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Tropical Cyclones and Monsoon Depressions

The analysis on tropical cyclones and monsoon depressions is made using the data of 1901-2010 period. A separate analysis during the satellite era (1961-2010) also has been made.

  • 1. Over the north Indian Ocean, significant decreasing trends are observed in the frequencies of cyclonic disturbances and tropical cyclones during the southwest monsoon season.
  • 2. Significant increasing trend in severe tropical cyclones is observed during the post-monsoon season (October-December).
  • 3. During the satellite era (1961-2010), cyclonic disturbances, tropical cyclones and severe tropical cyclones over the north Indian ocean and Bay of Bengal show significant decreasing trends during the monsoon and post-monsoon seasons.
  • 4. Monsoon low pressure systems forming over the Bay of Bengal during the monsoon season contribute significantly to the seasonal rainfall over the central parts of the country.

Total Cloud Cover

The analysis on cloud cover is made using 195 surface stations data for the period 1951-2010.

  • 1. There is a general decrease in mean total cloud cover over most of India, but an increase in the Indo-Gangetic plains and northeast India.
  • 2. The annual mean total cloud cover shows significant decreasing trend of 0.44 % per decade, mainly contributed by the monsoon season, where the declining rate is 0.93 % per decade.
  • 3. Out of the total number of stations showing decreasing trends, 59, 38, 48, 58 and 33 % of the stations show significant decrease in total cloud cover for annual, winter, summer, monsoon and post monsoon seasons respectively.
  • 4. There is a negative relationship between total cloud cover and diurnal temperature range suggesting decrease in cloud cover may be responsible for increase in diurnal temperature range.
 
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