4. Heat and Cold Waves Over India

Using daily maximum temperature of 103 stations during the hot weather season (April–June or AMJ) for the period 1961–2010 and daily minimum temperature data of 86 stations during the cold weather season (December–February or DJF) for the period 1971–2010 from Indian main land, various statistical aspects of heat waves (HWs) and cold waves (CWs) have been examined. The trends in the season frequency of these extreme temperature events and their association with the El Nino/La Nina events have also been examined. During AMJ, HWs are generally experienced over the north, north-west, central, east India and north-east Peninsula [together called core HW zone (CHZ)] with highest frequency during May. During DJF, the CWs are generally experienced in the core CW zone (CCZ) that is nearly same as CHZ but includes Jammu and Kashmir and excludes coastal Andhra Pradesh with highest frequency during January. Appreciable increase (decrease) in the frequency of the HW days was observed during El Nino (La Nina) events. Exactly opposite association was observed in case of CW days. Appreciable decadal variation was observed in the frequency, spatial coverage and area of maximum frequency both in the HW/SHW and CW/SCW days. Significant increasing (decreasing) trends in the HW (CW) days were observed in many stations from CHZ (CCZ). The observed trends in HWs and CWs over India are in tune with similar trends observed over various other parts of the world. It is desired to have a proper operational service system in the country for the advance warning of the HWs and CWs and proper public information campaigns on the dangers of these extreme temperature events.