On Tuesday, Delhi recorded its highest minimum temperature since 1969 — a burning 35.2 degree Celsius at night.
In the grip of high temperatures during the day, where seven heatwave days have been recorded in June alone (so far), a rise in night temperatures is a new worry.
This month, so far, the city has seen six consecutive warm nights — temperatures surging during the day have not fallen below the 40 degree Celsius mark at night since May 12.
The highest minimum temperature between 1969 and 2024 was 34.9 degree Celsius recorded on May 23, 1972.
India Meteorological Department (IMD) officials said records before 1969 have not been analysed yet and there is no clarity whether the temperature recorded on Tuesday is the highest ever for Delhi.
Two stations in Haryana — Fatehabad and Mahendragarh — recorded warmer mornings than Delhi at 35.4 degree Celsius and 35.3 degree Celsius.
According to IMD, a warm night is one in which the night’s minimum temperature is above normal by a difference of 4.5 to 6.4 degree Celsius. A severe warm night is observed when the departure from normal is more than 6.4 degree Celsius. The precondition for both is that the temperature during the day must be 40 degree Celsius or more.
Delhi’s minimum temperature on Wednesday was 8 degree above normal. The maximum temperature was 43.6 degree Celsius — 5 degree above normal.
In a 24-hour cycle, the temperature falls to its lowest point in the early hours of the day, most commonly between 3 am and 5 am, an IMD official said.
Any respite from the heat is expected only during this time, especially given that Delhi and several parts of northwest India are facing a rain deficit of more than 90%, and maximum temperatures have consistently been over the 40 degree mark.
“The reason that more heat stroke cases are being reported now, despite the maximum temperature hitting its peak in the last week of May, is the rise in night temperatures. There is no respite at all,” a doctor at a Delhi government hospital said.
“Moreover, homes are also warmer than the outdoors at night. So people are out in the open when it is the hottest outside [during the day] and indoors when the dip in temperature is not very much,” the doctor said.
The IMD is yet to do a detailed study of temperature extremes in June. However, publicly available statistics on temperatures in June in Delhi show that this is the first time since 2011, when 12 days between June 1 and June 19 have seen a minimum temperature of more than 30 degree Celsius.
2018, which also saw an exceptionally warm June, saw 10 such days.
The normal minimum temperature for this time of the month is 27.5 degree Celsius.
Several studies have cited the ‘urban heat island’ effect as one of the causes of the increase in temperature in cities. The effect is a local phenomenon, seen when highly urbanised areas with dense construction and few green areas see higher temperatures than areas that are relatively more open, and greener.
In Delhi, for several decades, areas that lie alongside the Ridge and the greener Lutyens’ Delhi have recorded a lower maximum and minimum temperature as compared to more urbanised pockets.
Studies say that because of the concretised nature of these pockets, heat gets trapped and the variation in the temperature can range from 2 degree to 4 degree within a few kilometres.
A recent study by Centre for Science and Environment (CSE) also says that the rate of cooling down in cities is now slower than it was in between 2001 and 2010 — contributing to warmer nights, overall.
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