Delhi’s average maximum temperature has not breached the 40°C threshold yet this year. This holds true whether one is looking at the data from the Safdarjung weather station or from the gridded data of the India Meteorological Department (IMD). With the maximum expected to reach a high of only 36°C up to April 21, it is likely that a 40°C maximum will arrive this year later than in the past 10 years.
With back-to-back western disturbances keeping Delhi’s weather cloudy, the Safdarjung weather station, considered representative of the city’s weather, is yet to breach the 40°C threshold. The forecast from the Regional Meteorological Centre (RMC) of IMD based in Delhi shows that the city is unlike to breach that threshold up to April 21, the last date for which there is a forecast.
The last time the city took this long to breach the 40°C threshold was in 2014, when this happened on April 30. Data for April on RMC’s website, published for 2011 and later, shows that the 40°C threshold has usually been breached around the middle of April.
To be sure, a late arrival of maximums above the 40°C threshold is not a complete guarantee of the weather being consistently pleasant. The average maximum in April 2014 was 34.8°C, marginally higher than the 34.5°C average maximum in April 2015, when the 40°C threshold was breached on April 20, 10 days earlier than in 2014.
Similarly, the average maximum at Safdarjung up to April 15 this year was 36.3°C. This is not much cooler than the average for the same period in 2017 and 2021 (37.1°C in both years), when the 40°C mark was breached on two days up to April 15. To be sure, the first 15 days of April this year are much cooler on average than in 2022, another year when the 40°C threshold was breached by April 15. The average maximum for the first 15 days of April in 2022 was 39.9°C and the 40°C was breached on six days up to April 15.
To be sure, while the Safdarjung station has not breached the 40°C threshold, newer stations such as Najafgarh and Narela have.
However, these stations have a short history and we cannot track if it is usual for these places to breach the threshold at this time. The maximum at these stations could also reflect local heat.
One way to account for such local heat is to use the gridded database of the IMD. Each grid in this data is a box bound by two latitudes and longitudes 1° apart (roughly 111 kilometres apart) and covers an area bigger than the political boundaries of Delhi. The database gives an average maximum for this area after mathematically imputing values for places where stations were not historically present.
This data shows that the city last took longer than April 21 to breach the 40°C threshold in 2018, when this happened on April 25. While Safdarjung station breached the threshold in 2018 on April 17, that was the only day of breach at the station until April 26. This means that the gridded data is reasonably compatible with Safdarjung data.
For example, the next recent instance of a late 40°C breach in the gridded data is 2014, same as the Safdarjung station.
The gridded data also allows one to look at long term trends, since it gives data from 1951 onwards. This shows that a late arrival of a 40°C maximum is unusual. 20-year averages (this smoothens yearly variations) of the first day when 40°C was breached has been decreasing. The average for 1951-1970 was day number 113 of the year or April 23. The average for the last 20 years (2004-2023), on the other hand, was 109 or April 19. This year is clearly an aberration.
