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IMD is already sensing heat waves. What are they and why do they happen?

GS Paper 1

Syllabus: Disaster Management

 

Source: The Hindu

 

Direction: This article is in continuation of our previous article on heat waves in northern India.

  

Context: 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.

  

What is heat wave?

  • According to the IMD, a region has a heat wave if its ambient temperature deviates by at least 4.5-6.4° C from the long-term average. There is also a heat wave if the maximum temperature crosses 45° C (or 37° C at a hill-station).

  

Origin of Heat Wave in India:

A study published in Nature Geoscience offers some clues as to how different processes contribute to the formation of a heat wave.

  • In spring, India typically has air flowing in from the west-northwest. The Middle East is warming faster than other regions in latitudes similarly close to the equator, and serves as a source of the warm air that blows into India.
  • Air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan, so some of the compression also happens on the leeward side of these mountains, entering India with a bristling warmth.
  • The air flowing in over the oceans is expected to bring cooler air, but the Arabian Sea is warming faster than most other ocean regions.
  • The strong upper atmospheric westerly winds that come in from the Atlantic Ocean over to India during spring control the near-surface winds. 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.
  • Global warming tends to warm the upper atmosphere faster than the air near the surface. This in turn means that the sinking air is warmer due to global warming, and thus produces heat waves as it sinks and compresses.