India Meteorological Department has issued a heat wave warning for Delhi and adjoining areas for May 22, 2023 after maximum temperatures in several parts of the capital were recorded above 45 degrees Celsius.


A Heat Wave 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.

When does a heat wave occur?

Heat Waves typically occur between March and June, and in some rare cases even extend till July.

What is the Criteria for a heatwave to be declared?

  • As per IMD, heat wave is considered if the maximum temperature of a station reaches at least 40°C or more for Plains and at least 30°C or more for Hilly regions.
  • IMD uses following criterion to declare the occurrence of heat wave in the region:
    • Based on Departure from Normal Temperature:
      • Heat Wave – Departure from normal is 4.50°C to 6.40°C;
      • Severe Heat Wave: Departure from normal is >6.40degree C
    • Based on Actual Maximum Temperature:
      • Heat Wave – When actual maximum temperature ≥ 45°C;
      • Severe Heat Wave: When actual maximum temperature ≥47
    • If above criteria met at least in 2 stations in a Meteorological subdivision for at least two consecutive days then heat wave occurrence is declared on the second day.
  • Heat Wave in coastal region
    • When maximum temperature departure is 4.5°C or more from normal, Heat Wave may be described provided actual maximum temperature is 37°C or more

Why do heat waves occur?

  • Heat waves can be connected with climate change.
  • In May and June, the Bay of Bengal often witnesses Anti-Cyclonic Circulations which work towards trapping the heat in the lower atmosphere.

What are the health impacts of heat waves?

The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke. Heat Wave conditions can result in physiological strain, which could even result in death.