North American total cloud amount variations this century

Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 75 (1989): 175-194 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

175

NORTH AMERICAN TOTAL CLOUD AMOUNT VARIATIONS THIS CENTURY A. H E N D E R S O N - S E L L E R S School o[ Earth Sciences, Macquarie University, North Ryde, N.S. W. 2109 (Australia)

(Received July 4, 1988; revised and accepted October 26, 1988)

Abstract Henderson-Sellers, A., 1989. North American total cloud amount variations this century. Palaeogeogr., Palaeoclimatol., Palaeoecol. (Global Planet. Change Sect.), 75: 175-194. Records of mean monthly total cloud amount from 143 locations in North America have been assembled. Generally the low and middle latitude station records extend over the period 1900-1984 (U.S.A.) and 1900-1982 (Canada) but few Arctic stations have records before 1930 and some begin recording cloud amount as late as the 1960s. The low and middle latitude station records show a tendency for total cloud amount to increase over this century. Only one of the 77 continental U.S.A. stations does not show an increase. The high latitude stations record increasing total cloud amount in the summer (June, July, August) season but not in the annual mean. The records show the largest increase between about 1930 and 1950. They are temporally consistent but do not exhibit significant spatial coherence. The history of observing and reporting practice has been carefully examined; if any significant effect were to be expected from the changes documented it would be a decrease at the time when the greatest recorded increase occurs. Other factors associated with increased population are possible "explanations". The most likely cause of increased cloud amount (if the temporal trend is real) is anthropogenerated clouds in the form of jet aircraft condensation trails but the large (1930-1950) increase identified here pre-dates the introduction of widespread commercial airflights even in the U.S.A.

Introduction C l o u d s a r e a n i m p o r t a n t f e a t u r e of a n y p l a n e tary system. They often dominate the planetary a l b e d o (e.g. V e n u s , J u p i t e r ) b u t also c o n t r i b u t e a n a d d i t i o n a l f a c t o r t o t h e g r e e n h o u s e e f f e c t of i n f r a r e d a b s o r b i n g gases in t h e a t m o s p h e r e . I n t h e case of t h e E a r t h , w h e r e c l o u d s c o v e r a p p r o x i m a t e l y 50% of t h e surface, i t is n o t k n o w n w h i c h of t h e a l b e d o a n d t h e g r e e n h o u s e e f f e c t s d o m i n a t e s t h e c o n t r i b u t i o n of c l o u d i n e s s t o t h e planetary climate system. Still more important is t h e l i k e l y c h a n g e in c l i m a t i c c o n t r i b u t i o n which might be anticipated from any cloudiness changes. We do not even know whether clouds h a v e a p o s i t i v e o r n e g a t i v e f e e d b a c k e f f e c t on 0921-8181/89/$03.50

© 1989 Elsevier Science Publishers B.V.

s u r f a c e t e m p e r a t u r e ( H a n s e n e t al., 1984; M c G u f f i e a n d H e n d e r s o n - S e l l e r s , 1987; Cess a n d P o t t e r , 1987). In order to try to understand how cloudiness c h a n g e s w i t h i n t h e p l a n e t a r y s y s t e m i t is necess a r y t o d e v e l o p m o d e l s of t h e p h y s i c a l p r o c e s s e s i n v o l v e d . I t is a l s o n e c e s s a r y t o o b t a i n q u a n t i t a tive observational data upon which to base predictive models and against which such models can be tested. Satellites now provide one data s o u r c e for c l o u d s t u d i e s ; s u r f a c e o b s e r v a t i o n s undertaken by trained meteorological observers is a n o t h e r d a t a s o u r c e . T h e a d v a n t a g e of s a t e l l i t e d a t a is t h a t t h e y p r o v i d e g l o b a l c o v e r a g e and adequate temporal sampling. The adv a n t a g e of t r a d i t i o n a l m e t e o r o l o g i c a l o b s e r v a -

176

tions is that they offer a long historical data base. This paper describes a detailed study of such an historical data set of cloudiness records derived for the North American continent. T o t a l c l o u d a m o u n t s for N o r t h A m e r i c a Total cloud amount records from 143 meteorological stations located in the continental United States (77 stations) and Canada (66 stations) have been assembled from various sources for the time period 1900-1982 (Canada) and 1900-1984 (U.S.A.). These total cloud amounts seem to increase over this period (Fig. 1). The most pronounced increase is between about 1930 and the early 1950s. This increase is apparent in the annual average curves for the continental U.S.A. and mid-latitude Canadian stations (Figs. la and lb) but only in the summer (June, July, August) season for the higher latitude (north of 60°N) Canadian stations (Fig. lc). The national-scale averaging in Fig. 1 does not obscure regional departures from the upward trend as is illustrated in Fig. 2 which shows annually averaged time series for all the individual stations concerned. All but one of the 77 continental U.S.A. station traces followed the upward trend (Fig. 2a). The exception is San Francisco. Thirty one of the 39 mid-latitude Canadian stations also show an overall (1900-1982) increase in total cloud amount (Fig. 2b) but only 14 of the 27 high latitude stations show a net increase. Fig. 2c underlines the generally much shorter record for these stations (about half have records from the 1920s and 1930s and most of the rest only since the 1960s) and the lack of any clear trend in annually averaged total cloud amount (cf. Fig. lc which is for J / J / A only). Increasing total cloud amounts have previously been identified by Angell et al. (1984) for the United States for the 33 year period from 1950 to 1982 and increases in upper-level cloudiness have been hypothesized (as a factor " n o t yet ruled out") by Karl et al. (1986) for the period 1941-1980 for the U.S.A. and Canada. Lee and Johnson (1985) evaluated the ex-

pectancy of cloudless photographic days in the contiguous United States and found that fortyfour of forty-five cities had their annual monthly average of cloudless photographic days decrease in the more recent period, 1950-1982, compared with the period 1900-1936. The cloud climatology of the United States has been examined by Changnon (1981) with specific reference to the possible influence of jet contrails. He found an increasing trend in cloudiness over the century for much of the United States. The time series illustrated here have been constructed from different station reports, for the most part, and are for a longer period a n d / o r encompass a larger geographical area than for previous work. Moreover the increasing trend seems to begin before the 1940s or 1950s which was the starting point of all but one of the earlier studies. It therefore seems worthy of further examination, especially in view of the recent tentative identification of upward trends in other climatic parameters (Jones et al., 1988). A most important statement to make is that the " t r e n d " in total cloud amount suggested by Figs. 1 and 2 may be nothing more than part of a long-period ( - 80-100 yr) oscillation. Furthermore it is only a land-based record established from less than 150 stations. In using the term " t r e n d " these caveats are recognized and acknowledged. Other possible "explanations" of the variation seen include station selection introducing bias, for example an urbanization effect, changes in observing practice a n d / o r reporting procedure, changes in station locations, changes in the number of observations per day and, of course, an increase in cloud amount, perhaps predominantly in one type of cloud. For any of these explanations to be satisfactory they would have to pinpoint the period 1930-1950 although the continental U.S.A. mean total cloud amount seems to show a further increase from about 1965 to the mid 1980s. It must also be recalled t hat these cloud amount changes are small. T he curve in Fig. l a represenks about a one tenth (absolute) increase in total cloud amount for the U.S.A. (approximately 0.5-0.6 tentk~ between 1930 and 1950 and about 0.4 tenths since 1965). The mid-lati-

177 (a) U.S.A.

