Heat Wave

What is a heatwave?
The Indian Meteorology Department (IMD) defines a heatwave as an excess of five to six degrees C over the maximum daily temperature (over a 30-year period) of less than 40 degree C or an excess of four to five degree C over a normal historical maximum temperature of over 40 degree C.
  • The IMD declares a heat wave when the actual maximum temperature is above 45 degree C.
Why in news?
Meteorologists have said that an average rise of 1 degree Celsius in summer temperatures over most of India would mean more days of extreme heat as well as a higher likelihood of heat waves compared to last year.
Why there is increased frequency and duration of heat waves?
The IMD concurs that the frequency and duration of heat waves over the country are increasing and attributes it to increasing greenhouse gases due to anthropogenic activity and the El Nino — characterised by the warming of sea surface temperatures over the Pacific Ocean and correlated with droughts in India — that is also linked to more heat waves.
How heat waves are formed?
  • Heatwaves are caused by a system of higher atmospheric pressure, whereby air from upper levels of the atmosphere descends and rotates out.
  • As it descends, it compresses, increasing the temperature.
  • The outward flow, meanwhile, makes it difficult for other systems to enter the area, and the large size and slow speed of the hot air causes the heat wave to remain for days or even weeks. The longer the system stays in an area, the hotter the area becomes. The high-pressure inhibits winds, making them faint to nonexistent.
  • Because the high-pressure system also prevents clouds from entering the region, sunlight can become punishing, heating up the system even more.
  • The combination of all of these factors come together to create the exceptionally hot temperatures we call a heat wave.


Effects of a Heatwave
  • Power Outages due to increased use of air conditioning.
  • Hyperthermia
  • Low-humidity heat waves associated with droughts and fueled in part by climate change contribute to the dry conditions that are driving wild fires.
  • Meltdown of roads and railways and other critical infrastructure


  • Heatwaves are relative to an area’s climate – temperatures that would constitute a heatwave in one area might not in another location – and the health effects on the individual are also relative to a range of risk factors.

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