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A recent study published in the journal Climate Dynamics has offered the clearest explanation yet of how the threat of moist heat, a combination of high temperature and high humidity that interferes with the body’s ability to cool itself, is unfolding across India, especially Kerala.
Led by Akshay Deoras at the University of Reading in the UK, the research draws on more than 80 years of weather data to show how the southwest monsoon itself governs the timing and geography of moist heatwaves.
“Our research shows for the first time that the monsoon is the key driver of where and when this deadly risk develops,” Deoras said. “Because we can forecast these monsoon patterns weeks ahead, this creates real opportunities to prepare and protect people.”
For decades, heatwaves in India were defined by temperature thresholds. When the mercury crossed a certain point, warnings were issued and emergency measures were activated. But temperature alone does not capture what the human body experiences. The body cools itself through sweating. When humidity is high, sweat cannot evaporate efficiently. The cooling system begins to fail. Core body temperature rises. The heart works harder. In extreme cases, heatstroke can develop rapidly, sometimes within hours.
This is why scientists rely on wet-bulb temperature, which combines heat and humidity, rather than air temperature alone. It reflects the limits of human survivability far more accurately. Under conditions of high humidity, even moderate temperatures can become dangerous.
Deoras points out that awareness itself is a challenge. “We often find people being more aware of dry heatwaves in India, given the scorching summer season, but moist heat remains less known and is therefore more dangerous,” he said. “Outdoor public gatherings are notorious for causing heat exhaustion. Advance warning of a moist heatwave could allow organisers to reschedule activities and strengthen medical preparedness.”
Kerala, with its long coastline, dense vegetation and monsoon driven climate, has always lived with humidity. But the baseline is shifting. Warmer days are now accompanied by warmer nights, reducing the body’s ability to recover. Urban areas are retaining heat for longer. Coastal winds that once moderated temperatures are becoming less predictable. The result is a slow but steady accumulation of thermal stress.
The new study places Kerala within a larger national pattern shaped by the internal rhythms of the monsoon. At the centre of this is a large-scale atmospheric system known as the Boreal Summer Intraseasonal Oscillation. This system drives the active and break phases of the monsoon and, as the research shows, also determines where moist heatwaves are likely to occur.
During active phases of the monsoon, when heavy rainfall sweeps across central and northern India, regions such as the Indo-Gangetic plains face a sharp increase in moist heat risk. The study notes that under a specific phase of this oscillation, “heatwave occurrence over northern India rises by 125 per cent above normal levels”.
When the monsoon enters a break phase, the geography of risk shifts. Rainfall reduces, cloud cover thins and surface temperatures begin to rise. But the moisture in the air does not disappear. It lingers. This is when peninsular India, including Kerala, becomes particularly vulnerable. The heat builds without relief, and the humidity ensures that the body cannot cope.
This shifting pattern has been consistent across more than eight decades of data. It is not random. It is part of the monsoon’s internal rhythm. Crucially, it is also predictable. The same atmospheric system that drives these changes can be forecast up to four weeks in advance.
In Kerala, the lived experience of this science is already evident. In recent summers, the state has recorded instances of heat-related illness and deaths even when temperatures did not meet official heatwave criteria. Public health practitioners say this reflects a deeper problem in how heat risk is understood.
“Heat stress is not just about temperature. It is about how the body experiences heat,” said a senior official associated with Kerala’s State Disaster Management Authority. “In a place like Kerala, humidity plays a major role. We are seeing more cases where people feel severe stress even when temperatures are below heatwave thresholds.”
Researchers studying coastal heat trends echo this concern. A recent analysis of India’s coastal regions found that humid heat is rising steadily, with wet-bulb temperatures increasing over the past four decades. “The combination of high humidity and rising temperatures is reducing the window of safe outdoor work,” said a climate scientist involved in the study. “This is particularly visible in coastal states like Kerala.”
For communities that depend on outdoor labour, the implications are immediate. Fishworkers returning from sea report intense fatigue even during early hours. Construction workers in urban areas describe conditions where dehydration sets in rapidly despite frequent breaks.
“Earlier, the sea breeze would give some relief. Now even that feels warm,” said a fisher from Alappuzha. “You feel tired very quickly. It is not like before.”
The burden of this change is uneven. In Kerala, those most affected are often those with the least capacity to adapt. Low-income households without access to cooling, elderly people in poorly ventilated homes and workers in the informal sector are particularly vulnerable. Heat, in this context, becomes a question of equity as much as climate.
The national picture reinforces this concern. A recent assessment by the Council on Energy, Environment and Water found that a majority of Indian districts now face high to very high heat risk. These districts are home to a large share of the country’s population.
“India is entering an era where heat risk is becoming more complex and more widespread,” said a researcher with the organisation. “It is no longer just about extreme temperatures. Humidity, nighttime heat and duration of heat events are all becoming critical factors.”
Official data from recent years shows tens of thousands of suspected heatstroke cases across India during intense heat seasons, with hundreds of reported deaths. Public health experts believe these figures underestimate the true scale of the problem, as heat related mortality is often underreported or misclassified.
What makes moist heat particularly challenging is its invisibility. It does not always produce record breaking temperatures. It does not always trigger alarms. It is experienced in the body before it is recognised in data. Fatigue, dizziness, dehydration and a sense of suffocation become early warning signs.
Experts say India’s heat response systems need to evolve quickly. “We need to move beyond temperature-based alerts,” said a public health specialist working on heat action plans. “Heat index and wet-bulb temperature should be integrated into early warning systems, especially in humid regions.”
The predictive capability highlighted in the new study offers a way forward. Because the monsoon’s internal oscillations can be tracked weeks in advance, authorities have a window to prepare.
“Advance warnings could allow hospitals to increase staffing before moist heat related admissions rise, enable city authorities to open cooling centres and adjust school hours, and help power systems manage demand,” Deoras said.
Such measures are not yet routine. In Kerala, as in much of India, the policy response is still catching up with the science. The state has made strides in disaster preparedness in the wake of repeated floods and landslides. Heat, however, has not received the same level of attention.
Climate projections suggest that the trends now being observed will intensify. Temperatures are expected to rise further. Humidity levels are likely to remain high. Extreme heat events may become more frequent.
For Kerala, this represents a shift in how climate risk is understood. The state’s environmental narrative has long been shaped by water, by the excess of it during floods and the unpredictability of it during monsoon failures. Now, the air itself is becoming a source of risk.
The monsoon, once seen primarily as relief from summer heat, is revealing a more complex role. It can bring rain and respite. It can also create the conditions for a different kind of heat, one that does not burn the skin but overwhelms the body from within.
India now has the scientific tools to understand this emerging threat. The question is whether that understanding will translate into timely action. For states like Kerala, where humidity is a constant presence, the stakes are particularly high.