Heatwaves occur over India between March and June. Meteorologists declare a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
Over the hills, the threshold temperature is 30 degrees Celsius. When the day temperature jumps by 4 to 5 degrees above the normal maximum temperature of a location, it is declared as a heatwave.
A heatwave spell generally lasts for a minimum of four days. On some occasions, it can extend up to seven or ten days. The longest recorded heatwave spell, in recent years, was between 18 – 31 May 2015.
This spell had severely affected parts of West Bengal along with Odisha, Andhra Pradesh, and Telangana. A similar spell in 2014 was reported during June 2 – June 11.
The current heatwave spell commenced on May 22 and is likely to continue till May 29. Heatwave conditions occurring in May have been observed to last longer, as the season reaches its peak this month.
Whereas those reported in June often die down sooner, often due to the onset of Southwest monsoon over the location or in its neighbourhood.
Heatwaves are common over the Core Heatwave Zone (CHZ) — Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana, as categorised by India Meteorological Department.
Summer season reaches its peak by May 15 in India, when the day temperatures across north, west, and central India cross 40 degrees and hover close to 45 degrees then on.
This year, north India did not experience such temperatures till May 21. It was mainly because of the continuous inflow of Western Disturbances that influenced the weather in the north till as late as April. However, this year, its influence persisted till early May.
Since the event of severe heat has emerged immediately after the passing of Cyclone Amphan, experts confirm its role in leading to the present heatwave spell.
Cyclone Amphan, which was a massive Super Storm covering 700 kms, managed to drag maximum moisture from over the Bay of Bengal, entire South Peninsula, parts of Central India and to some extent, even from the Arabian Sea.
All the moisture, that was otherwise built during the thunderstorm and rainfall, got gradually depleted from over vast areas as the storm advanced towards West Bengal and Bangladesh between May 16 and 20.
It has now triggered dry north-westerly winds to blow over Rajasthan, Madhya Pradesh, Uttar Pradesh and Maharashtra causing severe heatwave.