- Fast-forming storms are intensifying in Kerala, especially during the pre-monsoon months, bringing a triple threat of heavy rain, lightning, and strong wind gusts.
- A recent study found that lightning and extreme rainfall are clustering in specific hotspots.
- Topography and urbanisation are worsening the impacts. Moisture-laden air rising along the Western Ghats strengthens these storms, while paved urban surfaces prevent water from soaking into the ground, increasing the risk of flooding.
On a late evening in the first week of February, after what passed for a usual balmy afternoon, the night sky clouded over the fringes of the Kochi metropolitan area. Thunder cracked, lightning followed, and within minutes a burst of rain flooded roads, slowing traffic to a crawl in several stretches. Those caught without umbrellas could not walk a few yards to the nearest bus stop, while autorickshaws refused to ply because water was rapidly pooling on the streets.
The downpour did not last long, but it was intense enough to overwhelm drains and leave behind pools of water. This rain was isolated and quite early, before the summer season of March to May. However, fast-forming thunderstorms are becoming an increasingly familiar risk in Kerala, especially during the pre- and post-monsoon months, explained Abhilash S, head of the department of atmospheric sciences at the Cochin University of Science and Technology (CUSAT). A recent study by his team mapping lightning strikes, wind gusts, and short-duration rainfall across Kerala state shows they cluster in distinct hotspots — causing flash floods in cities and landslides in the hills.
Rapidly building thunderstorms
Convective storms — the fast-building thunderstorms that often arrive with little warning — are a familiar feature of Kerala’s pre-monsoon weather. These are associated with thunder, lightning, heavy rain, hail, strong winds, and sudden temperature changes. They can occur year-round, but are most common in the summer.
However, these storms are posing a triple threat — bringing intense rainfall, dangerous lightning, and strong wind gusts, as the study appearing in the journal Geomatics, Natural Hazards and Risk shows. These rains can flood roads, disrupt power supply, damage homes, and disrupt livelihoods, says the study by Krishna Kumar E.K. of the CUSAT department of atmospheric sciences, along with Abhilash and their colleagues at the Advanced Centre for Atmospheric Radar Research at CUSAT.
The team analysed pre-monsoon rains during May from 2021 to 2024 and noted in their paper that over Kerala, convective storm clouds with brightness temperatures below 240 K (Kelvin) and gust winds above 30 kt (knots) are particularly hazardous as they bring the triple threat of heavy rainfall, lightning, and strong winds. A temperature of 240 K is equivalent to minus 33 degrees Celsius at the cloud top, and 30 knots is about 55.6 km per hour. A gust denotes a rapid shift in wind speed with variations of 10 knots (18.5 km per hour) or more between peaks and lulls.
Brightness temperature is a measure of the intensity of electromagnetic radiation emitted by an object. When satellite images show brightness temperatures below 240 K, it indicates cold cloud tops, typically associated with tall, powerful thunderstorms. These clouds can rise to heights of around 15 km in the upper troposphere. This indicates strong upward movement of heat and moisture in the atmosphere. Such low cloud-top temperatures are widely used as an indicator of storm intensity because only strong upward air currents can lift cloud tops to these cold altitudes.
Brightness temperature and wind gusts are widely used indicators for inferring the characteristics of convective storms. Examining these two signals together helps scientists understand the risks associated with convective storms, as they capture both how tall and powerful the storm clouds are and the impact of strong surface winds, the paper explained. The most threatening events occur when very cold cloud tops coincide with strong, gusty winds, indicating a tall, powerful storm system.
Earlier research has shown that deep convective clouds over Kerala frequently exhibit brightness temperatures below 240 K, indicating exceptionally cold and high cloud tops.
Impacts across Kerala
Despite their relatively small spatial scale — typically spanning 15–20 km in diameter and lasting only a few hours — isolated convective storms can cause significant damage within a short time, the study noted. Their impacts are usually localised, which limits the overall extent of destruction, but within the affected area, the effects can be severe.
Studies show that lightning activity associated with these storms peaks during the pre-monsoon season in Kerala. For instance, pre-monsoon thunderstorms produce the highest lightning flash densities, with Kottayam district recording the greatest frequency. More recent work confirms this trend, showing a state-wide increase in lightning strikes, with Kottayam again emerging as the most affected district.
