What has led to an early and warm summer this year in Delhi?

While February, March and April all record on an average three to four western disturbances, its distribution has been fairly uneven across these months, which ultimately has led to such a sharp spike in temperatures.

New Delhi: After recording Delhi’s wettest January in the last 121 years and its wettest February in the last eight years, it did come as a surprise that the capital failed to receive any rainfall in March this year. While the normal average rainfall mark for March is 15.9mm, the capital failed to see a single rainy day, with only weak and feeble western disturbances (WDs) seen across the northern plains during the month. Not only did these WDs fail to bring rainfall, but sufficient cloud cover too – one that can provide a slight cooling effect.

Delhi has been recording an extreme weather event each month since August 2020 now. (Raj K Raj/HT Photo)

As per India Meteorological Department (IMD) data, this was India’s warmest March in the last 121 years, with an average maximum temperature of 33.10 degrees recorded for the country, beating the previous highest of 30.09 degrees in March of 2010. For Delhi, this was the fourth-highest average maximum (32.9 degrees) between 1951 and 2022, behind only 2022 (33.1 degrees), 2004 (33.4 degrees) and 1953 (34.4 degrees), while the mean minimum temperature (17.6 degrees) this year was the second-highest for March between 1951 and 2022, behind the years 2010 and 1953 which jointly recorded a mean minimum average of 18.5 degrees.

While February, March and April all record on an average three to four WDs, its distribution has been fairly uneven across these months, which ultimately has led to such a sharp spike in temperatures. In February, Delhi recorded seven WDs, with three being particularly active and bringing sufficient rainfall. While the first active spell was seen from February 1 to 3, the second was from February 8 to 10. The third active spell meanwhile occurred from February 25 to 27. This was also Delhi’s last spell of rainfall, with the capital failing to receive a single drop of rain since.

While March saw feeble WDs, none were active and brought no rainfall to the capital. April so far has seen a similar fate and while a WD is expected across northwest India from April 12-14, it too is expected to bring slight rainfall only in parts of Punjab and the foothills, evading Delhi once again.

An uncomfortable March

From recording a maximum of 26.0 degrees at the start of the month to a high of 39.6 degrees Celsius by the end, Delhi saw a rise of nearly 14 degrees across the month at Safdarjung, which is considered as the base station for Delhi’s weather. It went even higher, touching the 42-degree at other stations. As per IMD data, the normal maximum temperature for the second half of March is between 30.6 degrees Celsius and 32.0 degrees Celsius, based on a long-period average from 1981 until 2010. However, Delhi by the end of the month was consistently recording temperatures five to eight degrees above normal.

The average monthly maximum temperature of 32.9 degrees Celsius was 3.3 degrees higher than the mean monthly average of 29.6 degrees, while the monthly average minimum temperature of 17.6 degrees was two degrees above the mean monthly normal of 15.6 degrees.

Safdarjung also recorded four consecutive days of 39-degrees or higher to end the month, recording a maximum of 39.1 degrees, 39.2 degrees, 39.6 degrees and 39.5 degrees From March 28 until March 31 respectively.

As per IMD data from 1951 until 2022, the 39.6 degree high on March 30 was also the joint second-highest single-day March temperature behind 2021, when it touched 40.1 degrees. In 1973 too, Delhi recorded a high of 39.6 degrees.

Delhi also ended the month with 13 days where the maximum temperature was 35 degrees or higher, dropping below this mark only twice after March 16 – first on March 22 (34.6 degrees) and on March 26 (34.5 degrees), both times on account of strong winds.

RK Jenamani, a scientist at IMD says cloud cover, rain and winds are the primary factors influencing temperatures in the summers and while rain spells can bring down the mercury by 6-8 degrees – a cooling effect that lasts several days by keeping the maximum temperature 2-3 degrees below normal for the next couple of days as well, cloud cover and strong winds can bring down the mercury by 2-3 degrees on the same day and by 1-2 degrees on a subsequent day.

“In March, we had three to four western disturbances, but none of these was active. The associated signs in the form of some cloud cover and windy conditions were seen, which was the only time the maximum came below 35-degrees in the second half of March,” he said.

