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The 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 longterm average.

  • February 21st, 2023 was the third hottest day in the national capital in more than five decades.


GS III- Environment (Climate change)

Dimensions of the Article:

  1. What is a heat wave?
  2. Recent findings about Heat waves
  3. How do heat waves occur and it’s Impacts?
  4. Formation of Heat waves in India
  5. How does air mass contribute to heat waves?
  6. Way Forward

What is a heat wave?

  • A heat wave is a period of abnormally high temperatures, more than the normal maximum temperature that occurs during the summer season in the North-Western and South Central parts of India.
  • Heat waves typically occur between March and June, and in some rare cases even extend till July.
  • Higher daily peak temperatures and longer, more intense heat waves are becoming increasingly frequent globally due to climate change.
  • Conditions for Heat wave according to the IMD:
    • A region deviating from its ambient temperature by atleast 4.56.4° C from the longterm average has a heat wave.
    • For Plains, there is also a heat wave if the maximum temperature crosses 45°C.
    • At a hillstation, if the maximum temperature crosses 37°C, it is considered to be heat wave condition.

Recent findings about Heat waves:

  • Increased Frequency and intensity:
    • Heat waves are expected to become longer and more intense and frequent over the Indian subcontinent.
    • In the year 2022, the heat waves started early and witnessed multiple heatwaves.
  • Extending southwards:
    • The northsouth pressure pattern set up by the La Niña led to the extension of heatwaves further south.
    • La Niña is a world affecting weather phenomenon in which a band of cool water spreads eastwest across the equatorial Pacific Ocean.
  • Impact of El Nino:
    • The last three years have been La Niña years, which has served as a precursor to 2023 likely being an El Niño year.
    • It is more likely that the heat waves tend to be confined to north and northwest India in El Niño years.

How do heat waves occur and its Impacts?

  • Heat waves is a local phenomenon 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.
  • Generally, heat waves occurs due to the following reasons:
    • Warmer air flowing in from elsewhere
    • Warm air being produced locally
  • Health Impacts
    • The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke.
    • It also causes heat cramps, fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.
    • The extreme temperatures and resultant atmospheric conditions adversely affect people living in these regions as they cause physiological stress, sometimes resulting in death.

Formation of Heat waves in India:

As per the latest study, heat waves are formed in India for the following reasons:

  • Warm Air from the North-west:
    • In the context of climate change, West Asia is warming faster than other regions in latitudes similarly close to the equator.
    • The region serves as a source of the warm air that blows into India.
    • As the air flows from West Asia, it passes through 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.
  • Warming of Arabian sea:
    • Generally, the air when flowing over the oceans is expected to bring cooler air.
    • But the air flowing over the Arabian sea, which is witnessing an unusual warming over the recent years compared to most other ocean regions, is also responsible for Heat waves.
  • Strong upper atmospheric westerly winds:
    • The strong upper atmospheric westerly winds, that come in from the Atlantic Ocean over to India during spring, control the nearsurface winds.
    • Any winds flowing from west to east, generally flows faster than the planet itself, which is also rotating from west to east.
    • The energy to run past the earth near the surface, against surface friction, can only come from above. This descending air compresses and warms up to generate some heat waves.
  • Declining lapse rate:
    • Global warming tends to warm the upper atmosphere faster than the air near the surface.
    • Lapse rate is the rate at which temperatures cool from the surface to the upper atmosphere — is declining under global warming.
    • This in turn means that the sinking air is warmer due to global warming, and thus produces heat waves as it sinks and compresses.
  • These events have accelerated the occurrence and intensity of Heat Waves over India in the last few years.

How does air mass contribute to heat waves?

  • The age of the air mass and how far it has travelled also affect the formation of heat waves.
  • The north-western heatwaves are typically formed with air masses that come from 8001,600 km away and are around two days old.
  • Heat waves over peninsular India on the other hand arrive from the oceans, which are closer (around 200400 km) and are barely a day old. As a result, they are on average less intense.

Way Forward:

  • Heat waves have a sophisticated anatomy and it certainly have certain implications that cannot be ignored.
  • The early warning technology should be leveraged to take advantage of the processes, modes of formation, location, and age of the air mass to improve the quality of warnings and also increase how soon they can be issued.
  • However, Sizeable investments in human and computational resources have already increased India’s forecast skills in the last decade. Mortality over India due to heat waves are substantially lower than those in other midlatitude regions.
  • Hence, we should further improve forecast warnings, issue them as soon as possible, and couple them with citywide graded heat action plans to protect the vulnerable.

-Source: The Hindu

December 2023