Notably, around 75% of lightning strikes occur on wet days, a combination that significantly elevates the risk of injury and mortality, particularly among outdoor workers and other exposed populations, the CUSAT study showed. Earlier estimates by the Centre for Earth Science Studies indicated that Kerala recorded around 70 lightning-related deaths annually.
In northern districts, these storms tend to deliver heavier rainfall and greater overall intensity than in the south. Coastal Kerala has emerged as a particular hotspot, with extreme rainfall events becoming more intense from Ernakulam northwards to Kasaragod, the northernmost district, the study shows.
Rainfall intensity increases from the Western Ghats to coastal areas, the CUSAT study noted. “Trivandrum district receives the highest rainfall, highlighting its vulnerability to flash floods and waterlogging during the non-monsoon season.”
Heavy pre-monsoon rains can be particularly damaging for crops in southwestern India. “Many fruit, vegetable, and plantation crops are at sensitive flowering or fruit-setting stages, and excess moisture at this time increases susceptibility to flower drop, fruit cracking, and fungal diseases,” Manjesh M., a horticulture expert and assistant professor at Christ University Bengaluru, explained to Mongabay-India. “If hailstorms strike, they can physically damage crops and reduce fruit quality, leading to economic losses.”
Topography plays a role
While these findings help explain why even brief storms can now flood roads, fields, and urban neighbourhoods in minutes, topography also plays a crucial role in shaping these risks. The CUSAT study showed convective storms occur more frequently in regions adjacent to the Western Ghats, with northern Kerala exhibiting the highest intensity and southern Kerala displaying a greater frequency. Areas along the Kottayam–Idukki border in central Kerala and the eastern parts of Malappuram in northern Kerala record stronger rainfall during convective storms, as the region’s rising terrain lifts moisture-laden air.
At the same time, districts such as Kottayam, Ernakulam, Idukki, and Pathanamthitta in Central Kerala stand out as especially vulnerable to lightning strikes — one of the deadliest but least predictable weather hazards in the state, the study shows.
As climate change continues to load the atmosphere with heat and moisture, planners and architects note that understanding where and how these convective storms strike will be key to improving early warnings, disaster preparedness, and resilience. Cities with hard surfaces are especially vulnerable, for instance.
“In cities, most rainwater is rapidly drained away,” said Reshmi M.K., a habitat design specialist and associate professor in the school of architecture, Christ University, Bengaluru. “Instead of seeping down to the ground, it overwhelms already inadequate infrastructure and turns intense convective rainfall into urban flooding,”
Vast tiled and paved surfaces — roads, pavements, and parking lots — leave little room for vegetation or for water to percolate into the soil. These hard, impervious surfaces block natural absorption and recharge, effectively transforming rainfall into runoff rather than a resource, she explained.
Towards resilience
Officials of the Kerala State Disaster Management Authority (KSDMA) acknowledged the value of detailed studies of clouds and heavy rains and noted that the state is prepared to face them. KSDMA member secretary Shekar Lukose Kuriakose, a geographer, told Mongabay India that the state was among the first in India to formally recognise the destructive potential of lightning and gustnadoes — short-lived, tornado-like wind events associated with thunderstorms. “In 2015, Kerala notified lightning and strong winds — winds weaker than cyclones but capable of causing serious damage to life and property — as state-specific disasters,” he said.
In 2022, Kerala published a Lightning Action Plan that mapped monthly cloud-to-ground lightning density and fatalities. “For early warning, KSDMA subscribes to the granular lightning and Dangerous Thunderstorm Alert (DTA) system operated by Earth Networks that provides real-time cloud-to-ground lightning alerts and thunderstorm warnings linked to convective systems,” Kuriakose said. Meanwhile, the India Meteorological Department has introduced the Damini app.
“KSDMA has advised the Local Self Government Department to adopt structural design criteria from ASCE 7-10 or IS 875 (distinct structural load standards) while approving lightweight purlin roofs,” Kuriakose said. The advisory follows repeated incidents in which such roofs were blown away during intense, localised thunderstorm wind events.
Read more: Community rain monitoring helps disaster preparedness in Kerala
Banner image: Heavy rains in Kochi, Kerala. Representative image. (AP Photo/ R S Iyer)
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