- Heatwaves in the Indo-Gangetic Plain are driven both by external large-scale weather systems and local land and atmospheric conditions, but local conditions are found to be a stronger driving factor.
- Heatwave prediction should not rely solely on short-term temperature forecasts, finds recent research.
- Both factors are expected to become more influential as the climate warms, but for different reasons, and the local land-atmosphere processes are likely to take on a particularly important role in shaping future heatwaves over the Indo-Gangetic Plains.
Not all heatwaves in the Indo-Gangetic Plain are caused by large-scale weather systems; many are driven by local land and atmospheric conditions, a new study from the Indian Institute of Technology (IIT) Bombay, reports.
Heat accumulates more because of local surface heating and air compression processes than because of warm air moving in from other regions, the study says. Recognising these local precursors of extreme heat events is essential for improving early warning systems and helping communities prepare.
The scientists examined the role of anticyclones — large-scale high-pressure systems in the atmosphere, often spanning hundreds of kilometres — in the formation of 10 major pre-monsoon heatwave episodes between 2010 and 2024. The anticyclones have previously been thought to transport warm air into the Indo-Gangetic Plains.
The new study, however, shows that even under the same large-scale anticyclones, two very different types of heatwaves can form, depending on local factors.
These two types of heatwaves are moist and dry heatwaves.
Moist heatwaves develop in areas that received pre-monsoon showers, which wet the soil, Karthikeyan Lanka, Associate Professor at IIT Bombay’s Centre of Studies in Resources Engineering (CSRE), Centre for Climate Studies, and one of the study authors, says. When strong sunshine follows, the soil releases moisture into the air. At night, this moisture forms low clouds that act like a blanket, trapping the day’s heat close to the ground and preventing the land from cooling, Lanka explained to Mongabay-India. Over several days, heat steadily builds up and a heatwave sets in, with temperatures made more dangerous along with the high humidity.
Dry heatwaves form in regions that remain dry and cloud-free, where sunlight heats the ground directly and temperatures rise to extreme levels without the added burden of humidity.
The common understanding has been that persistent high-pressure systems settle over northern India, and that hot, dry air travelling in from the arid regions of northwestern India pushes temperatures to dangerous levels across the plains, says Lanka.
In the IIT study, “rather than looking only at the large-scale weather backdrop, we tracked, step by step, where the heat was actually coming from during each event,” says Lanka. “We asked a simple question: is the air over the plains getting hot because warm air is being carried in from outside the region, or because of what is happening locally associated with the sunlight heating the ground, the moisture in the soil, the clouds overhead, and the way the land and the lower atmosphere interact with each other?”
The main new finding is that the contribution of hot air being transported from other regions “is surprisingly small,” says Lanka. Most of the heat build-up happens locally, within the same column of atmosphere sitting over a particular area.
The large-scale high-pressure system still matters, because it provides the calm, sinking, cloud-free conditions that allow heat to accumulate in the first place, says Lanka. “But once that background is in place, it is the local conditions that decide where a heatwave will actually take hold. This helps explain something that has long been puzzling: why heatwaves often affect specific pockets of the plains rather than the entire region, even when the same large-scale weather system covers all of it.”
The significance of this work “is that it changes how we should think about where and why heatwaves strike,” says Lanka. “It moves the focus from distant weather systems towards the conditions right under our feet and above our heads i.e., the moisture in the soil, the clouds at night, the sunshine during the day, and how these interact.” These local conditions can be monitored, which means it should become possible to give more accurate, location-specific warnings of when and where a heatwave is likely to develop. As heatwaves grow more frequent and severe with climate change, this kind of targeted early warning will be increasingly important for protecting lives, livelihoods, and public health.
In the new study, the scientists document that both local land-atmosphere interactions and large scale atmospheric flow, such as high pressure, are very important for generating and maintaining heat waves over India, says Madhavan Nair Rajeevan, Vice Chancellor, Atria University, Bengaluru, and former Secretary, Ministry of Earth Sciences.
However, the large-scale atmospheric flow conducive for large scale descent (subsidence) is very vital. Local processes maintain and strengthen the heat wave physics, says Rajeevan, who was not associated with the study.
