e y of t h e c a t c h m e n t s o f Lakes V i c t o r i a , Kyoga and A l b e r t , Vol. I : Meteorology and h y d r o l o g y of t h e b a s i n , Geneva.