6.66.4 6.2 6.05.85.65,45.2 5.0 4.8 4.6 4.4 4.2 4.0 1900

(Annual)

19'10

19120

19130

(b) CANADA - MiD LATrrUDE

19~10

19150

1960

19170

19180

1950

1960

1970

1980

19160

19'70

19180

(Annual)

7.0 5.8 5.6 m

i

6.4 6.2 6.o.

5.8 5.6 5.4 1900

1910

1920

1930

1940

(©) ARCTIC (Juno/Jully/Au~t) 8.07.57.06.58.05.5 5.0 4.5 4,0 3.5 3.0 2.5 2.0

1900

19'10

19'20

i

1930

i

1940

i

1950

Fig. la. Continental U.S.A. mean annual total cloud amount, 77 station average for the period 1900-1984. The smoothed curve is of 20-year filtered values, b. Mid-latitude Canadian mean annual total cloud amount, 39 station average for the period 1900-1982. The smoothed curve is of 20-year filtered values, c. High-latitude (north of 60 o N) Canadian 27 station three month mean summertime (June, July, August) total cloud amount. The smoothed curve is of 20-year filtered values. (The stations included are listed, with locations, in Tables I - I I I and are mapped in Fig. 6).

178

(a) U.S.A. 8.0

1900

1920

1940

1960

1980

Year

(b)

CANADA (Mid latitudes) 8.5 8.0

\ 6.5.

i~

6.0'

5.5. 5.0"

_

4.5.

-_-

~

~ -

4.0. 3,5" 3.0" 2.5' 1900

i

i

1920

1940

i

-1960

i

!

1980

Year

(c) 8.5

CANADA (Arctic)

8.0 7.5 ~

==:~,~_ ~ - - - - - - - - - ~ / ~

7.0

.......... ~

6.6' 6.0. 5.5'

-=~-~-zp

6.0"

/~

4.54.03.53.0 2.6

..... 1900

!

i 1920

i

! 1640

! 1960

!

I 1980

Year

Fig. 2. A n n u a l m e a n total cloud a m o u n t curves for all t h e individual s t a t i o n s s t u d i e d superposed: a. Continental U.S.A. (77 stations), b. Mid-latitude C a n a d i a n (39 stations), c. H i g h - l a t i t u d e C a n a d i a n (27 stations). (Note t h a t m o s t of t h e high-latitude C a n a d i a n stations h a v e records shorter t h a n t h e 83-yr period available at t h e o t h e r N o r t h American stations. Also t h e a n n u a l m e a n curves in (c) do not show an increasing t r e n d with time cf. Fig. lc.

179 TABLE I

T A B L E I (continued)

Location, y e a r s in record a n d difference in t e n t h s of cloud a m o u n t between last a n d first a n n u a l m e a n s for U.S.A. stations

Station

Station

Location (N, W)

Years

Difference (tenths) (last-first)

30041 ' , 88015 , 32°18 ', 86o24 '

1900, 1984 1900, 1984

0.95 O.8O

33°26 ', 112001 ,

1900, 1984

1.20

35°20 ', 95023 '

1900, 1984

40 %8', 124010 ,

0.77

Calilornia Eureka I _ ~ Angeles S a n Diego S a n Francisco

Detroit Marquette S a u l t Ste. M a r i e

33°56 ', 118o23 , 32°44 ', 117o10 , 37o37 ' , 122o23 ,

1.15 O.68 0.59 -0.12

38°17', 104o31 ,

1900, 1984

0.63

44°55 ', 67o01 ,

1900, 1950 1.61

39°11 ', 76040 '

1900, 1984 0.83

41°17 ', 70o05 ,

1900, 1969 1.63

42°14 ', 83032 , 46°32 ', 87033 , 4 6 ° 2 8 ', 84022 ,

1900, 1984 1.76 1900, 1977 0.51 1900, 1984 1.32

32°21 ', 90o53 '

1900, 1965 0.28

Columbia St. Louis

38°58 ', 92022 , 38°45 ', 90023 ,

1900, 1983 1900, 1984

41°16 ', 72o42 ,

1900, 1950

1.98

Montana

1900, 1984

2.21

Havre Kalispell

Mississippi Vicksburg

Missouri

District of Columbia Washington *

1900, 1984 0.41

Michigan

1984 1984 1984 1984

Connecticut New Haven

32°28 ', 93°49 '

Massachusetts

1900, 1900, 1900, 1900,

Colorado Pueblo *

Difference (tenths) (last-first)

Maryland

Nantucket *

Arkansas Fort S m i t h

Eastport Baltimore

Arizona Phoenix

Years

Maine

Alabama Mobile Montgomery

Louisiana Shreveport

Location (N, W)

38 °51', 77 o 03'

1.53 1.51

98°34 ', 109040 , 1900, 1984 1.48 48°18 ', 114016 '- 1900, 1984 1.66

Nebraska

Flor/da Jacksonville Key West Pensacola Tampa

3 0 ° 2 5 ', 81039 , 24°35 ', 81042 , 30 o 28', 87 °12' 27 °58', 82o32 ,

1900, 1900, 1900, 1910,

1984 1984 1983 1984

1.29 1.28 1.13 1.38

Georg/a Macon *

32°42 ', 83039 '

1900, 1984

0.65

Idaho Boise

43o34 ', 116°13 '

1900, 1984

1.24

Lincoln North Platte *

40°51 ', 96o46 , 41°08 ', 100041 ,

1900, 1984 1900, 1984

1.40 1.69

Nevada Reno Winnemucca

3 9 ° 3 0 ', 119o47 , 1906, 1984 1.49 4 0 ° 5 4 ', 117048 , 1900, 1984 0.38

New Jersey A t l a n t i c City

39°27 ', 74035 ,

1900, 1984 2.07

33024 ', 104032 ,

1905, 1982

42°13 ', 75o59 , 43°07 ', 77o40 '

1900, 1984 0.68 1900, 1984 1.62

35°13 ', 8 0 ° 5 6 ' 35o13 ', 75o41 ,

1900, 1984 0.93 1903, 1984 0.81

New Mexico Roswell

1.29

I/linois Chicago Peoria *

41°47 ', 87045 , 40°40 ', 89°41

1900, 1984 1905, 1984

1.68 2.06

New York

Ind/ana Evansville Indianapolis

38 o 03', 87 ° 32' 39°44 ', 86010 '

1900, 1984 1900, 1984

1.73 1.68

North Carolina

41°32 ', 93039 '

1900, 1984 1900, 1973

1.18 2.36

North Dakota

4 2 ° 2 4 ', 9 0 o 4 2 ,

Fargo Williston

46°54 ', 96o48 , 4 8 ° 0 9 ', 103037 '

1900, 1984 1.34 1900, 1984 2.28

37 °46', 99 ° 58'

1900, 1984

1.41

38°02' , 84o36 ,

1900, 1984

1.89

Ohio Columbus Toledo

4 0 ° 0 0 ', 82053 ' 41°34 ', 83o28 '

1900, 1984 2.03 1900, 1984 1.67

Iowa Des Moines Dubuque

Kansas Dodge C~ty

Kentucky Lexington *

Binghampton * Rochester

Charlotte * Hatteras *

180 TABLE I (continued) Station

T A B L E II Years

Difference (tenths) (last-first)

35o24 ' , 97036 '

1900, 1984

0.61

45036 ', 122°36 ' 43013 ", 123020 ,

1900, 1984 1900, 1964

1.13 1.20

4 2 ° 0 5 ', 8 0 0 0 5 , 39°5T, 75°15 '

1900, 1983 1900, 1984

0.90 1.75

41°10 ', 71°35 '

