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Tuesday, July 05, 2022

Explained: Why is it so hot across India?

March was exceptionally hot, and April will end on a similar note. The country has already seen four heatwaves early in the summer of 2022, and no immediate respite is expected either

Written by Sanjana Bhalerao , Anjali Marar , Edited by Explained Desk | Mumbai, Pune |
April 27, 2022 4:56:45 pm
Explained: Why is it so hot almost everywhere in India?Bengal is reeling under heat wave conditions. A yellow warning of heat wave has been issued in several districts. A visual from Kolkata, Wednesday (Express photo by Shashi Ghosh)

Severe heat conditions have been consistently reported over large parts of India since the beginning of the summer season in March this year. Maximum temperatures in west Rajasthan and Vidarbha in Maharashtra have remained between 40 degrees and 45 degrees Celsius throughout the last two months.

Although March marks the seasonal transition from winter to summer, this year the month saw two spells of heatwaves.

In an unusually early summer, the country has already seen as many as four heatwaves since March

There have been at least 26 heatwave days since the beginning of March, and as many as four spells of heatwaves in a little over a month and a half. The last of these heatwaves remains ongoing.

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* The first of these lasted from March 11 to 19, and affected Kutch-Saurashtra, northern Konkan and Madhya Maharashtra, Rajasthan, Delhi, Punjab, Himachal Pradesh, Jammu, West Madhya Pradesh, West Uttar Pradesh and interior Odisha.

* The next spell, which commenced on March 27, was long — and abated only on April 12. It hit all the regions that were affected by the first heatwave, and also spanned Uttarakhand, Jharkhand, Chhattisgarh, Rayalaseema and East Madhya Pradesh, covering a very large patch of the country’s north, west, and centre.

* After a brief respite of a few days, a new heatwave started on April 17. This was relatively short, lasting until April 20, and was largely restricted to Delhi, Rajasthan, East Uttar Pradesh, Bihar, and Vidarbha.

* A fourth heatwave spell commenced on April 24 over Kutch-Saurashtra and Rajasthan, remains ongoing. It is expected to spread in the coming days to Delhi, Haryana, Chandigarh, Punjab, Uttar Pradesh, Madhya Pradesh, Bihar, Gangetic West Bengal, interior Odisha, Chhattisgarh, northern Gujarat, and Vidarbha.

Although March marks the seasonal transition from winter to summer, this year the month saw two spells of heatwaves (Express photo by Shashi Ghosh)

Heatwaves bring very hot days, but there is a technical definition for this meteorological phenomenon

The India Meteorological Department (IMD) declares a heatwave for a region when the maximum temperature reaches at least 40 degrees Celsius in the plains, at least 37 degrees Celsius along the coast, and at least 30 degrees Celsius in hilly regions.

Alternatively, a heatwave is declared when the maximum temperature rises by between 4.5 degrees Celsius and 6.4 degrees Celsius above normal. A severe heatwave is declared when the maximum temperature departs from normal is more than 6.4 degrees Celsius.

A third condition for a heatwave arises when an area records a maximum temperature of more than 45 degrees Celsius and up to 47 degrees Celsius on any given day.

There have been at least 26 heatwave days since beginning of March (Express photo by Shashi Ghosh)

There are months in the year and areas in the country that are the most vulnerable to heatwaves

Heatwaves occur most commonly in the summer months of April-June, with their severity and frequency peaking in May.

The most heatwave prone areas — known as the Core Heatwave Zone (CHZ) — are Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh, and Telangana.

Heatwaves can last between 4 and 10 days, and occasionally for longer. Heatwave spells in May are longer than those that occur in April and June, mainly because of the absence of rainfall.

Studies have shown that the CHZ experiences more than six heatwave days every year during these March to June months.

Many places in the northwest (Rajasthan) and cities along eastern coast (Andhra Pradesh, Odisha) report up to eight heatwave days in a season.

Regions in the extreme north (Himachal Pradesh, Kashmir, Ladakh), northeast, and southwest (Karnataka, Kerala, Goa) are less prone to heatwaves.

The IMD declares a heatwave for a region when the maximum temperature reaches at least 40 degrees Celsius in the plains (Express photo by Shashi Ghosh)

There are reasons why March and April have been so hot this year — an early onset of summer, the unusually long heatwaves, and the absence of periodic thundershowers

The usual light-intensity rainfall, hail and lightning have remained absent by and large over many areas of the country since March. The country’s all-India rainfall ended at minus 70.7 per cent in March.

March 2022 was India’s third warmest March since 1901. The monthly maximum temperature was 32.65 degrees Celsius against the normal of  31.24 degrees, the IMD said.

Normally, passing western disturbances – the eastward propagating stream of winds originating from the Mediterranean Sea – interact with moist winds blowing from southern India to trigger thunderstorms. The intermittent light rain and thunder helps keep a check on the heat.

Heatwaves can last 4 to 10 days, and occasionally for longer (Express photo by Shashi Ghosh)

This summer, the western disturbances have not been sufficiently strong, IMD officials have said. Even though five western disturbances have been recorded since March, three of these were feeble and passed over the extreme northern regions of the country, failing to cause significant changes in weather, a senior IMD scientist said.

Over south peninsular and northeast India, however, April has been wet. As a result, the summer has been relatively cool in these areas. The rainfall departures from normal during the period March 1-April 26 were 50.4 per cent and 49.7 per cent over south peninsular and northeast India respectively.

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Unfortunately, no major respite from the heat is expected anytime soon

The summer of 2022 has produced one of the longest heatwave spells over India in recent years. The last time the country saw a prolonged spell of high temperatures was between May 18 and 31, 2015, affecting parts of West Bengal along with Odisha, Andhra Pradesh, and Telangana. Another long spell was recorded between June 2 and June 11, 2014.

The summer of 2022 has produced one of the longest heatwave spells over India in recent years (Express photo by Shashi Ghosh)

This situation is not expected to change soon. Rajasthan, Delhi, Haryana, Chandigarh, Madhya Pradesh, Vidarbha, Uttar Pradesh, Jharkhand, interior Gangetic West Bengal, and Odisha must brace for hotter days until Sunday (May 1) at least. The maximum temperature is expected to rise by 2 degrees Celsius until May 1 before falling marginally.

However, the heatwave over Bihar and Chhattisgarh will abate after April 30.

On the other hand, temperatures over Telangana and parts of Andhra Pradesh could rise in the first week of May.

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