Is 40°C a new normal for summer in the country?
HT has analysed India Meteorological Department’s gridded temperature data-set to answer the question. Here is what the data show:
A western disturbance may have cooled temperatures across the country for the moment -- a phenomenon many scientists consider anomalous -- but it was preceded by reports of heatwaves from places that are normally not in the news for such a reason. An HT report by Jayashree Nandi on April 21 showed that many cities in northeastern and eastern India, including Kolkata, Agartala, Imphal and Shillong, were recording maximum temperatures close to their all-time high last week. However, what is interesting about the latest heatwave reports is that some stations in Imphal reported a maximum close to 40 degrees Celsius, a threshold usually associated with plains and deserts. This raises an interesting question. Are more places reaching this threshold generally used as one of the heatwave metrics in the plains? HT has analysed India Meteorological Department’s gridded temperature data-set to answer the question. Here is what the data show.
The area that sees temperatures reaching at least 40 degrees has not expanded
The weather office provides temperatures for 900 grids – a box bound by one degree latitude and longitude – for India. This means that all grids do not have the same area. A long-term analysis of the number of grids, or the area covered under them which saw a maximum temperature greater than or equal to 40 degrees Celsius does not show a consistent trend in the period between 1951 and 2022.
To be sure, tracking temperatures in the gridded dataset is not the same as tracking station-wise readings as the former captures a much larger area. For example, in the gridded dataset, Delhi’s highest maximum up to April 2023 is 48.07 degrees recorded on June 10, 2019. However, two new weather stations in Delhi recorded maximums of slightly above 49 degrees last year. While the gridded dataset has an advantage of capturing temperatures over a larger contiguous area, it might miss rise in temperatures due to factors such as increase in built-up areas in cities that increasingly have more people living there.
See Chart 1: Area that reached 40°C threshold in a year (sq. km)
But there is a clear increase in area on days when the temperature has reached the 40 degree threshold
What the data does show is that places that already see such temperatures are experiencing it more frequently. This is best understood by comparing the long-term trend in area-days when temperatures were 40 degrees or higher. Simply speaking, this is the sum of product of the area of such a grid and the number of days during the year when this threshold was breached. This number has increased consistently in the past two decades. Moreover, this number has not increased just because the 40 degree threshold has become more frequent than it was in desert-like areas. As the accompanying maps show, this pattern is also visible in places where the temperature crossed 40 degrees rarely.
See Chart 2: Area-Days of 40°C threshold in a year (sq. km*days)
Maps: Avg. no. of days when grid reached max temp >= 40°C
The 40 degree threshold arrives earlier and leaves later than it did before
The increase in area-days when the temperature is 40 degrees or higher is not just on account of peak summer becoming hotter . Data also show that the temperature crosses 40 degrees much earlier in the year and this threshold is breached even after summer has peaked. The arrival of this threshold is now about two weeks early and its departure about three weeks later. For example, in the decade ending 1960, the first day on which any grid in India crossed this threshold was, on average, the 92nd day of the year, or April 2. For the decade ending 2020, this date has now shifted to March 21, the 80th day of the year. Similarly, the last day on which any place in India crossed the threshold has been pushed back by about three weeks, from August 9 to 29. It gets hotter sooner, and stays that way longer -- but there are always exceptions such as this year, if only for a brief interregnum.
See Chart 3A, 3B: Day no. of year when any place in India crossed the 40-degree threshold (First day and last day)
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