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  Jun 09, 2023


Q. What is a Heatwave and when is it declared?


  • Heatwaves occur over India between March and June.
  • IMD declares a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
  • Over the hills, the threshold temperature is 30 degrees Celsius.

Q. How are they formed?


  • Heatwaves form when high pressure aloft (3,000–7,600 metres) strengthens and remains over a region for several days up to several weeks.
  • This is common in summer (in both Northern and Southern Hemispheres) as the jet stream ‘follows the sun’.
  • On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.
  • Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly.
  • Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds.
  • Reduction of clouds increases shortwave radiation reaching the surface.
  • A low pressure at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming.
  • Alternatively, the surface winds could blow from the hot continental interior towards the coastal zone, leading to heat waves.

Q. What are the criteria used to declare heatwave?


To declare heatwave, the below criteria should be met at least in 2 stations in a Meteorological subdivision for at least two consecutive days and it will be declared on the second day.

  1. a) Based on Departure from Normal
  • Heat Wave: Departure from normal is 4.5°C to 6.4°C
  • Severe Heat Wave: Departure from normal is >6.4°C
  1. b) Based on Actual Maximum Temperature (for plains only)
  • Heat Wave: When actual maximum temperature ≥ 45°C
  • Severe Heat Wave: When actual maximum temperature ≥47°C

Q. 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 and 31 May 2015.

Q. What are the Impact of Heat Waves?

Heat Strokes: The very high temperatures or humid conditions pose an elevated risk of heat stroke or heat exhaustion.

Older people and people with chronic illness such as heart diseaserespiratory disease, and diabetes are more susceptible to heatstroke, as the body’s ability to regulate heat deteriorates with age.

Increased Healthcare Costs: Effects from extreme heat are also associated with increased hospitalisations and emergency room visits, increased deaths from cardio-respiratory and other diseases, mental health issues, adverse pregnancy and birth outcomes, etc.

Lessens Workers’ Productivity: Extreme heat also lessens worker productivity, especially among the more than 1 billion workers who are exposed to high heat on a regular basis. These workers often report reduced work output due to heat stress.

Risk of Wildfires: The heat domes act as fuel to wildfires, which destroys a lot of land area every year in countries like the US.

Prevents Cloud Formation: The condition also prevents clouds from forming, allowing for more radiation from the sun to hit the ground.

Effect on Vegetation: The trapping of heat can also damage crops, dry out vegetation and result in droughts.

Increased Energy Demands: The sweltering heat wave also leads to rise in energy demand, especially electricity, leading to pushing up rates.

Power Related Issues: Heat waves are often high mortality disasters.

Avoiding heat-related disasters depends on the resilience of the electrical grid, which can fail if electricity demand due to air conditioning use exceeds supply.

As a result, there is the double risk of infrastructure failure and health impacts.

  • Initiatives Taken:
    • Global:
      • Global forums dealing with climate change issues—such as the World Health Organization, World Economic ForumFirst Global Forum on Heat and Health, and the Global Forum for Environment-OECD—also focus on heat waves by investing in research on health risks of extreme heat, climate and weather information, advice on surviving heat waves, partnerships and capacity building, and communications and outreach.
    • Indian:
      • The National Disaster Management Authority (NDMA) has issued guidelines on dealing with heatwaves.
        • However, India does not recognise heatwaves as a disaster under its Disaster Management Act (2005).