- Email: [email protected]
Atmospheric turbidity over India from solar radiation measurements
Solar Energy, Vol. 14, pp. 18~;-195. Pergamon Press. 1973. Printed in Great Britain
ATMOSPHERIC
TURBIDITY OVER INDIA RADIATION MEASUREMENTS
FROM
SOLAR
A. M A N I ? O. CHACKO~ and N. V. IYER~
(Received 23 August 1971) Abstract- Atmospheric turbidity and pollution have been studied in India for many years. This has been done by means of spectral measurements of direct solar radiation using Angstr6m pyrheliometers and Volz sun photometers. Radiation attenuation due to dust and haze is a phenomenon of great importance in India during the premonsoon months. Most of the dust content is removed by precipitation during the monsoon, but the haze region is established again as winter sets in over north India. An examination of atmospheric turbidity data over India during the last ten years shows that turbidity has doubled at almost all stations since 1961. This would appear to be associated with increasing pollution caused by rapid industrialization in the various areas under study. R 6 s u n ~ - L a turbidit6 et la pollution atmosphdriques ont 6t6 6tudides depuis de Iongues annges en lnde/~ partir de mesures spectrales de la radiation solaire directe, en utilisant des pyrhdliom6tres Ptngstrtim et des photom/~tres Volz. Un phdnom~ne de grande importance en lnde est I'attdnuation de la radiation, due h la poussi~re et ia brume pendant les topis prdcddant la mousson. La plupart de la poussi/~re disparalt avec les prdcipitations pendant la mousson mais la brume se reforme au d6but de I'hiver sur le Nord de I'Inde. Un examen de la turhidit6 atmosphdrique de l'Inde au cours des 10 derni6res anndes montre que la turbidit6 a doubl6 depuis 1961 b. presque toutes les stations, il semble que ceci spit lid ~ une pollution croissante, causde par I'industrialisation rapide des diverses r~gions 6tudi6es. Resumen-La turbidez y la contaminaci6n atmosf6rica han venido siendo estudiadas en la India durante muchos afios a partir de medidas de la radiaci6n solar directa, mediante el uso de pirheli6metros ,~ngstr6m y fot6metros solares Volz. La atenuaci6n de la radiaci6n como consecuencia del polvo y la nieblina es un fen6meno muy importante que se produce en la India durante los meses premonz6nicos. E1 contenido de polyp el eliminado en su mayor parte por la precipitaci6n durante el monz6n, pero la regi6n de nieblina vuelve a establecerse al incidir el invierno en el norte de la India. Un examen de la informaci6n relativa a la turbidez atmosf6rica en la India durante el tiltimo decenio muestra qu 6sta se ha duplicado en casi todas las estaciones desde el afio 196 I. Ello parece relacionarse con la contaminacidn producida pot la industrializaci6n r~ipida en las diversas ~ireas objeto de estudio. INTRODUCTION
IT Is well known that the atmosphere over the tropics is generally more turbid than that over the temperate latitudes, and that turbidity on high mountain peaks is markedly less than that observed at sea-level stations. Angstr6m[1] reported a mean value for the tropics for his turbidity coefficient/3 of approximately 0.12, which rapidly decreases with increasing latitude, and evolved an empirical relationship to correlate the variations of/3 with latitude and altitude. While this may be accepted as generally valid,/3 should also be expected to vary with seasonal and local changes in meteorological factors and long-term secular variations in the composition of the atmosphere. Values of Angstr6m's turbidity coefficient/3 have been determined at a number of stations in India since the IGY and the results published[2-5]. The earlier computations of /3 had been made on the assumption that the wavelength exponent a in ,~ngstr6m's empirical formula is a constant and has a value of 1.3. A reexamination * India Meteorological Department, New Delhi, India. t India Meteorological Department, Poona, India. 185
186
A. MANI, O. CHACKO and N. V. IYER
of the observational data showed that this assumption was not applicable at all stations in all seasons, particularly at Delhi (28°N) during the summer months and that a should be determined as far as possible with/3 in every case. Serious errors, however, exist in the evaluation of a and fl from pyrheliometric measurements. These are inherent in the very measurement of radiation integrals for specified spectral regions using broad-band pass filters. Routine filter measurements of solar radiation at network stations are also subject to large observational errors depending on the skill of the observers. While too much importance cannot therefore be attached to values of a and /3 derived from routine pyrheliometdc filter measurements of solar radiation, a study of fl can yield numerical values which can be compared over time and space and which are of practical value in synoptic or climatological studies of scattering by haze. The results can also be considered to provide a broad view of the transmission conditions of the atmosphere and to give information on the dependence of the atmospheric turbidity on air masses, seasons, latitude, altitude, and local atmospheric conditions. Angstr6m's turbidity coefficient fl refers to a wavelength 1.0/.tm outside the visible spectrum, and therefore its actual magnitude is of only abstract interest. The main advantage is that fl gives a direct idea of the fraction of incident solar radiation scattered and absorbed by aerosols and when multiplied by 2 gives a direct measure of the amount of radiation scattered in the visible region. Schuepp's turbidity coefficient B [6], on the other hand, refers to the wavelength 0.5 ~ in the central part of the visible spectrum. The two coefficients are related by the formula B = ft. 2'~ . log e. ,~ngstr6m[7] found that the values of fl are related to B by a constant factor which is practically independent of a and only slightly differs from unity, i.e., B = 1.05 fl when a = 1.3. Measurements of B using Schuepp's method involve rather complicated computations and a very high degree of precision in measurement. Values of a,/~, and B were therefore computed using Angstr6m's simplified method[l], A simple direct method for the measurement of B exists, that developed by Volz [8] using his sunphotometer. The paper presents results of measurements of atmospheric turbidity with pyrheliometers and sunphotometers and discusses the seasonal, annual, and secular variations at 12 stations in India. OBSERVATIONS Values of the intensity of direct solar radiation at normal incidence for the whole spectrum and with the three filters RG2, OG1, and RG8 are obtained using Angstr6m pyrheliometers of all 12 principal radiation stations in India whenever weather conditions permit and at all main synoptic hours of observation from sunrise to sunset. Measurements of the Schuepp's turbidity coefficient B using Volz sunphotometers are made at 4 stations at the regular synoptic hours. Direct solar radiation measurements can be made only when the skies are relatively clear and the turbidity data therefore relate to a particular meteorological condition. The data also give only day-time turbidity values and do not represent all conditions, particularly those in winter when strong temperature inversions exist at night. No
Atmospheric turbidity ov©r India
187
observations are generally possible during the four monsoon months from June to September when the skies are overcast with low clouds and the data are limited to the remaining eight months October to May. RESULTS
(a) Turbidity m e a s u r e m e n t s at Poona. Table l lists the mean monthly value of B, a, Bcale., and Bobs. at Poona from October 1968 to May 1969. The observations were specially made to verify whether or not careful measurements taken by skilled observers Table 1. Mean monthly values of atmospheric turbidity parameters at Poona Months October November December January February March April May Mean