1900, 1976

0.43

Calgary Edmonton Fort Chipewyan Medicine Hat

3 2 ° 5 4 ', 8 0 0 0 2 ' 33'57'. 81007 '

1900, 1984 1900, 1984

0.85 0.80

Saskatchewan Battleford * Prince Albert * Swift Current *

Station

Oklahoma Oklahoma City

Oregon Portland Roseburg *

Pennsylvania Erie Philadelphia *

Rhode Island Block Island

South Carolina Charleston Columbia

South Dakota Huron

4 4 ° 2 3 ', 98013 '

1900, 1984

1.84

35049 ' , 83059 '

1900, 1984

1.28

3 5 ° 1 4 ', 2 7 ° 4 6 ', 2 9 ° 1 8 ', 2 9 ° 3 2 ',

1900, 1900, 1900, 1900,

1984 1984 1950 1984

1.60 1.83 0.63 1.47

40046 ' , 111058 '

1900, 1984

2.28

4 4 ° 2 8 ', 73009 '

1906, 1984

1.34

3 6 ° 5 3 ', 76012 , 37039 ' , 77020 ,

1900, 1984 1900, 1984

2.37 1.27

Tennessee Knoxville

101042 , 97027 ' 94°48 ' 98°28 '

Utah Salt Lake City

Vermont Burlington *

Virginia Norfolk Richmond

Washington Seattle Spokane Walla Walla

4 7 ° 3 6 ', 122020 , 4 7 ° 3 7 ', 117°31 ' 4 6 ° 0 2 ', 118020 '

1900, 1984 1900, 1984 1900, 1984

1.16 0.44 1.62

3 8 ° 5 3 ', 79051 ' 3 9 ° 1 6 ', 81034 ,

1900, 1984 1900, 1961

1.14 2.15

4 2 ° 5 7 ', 8 7 ° 5 4 '

1900, 1984

1.13

4 1 ° 0 9 ', 104°49 ' 42°49 ', 108o44 ,

1900, 1984 1900, 1984

1.46 0.98

West Virginia Elkins Parkersburg

Wisconsin Milwaukee

Years

Difference (tenths) (last-first)

50 °41', 5 4 ° 1 8 ', 4 9 ° 1 7 ', 4 8 ° 2 4 ',

120 o 29' 130°18 ' 123005 ' 123019 '

1900, 1909, t900, 1900,

1979 1980 1980 1980

- 0.20 0.67 1.31 0.86

5 1 ° 0 2 ', 114002 ' 53033 ' , 113030 ' 58°42 ', 110010 ' 5 0 ° 0 5 ', 110037 '

1900, 1900, 1900, 1900,

1980 1980 1940 1982

0.78 0.87 - 1.88 0.16

52°41 ', 108020 ' 5 3 ° 1 0 ', 1 0 6 ° 0 0 ' 5 0 ° 2 0 ', 107045 '

1900, 1982 1902, 1982 1900, 1982

0.58 0.21 1.54

1900, 1900, 1900, 1900,

1982 1957 1982 1982

0.72 0.35 0.01 0,64

4 9 ° 2 5 ', 82026 ' 4 2 ° 5 9 ', 81013 , 4 5 ° 2 6 ', 75042 , 4 5 ° 1 9 ', 8 0 0 0 0 , 4 8 ° 2 7 ', 89012 ' 4 4 ° 3 0 ', 81021 ' 43040 ', 7 9 0 2 4 , 4 8 ° 3 5 ', 85016 ~

1910, 1900, 1900, 1900, 1900, 1906, 1900, 1900,

1982 1982 1982 1949 1969 1962 1982 1976

1.07 2.92 1.27 0.31 0.78 0.20 1.29 - 1.48

4 9 0 2 4 ', 63035 ' 48°31 ', 6 8 ° 1 9 '

1900, 1954 1900, 1951

1.32 -0.84

5 0 ° 4 0 ', 59019 ' 4 5 ° 3 0 ', 7 3 ° 3 5 ' 46048 ', 71013 ,

1912, 1978 1900, 1982 1900, 1982

-0.09 0.48 -0.09

4 7 ° 0 3 ', 65029 , 4 5 ° 5 7 ', 6 6 o 3 6 , 4 5 ° 1 7 ', 6 6 o 0 4 '

1900, 1982 1,900, 1982 1900, 1982

0.91 0.65 0.38

4 4 ° 3 9 ', 63036 , 43o57 ', 6 0 0 0 6 '

1900, 1982 1900, 1982

0.92 0.39

British Columbia Kamloops Prince Rupert Vancouver Victoria

A lberta

Churchill * 58045 ', 94o07 ' Minnedosa 5 0 ° 1 5 ', 9 9 0 5 0 , N o r w a y H o u s e * 5 3 ° 5 8 ', 9 7 ° 5 2 ' Winnipeg * 4 9 ° 5 3 ', 9 7 0 0 7 '

Ontario Kapuskasing * London * Ottawa * Parry Sound * Port Arthur * Southampton * Toronto * White River *

Quebec Anticosti * Father Point * Harrington Harbour * Montr6al * Qu6bec *

,New Brunswick Chatham * Fredericton * St. J o h n *

Nova Scotia

Wyoming Cheyenne * Lander

Location ( N, W )

Manitoba

Texa~" Amarillo Corpus Christi Galveston San Antonio *

L o c a t i o n , y e a r s in r e c o r d a n d d i f f e r e n c e in t e n t h s of c l o u d a m o u n t b e t w e e n l a s t a n d f i r s t a n n u a l m e a n s for m i d - l a t i tude Canadian stations

Location (N, W )

* I n d i c a t e s t h e 15 s t a t i o n s for w h i c h b o t h s u n r i s e t o s u n s e t a n d four i n d i v i d u a l ( r o u n d t h e clock) t o t a l c l o u d a m o u n t v a l u e s were a v a i l a b l e for t h e p e r i o d 1951-1970. T h e comp a r i s o n of t h e s e p a i r e d records is d e s c r i b e d on p. 183.

Halifax * Sable Island *

181 TABLE II (continued) Station

Nova Scot/a Sydney * Yarmouth *

Location (N, W)

Difference (tenths) (last-first)

46°10 ', 60010 , 1900, 1982 - 0.23 43°50 ', 66002, 1900,1982 0.63

Prince Edward Island Charlottetown * 46°14 ', 63°10 ' Newfoundland Belle Isle * St. George * St. Johns *

Years

1900, 1982

0.03

51°53', 55022, 1900,1969 0.87 48°27 ', 58°30" 1900,1945 -0.54 47°34 ', 52o42, 1900,1982 1.07

* Indicates the 30 stations for which four observations per day were available for the period 1955-1982. The resultant diurnal analysis is described on p. 182 and illustrated in Fig. 4.

t u d e C a n a d i a n s t a t i o n s (Fig. l b ) t o g e t h e r s h o w a b o u t 0.4 t e n t h s i n c r e a s e b e t w e e n 1930 a n d 1950 and the high latitude Canadian stations' summ e r t i m e t o t a l c l o u d a m o u n t (Fig. l c ) i n c r e a s e s b y over 1 t e n t h i n t h e s a m e p e r i o d b u t , as Fig. 2c u n d e r l i n e s , t h i s c o u l d be t h e r e s u l t of i n t r o d u c i n g a d d i t i o n a l s t a t i o n s i n t o t h e record. Search for an explanation of the apparent upward variation in total cloud amount

