Weatherbee | It’s only March and we’ve already crossed 40°C in some places – it’s unseasonably hot and also unusual

Mar 27, 2024 06:52 PM IST

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The number of days when any place in India breached 40°C in March and the sum of areas where the threshold was breached has rapidly increased since the 2000s

Large parts of southern India crossed the 40°C threshold on March 26, with some state administrations issuing alerts and asking residents to take precautions. How unusual is this? As the previous edition of Weather Bee had reported, temperatures in southern India have been above normal almost throughout March 2024. Therefore, even if unusual, it is not surprising that temperatures have stayed above normal in southern India in the absence of rain. Perhaps a more interesting question to ask is if maximums of 40°C or higher have become more common in March in India. This is because such maximums — if they are 4.5°C or more above normal — qualify the day as a heat wave.

Mumbai, India - March 20, 2024: A boy Jumping in the afternoon heat as temperature soars at Dadar chowpatty in Mumbai, India, on Wednesday, March 20, 2024. (Photo by Satish Bate/ Hindustan Times)(Hindustan Times)

Before looking at the historical trends in maximum temperatures in March, it is useful to see how often the 40°C threshold has been breached in March 2024. India Meteorological Department’s (IMD) gridded database shows that March 26 is not the first day this happened. The 40°C threshold had been breached on nine days before March 26: every day from March 12 to March 16; March 18; and every day from March 23 onward. March 26 was simply a day when this breach covered a much larger area: 10 times the maximum area covered on any day before March 26.

To be sure, these numbers need to be read with the fact that IMD’s gridded dataset averages temperature for a very large area. Each grid in this dataset is an area bound by two latitudes and two longitudes one degree apart, which is an area roughly 111 kilometres long and wide at the equator. This means that if only a small fraction of a grid breaches the 40°C threshold—due to local factors such as an urban heat island effect — it is unlikely that the grid overall will breach the threshold.

With the caveat about IMD’s gridded temperature data in mind, how usual is it for a 40°C breach to be recorded on 10 days in March? Not very. This threshold has been breached in March (up to March 26) in only 40 of the 74 years since 1951, the earliest year for which IMD has developed temperature-gridded data. This makes the 10-day breach in March 2024 the fifth longest since 1951.

However, the breach has not covered as large an area this time as in the top four years or in other years when the breach happened in 10 days. If the area over which the threshold was breached on every day in March is added up, 2024 would be ranked “only” 13th highest. These two trends read together are in keeping with the fact that March 2024 has been exceptionally warm in southern parts of India, but not as much in the northern half.

The two trends described above also imply that the 40°C threshold was breached quite regularly in March 2024; just not more than every year. This is expected because even the global average temperature does not increase in every successive year. It is the global temperature averaged over a longer period — such as a decade — that shows global warming. This is also the case with 40°C breaches in March in India. Long-term averages show that both the number of days when any place in India breached the 40°C threshold in March and the sum of areas where the threshold was breached has rapidly increased since the late 2000s. Therefore, while such breaches in March 2024 might be rare historically, they are the natural next point in a curve inching upwards.

Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.