What hit Uttar Pradesh this week, killing at least 111 people across 25 districts, was not a single storm but a collision of weather systems that should not ordinarily be in the same place at the same time. A typically winter system from the west that should already have retreated for the season met moisture pulled inland from the seas on either side of the subcontinent, over a land surface baked by weeks of intense summer heating. The combination, with this timing and intensity — is part of what scientists say is an emerging pattern of pre-monsoon thunderstorms growing more frequent and more violent as the climate warms.
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The immediate trigger, as IMD director general M Mohapatra noted on Thursday, was a passing western disturbance compounded by an unusually strong moisture incursion from the Bay of Bengal (PTI) The immediate trigger, as IMD director general M Mohapatra noted on Thursday, was a passing western disturbance compounded by an unusually strong moisture incursion from the Bay of Bengal. But the physics of why that combination produced 130 kmph winds — gusts ordinarily associated with cyclones, not inland thunderstorms — ran through multiple layers of the atmosphere over UP on Wednesday afternoon.
A western disturbance is an eastward-moving low-pressure system that sweeps across northern India in winter and spring, drawing moist air across the region at upper levels. When it meets warm, moisture-laden air pushed inland from the Bay of Bengal at lower levels — over a land surface baked by 40°C-plus heat — the atmosphere becomes severely unstable. Warm surface air rises rapidly; as it does, it cools, the water vapour it carries condenses into cloud, and the condensation releases latent heat that accelerates the rising air further. This self-reinforcing process — convection — is the engine of every thunderstorm. On Wednesday, the ingredients that drive such a phenomenon came together in UP.
“Maximum temperatures are increasing, due to global warming, along with that the moisture carrying capacity of the air is also increasing. We are now seeing enormous clouds, with the top of the cloud reaching up to 16 km in height. These are cumulonimbus clouds with capability of causing massive thunderstorm activity. It is not surprising that the wind speed went up to 130 kmph in some districts,” said Mahesh Palawat, vice president, climate and meteorology at Skymet Weather.
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Sixteen kilometres is roughly the upper limit of the troposphere over India — the weather-bearing layer of the atmosphere. Cumulonimbus clouds that reach that height are punching against its ceiling, and represent the most powerful storm systems the atmosphere is capable of producing.
The wind damage was driven by what meteorologists call a squall line — a coherent, sustained line of severe thunderstorms that swept from north-west UP towards the east. M Rajeevan, former secretary at the ministry of earth sciences, said the system bore the hallmarks.
“It could be a line of thunderstorms called a squall line. These are severe thunderstorms with strong winds and lightning. Most deaths occur due to lightning. Predicting squall lines is possible at least 12 to 24 hours in advance. Then we can do nowcasting using radars. But the real problem is that these systems are short-lived and of small spatial scale. Therefore predicting is a little difficult. Most of the problem is due to timely communication of warnings and response by people. That is the most challenging part,” he said.
What turns ordinary thunderstorms into an organised squall line is wind shear — the change in wind direction and speed with altitude. Without it, a thunderstorm collapses within an hour, choked by its own downdraft. With strong shear, the storm tilts; its updraft and downdraft separate; and the system can sustain itself for many hours, sweeping across hundreds of kilometres.
On Wednesday, the contrast between upper-level westerlies brought in by the western disturbance and lower-level easterlies carrying Bay of Bengal moisture, imaginably, provided the shear that organised the storms into a line.
Experts underscored that the conditions that produced Wednesday’s storm are becoming more common.
“Thunderstorms are becoming more intense because of strong convection. As the average temperature rises, there is more moisture due to higher water vapour holding capacity and hence more intense thunderstorms,” said OP Sreejith, scientist and head of the climate monitoring and prediction group at IMD.
He added that exposure was rising as well — more people working outdoors are caught in the path of these increasingly violent storms.
The physics behind Sreejith’s point is straightforward. Warmer air can hold more water vapour — roughly 7% more for every degree Celsius of warming, a relationship known as the Clausius-Clapeyron equation. As average temperatures rise, the air above India carries a heavier load of moisture; when a trigger like a western disturbance sets that moisture rising and condensing, the storms it produces are more energetic, and dump more water in shorter bursts, than the storms a cooler atmosphere would have generated.
Last year offered a near-identical case. “Last year in May we saw similar heavy dust storms and thunderstorms with wind speed reaching 100 kmph,” Palawat said. HT reported on May 22 last year that at least 56 people in three states had died in incidents connected with a thunderstorm that ripped through swathes of northern India.
“With higher temperatures, we should expect thunderstorms to be more violent,” Rajeevan concurred.
Thunderstorms and lightning have emerged as among the most lethal severe-weather hazards in India. More than 2,500 people die from lightning strikes every year, according to an analysis by the Council on Energy, Environment and Water (CEEW).
The think tank found that recorded lightning strikes rose from approximately 14.8 million in 2019–20 to 18.5 million in 2020–21, a 34% jump in a single year, which it attributed primarily to extreme climate events driven by global warming, alongside the depletion of water bodies, deforestation and environmental degradation.
The challenge, as Rajeevan noted, is not the science of prediction. It is the gap between the radar and the field.