S t a t i o n selection S t a t i o n s were c h o s e n for l e n g t h of r e c o r d a n d i n o r d e r t o give a d e q u a t e s p a t i a l coverage i n 1900. T h e y were selected i n c o n n e c t i o n w i t h e a r l i e r s t u d i e s ( H e n d e r s o n - S e l l e r s , 1986; M c G u f fie a n d H e n d e r s o n - S e l l e r s , 1988) w h i c h focussed o n t w o p e r i o d s i n t h e first h a l f of t h e c e n t u r y : 1901-1920 a n d 1934-1953. T h e s t a t i o n l o c a t i o n s are listed i n T a b l e s I - I I I . M a n y s t a t i o n s h a v e a v a r i e d h i s t o r y of m i n o r r e l o c a t i o n s ( u p t o 11 h a v e b e e n recorded) b u t t h e s e a r e n o t t e m p o r a l l y c o i n c i d e n t . T h e e a r l y p a r t of t h e r e c o r d is the most heterogeneous. For example, years 1900-1904 of V a n c o u v e r , B r i t i s h C o l u m b i a , are taken from the adjacent station (New Westminster, 4 9 ° 1 3 ' N , 1 2 2 ° 5 4 ' W ) w h i c h V a n c o u v e r s e e m s t o r e p l a c e i n 1905. H a r r i n g t o n H a r b o u r , Qu6bec, m o v e d f r o m 5 0 ° 3 0 ' N , 5 9 ° 2 9 ' W t o its p r e s e n t

l o c a t i o n i n 1912 a n d St. George, N e w f o u n d l a n d , m o v e d f r o m 4 8 ° 4 0 ' N , 5 8 ° 2 7 ' W t o its p r e s e n t l o c a t i o n i n 1911. T h e r e a r e also n a m e c h a n g e s t h r o u g h o u t t h e r e c o r d which, i n general, do n o t i m p l y location changes. For example, Battleford, Saskatchewan, was originally n a m e d N o r t h Battleford, Years 1900-1920 for Churchill, Manitoba, are from Fort Churchill at the same l o c a t i o n a n d f r o m 1941 d a t a f r o m P o r t A r t h u r , Ontario, have been replaced by data from Fort W i l l i a m . S o m e s t a t i o n s h a v e c e r t a i n l y b e e n affected b y i n c r e a s i n g u r b a n i z a t i o n b u t t h e r e are also s t a t i o n s for w h i c h t h i s is less o b v i o u s l y t r u e . I t is h a r d t o see w h y u r b a n i z a t i o n a l o n e w o u l d

TABLE III Location, years in record and difference in tenths of cloud amount between last and first annual means for Canadian high-latitude stations Station

Yukon Dawson Mayo Landing Teslin Whitehorse Watson Lake

Location (N, W)

Years

64°04 ', 139020, 63°34', 135052, 60°20 ', 132045, 60040', 135005' 60°02', 128°47'

1901, 1982 1.30 1929,1982 0.77 1944, 1982 -1.39 1941, 1982 -0.29 1941, 1982 -0.41

North West Territories Hay River 60051' , 115020' Aklavik 68°12', 135001, Arctic Bay 73°02 ', 85011' Chesterfield 63°21', 90042' Ford Good Hope 66025 ' , 128o53' Holman Island 70°44 ', 117044, Nottingham Island 63°06 ', 78o00 , Alert 82°30 ', 62020, Baker Lake 64°20 ', 96°00 ' Cambridge Bay 69°07 ', 105003, Clyde 70°27 ', 68°33' Coppermine 67°50 ', 115005' Coral Harbour 64°10 ', 83015, Fort Reliance 62°43', 109010' Fort Simpson 61°52', 121°35' Fort Smith 60°00 ', 111°56' Frobisher Bay 63°45', 68033' Mould Bay 76°14 ', 119°20 ' Norman Wells 65°18', 126042, Resolute 74°43', 94059, Yellowknife 62°14 ', 114002, Eureka 80°00 ', 85056'

Differences (tenths) (last-first)

1900, 1982 0.99 1929, 1960 0.81 1938,1976 -0.96 1932,1981 - 1.50 1916, 1972 1.29 1940,1968 -2.12 1931,1970 1955,1982 1950,1982 1932, 1982 1955,1982 1931, 1982 1955,1982 1955,1982 1916, 1982 1916, 1982 1955,1982 1955, 1982 1955,1982 1955,1982 1955, 1982 1955,1982

-0.88 1.62 -0.58 1.48 1.28 0.84 -0.20 -0.62 0.92 0.63 0.73 -0.71 0.12 -0.28 0.20 -0.01

182

affect the record of cloudiness between 1930 and 1950 particularly. In general the stations are not "class-l" type stations but, usually, "class-2" i.e. they are stations at which observations are currently made about once every 6 hours rather than once an hour or once every three hours. The temptation to switch to nearby stations offering a more complete record was resisted intentionally to avoid additional disturbance in the time series. This is not to suggest t ha t the record of observations is homogeneous. It is not.

(a)

~ ,~~--~

1900

Timing and number o[ observations A number of changes have taken place in the number of observations made per day and, consequently, in the timing of the observations. Figure 3 illustrates these changes for four example years: 1900, 1925, 1951 and 1982. In each map the station location is marked by a symbol representing the number of observations made per day using the okta scale. Inset in the maps are examples of the timing of different stations' reports illustrated by a 24 h clock face. The

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183

times shown are local time or close to local time. T h e inset stations have been chosen to be as typical as possible and generally represent observing practice for their region fairly well. The exception is Swift Current in 1982. This is the only mid-latitude Canadian station studied not making four observations per day in 1982. Generally the continental U.S.A. stations tend to switch from two observations per day to four observations per day between 1925 and 1951. T h e Canadian stations are less consistent; some begin the century with three observations per day but most have only two reports per day by 1925. Many of the high latitude Canadian stations do not begin until as late as the 1960s (see Fig. 2c). In the period 1930 to 1955 the additional mid-day (12-14 local time) observation is generally, but not always, higher than the morning and evening observation but the nighttime observation is lower. It has been possible to examine the probable impact of introducing additional observing times into the daily reports because data for 30 Canadian stations (starred in Table II) were available as monthly means for specific observing times for the period 1955-1982. All 30 stations reported four times a day (or six hourly intervals) over this period at local times ranging between 08, 06, 12 and 18 to 02, 08, 14 and 20. Thus the first of the four observations (which is almost always at 00, 01 or 02) is certain to be a nighttime report and the last of the four could be a late evening report. The earlier record (Fig. 3) may have contained observations at only the second and fourth times now used and very early in the century a few stations recorded only one observation a day which was usually made at around 08 or 09 local time. Figure 4a shows the 30 station average situations for these four observational times and for each of the months. The first observation is consistently lower than the other three and the fourth is slightly lower than the morning (second) and mid-day (third) observations. This pattern is consistent over all the stations considered even though the pattern of seasonal and diurnal cloud amounts varies considerably and the variability from month to month and station to

station is also considerable (Fig. 4). Overall, for the 30 stations and for the 12 months (i.e. 360 values in all) the s t a t i o n / m o n t h diurnal (4 values) mean is greater than the first observation mean on 38 occasions with the mean deviation being - 1.2 tenths; the third observation exceeds the s t a t i o n / m o n t h diurnal mean on 11 occasions, with the mean difference being +0.91 tenths and the fourth observation exceeds the s t a t i o n / m o n t h diurnal mean only once [Norway House in August] with a deviation of +0.77 tenths. The conclusion is consistent with the general expectation t h a t cloud amount is a maximum (on average) in mid-afternoon. Furthermore it suggests t h a t a 08 or 09 local time observation of total cloud amount is a good predictor of the diurnal mean value. More importantly, in the context of this study, it seems t h a t the one (09) or much more generally the two observations (07, 19) t h a t were common before 1925 (Fig. 3) would be very little changed by the addition of 01 and 13 h observations. If there were any effect it would be a very slight decrease in mean total cloud when all four observations were used. There is a second alteration in the record of continental U.S.A. total cloud amount between about 1945 and 1952. During this time a minority of the U.S.A. stations switch recording practice from recording the "daylight only average total cloud amount" to the "24 h average total cloud amount". For many stations this switch is the direct consequence of the introduction of two additional observing times which include a near midnight observation as depicted in Fig. 3. However, there are a few stations for which it is not possible to ascertain the exact timing or number of observations included in these two averages. For this reason it seemed worthwhile investigating the difference between "sunrise to sunset" mean total cloud amount as recorded and the average of four individual timed observations. It was possible to retrieve such comparative data for 15 continental U.S.A. stations for the period 1951-1970. T he stations for which a double record was available are starred in Table I. For these 15 stations the mean monthly "sunrise

