Climatology and Decadal Variation
Frequency of the CW/SCW Days
Figure 8a, b shows the spatial variation in mean number (frequency) of CW and SCW days, respectively, during the cold weather season (DJF) over the country. It is seen (Fig. 8a) that on an average > 2 CW days per season were experienced by most of the areas except southern Peninsula and north-east India. This region hereafter called the core CW zone (CCZ) and nearly same as CHZ expect that CCZ includes Jammu and Kashmir and excludes coastal Andhra Pradesh. Many areas over north and north-west of CCZ experienced > 6 CW days and 1-3 SCW days (Fig. 8b).
Month wise distribution of CW days showed (figures not given) that during December, CW days of > 2 were experienced by most areas of Rajasthan, some northern parts of Gujarat and Madhya Pradesh and western parts of Jammu and Kashmir. During January, CW days of > 2 were experienced by many areas of Punjab, Rajasthan, Madhya Pradesh, western part of J & K, southern parts of UP, northern parts of Maharashtra and some areas of Bihar, Jharkhand, Chhattisgarh and north Telengana. During February, the case was nearly similar to December except for reduced frequency of CW days over west J & K. During in all the 3 months, most of the Rajasthan experienced CW days of > 2 days with some areas of > 4 days. Month wise, SCW days were experienced mainly during January and February (about 1 -2 days in an average and mostly over north and north-west areas of CCZ).
The decade wise spatial distribution of season average of CW days (Fig. 9a-d) shows overall decrease in the average CW days over the country going from 19711980 to 1991-2000 with systematic and noticeable decrease over north, north-west and north-east India and slight increase over some southern parts of Central India. However, during recent decade (2001-2010), there was slight increase in the spatial coverage and frequency of CW days compared to the previous decade (1991 - 2000). During 1971-1980, many areas of north and north-west India reported > 6 CW days. However, during the subsequent decade (1981-90), the areas of > 6 CW days were mainly restricted to north-west India. During subsequent decades, 19912000 and 2001-2010, the areas of CW days of > 6 days showed further decrease over north-west India and an increase over southern parts of Central India. Decrease was also observed in the CW frequency over eastern parts of north-east India from about 2-6 days during the first two decades to less than 2 days during the recent two days.
The decadal variation of the average SCW days/season showed (figures not presented) relatively higher seasonal frequency of SCW days than its long-term
Fig. 8 a Average number of CW days over India during the DJF season computed using the CW information for the period of 1961-2010. b Same as a, but for SCW days
climatology during the first decade (1971-1980) over north and north-west area of CCZ. However, during the subsequent decades, an overall decrease in the average number of SCW days was observed over the region with lowest during 1991-2000 as in the case of CW days.
Fig. 9 Average number of CW days over India during the cold weather season (DJF) for the 4 decades viz. a 1971-80. b 1981-90. c 1991-2000 and d 2001-2010