DMPQ-What is a heat wave and what might be the reasons for high frequency of heat waves?

India Meteorological Department (IMD) uses temperature data from surface observatories to track heat waves. IMD also uses various numerical weather prediction models to track and predict occurrence and intensity of Heat Waves.

The following criteria are used for defining severity of the heat wave:

  1. When normal Tmax is less than or equal to 40°C and
  • ActualTmax is greater than Normal Tmax by 5°C / 6°C: heat wave
  • ActualTmax is greater than Normal Tmax by 7°C: severe heat wave


  1. When normal Tmax is greater than 40°C and
  • ActualTmax is greater than Normal Tmax by 4°C / 5°C: heat wave
  • Actual Tmax is greater than Normal Tmax by 6°C: severe heat wave

Latest studies show an increase in temperatures as well as occurrence of Heat Waves in many parts of country during the recent years. One of the reasons for the increase in heat waves is global warming associated with the increase in greenhouse gasses like Carbon dioxide, Methane etc. in the atmosphere. In India, rise in maximum temperatures as well as Heat Waves are found to be more in the months of April, May & June. Core Heatwave Zone (CHZ) is the most prone area for Heat Wave (HW) & Severe Heat Wave (SHW) with highest frequency of occurrence during the month of May. The CHZ covers States of Punjab, Himachal Pradesh, Uttarakhand, Delhi, Haryana, Rajasthan, Uttar Pradesh, Gujarat, Madhya Pradesh, Maharashtra, Chhattisgarh, Bihar, Jharkhand, West Bengal, Odisha, Andhra Pradesh and Telangana.

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