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Sophisticated Anatomy Of Heat Waves


Recently, 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.


GS I: History

Dimensions of the Article:

  1. What is a heat wave?
  2. How do heat waves occur?
  3. The Role of Air Mass in Heat Waves

What is a 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).
  • Heat waves are expected to become longer and more intense and frequent over the Indian subcontinent.
  • In 2022 itself, the heat waves started early and were more numerous. They also extended further south into peninsular India due to a north-south pressure pattern set up by the La Niña, a world-affecting weather phenomenon in which a band of cool water spreads east-west across the equatorial Pacific Ocean.
  • The last three years have been La Niña years, which has served as a precursor to 2023 likely being an El Niño year.
  • As we eagerly await the likely birth of an El Niño this year, we have already had a heat wave occur over northwest India. Heat waves tend to be confined to north and northwest India in El Niño years.

How do heat waves occur?

Heat waves are a phenomenon that can be formed in two ways:
  • Local phenomenon: When the air is warmed by higher land surface temperature or compressed along the way, producing hot air near the surface.
  • Air-flow phenomenon: When warmer air flows in from elsewhere, either due to global warming or due to the direction of air-flow.
Factors contributing to the formation of heat waves in India:
  • Air-flow from West Asia: West Asia is warming faster than other regions in latitudes similarly close to the equator, serving as a source of warm air that blows into India.
  • Air-flow over mountains: Air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan, with some compression happening on the leeward side, entering India with warmth.
  • Warming of the Arabian Sea: The Arabian Sea is unfortunately warming faster than most other ocean regions.
  • Upper atmospheric westerly winds: Strong winds from the Atlantic Ocean over to India during spring control the near-surface winds, which generates heat waves due to the energy needed to run past the earth near the surface.
  • Decline in lapse rate: Global warming tends to warm the upper atmosphere faster than the air near the surface, meaning that sinking air is warmer due to global warming, which produces heat waves as it sinks and compresses.

The Role of Air Mass in Heat Waves:

  • Age of the air mass: The age of the air mass is a factor that affects the formation of heat waves. North-northwestern heat waves are typically formed with air masses that come from 800-1,600 km away and are around two days old.
  • Travel distance of the air mass: The distance that the air mass has travelled is also a contributing factor. Heat waves over peninsular India arrive from the oceans, which are closer (around 200-400 km) and are barely a day old. As a result, they are on average less intense.
The Anatomy of Heat Waves and the Importance of Early Warning Systems:
  • Heat waves have a complex structure, which makes it difficult to predict them accurately.
  • However, early-warning systems can use the age and distance of the air mass, as well as the mode of formation and location of the heat wave, to improve the quality of warnings and increase the lead time for issuing warnings.
  • India has made significant investments in human and computational resources to improve its forecast skills in the last decade.
Protecting the Vulnerable from Heat Waves:
  • Despite lower mortality rates in India compared to other regions, it is important to further improve forecast warnings, issue them as soon as possible, and couple them with city-wide graded heat action plans to protect the vulnerable.

-Source: The Hindu

December 2023