A Heatwave is a period of abnormally high temperatures, more than the normal maximum temperature that occurs during the summer season in the North-Western parts of India.
Heat Waves typically occur between March and June, and in some rare cases even extend till July.
Criteria for Heat waves
The Indian Meteorological Department (IMD) has given the following criteria for Heat Waves:
Heat Waves need not be considered till the maximum temperature of a station reaches at least 40°C for Plains and at least 30°C for Hilly regions.
When the normal maximum temperature of a station is less than or equal to 40°C:
Heatwave Departure from normal is 5°C to 6°C
Severe Heatwave Departure from normal is 7°C or more.
When normal maximum temperature of a station is more than 40°C:
Heat Wave Departure from normal is 4°C to 5°C
Severe Heatwave Departure from normal is 6°C or more
When actual maximum temperature remains 45°C or more irrespective of normal maximum temperature, heat waves should be declared.
How long can a heatwave spell last?
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.
Areas of Heat waves in India
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.
Several recent studies indicate that CHZ experience more than six heatwave days per year during these four months.
Many places in the northwest and cities along with the southeastern coast report eight heatwave days per season.
However, the regions in the extreme north, northeast, and southwestern India is lesser prone to heatwaves.