The Indian Meteorological Department (IMD) has already started sensing the first signs of heat waves for this summer season.

What is the news?

• The IMD warned that the maximum temperatures over northwest, west, and central India would be 3-5° C higher than the long-term average in this week.
• If the heat waves had played out, they would have been the earliest these regions would have experienced this deadly phenomenon.

What are Heat Waves?

• Heatwaves generally occur over India between March and June.
• IMD declares a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
• Over the hills, the threshold temperature is 30 degrees Celsius.

How are they formed?

• Heatwaves form when high pressure aloft (3,000–7,600 metres) strengthens and remains over a region for several days up to several weeks.
• This is common in summer (in both Northern and Southern Hemispheres) as the jet stream ‘follows the sun’.
• On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.
• Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly.
• Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds.
• Reduction of clouds increases shortwave radiation reaching the surface.
• A low pressure at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming.
• Alternatively, the surface winds could blow from the hot continental interior towards the coastal zone, leading to heat waves.

Following criteria are used to declare heatwave:

To declare heatwave, the below criteria should be met at least in 2 stations in a Meteorological subdivision 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 context: El Nino + heat waves

• The last three years have been La Nina years, which has served as a precursor to 2023 likely being an El Nino
• The El Nino is a complementary phenomenon in which warmer water spreads west-east across the equatorial Pacific Ocean.
• As we eagerly await the likely birth of an El Nino 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 Nino years.

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.

How do different processes contribute to the formation of a heat wave?

• The direction of air flowing in from the west-northwest, warming in the Middle East, and compression over mountains of Afghanistan and Pakistan cause warm air to enter India.
• The warming Arabian Sea also contributes to the warming trend.
• Upper atmospheric westerly winds control near-surface winds, which rotate faster than the planet itself.
• Additionally, the lapse rate, or the rate at which temperatures cool from surface to upper atmosphere, is declining due to global warming.

Regional occurrences

• The other factors that affect the formation of heat waves are the age of the air mass and how far it has traveled.
• 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, which are closer (around 200-400 km) and are barely a day old.
• As a result, they are on average less intense.

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