TE analyzed a gridded temperature dataset from the India Meteorological Department to answer this question. Here’s what the data shows:
Disturbances from the West may have temporarily lowered temperatures across the country — a phenomenon that many scientists consider anomalous — but not before reports of heat waves coming from places that don’t usually make the news. The TE report on April 21 showed that many cities in northeastern and eastern India, including Kolkata, Agartala, Imphal and Shillong, recorded near-record maximum temperatures last week.
What’s interesting about the latest heatwave report, however, is that some sites in Imphal reported maximum temperatures approaching 40C, a threshold typically associated with plains and deserts. This raises an interesting question. Are more places reaching this threshold commonly used as one of the heat wave indicators for the plains?TE analyzed a gridded temperature dataset from the India Meteorological Department to answer this question. Here’s what the data shows.
See no expansion of areas where temperatures reach at least 40 degrees
The Bureau of Meteorology provides India with temperatures in 900 grids – a box bounded by 1 degree latitude and longitude. This means that all grids are not the same area. Long-term analyzes of the number of grids or areas covered by grids with maximum temperatures greater than or equal to 40°C do not show a consistent trend for the period 1951-2022.
To be sure, tracking temperature in a gridded dataset is not the same as tracking station readings because the former captures a larger area. For example, in the gridded dataset, the highest air temperature in Delhi as of April 2023 is 48.07 degrees recorded on June 10, 2019. However, two new weather stations in Delhi recorded a maximum temperature of just over 49 degrees last year. Although a gridded dataset has the advantage of capturing temperatures over a larger continuous area, it may miss temperature rises due to factors such as increased built-up areas in cities as more and more people live in them.
But on days when the temperature reaches the 40-degree threshold, the area increases significantly
The data does suggest that places that have already seen such temperatures are experiencing them more frequently. This is best understood by comparing the long-term trends in the number of days in an area when the temperature is 40 degrees or higher. Simply put, this is the sum of the area of such a grid multiplied by the number of days of the year that exceed that threshold. This number has been growing over the past two decades.Also, this number did not increase because the watershed reached 40 degrees more often than in desert areas. As the accompanying map shows, this pattern is also evident in places where temperatures rarely exceed 40 degrees.