What are Heatwaves?

 

India is gripped in the wrath of a long spell of heatwaves that too in the early month of April.

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.

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.

Following criteria are 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

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.

Impact of heatwaves

• It can be a natural disaster, and a danger because heat and sunlight may overheat the human body.
• Severe heatwaves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power outages due to increased use of air conditioning.