1968 1968 1968 1969 1969 1969 1969 1969
fl
a
B~.l~
Bo.~
0.044 0.050 0.046 0.070 0.072 0.058 0.107 0.122
0.9 1.2 0.9 0.8 0-9 0.9 1.1 1.2
0.070 0.081 0-086 0-106 0.100 0.102 0.192 0.231
0.070 0.075 0.070 0.103 0.108 0. 118 0.137 0.162
0.071
1.0
0.121
0.105
can lead to consistent and reliable results. T h e results show they can. a shows no seasonal variations and is remarkably constant with a mean value 1 . 0 _ 0-2. This is not very different from the value generally accepted as valid over a wide range of conditions and indicates that the wavelength dependence of the scattering of the atmosphere over Poona has the classical ~X-1'~ proportionality, and the size of the scattering particles consequently is about the same as that found for high altitude stations where Junge's power law holds good. While the wavelength exponent ~ is constant, turbidity shows a striking seasonal variation, with maximum values in the dry, hot premonsoon " s u m m e r " months, April to May, and minimum during the "winter" months, October to November. Figure i and Fig. 2(a) show the annual march of B and fl at Poona for the years 1959, 1964, and 1969. T h e r e is a steady increase in turbidity from October to May. So while the particle concentration increases in summer to three to four times the winter value, the particle size distribution remains basically unchanged throughout the year and is independent of ft. T h e pronounced maximum in/3 in the hot dry summer months indicates that the source of the aerosols is of entirely local origin, and that turbidity depends on conditions in the first few km of the atmosphere and the dispersion of dust from the ground to higher levels. Even during these highly-turbid months the turbidity can show remarkably sudden falls in value, for example, immediately after a thunderstorm when the sky, which was a drab white, turns a startling blue with values of B as low as 0-006. Rainout within clouds and washout below clouds are known to be especially effective in the wet removal of aerosols from the atmosphere. Turbidity shows a marked fall as soon as the monsoon set in, although it rises again whenever the monsoon is weak and the
188
A. MANI, O. C H A C K O and N. V. IYER 0.20 0.1g
ii /I
0"16
t
I
// m.
I
0.14
iI
0"12
I '~
0.10
~
0.OlJ
/
I
I
I
I
I
1969
0.06
0,04 0-02
I F
I M
I A
F i g , !, T u r b i d i t y
I M
I I J d Months
coemcint/3
0.10
A / \
/ / ....
0-08
0-06
/
/
/
I A
I $
a t P o o n a in 1 9 6 4 a n d 1 9 6 9 .
\
1959 \
Poono
\~
0-02
•._u ~
0
1969
-----
/
j,/
0.04 ~.
I 0
/
//
%% •
sI
(a) f~s~
0-12
.~
/
OqO
/ /
I-
/
/
/
~ ~ ~
/
1959 0elhl ~
----
1969
~ " ~
/
/--~
x
0.08 0"0
0-04 0.02 I F
I M
I A
I M
i d
I d
I A
l $
I 0
I N
0
Months
Fig. 2. Turbidity coefficient/3 at Delhi and Poona in 1959 and 1969.
Atmospheric turbidity over India
189
effectiveness of the rainout or washout process is reduced. T h e values remain low throughout winter. (b) Turbidity m e a s u r e m e n t s at Delhi. Conditions o v e r Delhi (28°N) in close proximity to the Rajasthan desert are completely different from those over Poona (18°N) a hill station 555 metres above sea level. T h e values of/3 and B at Delhi are about two to three times those over Poona and the seasonal variations are also more pronounced, with maximum values in summer and minimum in winter [Fig. 2(b)]. As a result of the later arrival of the monsoon over Delhi, the fall in turbidity occurs in July at D e l h i - n o t in June as at Poona. The passage of western disturbances during winter also contributes to the low values in winter over Delhi. An examination of the values of a during the different seasons shows that in winter it is not far from the classical value of 1.3, but in summer it is often zero or negative. T h e marked seasonal variation in both fl and a shows that both the particle concentration and the particle size distribution vary with the seasons at Delhi and that smaller particles are more numerously represented in winter when the turbidity is low and large numbers of particles of large size predominate in summer when the turbidity is high. Studying two years" data at Delhi, Rangarajan[9] came to the conclusion that aerosol scattering over north India in summer is independent of wavelength and aerosol particles have a particle size distribution different from that in temperature latitudes. He concluded that, since the increase of turbidity in summer is caused by the introduction on a massive scale of large and giant particles into the atmosphere by dustraising winds and dust storms over central and north India (reducing visibility to a few km or less), the larger than normal aerosol was responsible for bringing down the values of c~to near zero. Ramanathan and Karandikar[10] had shown earlier that ~ is either zero or even negative in summer over Delhi, i.e., the depletion of solar radiation by haze scattering was larger for 0.448/zm than at 0.330/zm. T h e y also found that the depletion, and therefore the scattering by haze was for the most part independent of wavelength. An increase in haze scattering at longer wavelengths and the negative value ofc~ was ascribed to an increase in mean particle size in summer over north India, when light of higher wavelength is scattered more than that of shorter wavelength. That a deep dense layer of dust and haze lies over central and north India during the hot premonsoon months with a maximum concentration from March to June is well known. Kendrew [ 11 ] states +'the sky is almost cloudless, but it cannot be described as clear or blue, since there is a constant dust haze, a grey pall through which the sun shines as a pale disc". Bryson et al.[12], who revived interest in the phenomenon and made an extensive study, called it ++a great brown sea lapping against the Himalayas" extending as it does across the whole of north India, with its top at about 10 km over the Rajasthan desert, lowering to about 5 km between Delhi and Calcutta. I A F aircraft observations made in 1963 showed that the haze varies from day to day in density, depth, and spatial distribution, being sometimes layered and sometimes uniform up to 10 kin. These observations were confirmed during the dust patrol organized by Bryson and his colleagues over India in 1966 (Peterson[13]). T h e y found the thickest layer to be confined to 3 km with dust concentrations as high as 1070/.~g/m '~ in the first few metres of the atmosphere and 700-800/zg/m a at higher levels with less dense layers up to 5 km over north and central India in April and May. T h e r e was no gradual decrease of dust concentration with height. Another interesting feature was the high altitude layers
190
A. M A N I , O. C H A C K O
arid N. V. I Y E R
of various thicknesses up to 7-8 km, above which the quantity of aerosol mass was less than 15/~g/m 3. A mineralogical analysis showed the dust consisted mainly of quartz with feldspar, mica, and carbonate. Kondratyev et a/.[14] have reported, from an analysis of the profiles of direct solar radiation fluxes from ten balloon flights, layers in the troposphere and stratosphere where increased concentration of attenuation components (water vapour and aerosols) are grouped. Over middle Soviet Union they reported aerosol layers below 2 - 4 km and between 6-10 kin. Aerosol layers were usually found above and below the tropopause and mostly over the inversions and between them in the stratosphere. (c) Turbidity at coastal stations. The annual march of/3 at Bhavnagar (22°N), a coastal station, is shown in Fig. 3. All coastal stations show fairly high values of
044 /
Bhovnogor
A / ~ ~
(1967-1969) Shillong -1969)
.....