184

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185 to sunset" total cloud a m o u n t has been comp a r e d with the " f o u r observations average" t o t a l cloud amount. O u t of t h e t o t a l of 180 comparisons there were only seven occasions w h e n t h e sunrise to sunset (average over 20 years) was less t h a n the four-observation mean. Generally, then, switching to a four-observation average reduced the reported m o n t h l y m e a n t o t a l cloud a m o u n t . F o r the 15 stations for which an intercomparison was possible, m o s t of the differences (decreased report for t h e four-observations) were in t h e range 0.4-1.0 t e n t h s of s k y cover w i t h t h e average difference for all 15 stations being 0.66 t e n t h s of s k y cover. T h e s e results are consistent w i t h the C a n a d i a n analysis illustrated in Fig. 4. Overall during the period 1930-1955 m a n y stations in t h e s t u d y included additional observing times. These sometimes included a near-noon time, although this was often already included in the " d a y l i g h t average" previously recorded and almost always a n e a r - m i d n i g h t observation. T h e overall effect of these changes is likely to h a v e been a reduction in the t o t a l cloud a m o u n t recorded. I f a n u m b e r of stations switched averaging m e t h o d a t a b o u t the s a m e t i m e (as was the case for t h e U.S.A. stations in the early 1950s) a decrease in the overall continental m e a n could, perhaps, be anticipated.

Observing practice Historical observations o[ cloudiness In Shaw's (1926) M a n u a l of Meteorology the observing and recording of q u a n t i t a t i v e assessm e n t s of t o t a l cloud a m o u n t is described in C h a p t e r 1 entitled " T h e ' W o r l d as K n o w n to the Ancients' ". N o r t h American meteorological practice derived directly f r o m already established E u r o p e a n observational m e t h o d s (espe-

cially French and British). D o c u m e n t a r y evidence p e r t a i n i n g to such cloud observations is available f r o m as long ago as 1837 when cloud a m o u n t was observed a n d recorded on a scale of 0 - 1 0 a t t h e R o y a l Greenwich Observatory, U.K. T h e s a m e scale a p p e a r s on the printed forms used b y t h e R o y a l Engineers in t h e 1860s and t h e m o r e a b u n d a n t r e t u r n s f r o m second order stations f r o m a b o u t 1875. T h e Telegraphic W e a t h e r R e p o r t s of 1860 also include " C l o u d (1-9) p r o p o r t i o n " , as do l a t e r British Daily W e a t h e r Reports. Foreign reports in the old D a i l y W e a t h e r R e p o r t s exclude this item, b u t t h a t m a y h a v e been a m a t t e r of policy (or space) r a t h e r t h a n a lack of observations. T h e use of 0 - 9 r a t h e r t h a n a 0 - 1 0 scale is also r e p o r t e d b y F i t z r o y (1863) in Appendix M which describes t h e " W e a t h e r Register for Oct o b e r 1862". T h e scale seems to h a v e represented t e n t h s of s k y covered b y clouds w i t h 9 a n d 10 t e n t h s being r e p o r t e d as 9 in the telegraphic code where only one digit was used. T h e r e is some a m b i g u i t y a b o u t the scale however. T h e f r o n t of t h e 1855 " I n s t r u c t i o n s to Marine Observers" (written on behalf of R e a r Admiral Fitzroy) s t a t e s t h a t t h e scale is used to indicate t h e " p r o p o r t i o n of t h e s k y d e a r , thus p o i n t 8 indicates 8 t e n t h s of sky clear". T h i s confusion between clear and cloud seems to h a v e been an error of inversion which was later corrected. On t h e o t h e r h a n d t h e scale in Fitzroy's (1863) appendix gives an example in which cloud code 7 as well as cloud code 9 is recorded in association w i t h an " o v e r c a s t " r e p o r t a l t h o u g h rain is always associated w i t h larger cloud code n u m b e r s as m i g h t be expected. Overall it seems reasonable to assume t h a t the scale was used to records tenths of sky covered b y cloud. B y 1919 Brooks produced global estimates of

Fig. 4a. Monthly mean cloud amounts averaged for each of four observing times per day (usually between 00, 06, 12, 18 and 02, 08, 14, 20) for 30 Canadian stations (starred in Table II) for the period 1955-1982 for which individual observing time data were available. The monthly means are shown for all 12 months and (inset across the local time axis) the cloud amount for the four canonical months are repeated, b. As (a) except for one station only Toronto (01, 07, 13, 19 Eastern Standard Time). The error bars on each monthly mean are ± 1 standard deviation for that month over the 28 years, c. As (b) for Qu6bec (01, 07, 13, 19, Eastern Standard Time) d. As (b) for Churchill (00, 06, 12, 18, Central Standard Time) e. As (b) for St. Johns (02, 08, 14, 20 Newfoundland Standard Time) f. As (b) for Sable Island (02, 08, 14, 20, Atlantic Standard Time) (Note the wide range of diurnal and seasonal cycles and the usefulness of the second observation as a predictor of the diurnal mean).

186

cloudiness (revised 1921) which implies fairly widespread quantitative reporting by t hat time and also suggests that tenths of sky cover was widely, if not universally, employed at t hat time (Brooks, 1927). Shaw's (1936, first edition 1928) Comparative Meteorology includes twenty-six figures showing the monthly and annual cloud distribution for the two hemispheres as "compiled for us in the Meteorological Office by C.E.P. Brooks in 1919 and revised in 1921" (Shaw, 1936, p. 145). These figures reference fifty sources, mostly charts, tables and diagrams, as the authority for the cloudiness figures underlining th a t observation and recording were widespread. In the early 1900s cloud amount seems to have been observed and recorded in tenths of sky cover fairly widely. The first revision to the synoptic code in 1921 required the inclusion of cloud amount reported in tenths. In 1929, the synoptic code was again revised to include more cloud information and nephoscope observations. The observation and reporting units remained as tenths. However, at the Conference of the International Meteorological Organization, held in Washington D.C. in 1947, it was recommended th at amount of cloud be reported in eighths instead of tenths; once again to save a digit. The eighth group of code letters NsChsh ~ was to be repeated until the sky state was fully described. This change of procedure was brought into force with the introduction of the revised International Code (Washington) on 1 January 1949. This revision in the reporting procedure for cloud amount was adopted by the western world in January 1949. However, most of the eastern block countries postponed the adoption of the code until 1950 and the U.S.S.R. did not implement the revised code until February 1953. Furthermore only some nations changed observing procedure, although all adopted the new reporting practice.

