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Annual and Seasonal Mean TCC Variation

Spatial distribution of long-term mean TCC over India based upon 1951-2010 period is shown in Fig. 3. The patterns of annual mean TCC indicate region of the highest clouding and lower CV over the Western Himalayas, north-east and over south peninsula (Fig. 3a), where some stations such as Gangtok, Kodaikanal, Alleppey, Thiruvananthapuram have >65 % mean cloud cover. The Western and Central India are having the lowest annual mean clouding and the highest coefficient of variation, where some stations such as Phalodi, Hardoi, Jaisalmer and Sri Ganganagar have <25 % mean cloud cover. Similar to annual CV, variations in seasonal CV are the highest (the lowest), where TCC is the lowest (highest). Winter season TCC is the highest in the Western Himalayas, extreme north-east and extreme south peninsula, while rest of the country is having the lowest TCC as shown in Fig. 3b. Summer season TCC is the highest over the Western Himalayas, north-east, south peninsula and along the eastern coastline as shown in Fig. 3c. Highest TCC over India is during the monsoon season when entire country except western Rajasthan, parts of Punjab and Jammu and Kashmir is having cloud cover above 60 %. Stations in Western Ghats, Chhattisgarh and north-east India have the highest clouding. The highest monsoon season mean TCC is in Mahabaleshwar (94.7 %), while the lowest TCC is in Sri Ganganagar (28.6 %). Postmonsoon season TCC is highest in the south peninsula and north-east India. While Kodaikanal, Thiruvananthapuram and Nagapattinam are having >70 % TCC, stations in the west and Central India such as Hardoi, Sri Ganganagar, Jaisalmer and Phalodi have lower than 10 % mean TCC for the postmonsoon season.

Spatial Patterns of Annual and Seasonal TCC Trends

Annual

Out of 195 stations selected for study, 125 stations are exhibiting decrease in TCC out of which trends for 74 stations are significant at the 95 % level of confidence as given in Table 1. However, the total cloud cover is significantly increasing at 28 stations, which are mainly located in the north-west and north-east India. Spatial

Table 1 Number of stations having decreasing/increasing trends of total cloud cover (TCC) during 1951-2010 for annual, winter, summer, monsoon and postmonsoon

Number of stations

Annual

Winter

Summer

Monsoon

Postmonsoon

Decreasing

125

118

112

141

112

Significantly decreasing

74

45

54

82

37

Increasing

70

77

83

54

83

Significantly increasing

28

24

32

18

11

distribution of annual TCC trends indicates overall decrease in cloud cover over the country except in the north-west and the north-east India as shown in Fig. 6a. The annual TCC trend values range between -6.2 % per decade and +6.9 % per decade over the country. Stations showing significant decreasing trends are mostly concentrated over the Central India, extreme south peninsula and along western and eastern coasts of India, where rate of decline is in the range of -4 % per decade to -7 % per decade. In contrast, many stations in the north-west India, Indo-Gangetic plains and north-east India have significant increasing trends in the range of +4 % per decade to +7 % per decade. The patterns of annual TCC trends obtained are consistent with trends reported earlier by Jaswal (2010).

 
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