O.i~' ~e" 0-10 .o
~ o-~ "12-
O-O2 I F
I M
I A
I M
I J
I J
I A
14s / S
I 0
I N
O
Months Fig. 3. A n n u a l
variation of turbidity
coefficient /3 at B h a v n a g a r and Shillong
( ! 9 6 7 - ] 969).
ranging from 0.04 to 0.14 although the atmosphere is comparatively dustfree in contrast to continental stations in the north. As pointed out by Yamamoto et al.[15], 13 increases with an increase in precipitable water content and should be high at humid coastal stations. (d) Turbidity at hill stations. As may be expected, turbidity at all hill stations is low. At Shiliong (26°N) 1500 m above mean sea level,/3 has only a value of 0.07 in summer, 0.02 in winter, and is almost zero during the monsoon months (Fig. 3). At Kodaikanal (10°N) 2340m above mean sea level,/3 was as low as 0.01 in February 1965. Measurements of turbidity made at Gulmarg (34°N) in Kashmir 3000 m above sea level in June gave values for ~ as high as 0.04-0.07. Turbidity measurements in the Everest region (28°N) 6200 masl also gave unexpectedly high values of the order of 0.06 (/~ngstr6m and Drummond [ 16]). Bishop's observations at the Silver Hut glacier were made in April and May when a dense pall of dust lies over central and north India and extends to 6-10 km into the atmosphere. All north Indian mountain stations in the south are comparatively free from turbidity. Even at Kodaikanal, as at Manna Loa,
Atmospheric turbidity over India
191
the skies are a deep blue in the mornings with the haze lying like a brown blanket over the plains below. In the afternoons the haze rises as a result of increased turbulent convection and the dispersion of dust to higher levels, and skies become hazy. (e) Transmissivity of the atmosphere. The same monthly values of the transmission coefficient q computed from ! = !o. qm where I and Io are the values o f the direct solar radiation received at the surface and outside the atmosphere and m is the optical air mass, show that the atmosphere is more transparent during winter and monsoon than during the summer months. Only about 64 per cent of the incoming direct solar radiation is transmitted during June over Poona, while 77 per cent is transmitted in December. At Delhi the corresponding figures are 55 and 76 per cent. As much as half the solar radiation is absorbed and scattered over Delhi, Jodhpur, and Ahmedabad in summer. SPATIAL DISTRIBUTION
O F /3 O V E R I N D I A
The geographical distribution of/3 during the winter, summer, and postmonsoon seasons and for the whole year is shown in Fig. 4. Yamamoto's method[l 5] of graphic representation has been followed since atmospheric turbidity changes occur generally in mesoscale and the drawing of isopleths of ,8 is not quite justified, particularly when one considers the sparseness of both stations and data. The diameter of each circle indicates seasonal average value of/3 for that station. It will generally be seen that for the whole year,/3 values are higher over central and north India than over the peninsula and over the subcontinent. As a whole, they are highest during the hot, dry summer months and least during winter and post monsoon months. Calcutta is an exception with high turbidity values, obviously the result of high local industrial pollution. The seasonal variations of 13 indicate that dust emanating from the earth's surface constitutes a major portion of the aerosol over the subcontinent. It is to be expected that the time of maximum dust content will correspond to the time of largest temperature gradient in the lower atmospheric layers and the maximum dispersion of dust from the ground to the higher layers of the atmosphere.
TURBIDITY
IN A I R M A S S E S
The air over western, central, and northern India in the summer months is tropical continental air To, with its source region in southwest Asia. It is the driest and hottest air over the Indian subcontinent, with marked instability, intense insolation, and turbulence, leading to the development of dust-raising winds and dust storms which persist for days. The haze persists as a dirty white milky canopy over a large part of north India when upper winds weaken. The summer monsoon sets over the peninsula in early June. The moist, southwesterly surface winds move rapidly northward across the country until by July virtually all of India is influenced by the system. With the establishment of the monsoon a highly-humid, cool, and conventively-indifferent equatorial maritime air lies over south and central India during the monsoon months, June to September, characterized by mostly cloudy to overcast skies, frequent rain, and drizzle, in the north this air mass becomes modified as it turns west round the seasonal monsoon trough to the north. The beginning of the cold season is marked by the retreat of the monsoon, which
S E V d , 14, No. 2 - G
192
A. MANI, O. CHACKO and N. V. IYER
Wintgr
© SHLO
v©
o
TRV
post mnl.~n
Annuol
©
© JDPO DLH
SHLo
~,HM
CAL(~J~
~_PNANGP~s K
OLH
~AHM
CALf ~ a ~
~PNA
~ TRV ~.J
SHLO
~VSK MDS
TRV
Fig.4.Distribution of turbidity coefficient/3. occurs at a more leisurely pace than the onset, the mean date ranging from early September in the north to late October in the south. During the winter months, December to February, the modified cold dry air over north and central India is of continental polar origin Pc characterized by convective stability, clear skies and good visibility, except for local mist in the morning and occasional haze in the afternoon. The winter haze is light brown in colour, generally settles during the night, rises up in the forenoon, and gets dissipated by thermal turbulence and mixing with upper winds by noon. Representative values of the atmospheric tubididty coefficient/3 for the three airmasses T~ over western and north India in summer, P~. over north and central India in summer, and E,, over south and central India during the monsoon months, have been
Atmospheric turbidity over India
193
calculated from values of typical stations in the north, centre and south of the subcontinent. Te/3 = 0.20 E,,/3 = 0.05 P,/3 = 0.05 SECULAR VARIATIONS OF TURBIDITY COEFFICIENT
Figure 1 and Fig. 2(a) and (b) give two sets of curves showing the annual march of /3 at Delhi and B and fl at Poona in 1959, 1964, and 1969 respectively. While the month-to-month variations are basically similar, both stations show a two to eight fold increase in B and/3 since 1959. Figure 5 shows the year-by-year variations in/3 at both Poona and Delhi from 1958 to 1969. While there are marked changes in/3 from year-to-year, the general trend is a two-fold increase with the years, presumably associated with the increase in industrialization, urbanization, and traffic at both stations since 1958. Figure 6 shows the year-by-year variation of direct solar radiation I, global solar radiation G, and diffuse solar radiation D at Poona from 1958 to 1968. Figure 7 shows similar values for Delhi. Here again the pattern is similar, a small decrease in the intensity of direct solar radiation over the years at both stations and a slight increase in diffuse solar radiation with time. While there is no marked decrease in the global solar radiation at Poona, at Delhi there is a steady decrease with the years. Figures 5, 6, and 7 show marked increases in turbidity occur in 1963 and 1966. These would appear to be associated with the increase in particulate matter injected into the atmosphere by Mount Agung and Taal volcano eruptions of 1963 and 1965. Raghavan and Yadhav [17], in a study of the depletion of solar radiation by particulate matter in the atmosphere over Delhi, reported unexpectedly high values in 1962 and 1964-1965. Kondratyev et al.[ 14] also reported similarly high values.