Present observing practice In making the observation of cloud amount, the observer must stand in a position affording an uninterrupted view of the whole sky. Malberg (1973) suggested that surface-based ob-

servations are typical of a circular area of radius 50 km centred at the observing site but the results of S6ze et al. (1986) indicate t hat this is probably an overestimate; a radius of - 30 km being a more appropriate climatological value. The estimate of total cloud amount records the amount of the whole sky dome obscured by cloud. Thus, the surface observer includes in his estimate of cloud amount the sides of clouds since they can (when vertically extended) obscure part of the sky dome (Merritt, 1966). This inclusion of cloud sides in the estimate of cloud amount has led to the assertion t hat surface observers overestimate total cloud amount as compared with estimates derived from satellite retrievals. This is not necessarily the case, but the verity of the assertion does not, in any case, affect this present study. The U.K. observing procedure, as described by Her Majesty's Stationery Office's publications now states that cloud amount estimates shall be made in eighths. It further suggests that it is "convenient to imagine the sky divided into quadrants by two arcs drawn at right angles through the zenith. Each quadrant then represents two eighths of the total sky. If we choose the most appropriate of the figures--0 = clear or almost clear of cloud; 1 = about half-covered; 2 = completely or almost completely covered

T A B L E IV C o n v e r s i o n f r o m t e n t h s t o o k r a s of c l o u d cover a n d associa t e d o k t a r e p o r t i n g codes Code Figure

F r a c t i o n covered in t e n t h s

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0 1

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187

with cloud--for each separate quadrant then the total amount of cloud for the whole sky is simply obtained by adding the amounts in the separate quadrants" (HMSO, 1977, p. 53). On the other hand, the U.S. observing procedure continues to use fractional estimates of sky cover made in tenths but the reports made in the international synoptic code are of the fraction of sky covered in oktas (eighths). The conversion is made according to Table IV so t h a t when no clouds are present okta code figure zero is reported; when any particles of cloud exist up to 1-tenth cloud cover okta code figure 1 should be recorded; when nine tenths of cloud cover, or an overcast with small openings, exists, okta code number 7 is reported and when the celestial dome is completely overcast okta code figure 8 is reported (U.S. Dep. Commer., 1979, p. B3-1). This means that all but the synoptic coded observations of cloud in the U.S.A. and Canada are archived in tenths of sky cover, whilst those in Europe and many other countries are now archived in oktas, but were, until the early 1950s, archived in tenths. If the record studied had included European or other national records for which the period 1947-1953 represented a switch from the tenths to oktas observing scale this might have been a plausible explanation of any discontinuity of the record. However, the North American records analyzed here were not, as far as can be established, disrupted by this W.M.O. modification because observation continued (and continues now) to be undertaken in tenths of sky cover. Data sources

Historical cloud observations are not included in collected meteorological summaries such as the World Weather Record and thus archive data sources must be sought if the historical record of cloud amount is to be analyzed. It is necessary in a study such as this to obtain as good and consistent a spatial coverage as possible. A pr~selection can generally be made of the stations which possessed continuous meteorological records in the archive, but after consultation of the records themselves it is often neces-

sary to modify the station list quite considerably. The most satisfactory source of archive material for the continental United States was found to be the Climatological Record Book. These records were much more consistent in method and in number of observations made per day than contemporaneous European cloud observations. The data have been fully described in Henderson-Sellers (1986). Canadian meteorological reports date back to the 1850s (cf. Hamilton, 1986). The source of all the Canadian data used here is the Monthly Record of Meteorological Observations for Canada issued annually. Originally produced in 1873 by the Department of Marine and Fisheries and termed the "Canadian Meteorological Service Report", it was renamed in 1916 and became the responsibility of the Canadian Meteorological Service. The report retained its annual and national format until 1977, when reporting stations were divided geographically so that six reports were issued each year for western, eastern and northern Canada for the two half years J a n u a r y / J u n e and July/December. Sixty-six Canadian stations were selected altogether (described in Henderson-Sellers and McGuffie, 1988) but about ten of these (mostly the Arctic stations) do not have observations before - 1930 and others begin observing as late as the mid 1950s (i.e. there are only 41 throughout most of the study period). The stations selected are almost all Class II observing stations and, although it would have been possible to replace some of the selected stations with adjacent Class I (the most complete reports) stations for part of the record, it was deemed more appropriate to retain consistency throughout. In Canada cloud observations have been made in tenths throughout but recording was in either tenths or percentages. Times of observations vary through the study period, and generally the number of observations made during each 24-h period increases through the century, although even in 1900 about half the stations made three observations per day (usually at 8 or 9 a.m., 3 or 4 p.m. and 8 or 9 p.m.), the others

188

having two reports. By the 1950s there are often 4 or 5 observations. There is some confusion in the earlier records concerning the time of the observations. Often the record states that observing times are 75th meridian times rather than local times but this is not always the case. Between 1900 and 1916, times appear to be 75th meridian times and after 1916 three or more time bases, including local time, are used. As with our previous studies, all the available data have been averaged into a monthly mean value of total cloud amount. Thus these ambiguities in the actual observing time are of no real significance. Typographical errors are rather common in the bound volumes. It is fairly easy to identify errors in transcription of the station location but the stated barometer height also varies at many stations by larger amoUnts than seems likely as a result of minor s t a t i o n / instrument relocation. These errors suggest t hat there may also be errors in the transcription of observations. In general it is impossible to identify such errors although some s t a t i o n / y e a r cloud amounts do seem to have been given in tenths even though the table heading states the values are percentages, When these errors were quite clear (for example, Prince Albert, Saskatchewan, being quoted as having a mean January cloud amount of 4.4%) they have been corrected. It is presumed th at observers working at the stations used in the historical study described earlier had enjoyed some degree of training and had accumulated experience; both of which are essential prerequisites for accurate recording. However, it is well established t ha t cloud observations, even by trained meteorologists, differ from individual to individual (e.g. Merritt, 1966) and it is possible th a t traits common in observations made by untrained observers may be present to some degree in these observations. Notwithstanding these caveats, it seems likely that the occurrence (and amount) at least of total cloud is generally well reported, at major meteorological and climatological stations. There does not seem to be any obvious reason for anticipating systematic bias in observer ability across the whole of North America. There is, however, a further difficulty to be

overcome in certain of these locations. This is the estimation of sky covered by jet aircraft condensation trails. There is no ambiguity in reporting procedure, inasmuch as condensation trails are considered as cloud if they exist in the sky dome at the time the observation and report is made. More specifically the fraction of the celestial dome covered by condensation trails and cloud masses t h a t have obviously developed from condensation trails, are included in the values reported for cloud amount as follows. (1) rapidly dissipating condensation trails are not included. (2) persistent condensation trails and cloud masses obviously developed from condensation trails are included. (3) in the SYNOP reporting code this occurrence is indicated by adding the contraction " C O T R A " at the end of the message (U.S. Dep. Commer., 1979, p. B3-1). The ease of identification of contrails and their persistence prompted early investigations of the potential radiative, and hence climatological, impact of aircraft condensation trails (Reinking, 1968; Kuhn, 1970). However, estimation of the amount of sky covered is difficult. As part of the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) executed throughout 1987 in Manhattan, Kansas, U.S.A. (Sellers et al., 1988), a number of all-sky cameras were operated. The pilot test period, between March and May of t hat year, has given rise to over 1200 all-sky images. These have not yet been fully analysed since retrieval of sky cover and cloud type and amount is a long and painstaking process. However, a preliminary survey shows that between 10 and 14% of these daylight photographs include jet contrails. This degree of man-made cloud would have a significant impact on cloud amount investigations in regions of considerable commercial aircraft overflights [cf. Changnon, 1981].