An equally hot April

April has continued where March left off, with already, five ‘heatwave’ days recorded in the month, which is the second-highest for April in the last decade, behind only April 2017, when six such days were recorded. Mercury touched the 40-degree mark for the first time on April 7, 2021, with five consecutive days recorded where the maximum was 40-degrees or higher – thereby also meeting IMD’s heatwave criteria.

For it to be declared a heatwave, the maximum temperature needs to be 40 degrees or higher, with the maximum also being 4.5 degrees or more above normal. It is a ‘severe heatwave’ when it is 6.5 degrees or more above normal.

On April 11, the fifth consecutive heatwave day for Delhi, the maximum touched 42.6 degrees Celsius at Safdarjung – seven degrees above normal for this time of the year, also making it the hottest April day in the last five years. The last time Delhi recorded a higher maximum in April was on April 21, 2017, when it touched 43.2 degrees. The 42.6 degree high is also the highest maximum recorded in the first half of April in the last 72 years, data from 1951 shows.

Why such extreme heat?

Jenamani says while Delhi recorded seven WDs in February, in comparison to a normal of three to four, the WDs in March and April have been inactive, not even causing enough cloudiness to provide some relief from the overcast skies. “In March, all WDs were unable to influence Delhi’s weather. In April too, so far, there are no chances of rain till April 20, despite a WD expected across the northern plains from April 12-14 and then from April 18-20,” he says. In both these spells, Delhi is only expected to see partly cloudy skies and a slight increase in wind speed, which may take the mercury down to 38-39 degrees, but not provide any substantial relief.

Navdeep Dahiya, an amateur weather, who runs Live Weather of India says while such a long spell of no rainfall across northwest India was rare, it can happen over a month. The last time Delhi recorded no rainfall in March was in 2018. While the normal rainfall mark for March is 15.9mm, it is 12.2mm in April.

“It can go an entire month without rainfall, but this spell has led to dry weather across the northern plains and without any significant activity, mostly clear skies too. This raises the temperature far quicker than normal and while some parts of north India may receive some light rain in April this week, Delhi may miss it again,” he says.

Delhi has been recording an extreme weather event each month since August 2020 now. Several rainfall records were broken last year, including Delhi receiving 1,512.4mm of annual rainfall, the second-highest ever, behind 1,534.3mm in 1933. In May 2021, Delhi received 119.3mm of rainfall in a single day, the highest ever for the month, while July saw Delhi receive 507.1mm of rainfall in total, the second-highest ever for the city. In August, Delhi recorded a “very heavy” rainfall spell of 138.8mm in a single day – the highest single-day rainfall for August since 2007. And in September, Delhi recorded 413.3mm of rainfall – an excess of 230%, making it the highest rainfall for the month since 1944 and the second highest in 121 years.

This was intertwined between ‘severe heatwaves’ in June, Delhi recording its second warmest February in 121 years (mean monthly maximum of 27.9 degrees) and March recording its second warmest day ever, when it touched 40.1 degrees Celsius on March 30.

Dipankar Saha, former head of the Central Pollution Control Board’s air laboratory, says attributing extreme events to climate change is difficult throughout one to two years, but a decade assessment could help pinpoint anomalies. “If the maximum or the minimum has been higher last decade as compared to the previous one, then it could certainly be due to climate change and man-made influences. Certainly, weather over north India has been particularly erratic over the last two years,” he said.

Abinash Mohanty, programme lead at the Council on Energy Environment and Water (CEEW) says while March was the warmest in the last 121 years, the above-normal temperature will continue in April too, providing no respite from the heat.

“There is no respite expected in the coming days due to the absence of western disturbances that bring cold winds due to pre-monsoonal activity. No such activity has also been found over southern regions resulting in the prolonged dry spell. Based on these trends, we are likely to see a very high number of heatwave days experienced this summer. The climatological anomalies coupled with large-scale landscape disruption have contributed to the spurt in climate extremes like heat waves,” says Mohanty, stating managing heatwaves better calls for investing in the restoration of ecosystem assets like wetlands and mangroves.

“An increase in urban green cover can substantially mitigate the risks arising from adverse climate extremes such as heatwaves. Further, we should mandate a cool-roof policy for areas considered as heatwave hot spots, and promote impact-based early warning on heatwaves, so that lives and livelihoods could be protected," he added.