Environment & Biodiversity

Heat Waves and their Anatomy

The Indian Meteorological Department (IMD) has already detected the first signs of summer heat waves.

What’s the latest?

  • The IMD forecasted that maximum temperatures in northwest, west, and central India this week would be 3-5° C higher than the long-term average.
  • If the heat waves had continued, they would have been the first time these areas experienced this deadly phenomenon.

What exactly are Heat Waves?

  • Heatwaves are common in India between March and June.
  • The IMD declares a heatwave event when the maximum (day) temperature in the plains exceeds 40 degrees Celsius.
  • The temperature over the hills is 30 degrees Celsius.

How do they come to be?

  • Heatwaves form when high pressure aloft (3,000-7,600 metres) builds up and remains over a region for several days to several weeks.
  • The jet stream ‘follows the sun’ during the summer (in both the Northern and Southern Hemispheres).
  • The high pressure area is located on the equator side of the jet stream, in the upper layers of the atmosphere.
  • Summer weather patterns change more slowly than winter weather patterns. As a result, the upper level high pressure moves slowly as well.
  • Under high pressure, the air sinks (warms and dries adiabatically), inhibiting convection and preventing the formation of clouds.
  • Cloud removal increases the amount of shortwave radiation reaching the surface.
  • A low pressure at the surface causes surface wind from lower latitudes to bring warm air, thereby amplifying warming.
  • Surface winds could also blow from the hot continental interior to the coastal zone, causing heat waves.

To declare a heatwave, the following criteria are used:

  • To declare a heatwave, at least two stations in a Meteorological subdivision must meet the following criteria for at least two consecutive days, and it will be declared on the second day.

(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

(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 

Recent events include El Nino and heat waves

  • The last three years have been La Nina years, indicating that 2023 will most likely be an El Nino year.
  • El Nino is a complementary phenomenon that occurs when warmer water spreads west to east across the equatorial Pacific Ocean.
  • While we wait for an El Nino to form this year, a heat wave has already occurred over northwest India.
  • In El Nino years, heat waves are typically confined to north and northwest India.

What causes heat waves in the first place?

  • Heat waves form for one of two reasons: warmer air is coming in from somewhere else, or something is producing it locally.
  • Local warming occurs when air is warmed by higher land surface temperatures or when air sinking down from above is compressed along the way, producing hot air near the surface.

How do various processes influence the formation of a heat wave?

  • Warm air enters India due to the direction of air flowing in from the west-northwest, warming in the Middle East, and compression over mountains in Afghanistan and Pakistan.
  • The Arabian Sea’s warming also contributes to the warming trend.
  • Westerly winds in the upper atmosphere control near-surface winds, which rotate faster than the planet itself.
  • Furthermore, as a result of global warming, the lapse rate, or the rate at which temperatures cool from the surface to the upper atmosphere, is decreasing.

Local occurrences

  • Other factors that influence the formation of heat waves include the age of the air mass and its distance travelled.
  • North-northwestern heatwaves are typically formed by air masses that are 800-1,600 km away and two days old.
  • Heat waves, on the other hand, arrive from the oceans, which are closer (around 200-400 km) and only a day old.
  • As a result, they are less intense on average.
And get notified everytime we publish a new blog post.