0-14 .....
0'12
New Delhi Poono
~. 0-I0 ,s J"
/."
._u o.o6 /
0"06
/ r - - -- ..i l i /
" i 0-04 ~ . v p
j
0.02 o
1958
i
I
1960
i
I
1962
i
I
1964
i
I
1966
i
Years
Fig. 5. Variation of turbidity, year by year ( ! 958-1969).
I
1'968
194
A. MANI, O. C H A C K O and N. V. IYER .E E
_
,~:
................
.
Poono
''
- ~~ t2O -
~-NE '8 ~ 0 -
G,oba,
-----
-I,.~
Diffuse radiation
- . . . . . Direct solar radiation
",
*"'"""
""'"'~ ..............
/ I
11'2
I10
0~"
II
ulO0
1.0
x/
io,m
66
5~0
i
1956
I
i
1960
I
i
1962
I
~
1964
I
I
i
1966
1960
0.6
.~-
0
Yeors
Fig. 6. Variation of global, diffuse and direct solar radiation, year by year, for Poona. .G
E
- /" "', "" "~ "'~ .
Delhi
.
.
.
.
.
.
----
.
lAX
, Global 1.2 Diffuse rodiotion Direct solor mdiotion
...........
i/~',,.
f,
/" 0.9
• -
._~
i
"~
/ ' ~ .\,
,=L
~o_~-,,ok, /
-i
~
/
r
901 19~
i
x..
'~ J
"
v
\/ i
I 1962
O
o.e _~
/~
\ / I 1960
..
I/
/\
'
oioo/"/ ~00_
/
I 1964
J
0.7 .g
\ I I~
L
0.6
i
~ o o
0.5 ~
Years
._~ 0
Fig. 7. Variation of global, diffuse and direct solar radiation, year by year, for Delhi. CONCLUSION
A study of the atmospheric turbidity from solar radiation measurements at a numbel of stations shows that (l) Turbidity over India shows a marked annual variation with very high values ir summer, about 2 to 3 times those in the postmonsoon and winter seasons. (2) in north and central India during summer the atmosphere is 3 to 4 times as
Atmospheric turbidity over India
195
turbid as the rest of the country. Values of tx are very low, often becoming zero or negative. Under these conditions haze scattering is independent of wavelength and neutral. Even at high altitude stations in the Himalayas the turbidity is high, as a result of the dense layer of dust that extends to 6- l 0 km into the atmosphere over north and central India in this season. (3) Reduction of turbidity as a result of rainout and washout in precipitation is observed at all stations, both in the premonsoon thunderstorm season and during the monsoon months. (4) An examination of turbidity values over India during the last ten years shows that/3 has doubled at almost all stations since 1958. This would appear to be associated with increasing pollution caused by rapid industrialization and urbanization in the areas under study. (5) Marked decreases in direct solar radiation and increases in turbidity are noticed in 1963 and 1965 and these are presumably associated with the increase of particulate material injected into the atmosphere by Mount Agung and Taal volcano eruptions in 1963 and 1965. REFERENCES [ 1] A. ,Angstr6m, Techniques of determining the turbidity of the atmosphere. Tellus 13, 214 ( 1961 ). [2] O. Chacko and V. Desikan, Atmospheric turbidity measurements over India. Ind. J. Met. Geophys. 16 649-664 (1967). [3] O. Chacko, V. Desikan and N. V. lyer, Atmospheric turbidity and aerosol content of the atmosphere over india. Proceedings o f the IQS Y Symposium N e w Delhi, pp. 666-672, (1966). [4] A. Mani and O. Chacko, Measurement of solar radiation and atmospheric turbidity with ,Angstr~im pyrheliometers at Poona and Delhi during the IGY. Ind. J. Met. Geophys. 14, 220 (1963). [5] A. Mani, O. Chacko and S. Hariharan, A study of Angstr~im turbidity parameters from solar radiation measurements in lndi~. Tellus 21,829-843 (1969). [6] W. Schuepp, Die Bestimmung der Komponenten der atmospharischen Trubung aus Aktinometermessungen. A rchs Met. Geophys. Biokim. 1,257 (1949). [7] A. ,~,ngstr/~m, The parameters of atmospheric turbidity. Tellus 16, 64 (1963). [8] F. Volz, Spektrale Measungen der sonnenstrahlung und Trubungsestimmung mit Selenzellen-Klein photometeren. Bet. D. Weterd. 2, 13 (1959). [9] S. Rangarajan, Studies on atmospheric ozone and solar radiation. Ph.D. Thesis, University of Poona, (1966). [10] K. R. Ramanathan and R. V. Karandikar, Effect of dust and haze on measurements of atmospheric ozone made with Dobson's spectrophotometers. Quart. J. R. Met. Soc. 75,257-267 (1949). [ I i ] W. G. Kendrew, The Climates o f the Continents, p. 473. New York, Oxford University Press (1942). [12] R. A. Bryson, C. W. Wilson and P. M. Kuhn, Some preliminary results from radiation sonde ascents over India. Symposium on Meteorology, Rotorua, New Zealand (1963). [l 3] J. T. Peterson, Measurement of atmospheric aerosols and infrared radiation over Northwest India and their relationship. Technical Report No. 38. The University of Wisconsin (January 1968). [14] K. Y. Kondratyev, G. A. Nikolsky, I. Y. Badinov and S. D. Andreev, Director solar radiation up to 30km and stratification of attenuation components in the stratosphere. Appl. Optics 6, 197-207 (1967). [15] G. Yamamoto, M. Tanaka and K. Arao, Hemispherical distribution of turbidity coefficient as estimated from direct solar radiation measurements. J. met. Soc. Japan. 46, 267 (1968). [16] A. Angstr6m and A. J. Drummond, Note on solar radiation in mountain regions at high altitude. Tellus 18.801-808 (1968). [ 17] S. Raghavan and B. R. Yadhav, Depletion of solar radiation by particulate matter in the atmosphere: A study with special reference to Delhi. Ind. J. Met. Geopys. 17,607 (1965).
