Simply put: The heat is on. Or is it?

It seems to have got exceptionally hot across the country rather early this year. What is the reason? Does the data show it’s extra hot this year to begin with?

Written by Amitabh Sinha | Updated: April 22, 2017 10:24:32 am

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The summer is yet to peak and several parts of the country have already experienced more than one spell of a heat wave. Currently, north and northwest India, including Delhi, are going through a heat wave with maximum temperatures at several places rising as high as 8 degrees Celsius above normal. More than 30 deaths across the country have been attributed to excessive heat, mainly in Maharashtra — the state that experienced a heat wave as early as in March.

So, are we in the midst of an unusually hot summer?

Not really. Such heat wave events occur every year at different times in different areas. It is true that average mean temperature, calculated for the entire country, was higher than normal in March — as it has been for April so far. But this is well within the variations that we see from year to year.

The country recorded an average mean temperature of 25.5 degrees Celsius in March, which was only 0.42 degrees above the normal of 25.08 degrees that is expected in the month. The first half of April was slightly hotter, with the deviation from normal being more than 1 degree Celsius in most parts of the country, except in the eastern and northeastern regions. However, temperatures are predicted to fall within the next couple of days — and the deviation from the normal, taken for the entire month, is likely to come down.

Incidentally, Maharashtra, which reported a number of heat-related deaths in March, did not see any abnormally high temperatures that month. The average mean temperature in the state in March was, in fact, what is expected during that month. In April, however, Maharashtra has recorded temperatures that are 1-2 degrees Celsius higher than normal.

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But isn’t every year getting warmer because of climate change?

It is true that India has not remained unaffected by the global trend of rising temperatures. Earlier this year, the India Meteorological Department (IMD) had announced that 2016 was the warmest year for India since 1901, with the annual average mean temperature being 0.91 degrees Celsius higher than the average of the 30-year period between 1961 and 1990. The IMD had also predicted that the summer of 2017 was likely to be warmer than normal, with many areas expected to see temperatures that are at least one degree Celsius above normal.

2016 was also the warmest year globally. The period 2011-2015 was the warmest 5-year period ever. However, it would still be erroneous to directly link heat wave events in the country with climate change as a whole. Most of these heat wave events — short, localised spells of high temperature — can be attributed to dynamic atmospheric conditions prevailing over the region.

Temperatures in Delhi are already well into the 40s. Are we going to see even hotter days?

Traditionally, the hottest days in India are recorded in May and June. The country starts getting warmer from February, and the southern states are the first ones to experience heat, following the Earth’s movement around the Sun. North and northwest India, the Gangetic plains, and eastern India start witnessing higher temperatures from April onward, and continue to be hot until June —by which time most of south India begins to receive monsoon rainfall. So, while Maharashtra may have reached its peak temperatures, north and northwest India might see even higher temperatures in the next two months. In between, short spells of heat waves, spread over 2 to 7 days, can occur anywhere, depending on atmospheric conditions.

And when will the current heat wave in north India come to an end?

Almost the whole of north and northwest India has been experiencing heat wave conditions since April 15. Even parts of Jammu and Kashmir, Himachal Pradesh, Uttarakhand and western Uttar Pradesh, which are relatively cool places, are facing a heat wave. The two meteorological stations in Delhi, at Safdarjung and Palam, have recorded 40-degree-plus temperatures every day since April 15. The Palam station, in fact, recorded these highs also on the 2 days preceding this period.

However, this spell is nearing its end now. In its bulletin on Thursday, the IMD has said that the heat wave was likely to be over by Saturday. A cyclonic air circulation building up over Pakistan has already resulted in rainfall in some parts of Jammu and Kashmir on Thursday. This circulation is likely to gain in strength over the next 2 days, and spread to the north and northwestern parts of the country, bringing in colder air from the area around the Mediterranean Sea. Accordingly, the IMD’s bulletin does not carry any heat wave warning after Friday.

Decoding the heat wave

# A heat wave is only declared when, for two consecutive days, the maximum temperature rises above 40 degree Celsius and is at least 4.5 degree Celsius above normal, or when maximum temperature exceeds 45 degree Celsius over two days. In hilly regions, the temperature should be above 30 degree Celsius with a departure of at least 4.5 degree Celsius.

# A severe heat wave is said to prevail when for two consecutive days, the maximum temperature exceeds 40 degree Celsius and the departure from normal is at least 6.5 degree Celsius. It can also be declared when the maximum temperature goes beyond 47 degree Celsius.


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