Heat Wave (HW)/Severe Heat Wave (SHW)
Climatology and Decadal Variation
Frequency of the HW/SHW Days
Figure 2a, b, respectively, depicts the spatial variation of mean number (frequency) of HW and SHW days over the country expressed as days per season for the AMJ season. Most areas of the country except north-east India and large parts of Peninsula have experienced on an average > 2 HW days (Fig. 2a). This region hereafter called the core HW zone (CHZ) covers states of Punjab, Himachal Pradesh, Uttarakhand, Delhi, Haryana, Rajasthan, Uttar Pradesh, Gujarat, Madhya Pradesh, Chhattisgarh, Bihar, Jharkhand, West Bengal, Orissa and Telengana and met subdivisions of Marathwada, Vidharbha, Madhya Maharashtra and coastal Andhra Pradesh. Many areas of west Rajasthan, Punjab, Haryana, Vidharbha, coastal Andhra Pradesh, northern parts of East Rajasthan, Madhya Pradesh, Orissa, Telengana and southern parts of Uttar Pradesh and Chhattisgarh have experienced on an average > 8 HW days. As shown in Fig. 2b, mean SHW days of 1-3 days were mainly experienced over isolated areas of north and eastern parts of the CHZ.
The decade wise spatial distribution of mean HW days for the AMJ season shows (Fig. 3a-e) that in all the 5 decades, on an average, > 8 HW days/season were experienced by many areas from eastern parts of Peninsula, north and north-west India, and some areas of Central India. The overall decadal mean
Fig. 2 a Average number of HW days over India during the hot weather season (AMJ) (1961— 2010). b Same as a but for the SHW days
Fig. 3 Mean number of HW days over India during the hot weather season (AMJ) for the 5 decades viz. a 1961-70. b 1971-80. c 1981-90. d 1991-2000 and e 2001-2010 patterns of HW days during the last two decades were similar to that of middle decade (1981-90), but with areas of > 8 HW days spread southwards over North India and northwards over the Peninsular. Among the 5 decades, the areas of > 8 HW days over the north and north-west India during the recent two decades (1991 - 2000 and 2001-10) were slightly more and relatively south than that during the first two decades (1961-70 and 1971-80).
The decadal variation of the SHW days (figure not presented) showed increase in the frequency and areal coverage in the average SHW over north and eastern parts of CHZ during the recent two decades compared to previous three decades.