ATMOSPHERIC
TURBIDITY OVER INDIA RADIATION MEASUREMENTS
FROM
SOLAR
A. M A N I ? O. CHACKO~ and N. V. IYER~
(Received 23 August 1971) Abstract- Atmospheric turbidity and pollution have been studied in India for many years. This has been done by means of spectral measurements of direct solar radiation using Angstr6m pyrheliometers and Volz sun photometers. Radiation attenuation due to dust and haze is a phenomenon of great importance in India during the premonsoon months. Most of the dust content is removed by precipitation during the monsoon, but the haze region is established again as winter sets in over north India. An examination of atmospheric turbidity data over India during the last ten years shows that turbidity has doubled at almost all stations since 1961. This would appear to be associated with increasing pollution caused by rapid industrialization in the various areas under study. R 6 s u n ~ - L a turbidit6 et la pollution atmosphdriques ont 6t6 6tudides depuis de Iongues annges en lnde/~ partir de mesures spectrales de la radiation solaire directe, en utilisant des pyrhdliom6tres Ptngstrtim et des photom/~tres Volz. Un phdnom~ne de grande importance en lnde est I'attdnuation de la radiation, due h la poussi~re et ia brume pendant les topis prdcddant la mousson. La plupart de la poussi/~re disparalt avec les prdcipitations pendant la mousson mais la brume se reforme au d6but de I'hiver sur le Nord de I'Inde. Un examen de la turhidit6 atmosphdrique de l'Inde au cours des 10 derni6res anndes montre que la turbidit6 a doubl6 depuis 1961 b. presque toutes les stations, il semble que ceci spit lid ~ une pollution croissante, causde par I'industrialisation rapide des diverses r~gions 6tudi6es. Resumen-La turbidez y la contaminaci6n atmosf6rica han venido siendo estudiadas en la India durante muchos afios a partir de medidas de la radiaci6n solar directa, mediante el uso de pirheli6metros ,~ngstr6m y fot6metros solares Volz. La atenuaci6n de la radiaci6n como consecuencia del polvo y la nieblina es un fen6meno muy importante que se produce en la India durante los meses premonz6nicos. E1 contenido de polyp el eliminado en su mayor parte por la precipitaci6n durante el monz6n, pero la regi6n de nieblina vuelve a establecerse al incidir el invierno en el norte de la India. Un examen de la informaci6n relativa a la turbidez atmosf6rica en la India durante el tiltimo decenio muestra qu 6sta se ha duplicado en casi todas las estaciones desde el afio 196 I. Ello parece relacionarse con la contaminacidn producida pot la industrializaci6n r~ipida en las diversas ~ireas objeto de estudio. INTRODUCTION
IT Is well known that the atmosphere over the tropics is generally more turbid than that over the temperate latitudes, and that turbidity on high mountain peaks is markedly less than that observed at sea-level stations. Angstr6m[1] reported a mean value for the tropics for his turbidity coefficient/3 of approximately 0.12, which rapidly decreases with increasing latitude, and evolved an empirical relationship to correlate the variations of/3 with latitude and altitude. While this may be accepted as generally valid,/3 should also be expected to vary with seasonal and local changes in meteorological factors and long-term secular variations in the composition of the atmosphere. Values of Angstr6m's turbidity coefficient/3 have been determined at a number of stations in India since the IGY and the results published[2-5]. The earlier computations of /3 had been made on the assumption that the wavelength exponent a in ,~ngstr6m's empirical formula is a constant and has a value of 1.3. A reexamination * India Meteorological Department, New Delhi, India. t India Meteorological Department, Poona, India. 185
186
A. MANI, O. CHACKO and N. V. IYER
of the observational data showed that this assumption was not applicable at all stations in all seasons, particularly at Delhi (28°N) during the summer months and that a should be determined as far as possible with/3 in every case. Serious errors, however, exist in the evaluation of a and fl from pyrheliometric measurements. These are inherent in the very measurement of radiation integrals for specified spectral regions using broad-band pass filters. Routine filter measurements of solar radiation at network stations are also subject to large observational errors depending on the skill of the observers. While too much importance cannot therefore be attached to values of a and /3 derived from routine pyrheliometdc filter measurements of solar radiation, a study of fl can yield numerical values which can be compared over time and space and which are of practical value in synoptic or climatological studies of scattering by haze. The results can also be considered to provide a broad view of the transmission conditions of the atmosphere and to give information on the dependence of the atmospheric turbidity on air masses, seasons, latitude, altitude, and local atmospheric conditions. Angstr6m's turbidity coefficient fl refers to a wavelength 1.0/.tm outside the visible spectrum, and therefore its actual magnitude is of only abstract interest. The main advantage is that fl gives a direct idea of the fraction of incident solar radiation scattered and absorbed by aerosols and when multiplied by 2 gives a direct measure of the amount of radiation scattered in the visible region. Schuepp's turbidity coefficient B [6], on the other hand, refers to the wavelength 0.5 ~ in the central part of the visible spectrum. The two coefficients are related by the formula B = ft. 2'~ . log e. ,~ngstr6m[7] found that the values of fl are related to B by a constant factor which is practically independent of a and only slightly differs from unity, i.e., B = 1.05 fl when a = 1.3. Measurements of B using Schuepp's method involve rather complicated computations and a very high degree of precision in measurement. Values of a,/~, and B were therefore computed using Angstr6m's simplified method[l], A simple direct method for the measurement of B exists, that developed by Volz [8] using his sunphotometer. The paper presents results of measurements of atmospheric turbidity with pyrheliometers and sunphotometers and discusses the seasonal, annual, and secular variations at 12 stations in India. OBSERVATIONS Values of the intensity of direct solar radiation at normal incidence for the whole spectrum and with the three filters RG2, OG1, and RG8 are obtained using Angstr6m pyrheliometers of all 12 principal radiation stations in India whenever weather conditions permit and at all main synoptic hours of observation from sunrise to sunset. Measurements of the Schuepp's turbidity coefficient B using Volz sunphotometers are made at 4 stations at the regular synoptic hours. Direct solar radiation measurements can be made only when the skies are relatively clear and the turbidity data therefore relate to a particular meteorological condition. The data also give only day-time turbidity values and do not represent all conditions, particularly those in winter when strong temperature inversions exist at night. No
Atmospheric turbidity ov©r India
187
observations are generally possible during the four monsoon months from June to September when the skies are overcast with low clouds and the data are limited to the remaining eight months October to May. RESULTS
(a) Turbidity m e a s u r e m e n t s at Poona. Table l lists the mean monthly value of B, a, Bcale., and Bobs. at Poona from October 1968 to May 1969. The observations were specially made to verify whether or not careful measurements taken by skilled observers Table 1. Mean monthly values of atmospheric turbidity parameters at Poona Months October November December January February March April May Mean
