Largely because this has been a rainy year, India experienced a relatively cool summer this year. While heat waves were indeed reported, they were confined enough that upward deviation from usual temperatures was not prolonged. This has finally changed in the past week, HT’s analysis of the gridded data of the India Meteorological Department (IMD) shows. India’s average maximum temperature was warmer than normal for a full week for the first time since July 25; and the upward deviation was
Largely because this has been a rainy year, India experienced a relatively cool summer this year. While heat waves were indeed reported, they were confined enough that upward deviation from usual temperatures was not prolonged. This has finally changed in the past week, HT’s analysis of the gridded data of the India Meteorological Department (IMD) shows. India’s average maximum temperature was warmer than normal for a full week for the first time since July 25; and the upward deviation was more than 0.2°C for more than five days for the first time since June 14. The reason? The opposite of what kept daytime temperatures lower than usual earlier: an early withdrawal of the monsoon from some parts and the unusually dry weather in the past week.
That this has been a relatively cool summer in India can be seen from the deviation of India’s average monthly temperature from the 1981-2010 average, which the IMD considers as the normal for temperature in India. India’s average maximum temperature was warmer than normal from January to April, but has been cooler than normal in every month thereafter.
The trend described above can also be seen in the deviation of India’s daily maximum temperature. Of the 148 days from May 1 to September 25, only 48 days were warmer than normal. If these warmer than normal days were concentrated in May, when even the normal is high, India would have seen far more prolonged heat waves affecting a much larger region than they did this year. However, this was not the case.
No day from May 1 to June 6 was warmer than normal for India on average. This was followed by eight days of warmer than normal temperatures for eight days, of which seven were warmer than normal by more than 0.2°C. This was the last time both these thresholds were crossed for such a long period. There was a nine-day stretch in July that was warmer than normal, but only three of those days crossed the 0.2°C threshold. Similar but shorter spells were observed in August, but the upward deviation in those spells was also largely under 0.2°C. This trend finally changed in the week ending September 25. All days in the week were warmer than normal, with six of them more than 0.2°C warmer than normal.
To be sure, not all parts of India were warmer than normal in the past week. As the accompanying map shows, large parts of western and peninsular India were cooler than normal. However, the impact of such regions – they make up 36% of the country’s area – was offset by the rest of the country, where the upward deviation was high.
The regional patterns in maximum temperature’s deviation also explain the reason behind them. Large parts of western and peninsular India have seen at least “light rain” – defined as rain of 2.5-7.5 mm in a day -- very recently. This is not the case with most of the northern half of India, where the monsoon’s weather systems have retreated a few days before schedule.
To be sure, the map of consecutive dry days alone may not explain why north-eastern states experienced a large upward deviation in maximum temperatures in the last week. Parts of the region have received at least “light rain” very recently, but are still much warmer than normal. The reason behind this is that this region is supposed to be rainier than it was in the past week. The 1971-2020 average – IMD considers this as the Long Period Average or LPA for tracking rain’s performance – of rain shows that some parts of north-eastern states are supposed to average “moderate rain” (7.6-35.5 mm rain per day) in the week ending September 25. Instead, the region has received only light rain where it has received any rain at all, warming it up compared to normal.
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