More Than Just the Temperature
When rain falls, the air temperature drops, but it also becomes saturated with moisture. This soaring humidity is the crux of the problem. Your body’s primary cooling mechanism is sweating. As sweat evaporates from your skin, it draws heat away. But when
the air is already full of water vapour, evaporation slows down dramatically or stops altogether. This means that even if the thermometer shows a seemingly moderate 30°C, the ‘feels like’ temperature, or heat index, can be well over 40°C. Your body struggles to cool down, leading to a dangerous buildup of internal heat, increasing the risk of heat exhaustion and heatstroke. This is why you can feel more drained and uncomfortable during a humid monsoon day than a dry, hot day in May.
The Critical Role of Wet-Bulb Temperature
To quantify this combined threat of heat and humidity, scientists use a metric called wet-bulb temperature. Imagine a thermometer with a wet cloth wrapped around its bulb. The temperature it shows as water evaporates from the cloth is the wet-bulb temperature. It essentially measures how effectively the air can cool something through evaporation. Researchers have identified a wet-bulb temperature of 35°C as the theoretical upper limit for human survival. Beyond this point, even a healthy person resting in the shade with unlimited water cannot adequately cool their body and risks fatal heatstroke within hours. While this threshold is rarely crossed, studies show that even lower wet-bulb temperatures, which are becoming more frequent across India, are linked to increased mortality, especially among vulnerable populations.
Cities That Never Cool Down
This problem is significantly worse in cities due to the Urban Heat Island (UHI) effect. Concrete buildings, asphalt roads, and other man-made surfaces absorb and retain far more heat than natural landscapes. During the day, these surfaces bake in the sun. At night, they slowly release this stored heat, preventing cities from cooling down effectively. The lack of green spaces and water bodies, which provide natural cooling through evapotranspiration, further compounds the issue. As a result, urban areas can be several degrees warmer than the surrounding countryside, a difference that persists even during the monsoon. This lack of nighttime relief means the body is under constant heat stress, day and night, making recovery difficult and increasing health risks.
New Research for a New Reality
Recognising this growing threat, researchers across India are intensifying their efforts to monitor and understand humid heat. Using a combination of satellite imagery, on-ground weather stations, and public health data, scientists are creating detailed heat stress maps for major urban centres. Organisations like the Centre for Science and Environment (CSE) and the Council on Energy, Environment and Water (CEEW) are tracking how factors like land use, building density, and relative humidity contribute to the heat index in cities like Delhi, Mumbai, and Ahmedabad. Studies have found that dangerously humid days are increasing in frequency across the Indo-Gangetic plain and coastal regions. This research is crucial for developing effective heat action plans that go beyond just daytime temperature warnings to include humidity, nighttime heat, and targeted interventions for at-risk groups like outdoor workers, slum dwellers, and the elderly.