1968 1968 1968 1969 1969 1969 1969 1969
fl
a
B~.l~
Bo.~
0.044 0.050 0.046 0.070 0.072 0.058 0.107 0.122
0.9 1.2 0.9 0.8 0-9 0.9 1.1 1.2
0.070 0.081 0-086 0-106 0.100 0.102 0.192 0.231
0.070 0.075 0.070 0.103 0.108 0. 118 0.137 0.162
0.071
1.0
0.121
0.105
can lead to consistent and reliable results. T h e results show they can. a shows no seasonal variations and is remarkably constant with a mean value 1 . 0 _ 0-2. This is not very different from the value generally accepted as valid over a wide range of conditions and indicates that the wavelength dependence of the scattering of the atmosphere over Poona has the classical ~X-1'~ proportionality, and the size of the scattering particles consequently is about the same as that found for high altitude stations where Junge's power law holds good. While the wavelength exponent ~ is constant, turbidity shows a striking seasonal variation, with maximum values in the dry, hot premonsoon " s u m m e r " months, April to May, and minimum during the "winter" months, October to November. Figure i and Fig. 2(a) show the annual march of B and fl at Poona for the years 1959, 1964, and 1969. T h e r e is a steady increase in turbidity from October to May. So while the particle concentration increases in summer to three to four times the winter value, the particle size distribution remains basically unchanged throughout the year and is independent of ft. T h e pronounced maximum in/3 in the hot dry summer months indicates that the source of the aerosols is of entirely local origin, and that turbidity depends on conditions in the first few km of the atmosphere and the dispersion of dust from the ground to higher levels. Even during these highly-turbid months the turbidity can show remarkably sudden falls in value, for example, immediately after a thunderstorm when the sky, which was a drab white, turns a startling blue with values of B as low as 0-006. Rainout within clouds and washout below clouds are known to be especially effective in the wet removal of aerosols from the atmosphere. Turbidity shows a marked fall as soon as the monsoon set in, although it rises again whenever the monsoon is weak and the
188
A. MANI, O. C H A C K O and N. V. IYER 0.20 0.1g
ii /I
0"16
t
I
// m.
I
0.14
iI
0"12
I '~
0.10
~
0.OlJ
/
I
I
I
I
I
1969
0.06
0,04 0-02
I F
I M
I A
F i g , !, T u r b i d i t y
I M
I I J d Months
coemcint/3
0.10
A / \
/ / ....
0-08
0-06
/
/
/
I A
I $
a t P o o n a in 1 9 6 4 a n d 1 9 6 9 .
\
1959 \
Poono
\~
0-02
•._u ~
0
1969
-----
/
j,/
0.04 ~.
I 0
/
//
%% •
sI
(a) f~s~
0-12
.~
/
OqO
/ /
I-
/
/
/
~ ~ ~
/
1959 0elhl ~
----
1969
~ " ~
/
/--~
x
0.08 0"0
0-04 0.02 I F
I M
I A
I M
i d
I d
I A
l $
I 0
I N
0
Months
Fig. 2. Turbidity coefficient/3 at Delhi and Poona in 1959 and 1969.
Atmospheric turbidity over India
189
effectiveness of the rainout or washout process is reduced. T h e values remain low throughout winter. (b) Turbidity m e a s u r e m e n t s at Delhi. Conditions o v e r Delhi (28°N) in close proximity to the Rajasthan desert are completely different from those over Poona (18°N) a hill station 555 metres above sea level. T h e values of/3 and B at Delhi are about two to three times those over Poona and the seasonal variations are also more pronounced, with maximum values in summer and minimum in winter [Fig. 2(b)]. As a result of the later arrival of the monsoon over Delhi, the fall in turbidity occurs in July at D e l h i - n o t in June as at Poona. The passage of western disturbances during winter also contributes to the low values in winter over Delhi. An examination of the values of a during the different seasons shows that in winter it is not far from the classical value of 1.3, but in summer it is often zero or negative. T h e marked seasonal variation in both fl and a shows that both the particle concentration and the particle size distribution vary with the seasons at Delhi and that smaller particles are more numerously represented in winter when the turbidity is low and large numbers of particles of large size predominate in summer when the turbidity is high. Studying two years" data at Delhi, Rangarajan[9] came to the conclusion that aerosol scattering over north India in summer is independent of wavelength and aerosol particles have a particle size distribution different from that in temperature latitudes. He concluded that, since the increase of turbidity in summer is caused by the introduction on a massive scale of large and giant particles into the atmosphere by dustraising winds and dust storms over central and north India (reducing visibility to a few km or less), the larger than normal aerosol was responsible for bringing down the values of c~to near zero. Ramanathan and Karandikar[10] had shown earlier that ~ is either zero or even negative in summer over Delhi, i.e., the depletion of solar radiation by haze scattering was larger for 0.448/zm than at 0.330/zm. T h e y also found that the depletion, and therefore the scattering by haze was for the most part independent of wavelength. An increase in haze scattering at longer wavelengths and the negative value ofc~ was ascribed to an increase in mean particle size in summer over north India, when light of higher wavelength is scattered more than that of shorter wavelength. That a deep dense layer of dust and haze lies over central and north India during the hot premonsoon months with a maximum concentration from March to June is well known. Kendrew [ 11 ] states +'the sky is almost cloudless, but it cannot be described as clear or blue, since there is a constant dust haze, a grey pall through which the sun shines as a pale disc". Bryson et al.[12], who revived interest in the phenomenon and made an extensive study, called it ++a great brown sea lapping against the Himalayas" extending as it does across the whole of north India, with its top at about 10 km over the Rajasthan desert, lowering to about 5 km between Delhi and Calcutta. I A F aircraft observations made in 1963 showed that the haze varies from day to day in density, depth, and spatial distribution, being sometimes layered and sometimes uniform up to 10 kin. These observations were confirmed during the dust patrol organized by Bryson and his colleagues over India in 1966 (Peterson[13]). T h e y found the thickest layer to be confined to 3 km with dust concentrations as high as 1070/.~g/m '~ in the first few metres of the atmosphere and 700-800/zg/m a at higher levels with less dense layers up to 5 km over north and central India in April and May. T h e r e was no gradual decrease of dust concentration with height. Another interesting feature was the high altitude layers
190
A. M A N I , O. C H A C K O
arid N. V. I Y E R
of various thicknesses up to 7-8 km, above which the quantity of aerosol mass was less than 15/~g/m 3. A mineralogical analysis showed the dust consisted mainly of quartz with feldspar, mica, and carbonate. Kondratyev et a/.[14] have reported, from an analysis of the profiles of direct solar radiation fluxes from ten balloon flights, layers in the troposphere and stratosphere where increased concentration of attenuation components (water vapour and aerosols) are grouped. Over middle Soviet Union they reported aerosol layers below 2 - 4 km and between 6-10 kin. Aerosol layers were usually found above and below the tropopause and mostly over the inversions and between them in the stratosphere. (c) Turbidity at coastal stations. The annual march of/3 at Bhavnagar (22°N), a coastal station, is shown in Fig. 3. All coastal stations show fairly high values of
044 /
Bhovnogor
A / ~ ~
(1967-1969) Shillong -1969)
.....
