The rain gauges in Delhi have shown almost nothing. Yet, Delhi-NCR has been on the edge of a violent stormy corridor through Thursday night. The impact has been far more severe in Uttar Pradesh, where around 111 deaths have been reported due to violent weather since Thursday.
While dust storms and thunderstorms are not unusual in May, the intensity and frequency of the recent spells have raised questions over what is driving the weather.
Simply put, warm and moist air closer to the ground interacted with the drier air higher up. This kind of atmospheric setup can make the air unstable and trigger sudden thunderstorm activity, strong gusty winds, lightning and dust storms.
What happened in Delhi-NCR early Friday?
Through Thursday night and early Friday, the IMD warned of thunderstorms, lightning and gusty winds across Delhi-NCR and parts of Haryana, Uttar Pradesh and Rajasthan.
Around 2.40 am, Delhi was placed under an orange alert, with sustained surface winds of 20-30 kmph gusting up to 60 kmph. The night before, winds upto 98 kmph were reported in the capital.
The severity was not just because of surface winds. Radar wind observations shared by the IMD from Lucknow early Friday showed a sharp change in wind direction with height, including that of moisture-bearing easterlies in the lower levels and drier westerlies above.
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In Uttar Pradesh, nowcasts from the Lucknow Met Centre warned of moderate thunderstorms with winds of 40-60 kmph, gusting up to 70 kmph in some western districts.
According to the IMD’s three-hourly rainfall bulletin issued Friday morning, Delhi’s main weather stations recorded no measurable rainfall between 8.30 am Thursday and 8.30 am Friday.
But strong winds were recorded across the city.
The highest wind speed was reported at Pusa, where winds touched 68 kmph around 2.30 am. Palam recorded 48 kmph at 2.45 am, Pitampura 41 kmph at 2.30 am, Mayur Vihar 39 kmph at 2.45 am, Safdarjung 37 kmph at 12.30 am, Jafarpur 35 kmph at 3 am, and Janakpuri 28 kmph at 2.45 am.
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The event was largely wind-driven over Delhi, with little or no rainfall recorded at the official stations during the period.
Why did strong winds occur despite almost no rain?
This May, Delhi has been drier than recent years with only 13.61 mm rainfall reported against the normal of 30.7 mm, as per IMD records.
Thunderstorms do not always bring widespread rain to every region they affect. In pre-monsoon storms, strong winds can occur because of downdrafts, which are currents of air that descend rapidly from storm clouds and spread out near the surface.
When dry air exists in the middle atmosphere, some of the falling rain evaporates before reaching the ground. This evaporation cools the air, making it heavier and causing it to rush downward. Once it hits the ground, it spreads outward as gusty winds.
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This is why Delhi can experience intense winds, dust, thunder and lightning, even if rainfall remains very light or absent at many stations.
What specifically triggered these back-to-back storms?
A senior IMD official from the National Weather Forecasting Centre, who did not wish to be named, said Friday’s gusty winds were linked to the interaction between moisture-carrying easterlies in the lower levels and dry westerlies in the mid-levels.
A day earlier, on Thursday, the storm activity was attributed to a slightly broader setup as a cyclonic circulation over the northwestern part of Uttar Pradesh in the lower atmosphere met easterly winds coming from southern Rajasthan, and westerly winds in the middle troposphere.
This mix created favourable conditions for thunderstorm development.
The lower-level easterlies supplied moisture. The dry westerlies above helped create instability. The cyclonic circulation acted as a trigger by disturbing the wind flow and allowing storm clouds to develop.
Is this common in May?
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May is a transition month in Northwest India. Surface temperatures over heated landmasses often rise very high during the day. This heating makes the lower atmosphere unstable.
When moisture enters this heated environment, thunderclouds can develop quickly. If there is also a trough, cyclonic circulation, wind discontinuity, or western disturbance nearby, storm activity can become more intense.
This is why May often sees dust storms, squalls, thunderstorms, lightning and occasionally hail over Delhi-NCR and adjoining regions.
In Delhi’s long-term climatology, May records the highest number of squall days.
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Safdarjung, Delhi’s base weather station, has a long-period average of 3.6 squall days in May. The average number of thunderstorm days also rises to 5.9 days in May, higher than the January-April range.
So the occurrence of storms in May is not unusual and certainly not unseasonal.
What’s the forecast for the weekend?
For Delhi, the IMD forecast mainly clear skies on Friday, turning partly cloudy towards afternoon or evening, with a possibility of thundery development towards evening or night. Strong surface winds are likely on May 17 and 18, with speeds of 20-30 kmph and gusts up to 40 kmph.
In Uttar Pradesh, mostly dry weather is likely over both west and east UP from May 15 to 18, while isolated rain or thundershowers are likely over east UP on May 19 and 20. Heatwave conditions are also likely over parts of the state, the IMD said.
