In the week of February 21, the India Meteorological Department (IMD) warned that the maximum temperatures over northwest, west, and central India would be 3-5° C higher than the long-term average. On February 21 itself, the national capital recorded its third hottest February day (33.6° C) in more than five decades.


What is heat wave?

  • A Heat Wave is a period of abnormally high temperatures, more than the normal maximum temperature.
  • 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.


What is the criterion for declaring heat wave?

  • As per IMD, heat wave is considered if maximum temperature of a station reaches at least 40°C or more for Plains and at least 30°C or more for Hilly regions.
  • IMD uses following criterion to declare the occurrence of heat wave in the region —
      • Based on Departure from Normal Heat Wave —
          • Heat Wave — Departure from normal is 4.50°C to 6.40°C;
          • Severe Heat Wave — Departure from normal is >6.40degree C
      • Based on Actual Maximum Temperature Heat Wave —
          • Heat Wave — When actual maximum temperature ≥ 45°C;
          • Severe Heat Wave — When actual maximum temperature ≥47
      • If above criteria met at least in 2 stations in a Meteorological sub-division for at least two consecutive days, heat wave occurrence is declared on the second day.
  • Heat Wave in coastal region —
    • When maximum temperature departure is 4.50°C or more from normal, Heat Wave may be described provided actual maximum temperature is 37°C or more


Why do heat waves occur in the first place?

  • Heat waves are formed for one of two reasons: because warmer air is flowing in from elsewhere or because something is producing it locally.
      • Air is warmed locally when the air is warmed by higher land surface temperature; or
      • because the air sinking down from above is compressed along the way, producing hot air near the surface.


Origin of heat waves in Indian context

  • Direction from which air is flowing in India —
      • In spring, India typically has air flowing in from the west-northwest.
      • In the context of climate change, the Middle East is warming faster than other regions in latitudes, and serves as a source of the warm air that blows into India.
  • Compression of air —
      • Air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan.
      • As a result, some of the compression also happens on the leeward side of these mountains, entering India with a bristling warmth.
  • Warmer Arabian Sea —
      • The air flowing in over the oceans is expected to bring cooler air, since land warms faster than the oceans.
      • However, this does not happen as the Arabian Sea is warming faster than most other ocean regions.
  • Role of upper atmospheric westerly winds —
      • The strong upper atmospheric westerly winds that come in from the Atlantic Ocean over to India during spring control the near-surface winds.
      • Any time winds flow from the west to the east, the winds are blowing faster than the planet itself, which is also rotating from west to east.
      • The energy to run past the earth near the surface, against the surface friction, can only come from above.
      • This descending air compresses and warms up to generate some heat waves.
  • Declining lapse rate (the rate at which temperatures cool from the surface to the upper atmosphere) —
      • It should be noted that the lapse rate is declining under global warming.
      • In other words, global warming tends to warm the upper atmosphere faster than the air near the surface.
      • Hence, the sinking air is warmer due to global warming, and thus produces heat waves as it sinks and compresses.
  • Other factors —
      • The other factors that affect the formation of heat waves are the age of the air mass and how far it has travelled.
      • The north-northwestern heatwaves are typically formed with air masses that come from 800-1,600 km away and are around two days old.
      • Heat waves over peninsular India on the other hand arrive from the oceans and are barely a day old. As a result, they are on average less intense.


Why heat waves phenomena have started to occur more frequently?

  • Due to the processes that contribute to the formation of a heat wave, and the ways in which global warming affects them, these events have started to occur once every few years, and are also more intense. Earlier, it used to be a once-in-decade phenomenon.
  • The area covered by these heat waves is also influenced by the background pressure patterns set up by El Niño and La Niña events, and of late it has been expanding. The El Niño is a complementary phenomenon in which warmer water spreads west-east across the equatorial Pacific Ocean.



Although, the mortality over India due to heat waves are substantially lower than those in other mid-latitude regions. However, we should also not become complacent, and further improve forecast warnings, issue them as soon as possible. Also, these forecasts should be coupled with city-wide graded heat action plans to protect the vulnerable.


SourceThe Hindu


QUESTION – What is the ‘heat wave’ phenomenon? How does the IMD declare a heat wave? What are the reasons behind the frequent occurrence of heat wave events in India?