Storms, squalls, and hail: What’s behind Delhi’s erratic May weather?

What caused these seemingly erratic and volatile conditions?

The Met Department said it was due to a convergence of atmospheric conditions that, while extreme, are not entirely out of place for the pre-monsoon month of May in Delhi.

According to the India Meteorological Department (IMD), such storm activity aligns with long-term climatological patterns for the region. Though this May, it said, has stood out in terms of frequency and ferocity.

Thunderstorms are not unusual in the pre-monsoon month.

In fact, climatically, May records the highest number of squall days in Delhi. The long-period average (1991–2020) shows 3.6 squall days in the month at Safdarjung — Delhi’s official weather station.

May also sees the average number of thunderstorm days in Delhi rise to 5.9 days. Between January and April, it is in the range 1.4 to 3.4 days.

Moisture aided storm formation

IMD officials also said more moist wind days than usual have contributed to the rise in storm activity.

Krishna Mishra, a senior IMD scientist, explained: “Easterlies or south-easterlies [wind patterns] from the Bay of Bengal and southwesterlies from the Arabian Sea are called moist winds because they transport moisture into Northwest India. Whenever these winds prevail in the lower tropospheric levels, either an east-west or a north-south trough will form.”

Such enhanced days of moist winds have led to frequent thunderstorm activity this month, while the typically dry northwesterlies or westerlies have been less dominant.

May is a transitional month, known for intense heat. So, a heated landmass, coupled with frequent western disturbances, can be conducive for thunderstorm formation. During this month, daytime temperatures often exceed 40°C, creating the right environment for violent convective activity once moist air arrives.

On Wednesday night, after Delhi recorded its hottest night of the season at 30.2°C, evening hailstorms, rain, and winds up to 79–80 kmph resulted in a sharp drop in temperatures. The Palam station recorded a 14°C fall within an hour — 37°C to 23°C — while the Lodhi Road reported a drop of 10.7°C.

A cyclonic circulation in neighbouring Haryana was cited as the cause of the storm, fed by moisture-laden winds from the Arabian Sea and Bay of Bengal, according to IMD. As the cloud mass moved south-southeastwards over Gurgaon, Noida, and Faridabad, it led to intense thunderstorm activity.
While there was no western disturbance over Northwest India, IMD officials stated that once the cyclonic circulation reached Eastern UP, an interaction with a western disturbance was observed, further triggering the storm activity.
A rainy May

On May 15, Delhi-NCR woke up to a thick haze — reminiscent of the pollution months — after dust-laden winds from North Pakistan travelled through Rajasthan, Punjab and Haryana and shrouded the region overnight.

On May 2, Delhi saw another powerful spell of rain and storm. The Safdarjung station recorded 77 mm of rainfall, the second-highest single-day total for the month since 1901. The downpour triggered waterlogging, power outages, and flight disruptions, and led to a tree collapse that killed four members of a family.

The month’s total rainfall has touched 105 mm, against the normal of 18.2 mm. No heatwave days have been declared so far this May, though the heat index (real-feel temperatures) touched 50.1°C on Wednesday — while the actual temperature stood at 39.8°C.

IMD defines a rainy day as one that sees more than 2.4 mm of rainfall. While Delhi’s long-period average for May is 2.7 rainy days, this month, stations like Palam and Ridge recorded 6 rainy days each. At Safdarjung, 3 rainy days have been recorded so far.

Climatologically, hail is also most likely in May in Delhi, though it still averages only 0.5 days for the month. This year, several parts of the city, including Lodhi Road, reported hailstorms.