India is bracing for another intense summer, with the India Meteorological Department warning of above-normal heatwave days in several regions. Temperatures are likely to cross 40C even in places that may not officially record heatwaves. But beyond the rising mercury, another factor is quietly worsening the situation: humidity.
What is moist heat?
Moist heat refers to the combined effect of high temperatures and elevated humidity levels. Humidity indicates how much water vapour is present in the air. As temperatures rise, especially during summer, the air can hold more moisture, making conditions feel heavier and more oppressive than dry heat.
Why humid heat feels worse
The human body relies on sweating to cool down. When sweat evaporates from the skin, it carries heat away, helping regulate body temperature. However, in humid conditions, the air is already saturated with moisture. This slows down evaporation, making it harder for the body to cool itself.
As a result, people tend to feel more discomfort in humid heat than in dry heat, even at similar temperatures. Scientists measure this combined stress using wet-bulb temperature, which reflects how effectively the body can cool itself. When both heat and humidity are high, the body struggles to release heat, increasing the risk of heat stress, exhaustion and heat stroke.
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Rising risk across India’s coasts
Recent research based on IMD data from 1981 to 2020 shows a clear rise in wet-bulb temperatures across seasons. The increase has been particularly sharp since the early 2000s, with coastal regions witnessing stronger heat stress due to the dual rise in temperature and humidity.
The east coast has seen a greater increase in humidity with each degree of warming compared to the west coast. This means even moderate temperature rises are now being felt more intensely.
“WBT levels are critical determinants of heat exhaustion and heatstroke; these findings have significant implications for human health and safety,” Mongabay-India P Vijaykumar, assistant professor at the University of Kerala, as saying.
Human impact already visible
The effects are already being felt on the ground. “I go for fishing at 5 am, but come back by 8 am, whether I get enough fish or not,” said 73-year-old fisherman Mariyani Miyelpillai. “I cannot manage this heat.”
Studies indicate that extreme humid heat events are increasing globally, with some regions nearing the limits of human tolerance. In India, densely populated districts face high to very high heat risk, according to recent assessments.
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“India is entering an era where heat risk is becoming more complex and more widespread,” a researcher told Down to Earth magazine. “It is no longer just about extreme temperatures. Humidity, nighttime heat and duration of heat events are all becoming critical factors.”
What needs to change
Experts say India must rethink how it prepares for extreme heat. Current warning systems focus largely on temperature, but that may no longer be enough.
“We need to move beyond temperature-based alerts. Heat index and wet-bulb temperature should be integrated into early warning systems, especially in humid regions,” a public health specialist working on heat action plans told Down to Earth.
Raising awareness is equally important. “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,” said Akshay Deoras of the University of Reading. “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.”
As heat patterns shift, understanding humidity’s role could be key to reducing health risks and protecting vulnerable populations.
