Heat, instability, 128 kmph winds: The science behind the dust storms that hit Delhi-NCR

Most of that destruction came with almost no rain.

The mechanisms behind the storms, and the unusual frequency of such events in this pre-monsoon season, point to a specific and increasingly documented set of atmospheric conditions over northwest India.

This intensified what meteorologists call the lapse rate: the rate at which temperature drops with height, or the temperature difference between the hotter ground and the colder sky above. The wider that gap, the more unstable the atmosphere and the more energy stored.

A pocket of warm surface air, once it begins to rise, remains warmer and lighter than the air around it at every level. It keeps moving upwards. It gets faster. This stored upward force is measured as CAPE (convective available potential energy) – essentially, how much explosive energy the atmosphere has created.

Research on a comparable Delhi pre-monsoon dust storm in May 2018 recorded a CAPE value of 2,696 joules per kilogram with a high index of -8.98, both indicating severe convective forcing (Chakraborty et al., Agricultural and Forest Meteorology, 2021). Values ​​above 2,500 J/kg are usually associated with severe storm conditions.

Even extreme CAPE does not guarantee a hurricane. Another volume – the CIN, or convection barrier – acts as a lid that helps keep the instability bottled up. High CIN values ​​generally suppress cloud growth, even if the CAPE – the energy that propels the updraft – is high.

By Tuesday, the CIN lid would have been steadily eroded by surface heating. By late afternoon, it was gone.

Ashwari Tiwari of IndiaMetSky said CAPE essentially acts as ‘juice’ for a storm of this intensity. “Essentially, this is an unstable environment and all it takes is a spark for thunderstorms to develop. The higher the energy, the more it will facilitate the development of larger and stronger storms,” ​​he said, adding that beneath it, warm moist air moves upward rapidly. “CAPE and CIN work against each other in this area,” he said.

The second component was moisture, which was carried into the warm column by cyclonic circulation – a counterclockwise spiral of winds – flowing from the northwest, near Pakistan. Humidity matters for a reason beyond simple humidity. This lifting lowers the condensation level – in other words, the height at which the rising air becomes cool enough to condense into clouds. The lower cloud base means that a pocket of rising warm air strikes the condensation early; At that point, condensation releases heat back into the rising air, causing it to rise even more rapidly.

So, surface heat provides stored energy; Moisture reduces the trigger threshold and increases updraft when a fire occurs.

A surface-level trough – a corridor of low pressure passing through Rajasthan, Haryana and Delhi – acts as a funnel, drawing moist air inward and upward into the base of the developing column. “High heat and humidity caused instability,” Tiwari said.

“A surface-level low is also forming, which will eventually turn into the monsoon axis,” he said.

IMD Director General M Mohapatra told HT that such storms are not uncommon during pre-monsoon, but their intensity may depend on instability in the atmosphere. “Generally, there are four factors that determine the type and intensity of such storms. The first is intense heat and we have seen it for the last two days. The second is moisture. The third is unstable atmosphere and the fourth is a trigger, which is usually a weather system. In this case, it was the cyclonic circulation. If there is enough moisture, we get adequate rains, but generally, we see dry storms over northwest India. Rains are more likely to occur late at night, when the temperature drops, ” he said.

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By 5.30 pm on Tuesday evening the lid was broken. A cumulonimbus – a deep, anvil-topped storm cloud capable of reaching 15 km into the atmosphere, further than most commercial aircraft can fly – developed within the next hour, its updraft pulling warm moist surface air upward at speeds of tens of meters per second. It rained at high altitude in the cloud.

dry storm

Here’s the counter-intuitive origin of Tuesday’s event.

In pre-monsoon the air below the cloud base over northwest India is extremely hot and dry. Precipitation falling from cumulonimbus falls into this layer and begins to evaporate before landing – a phenomenon called virga, visible as a brown curtain hanging below a cloud base that dissipates in mid-air.

As the precipitation evaporated, it took away heat from the surrounding air column, causing it to cool rapidly. Cold air is dense and heavy; This mass started sinking rapidly. Falling rain and snow particles dragged the air downwards with them, further accelerating the rate of descent. The result was a downdraft: a column of cold, dense air moving toward the surface like a piston falling from altitude. Research on the Delhi dust storm of May 2018 confirmed this downdraft-driven mechanism as a defining characteristic of this class of storm (Banerjee et al., Journal of Geophysical Research: Atmospheres, 2021).

At the surface, the descending column hit the ground and spread rapidly outward in all directions – like cold water poured on a flat table. This spreading mass is the outflow of the cold pool, the winds are moving around over the land, and as it moves, it lifts up the loose topsoil. The wall of dust was the visible footprint of a downdraft that had evaporated its own rain on its way down.

Overnight rainfall data makes the system legible.

Palam, where the highest evening wind speed of 120 km/hr was recorded, received only 0.1 mm of rain throughout the night. In other words, the most intense flows occurred exactly where there was almost no rain – the storm was strongest where it was driest. Further away from the center of outer flow, rainfall increased: Safdarjung recorded 9.6 mm rainfall between 11:30 pm and 2:30 am and a maximum wind speed of 74 km/h; Lodhi Road recorded 7.4 mm rainfall; Pusa, where the highest overnight wind speed was 128 km/hr, received 4 mm rainfall. “No significant drop in temperature was recorded as it was mostly a dry storm,” India Meteorological Department (IMD) scientist Krishna Mishra said about the evening storm.

The gradient in wind readings at stations also reflects the terrain. “There are limited constraints around the airport,” Skymet’s Mahesh Palawat said, explaining the risks to Palam. The open, flat ground offers no friction to slow the outflow.

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What the meteorological literature establishes, based on PV Joseph’s seminal study in Weather in 1980, is the mechanism: that the typhoon is driven not by surface winds but by downdrafts – the outflow of the cold pool of cumulonimbus cloud described above – which is why its force and its rainfall often reach different locations. According to historical records, Delhi averages about eight blinds per year (Joseph, Mausam, 1980). 101 km/h at Palam on Sunday was an isolated episode within the same pre-monsoon window.

The broader pattern of the 2026 pre-monsoon is more noticeable. Western disturbances – cyclonic storm systems that travel eastward along the subtropical jet, typically most active in winter – have doubled in frequency during June over the past 20 years, a band of westerly winds causing the jet’s delayed northward migration (Hunt, Weather & Climate Dynamics, 2024). More western disturbances in June mean there is more cyclonic circulation available to deliver moisture to the warmer plains, and trigger more frequent convection of the type that became active on Tuesday.

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This coincides with a well-established finding in the heatwave science literature: that northwest India’s extreme heat events – sustained 46–48 °C events – are linked to persistent upper-level anticyclones, a high pressure system that suppresses convection, seals heat beneath clear skies, and prevents moisture from penetrating.

When that system becomes firmly established, the temperature continues to rise uninterrupted for several days. When it is absent or weak, moisture entrains, and convective storms erupt at the lower boundary (Ratnam et al., Scientific Report, 2016).