Seasonality, temporal and spatial coherence The continental U.S.A. seasonal total cloud amount curves (Fig. 5a) are generally temporally

189

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190

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/ I I I I 1 I I 1900 1910 1920 1930 1940 1950 1960 1970 1980

Fig. 5. (continued).

coherent. T h e S e p t e m b e r / O c t o b e r / N o v e m b e r record is very m u c h m o r e variable t h a n the M A M and J J A records; the next m o s t variable being the D J F record. T h e greatest increase in t o t a l cloud a m o u n t is seen in the J J A record with a rise between 1930 and 1950 of around one t e n t h of sky cover. All the seasonal curves show a drop in total cloud a m o u n t between 1950 and 1953 (this is particularly clear in SON). One possible explanation of this feature is the widespread switch to full 24 h observations of cloud as c o m p a r e d with previous sunrise to sunset only observations. T h i s change in observing practice, discussed above, did not affect all the continental U.S.A. stations and the change was spread over a n u m b e r of years around 1950. T h u s it seems unlikely t h a t this factor is the only cause of this feature. In middle latitudes, Canadian station records exhibit a less seasonal range in variability (Fig. 5b). T h e increase in t o t a l cloud a m o u n t s occurs p r e d o m i n a n t l y in t h e M A M and J J A seasons with a m u c h smaller (and noisier) increase in S O N and very little change in D J F . T h e high latitude seasonal record (Fig. 5c) is considerably disturbed by the very small number of stations reporting in the first half of the century (cf. Fig. 2c). T h e greatest increase in total cloud a m o u n t s is seen in the J J A record (shown in Fig. lc). T h e annual record for these latitudes is similar to t h a t for the middle latit u d e Canadian stations inasmuch as there is no suggestion of increases in total cloud a m o u n t after a b o u t the mid 1950s. Spatial coherence has been investigated by calculating the absolute difference in m e a n annual total cloud a m o u n t for the longest time span in each station record. T h e years and the absolute difference (in tenths of sky cover) between the first year and the last are given in Tables I - I I I . A negative difference m e a n s t h a t cloud a m o u n t has decreased between the two end years of t h a t s t a t i o n ' s record. In the U.S.A. only one station (San Francisco) shows a negative difference. T h e remaining 76 station differences (all positive; m e a n 1.31) have been divided into the four quartile sets b y the median a n d the interquartile range values which

191

are 1.30 tenths, 0.83 tenths and 1.67 tenths respectively; the maximum and minimum values being 2.37 and 0.29 tenths. The distribution of these values does not show any particular spatial coherence with top and lowest quartile stations being practically co-located and widely dispersed (e.g. New Haven and Block Island; Salt Lake City and Winnemucca; Corpus Christi and Galveston). There is, perhaps, a slight tendency for third quartile stations to cluster in the northeast (Nantucket, Eastport, Burlington and Rochester) and second quartile stations in the southeast (Jacksonville, Key West, Pensacola and Mobile) but Binghampton is in the lowest quartile and Tampa in the third. There is no other grouping discernible. Eight of the mid-latitude Canadian stations show negative differences between the first and last records in the data set. However, these include some stations with very patchy records such as Fort Chipewyan, St. George and Father Point. The general trend in this area is for an increase in total cloud amount with a mean value of annually averaged increase of 0.77 tenths (taking only the stations with positive differences in Table II). All 39 stations have been split into four groups by the median and upper and lower quartiles: 0.58, 0.01 and 0.91. The lowest quartile contains the eight stations which show a decrease in cloud amount and no organization can be seen in this group. There is a slight tendency for the more westerly stations to show a greater increase in cloud amount than the area as a whole with Prince Rupert, Victoria, Edmonton and Calgary all appearing in the third quartile group and Vancouver in the top quartile. The central and eastern areas show no discernible grouping with relatively close locations such as Quebec (lowest quartile) and Ottawa (highest quartile) showing different degrees of change. Thirteen of the twenty seven Arctic stations show a decrease in total cloud amount but these are predominantly the stations with shorter records. The longer record stations such as Dawson, Hay River, Fort Simpson, Fort Smith and Fort Good Hope all show increased cloud amount. Moreover all of these stations lie to-

TABLE V Correlation coefficients a n d gradients for a linear fit to t h e a n n u a l l y a n d seasonally averaged d a t a for t h e full record a n d for t h e period of greatest change (i.e. 1929 to 1952) 1900-1982 (1984 in U.S.A.)

1929-1952

Gradient (tenths per decade)

Productmoment correlation

Gradient (tenths per decade)

Productmoment correlation

0.16 0.18 0.16 0.12 0.19

0.879 0.846 0.829 0.769 0.798

0.49 0.48 0.46 0.50 0.48

0.938 0.857 0.776 0.852 0.806

Canada (mid-latitude) Annual 0.11 0.848 DJF 0.08 0.633 MAM 0.12 0.725 JJA 0.13 0.763 SON 0.11 0.708

0.32 0.26 0.36 0.49 0.18

0.846 0.689 0.717 0.857 0.408

Arctic Annual DJF MAM JJA SON

0.44 0.37 0.39 0.50 0.29

0.866 0.538 0.696 0.823 0.527

U.S.A. Annual DJF MAM JJA SON

0.15 - 0.01 0.16 0.27 0.15

0.640 - 0.033 0.594 0.753 0.584

wards the west of the Arctic region of Canada and all occur within the top quartile of the stations except Fort Smith which lies in the third quartile. This may, slightly, reinforce the suggestion from the mid-latitude Canadian stations that clearer increases are seen in the west. Most of the other Arctic station records are too short to warrant detailed spatial analysis. The annual and seasonal trends in cloudiness have been assessed by computing the Pearson product-moment correlation coefficient and the gradient of a best-fit straight line (Table V). For the continental U.S.A. the correlations for the full data record are all positive and highly suggestive. The gradient of t h e u p w a r d trend in total cloud amount is between 0.12 tenths per decade (JJA) and 0.19 tenths per decade (SON). This eighty-four year trend would amount to increases between 1 and 2 tenths of cloud cover if continued to the end of this century. The mid-latitude Canadian station record is less well

192

(but always positively) correlated with date; the annual and J J A correlations being suggestive with associated increases of 0.11 and 0.13 t e n t h s of cloud cover respectively. Only the s u m m e r time (JJA) Arctic correlation is large enough to be of consequence with an associated gradient in

__

--'~Ji~

cloud a m o u n t of 0.27 t e n t h s per decade or 2.7 tenths in a century. T h e other seasons (and the annual case) show increases b u t less strong correlation w i t h D J F showing no association between cloud a m o u n t and d a t e at all. T a b l e V also lists the correlation coefficients

~

~

!

i~\\\

i

~ L A " ~ I A C'

(( / L

li

"

\ ~te~ths) IgOO 1940 1980

)

/

Io0~

Fig. 6. Annually averaged total cloud a m o u n t (20-yr filtered values) for t h e full period of record at each of t h e 143 N o r t h American stations studied (note t h a t s o m e ordinates rise to 6 t e n t h s and o t h e r s to 8 tenths. T h e origin is always 2 t e n t h s and 1900).