O.i~' ~e" 0-10 .o
~ o-~ "12-
O-O2 I F
I M
I A
I M
I J
I J
I A
14s / S
I 0
I N
O
Months Fig. 3. A n n u a l
variation of turbidity
coefficient /3 at B h a v n a g a r and Shillong
( ! 9 6 7 - ] 969).
ranging from 0.04 to 0.14 although the atmosphere is comparatively dustfree in contrast to continental stations in the north. As pointed out by Yamamoto et al.[15], 13 increases with an increase in precipitable water content and should be high at humid coastal stations. (d) Turbidity at hill stations. As may be expected, turbidity at all hill stations is low. At Shiliong (26°N) 1500 m above mean sea level,/3 has only a value of 0.07 in summer, 0.02 in winter, and is almost zero during the monsoon months (Fig. 3). At Kodaikanal (10°N) 2340m above mean sea level,/3 was as low as 0.01 in February 1965. Measurements of turbidity made at Gulmarg (34°N) in Kashmir 3000 m above sea level in June gave values for ~ as high as 0.04-0.07. Turbidity measurements in the Everest region (28°N) 6200 masl also gave unexpectedly high values of the order of 0.06 (/~ngstr6m and Drummond [ 16]). Bishop's observations at the Silver Hut glacier were made in April and May when a dense pall of dust lies over central and north India and extends to 6-10 km into the atmosphere. All north Indian mountain stations in the south are comparatively free from turbidity. Even at Kodaikanal, as at Manna Loa,
Atmospheric turbidity over India
191
the skies are a deep blue in the mornings with the haze lying like a brown blanket over the plains below. In the afternoons the haze rises as a result of increased turbulent convection and the dispersion of dust to higher levels, and skies become hazy. (e) Transmissivity of the atmosphere. The same monthly values of the transmission coefficient q computed from ! = !o. qm where I and Io are the values o f the direct solar radiation received at the surface and outside the atmosphere and m is the optical air mass, show that the atmosphere is more transparent during winter and monsoon than during the summer months. Only about 64 per cent of the incoming direct solar radiation is transmitted during June over Poona, while 77 per cent is transmitted in December. At Delhi the corresponding figures are 55 and 76 per cent. As much as half the solar radiation is absorbed and scattered over Delhi, Jodhpur, and Ahmedabad in summer. SPATIAL DISTRIBUTION
O F /3 O V E R I N D I A
The geographical distribution of/3 during the winter, summer, and postmonsoon seasons and for the whole year is shown in Fig. 4. Yamamoto's method[l 5] of graphic representation has been followed since atmospheric turbidity changes occur generally in mesoscale and the drawing of isopleths of ,8 is not quite justified, particularly when one considers the sparseness of both stations and data. The diameter of each circle indicates seasonal average value of/3 for that station. It will generally be seen that for the whole year,/3 values are higher over central and north India than over the peninsula and over the subcontinent. As a whole, they are highest during the hot, dry summer months and least during winter and post monsoon months. Calcutta is an exception with high turbidity values, obviously the result of high local industrial pollution. The seasonal variations of 13 indicate that dust emanating from the earth's surface constitutes a major portion of the aerosol over the subcontinent. It is to be expected that the time of maximum dust content will correspond to the time of largest temperature gradient in the lower atmospheric layers and the maximum dispersion of dust from the ground to the higher layers of the atmosphere.
TURBIDITY
IN A I R M A S S E S
The air over western, central, and northern India in the summer months is tropical continental air To, with its source region in southwest Asia. It is the driest and hottest air over the Indian subcontinent, with marked instability, intense insolation, and turbulence, leading to the development of dust-raising winds and dust storms which persist for days. The haze persists as a dirty white milky canopy over a large part of north India when upper winds weaken. The summer monsoon sets over the peninsula in early June. The moist, southwesterly surface winds move rapidly northward across the country until by July virtually all of India is influenced by the system. With the establishment of the monsoon a highly-humid, cool, and conventively-indifferent equatorial maritime air lies over south and central India during the monsoon months, June to September, characterized by mostly cloudy to overcast skies, frequent rain, and drizzle, in the north this air mass becomes modified as it turns west round the seasonal monsoon trough to the north. The beginning of the cold season is marked by the retreat of the monsoon, which
S E V d , 14, No. 2 - G
192
A. MANI, O. CHACKO and N. V. IYER
Wintgr
© SHLO
v©
o
TRV
post mnl.~n
Annuol
©
© JDPO DLH
SHLo
~,HM
CAL(~J~
~_PNANGP~s K
OLH
~AHM
CALf ~ a ~
~PNA
~ TRV ~.J
SHLO
~VSK MDS
TRV
Fig.4.Distribution of turbidity coefficient/3. occurs at a more leisurely pace than the onset, the mean date ranging from early September in the north to late October in the south. During the winter months, December to February, the modified cold dry air over north and central India is of continental polar origin Pc characterized by convective stability, clear skies and good visibility, except for local mist in the morning and occasional haze in the afternoon. The winter haze is light brown in colour, generally settles during the night, rises up in the forenoon, and gets dissipated by thermal turbulence and mixing with upper winds by noon. Representative values of the atmospheric tubididty coefficient/3 for the three airmasses T~ over western and north India in summer, P~. over north and central India in summer, and E,, over south and central India during the monsoon months, have been
Atmospheric turbidity over India
193
calculated from values of typical stations in the north, centre and south of the subcontinent. Te/3 = 0.20 E,,/3 = 0.05 P,/3 = 0.05 SECULAR VARIATIONS OF TURBIDITY COEFFICIENT
Figure 1 and Fig. 2(a) and (b) give two sets of curves showing the annual march of /3 at Delhi and B and fl at Poona in 1959, 1964, and 1969 respectively. While the month-to-month variations are basically similar, both stations show a two to eight fold increase in B and/3 since 1959. Figure 5 shows the year-by-year variations in/3 at both Poona and Delhi from 1958 to 1969. While there are marked changes in/3 from year-to-year, the general trend is a two-fold increase with the years, presumably associated with the increase in industrialization, urbanization, and traffic at both stations since 1958. Figure 6 shows the year-by-year variation of direct solar radiation I, global solar radiation G, and diffuse solar radiation D at Poona from 1958 to 1968. Figure 7 shows similar values for Delhi. Here again the pattern is similar, a small decrease in the intensity of direct solar radiation over the years at both stations and a slight increase in diffuse solar radiation with time. While there is no marked decrease in the global solar radiation at Poona, at Delhi there is a steady decrease with the years. Figures 5, 6, and 7 show marked increases in turbidity occur in 1963 and 1966. These would appear to be associated with the increase in particulate matter injected into the atmosphere by Mount Agung and Taal volcano eruptions of 1963 and 1965. Raghavan and Yadhav [17], in a study of the depletion of solar radiation by particulate matter in the atmosphere over Delhi, reported unexpectedly high values in 1962 and 1964-1965. Kondratyev et al.[ 14] also reported similarly high values.
