New Delhi: Rain is usually seen as relief from the heat. But new research suggests that, in North India’s pre-monsoon season, it can sometimes do the opposite by setting the stage for more dangerous heatwaves.
An IIT Bombay study led by research scholar Manali Saha and assistant professors Vishal Dixit and Karthikeyan Lanka, published in the Geophysical Research Letters on 16 April, finds that while large-scale weather systems, such as anticyclones, do not determine where the heatwaves will actually strike across the Indo-Gangetic Plains. Instead, it is the atmospheric conditions and geography of the region that trigger extreme heat.
“Most people explain India’s heatwaves through large-scale weather patterns. But we kept wondering: if it’s all about big circulation systems, why don’t heatwaves hit everywhere in the affected region?” Lanka wrote on Linkedin.
This question led the team to isolate the triggers behind heatwave onset in a region. Their findings challenge the conventional definitions of heatwaves.
One of the key findings of the research is that heat advection — the horizontal transport of heat — barely matters, but it is the local land-atmosphere interactions dominate. These include diabatic heating, where the land surface warms the air above it, and adiabatic compression, where sinking air heats up.
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Rainfall and heatwaves
One of the more counterintuitive findings by the IIT Bombay study is the role of rainfall. While rain is often seen as a cooling force, the study finds that pre-monsoon showers can, under certain conditions, actually result in a heatwave or a pre-heatwave-like situation.
Moist heatwaves tend to develop in areas that have recently received rainfall, as wet soil increases evaporation, adding moisture to the air, while nighttime cloud cover traps heat near the surface, preventing
In contrast, dry heatwaves form under entirely different conditions — clear skies, parched soils, and intense daytime solar heating. With no moisture to buffer the heat, temperatures rise sharply through the day.
The study also highlights that two neighbouring regions experiencing the same large-scale weather system can have vastly different outcomes— one becoming a heatwave hotspot, the other remaining relatively unaffected.
By comparing heatwave-affected areas with nearby regions that did not experience extreme heat, the research shows that these local differences are critical to understanding heatwave behaviour.
The implications are significant for forecasting and public safety as pre-monsoon rain can lead to a sustained heat build-up for over four to five days.
“Recognising these local precursors is essential for improving early warning systems,” the study notes, pointing to factors such as soil moisture, pre-monsoon rainfall, humidity, and cloud cover as key indicators.
(Edited by Insha Jalil Waziri)