A separate study from University of Hamburg, Germany, also looks at the influence of local factors on heatwaves, focussing on South Asia (northern India and parts of Indo Gangetic plain, Pakistan, Afghanistan), as it remains a major hotspot for heatwave activity.
Multiple factors driving heatwaves over northwestern India include changes in large-scale weather systems, including western disturbances, which can reduce moisture and create dry conditions weeks before the heat arrives, says Abhirup Banerjee, Postdoctoral Researcher, at the Institut für Meereskunde, University of Hamburg and co-author of the study. These dry spells then allow the land surface to heat up more rapidly, making the resulting heatwaves more intense and longer-lasting.
Talking about the IIT Bombay study, Banerjee says that it demonstrates that during the heatwave event itself, local surface-level processes are the primary drivers of rising temperatures. “This complements our work, which focuses on the ‘pre-conditioning’ phase — how large-scale factors set the stage for these events well in advance.”
A key implication of this research is that heatwave prediction should not rely solely on short-term temperature forecasts, says Banerjee. “To improve early warning systems, we must monitor ‘slower’ precursors — such as soil moisture, circulation anomalies, and local land-surface conditions — alongside large-scale atmospheric patterns. Integrating these factors could allow us to anticipate and prepare for extreme heat much earlier than we currently do.”
Factors influencing heatwaves
Which factors influence heatwave formation more – local or global factors? The two play different roles, and both are needed, says Lanka of the IIT Bombay study. Large-scale weather systems, such as the high-pressure anticyclones that sit over northern India during the pre-monsoon months, set the stage for a heatwave. They calm the winds, push the air downwards, and clear the skies, allowing sunlight to reach the ground unhindered. “Without this background setup, a heatwave would not form in the first place,” he says.
“But our study shows that this background alone is not enough,” adds Lanka. Within the same large-scale system, some areas of the Indo-Gangetic Plains develop severe heatwaves while neighbouring areas, just a short distance away, do not. The difference comes down to local conditions, he adds.
The IIT Bombay research finding matters for forecasting, as “once the large-scale setup is known, the more useful question for day-to-day warnings would be to find out those localised regions where conditions align in a way that will actually trigger a heatwave,” says Lanka.
Banerjee agrees, saying that while large-scale atmospheric factors are often responsible for creating the initial conditions for a heatwave across a broad region, local dynamics-such as land-surface moisture, urbanisation, and forest cover are what determine the intensity and persistence of the heat.
Both factors are expected to become more influential as the climate warms, but for different reasons, and the local land-atmosphere processes are likely to take on a particularly important role in shaping future heatwaves over the Indo-Gangetic Plains, says Banerjee of University of Hamburg.
“In my opinion both are very important – large scale atmospheric circulation and local interactions,” says Rajeevan.
Climate change context
Large-scale circulation patterns, such as the anticyclones that create favourable conditions for heatwaves, are themselves changing under global warming, says Lanka. There is growing evidence that these high-pressure systems may become more persistent or more intense in a warmer climate, which would mean the large-scale stage is set for heatwaves more often than before, he says. “However, projecting exactly how these circulation patterns will change is still an active area of research, and there is considerable uncertainty in the details for our region.”
Local conditions, on the other hand, are expected to amplify heatwaves in a more direct way. I would call this a land-atmosphere “trap”, says Lanka: A warmer atmosphere draws moisture out of soils faster, which means soils dry out more quickly once the pre-monsoon showers end. Drier soils, in turn, cause the incoming sunlight to heat the ground more intensely, feeding back into stronger and longer-lasting dry heatwaves.
Climate change is affecting both the land and the atmosphere simultaneously, says Banerjee. The land is becoming more prone to rapid drying, while large-scale weather patterns are shifting in ways that favour heat extremes. It is the combination of these two factors that is fundamentally changing heatwave behaviour in the region.
Banner image: A man sits in front of a water cooler at a roadside cooling station set up by the government to provide relief from the intense summer heat in New Delhi, India, Wednesday, May 20, 2026. (AP Photo/Manish Swarup)
Read more: Workers groups are pushing for legal protections against deadly heat
Credits
