What is a heatwave?

April 18, 2024

Recently, India Meteorological Department (IMD) has predicted an increase in the maximum temperature and the frequency of heatwave conditions in the forthcoming days over eastern and southern India.

About Heat wave:

  • The definition of a heatwave depends on the physiography of regions.
  • Qualitatively, heat wave is a condition of air temperature which becomes fatal to human body when exposed.
  • Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
  • The IMD will declare a heatwave:
    • if the maximum temperature recorded at a station is 40 degrees Celsius or more in the plains, 37 degrees Celsius or more in the coast, and 30 degrees Celsius or more in the hills.
    • a heatwave’s severity is determined by its departure from normal temperature.
    • there is a ‘normal heatwave’ when the departure is by 4.5-6.4 degrees Celsius and a ‘severe heatwave’ if the departure is greater.
  • Heat wave declaration could also be based on actual maximum temperature:
    • A ‘heatwave’ is when this figure is greater than 45 degrees Celsius and a ‘severe heatwave’ when greater than 47 degrees Celsius.
    • The IMD takes these two ‘routes’ only when at least two stations in a meteorological subdivision report such a high maximum or when at least one station has recorded a corresponding departure from the normal for at least two consecutive days.
  • Governments at various levels — State, district, and city — have prepared Heat Action Plans (HAPs).
  • HAPs aim to increase preparedness and lower the adverse impacts of extreme heat by outlining strategies and measures to prepare for, address and recover from heatwaves. 
  • The National Disaster Management Authority and IMD are reported to be working with 23 States to develop HAPs.
  • There is no centralised database on HAPs, but at least 23 HAPs exist at the State and city level, with a few States, such as Odisha and Maharashtra, laying out district-level HAPs.