193 a n d gradients for the period of t h e records which shows the m o s t rapid change in cloud a m o u n t : t w e n t y - t h r e e years f r o m 1929 to 1952. Generally t h e correlations w i t h d a t e are stronger and the trends are considerable. T h e U.S.A. stations in all seasons and annually, and t h e m i d and high latitude C a n a d i a n stations for the J J A season indicate a t o t a l increase in cloud a m o u n t over this period of around 1 t e n t h of cover. T a b l e V underlines t h a t the continental U.S.A. has suffered an u p w a r d trend in cloud a m o u n t this century w i t h most, b u t b y no m e a n s all, t h e increase occurring between 1930 a n d the early 1950s. C a n a d i a n cloudiness is less seasonally coherent b u t in t h e s u m m e r t i m e ( J J A ) t h e increase in cloud a m o u n t has m a t c h e d t h a t in the U.S.A. in b o t h mid and high latitudes. T h e increases, if sustained, a m o u n t to between one a n d two t e n t h s in a century.

Summary Of the 143 station records examined a l m o s t all of the U.S.A. and low latitude C a n a d i a n records are spatially consistent and t e m p o r a l l y coherent: showing a general t e n d e n c y for t o t a l cloud a m o u n t to increase over the eighty-plus y e a r record with m o s t of the increase occurring between 1930 and 1950. T h i s result is seen clearly in the 20-year filtered annually averaged trends (Fig. 6). Few Arctic station records e n c o m p a s s t h e period of rapid increase in t o t a l cloud a m o u n t a n d m a n y of the annually averaged curves for the Arctic fail to show the increasing t r e n d seen in lower latitudes (Fig. 6 cf. Fig. lc and Fig. 5c). T h e record of t o t a l cloud a m o u n t examined here is continuous b u t n o t homogeneous. T h e heterogeneities are the result, primarily, of the addition of m o r e observations and, especially, n i g h t t i m e observations into the recent (post 1945) record. All o t h e r identified changes in observing practice are of m u c h smaller consequence t h a n these two effects. I t is difficult to see how this increase in the n u m b e r of observations m a d e per d a y could h a v e caused the positive variations in the record. A minority of the stations h a v e suffered m i n o r relocations a n d / o r h a v e short breaks in t h e re-

cord. M o s t are likely to h a v e been subject to the effects of increasing populations: u r b a n t h e r m a l waste, aerosols, increased agriculture and perh a p s soil erosion, aircraft condensation trails. I t is difficult to see w h y these effects would i m p a c t t h e record p a r t i c u l a r l y in the period 1930-1950. T h e r e does not seem to be a n y significant sub-continental scale spatial homogeneity, except t h a t t h e high l a t i t u d e stations do not show an a n n u a l l y averaged increase. T h e r e is a slight suggestion of greater (or m o r e sustained) increases on t h e western side of Canada. Seasonal trends are coherent in t h e U.S.A. b u t m u c h less so in Canada. T h e C a n a d i a n s u m m e r season ( J J A ) does however show trends in t o t a l cloud a m o u n t similar to those sustained t h r o u g h o u t the y e a r in t h e U.S.A. T h e increasing t r e n d in t o t a l cloud a m o u n t h a s now continued for over eighty years in the v a s t m a j o r i t y of N o r t h America s o u t h of 6 0 ° N a n d stations w i t h long records in the region n o r t h of 6 0 ° N also show t h e trend. If t h e trend is sustained to t h e end of this century, t o t a l cloud a m o u n t will h a v e increased on a continentai scale b y between one a n d two t e n t h s of cover in one h u n d r e d years. Such a trend, if it is shown to be real a n d secular, is widespread enough to be considered a global-scale change.

References Angell, J.K., Korshover, J. and Cotton, G.F., 1984. Variation in United States cloudiness and sunshine, 1950-82. J. Climatol. Appl. Meteorol., 23: 752-761. Brooks, C.E.P., 1927. The mean cloudiness over the Earth. Mem. R. Meteorol. Soc., 1(10): 1-138. Cess, R.D. and Potter, G.L., 1987. Exploratory studies of cloud radiative forcing with a general circulation model. Tellus, 39A: 460-473. Changnon, S.A., 1981. Midwestern cloud, sunshine and temperature trends since 1901: possible evidence of jet contrail effects. J. Appl. Meteorol., 20: 496-508. Fitzroy, R., 1863. The Weather Book: A Manual of Practical Meteorology. Longman, Green, Longman, Roberts and Green, London, 2nd ed., 463 pp. Hamilton, K., 1986. Early Canadian weather observers and the "year without a summer", Bull. Am. Meteorol. Soc. 67: 524-532. Hansen, J., Lacis, A., Rind, D., Russell, G., Stone, P., Fung, I., Ruedy, R. and Lerner, J., 1984. Climate sensitivity: analysis of feedback mechanism. In: J.E. Hansen and T. Takahashi (Editors), Climate Processes and Climate

194 Sensitivity. Am. Geophys. Union, Washington, D.C., pp. 130-163. Henderson-Sellers, A., 1986. Increasing cloud in a warmer world. Clim. Change, 9: 267-309. H.M.S.O., 1977. The Marine Observer's Handbook. HMSO, London, 10th ed., 157 pp. Jones, P.D., Wigley, T.M.L., Folland, C.K., Parker, D.E., Angell, J.K., Lebedeff, S. and Hansen, J.E., 1988. Evidence for global warming in the last decade. Nature, 332: 790. Karl, T.R., Kukla, G. and Gavin, J., 1986. Relationship between decreased temperature range and precipitation trends in the United States and Canada, 1941-80. J. Climatel. Appl. Meteorol., 25: 1878-1886. Kuhn, P.M., 1970. Airborne observations of contrail effects on the thermal radiation budget. J. Atmos. Sci., 27: 937-942. Lee, J.E. and Johnson, S.D., 1985, Expectancy of cloudless photographic days in the contiguous United States. Photogr. Eng. Remote Sens., 51: 1883-1891. Malberg, H., 1973. Comparison of mean cloud cover obtained by satellite photographs and ground based observations over Europe and the Atlantic. Mon. Weath. Rev., 101: 893-897. McGuffie, K. and Henderson-Sellers, A., 1987. Will clouds produce a negative feedback in a CO2-warmed world? In:

Proc. IAHS, Symp. " T h e Influence of Climate Changes and Climatic Variability on Hydrology and Water Resources", I.U.G.G., Vancouver, August 1987. McGuffie, K. and Henderson-Sellers, A., 1988. Is Canadian cloudiness increasing? Atmos. Ocean, 26: 608-633. Merritt, E.S., 1966. On the reliability and representativeness of sky cover observations. J. Appl. Meteorol., 5: 369. Reinking, R.F., 1968. Insolation reduction by contrails. Weather, 23: 171-173. Sellers, P.J., Hall, F.G., Asrar, G., Strebl, D.E. and Murphy, R.E., 1988. The First ISLSCP Field Experiment (FIFE), Bull. Am. Meteorol. Soc., 69: 22-27. S~ze, G., Drake, F., Desbois, M. and Henderson-Sellers, A., 1986. Total and low cloud amounts over France and southern Britain in the summer of 1983: comparison of surface-observed and satellite retrieved values. Int. J. Remote Sens., 7: 1031-1050. Shaw, N., 1926. Manual of Meteorology, Volume I: Meteorology in History. Cambridge Univ. Press, Cambridge, 339 pp. Shaw, N., 1936. Manual of Meteorology, Volume II: Comparative Meteorology. Cambridge University Press, Cambridge, 2nd ed., 472 pp. U.S. Department of Commerce, 1979. Federal Meteorological Handbook No. 2. Synoptic Code, U.S. Dep. Commer., U.S. Dep. Def., U.S. Dep. Transp.