A long-term analysis of maximum temperatures in India is only possible using the gridded dataset of the India Meteorological Department (IMD), which gives this daily data since 1951. This shows that March 10 is early for a breach in the 40°C threshold. The average for the 1981-2010 period – the average for this period is used as the climatological normal for temperatures in India -- shows that the usual date for the breach is March 25.

How early is a March 10 breach in the 40°C threshold? The sixth earliest since 1951. The earliest breach of the threshold was last year, when it happened on February 29; and the latest recorded breach of the threshold was in 1951, when it happened on April 15.

To be sure, while March 10 is not the earliest breach in the 40°C threshold, the area that breached the threshold on the day is far bigger than the area covered by the first breach in other years when it happened early. Around 2.1% of India’s area breached the threshold on March 10 this year (which increased to 2.6% on March 11). The highest area the 10 earliest breaches of the threshold covered (other than 2025) was 1.1%. This happened in 2009 when the breach happened on March 3.

Clearly, the 40°C breach is unusual in more ways than one this year. This is in line with long-term trends. For example, the 10-year rolling average of the first day of breach has decreased from 92nd day of the year (or April 2) in the decade ending 1960 to 75th day of the year in the decade ending 2025 (or March 16). In other words, the breach is now recorded 17 days earlier than it did in the 1950s. HT discussed this trend in detail in 2023. Not only does the breach happen earlier in the year than in the past, the last day on which the breach happens also now comes later than in the past. As expected, the number of days on which the breach happens over the course of the year has also increased.

To be sure, the results described above must be read with the fact that IMD’s gridded data set gives the maximum temperature for an area of dimension 1° x 1° (roughly 100 km x 100 km). This means that an isolated place breaching the threshold because of hyper-local factors – such as a heat island effect produced by concretisation – may not get recorded in this data set. However, that also allows one to look at broader trends in weather over time rather than the impact of local factors. This broad trend is in line with long-term warming. A temperature threshold associated with the unpleasant warmth of an Indian summer now comes earlier and stays longer.