Number Theory: Why a minor deviation in April’s heat felt far hotter
India’s average maximum temperature for April was 35.83 degrees Celsius, 0.86 degrees higher than the 1981-2010 average, considered the normal, according to the gridded dataset of the India Meteorological Department (IMD). In IMD’s own analysis until April 28, the maximum temperature was the fourth highest since 1901 and 1.12 degrees above normal. This difference in figures is because the gridded dataset uses a somewhat different list of stations and mathematical operations. Calculating the precise rank of April 2022 is also besides the point.
Temperature since March, and particularly in April, has demonstrated what even small deviations from normal can look like. The following four charts explain why that is the case.
1. Most regions don't need a big deviation to get very hot in April
So, why did April feel so hot despite the maximum temperature moving just 0.86°C above normal? One reason is that this deviation is a nationwide average – and like all averages, this number ends up masking several strong region-specific deviations. This means that if some regions did not experience a harsh summer, they brought down the national average.
Large swathes of India – states in the northwest and central regions, and Uttar Pradesh – experienced considerable deviations above normal by two-to-five degrees, and a small region in the north-west even experienced deviations above five degrees. Only a few areas saw a deviation below normal, and even fewer – mostly in the northeast – experienced deviations below normal of more than two degrees. To be sure, a large deviation is not even needed when the normal itself is high, which is the case with Indian summers. Except for the extreme north, the northeast, and parts of the western coast, the normal for April itself is higher than 35°C. For central and north-western regions and large parts of the Gangetic plains, this average is higher than 37.5°C, and for a small part of central India, it is even higher than 40°C. Therefore, even with a small deviation, large parts of India witnessed an average maximum of above 40°C.
Even small deviations in late April can lead to high temperatures
Apart from regional variations, there is also the deviation over time that one needs to account for. Just as parts of India have a high 1981-2010 average for April as a whole, all of India gets warmer on the “normal” chart itself later in April, as the chart below shows. This year, this transition happened much faster. On the normal chart, the maximum goes up from 30 degrees to 35 degrees in over a month: from March 7 to April 14. In 2022, this five-degree jump happened in a one-week span between March 8 and March 16. From March 19 to March 27, the average hovered in the 34-35°C range. After that period, it has largely stayed above the 35°C line. This is to say that India has endured high temperatures this year for a much longer period than what people are used to at this time of the year. The highest average maximum that occurs on India’s normal chart is 36.81 degrees, which is the normal for May 26. The average maximum in India was higher than this value continuously for the four-day period of April 26-April 29, a full month before such a temperature would be expected.
India has seen very rapid warming post 1980s…
Why was this April so hot? Individual days, months, weeks, or even years can be warmer than expected because of local factors. However, if one looks at a longer timescale, it becomes clear that there has been very rapid warming since the 1980s. Compared to the pre-industrial average (the 1851-1900 average) India had become only 0.4 degrees warmer up to 1980, according to estimates from Berkeley Earth, a US-based non-profit. This deviation has increased by an extra 0.6 degrees in half that time, with India becoming a full degree warmer by 2020. Therefore, scientists have attributed the hot conditions prevailing since March partly to global warming.
…and local factors made it worse this April
Apart from the long-term trend of global warming, local factors have also had a role to play in the heat prevailing in India since March, particularly in its north-western regions. While March rainfall was low even for the country as a whole – it was the sixth lowest since 1901 according to IMD’s gridded dataset – in April, this problem was concentrated in the north-western and central regions. Seven regions – Gujarat, Rajasthan, Punjab, Haryana, Delhi, Madhya Pradesh, and Uttar Pradesh – received less than half a millimetre of rainfall. All of them except Gujarat received at least 3mm rainfall in April in the 1961-2010 period. To be sure, the average rainfall for India in April had a 20.8% surplus – and this again hides the extreme region-wise variation. All of this was on account of rainfall in north-eastern states of Assam, Nagaland, Arunachal Pradesh, Sikkim, and Meghalaya and in southern states of Tamil Nadu and Karnataka, where rainfall was 1.4 to 2.7 times the 1961-2010 average.