0-14 .....
0'12
New Delhi Poono
~. 0-I0 ,s J"
/."
._u o.o6 /
0"06
/ r - - -- ..i l i /
" i 0-04 ~ . v p
j
0.02 o
1958
i
I
1960
i
I
1962
i
I
1964
i
I
1966
i
Years
Fig. 5. Variation of turbidity, year by year ( ! 958-1969).
I
1'968
194
A. MANI, O. C H A C K O and N. V. IYER .E E
_
,~:
................
.
Poono
''
- ~~ t2O -
~-NE '8 ~ 0 -
G,oba,
-----
-I,.~
Diffuse radiation
- . . . . . Direct solar radiation
",
*"'"""
""'"'~ ..............
/ I
11'2
I10
0~"
II
ulO0
1.0
x/
io,m
66
5~0
i
1956
I
i
1960
I
i
1962
I
~
1964
I
I
i
1966
1960
0.6
.~-
0
Yeors
Fig. 6. Variation of global, diffuse and direct solar radiation, year by year, for Poona. .G
E
- /" "', "" "~ "'~ .
Delhi
.
.
.
.
.
.
----
.
lAX
, Global 1.2 Diffuse rodiotion Direct solor mdiotion
...........
i/~',,.
f,
/" 0.9
• -
._~
i
"~
/ ' ~ .\,
,=L
~o_~-,,ok, /
-i
~
/
r
901 19~
i
x..
'~ J
"
v
\/ i
I 1962
O
o.e _~
/~
\ / I 1960
..
I/
/\
'
oioo/"/ ~00_
/
I 1964
J
0.7 .g
\ I I~
L
0.6
i
~ o o
0.5 ~
Years
._~ 0
Fig. 7. Variation of global, diffuse and direct solar radiation, year by year, for Delhi. CONCLUSION
A study of the atmospheric turbidity from solar radiation measurements at a numbel of stations shows that (l) Turbidity over India shows a marked annual variation with very high values ir summer, about 2 to 3 times those in the postmonsoon and winter seasons. (2) in north and central India during summer the atmosphere is 3 to 4 times as
Atmospheric turbidity over India
195
turbid as the rest of the country. Values of tx are very low, often becoming zero or negative. Under these conditions haze scattering is independent of wavelength and neutral. Even at high altitude stations in the Himalayas the turbidity is high, as a result of the dense layer of dust that extends to 6- l 0 km into the atmosphere over north and central India in this season. (3) Reduction of turbidity as a result of rainout and washout in precipitation is observed at all stations, both in the premonsoon thunderstorm season and during the monsoon months. (4) An examination of turbidity values over India during the last ten years shows that/3 has doubled at almost all stations since 1958. This would appear to be associated with increasing pollution caused by rapid industrialization and urbanization in the areas under study. (5) Marked decreases in direct solar radiation and increases in turbidity are noticed in 1963 and 1965 and these are presumably associated with the increase of particulate material injected into the atmosphere by Mount Agung and Taal volcano eruptions in 1963 and 1965. REFERENCES [ 1] A. ,Angstr6m, Techniques of determining the turbidity of the atmosphere. Tellus 13, 214 ( 1961 ). [2] O. Chacko and V. Desikan, Atmospheric turbidity measurements over India. Ind. J. Met. Geophys. 16 649-664 (1967). [3] O. Chacko, V. Desikan and N. V. lyer, Atmospheric turbidity and aerosol content of the atmosphere over india. Proceedings o f the IQS Y Symposium N e w Delhi, pp. 666-672, (1966). [4] A. Mani and O. Chacko, Measurement of solar radiation and atmospheric turbidity with ,Angstr~im pyrheliometers at Poona and Delhi during the IGY. Ind. J. Met. Geophys. 14, 220 (1963). [5] A. Mani, O. Chacko and S. Hariharan, A study of Angstr~im turbidity parameters from solar radiation measurements in lndi~. Tellus 21,829-843 (1969). [6] W. Schuepp, Die Bestimmung der Komponenten der atmospharischen Trubung aus Aktinometermessungen. A rchs Met. Geophys. Biokim. 1,257 (1949). [7] A. ,~,ngstr/~m, The parameters of atmospheric turbidity. Tellus 16, 64 (1963). [8] F. Volz, Spektrale Measungen der sonnenstrahlung und Trubungsestimmung mit Selenzellen-Klein photometeren. Bet. D. Weterd. 2, 13 (1959). [9] S. Rangarajan, Studies on atmospheric ozone and solar radiation. Ph.D. Thesis, University of Poona, (1966). [10] K. R. Ramanathan and R. V. Karandikar, Effect of dust and haze on measurements of atmospheric ozone made with Dobson's spectrophotometers. Quart. J. R. Met. Soc. 75,257-267 (1949). [ I i ] W. G. Kendrew, The Climates o f the Continents, p. 473. New York, Oxford University Press (1942). [12] R. A. Bryson, C. W. Wilson and P. M. Kuhn, Some preliminary results from radiation sonde ascents over India. Symposium on Meteorology, Rotorua, New Zealand (1963). [l 3] J. T. Peterson, Measurement of atmospheric aerosols and infrared radiation over Northwest India and their relationship. Technical Report No. 38. The University of Wisconsin (January 1968). [14] K. Y. Kondratyev, G. A. Nikolsky, I. Y. Badinov and S. D. Andreev, Director solar radiation up to 30km and stratification of attenuation components in the stratosphere. Appl. Optics 6, 197-207 (1967). [15] G. Yamamoto, M. Tanaka and K. Arao, Hemispherical distribution of turbidity coefficient as estimated from direct solar radiation measurements. J. met. Soc. Japan. 46, 267 (1968). [16] A. Angstr6m and A. J. Drummond, Note on solar radiation in mountain regions at high altitude. Tellus 18.801-808 (1968). [ 17] S. Raghavan and B. R. Yadhav, Depletion of solar radiation by particulate matter in the atmosphere: A study with special reference to Delhi. Ind. J. Met. Geopys. 17,607 (1965).