Why India Must Wake Up To Wet-Bulb Temperatures

New Delhi: In many parts of India, the scorching days of May and June have given way to something even more perilous - humid heat that persists even at night. This is the kind of weather that leaves you drenched in sweat but offers no relief. What we are facing now is a phenomenon known as wet-bulb temperature. More people need to understand what that really means.

Most of us are familiar with the idea of heatwaves - days when the temperature crosses 40 or even 45 degrees Celsius. But wet-bulb temperature is different. It combines both heat and humidity to show how hot it actually feels to the human body. When humidity is high, sweat does evaporate easily. 

When sweat does not evaporate, the body can not cool down. At a wet-bulb temperature of around 30 to 32 degrees Celsius, even healthy people can struggle to stay safe. Experiencing 35 degrees Celsius over long periods makes human survival nearly impossible, even in the shade. In many parts of India, vulnerable communities that lack access to cooling solutions face this reality.

Wet-bulb temperature tends to rise sharply during the onset of the monsoon, especially in regions where humidity increases but rainfall remains inconsistent. This happens because the monsoon winds bring moisture, making the air feel much hotter even if the actual temperature is not extreme. Studies show that humid heat extremes have been increasing in India during monsoons, with more people exposed to dangerous wet-bulb conditions over the years. A delayed monsoon can make things worse, as high humidity combines with lingering heat, leading to prolonged heat stress.

What makes this kind of heat especially dangerous is that it does not feel extreme in the way we expect. A thermometer might show 38 degrees Celsius, which does not seem alarming - but with high humidity, it becomes deadly. In Delhi, for example, the ‘real feel’ temperature recently touched 51.9 degrees Celsius, far higher than the actual air temperature. 

That is the kind of invisible danger we are dealing with. People do not realise they are in trouble until they are dizzy, dehydrated, or worse. This is the silent side of climate change. We have seen firsthand how it is impacting those who have the least protection - migrant workers, women working as domestic help, children in tin-roof classrooms, and the elderly living in single-room homes with no ventilation.

At SEEDS (Sustainable Environment and Ecological Development Society), our work traditionally focused on disasters like floods and earthquakes - events that come suddenly and demand immediate response. Over the past 15 years, heat has emerged as one of the most pressing risks that our teams have heard communities discuss. What truly brought this home was a conversation with a schoolteacher in Delhi who said half her students were missing class due to heat-related illness. That was when we realised this was not just a matter of discomfort. It was a crisis hiding in plain sight.

This prompted a fundamental question: where does this heat really hurt? The answer led directly to people’s roofs. Approximately 80 per cent of the sun’s heat enters through the roof, and most low-income homes are topped with materials such as tin, asbestos, or concrete slabs that effectively trap this heat well into the night. When indoor temperatures remain dangerously high after sunset, the body never gets a chance to cool down. This is where heat turns deadly. That is why our solutions focus on low-cost cool roofs, better water access, shaded public spaces, and most importantly, raising awareness.

However, awareness alone is not enough. In many households, domestic workers are not allowed to rest or even drink cool water during working hours. Construction workers keep working through the heat because missing a day means losing wages. Rest breaks and fair compensation during extreme heat events have become critical points of advocacy. Even simple measures like heat-rest mandates can save lives if implemented seriously.

The next challenge is bridging the gap between scientific knowledge and practical action. This year, we worked closely with governments to help them understand not just the risk, but how to respond in ways that make sense locally. A solution that works in Chennai may not work in Varanasi. People must be at the centre of the response.

Using AI and satellite data, we can now identify areas where heat stress is highest, down to individual rooftops. Wet-bulb temperatures are not just weather statistics. They are deeply personal. When combined with data on health, income, housing, and access to water, we can design hyper-local responses. For some, it might mean retrofitting a roof. For others, simply creating a shaded place to rest during work hours can make all the difference.

An important thing to keep in mind, though, is that data alone does not drive change. People do. Building true heat resilience depends on the involvement of communities themselves. Through WhatsApp groups, local meetings, and heat action kits - simple items like caps, water bottles, and umbrellas paired with practical guidance - protective information reaches those most at risk. ASHA and Anganwadi workers, already trusted voices in their neighbourhoods, help carry these messages forward. Most critically, local leadership makes sustained action possible. Community champions ensure that information is not only received but acted upon, building long-term ownership of heat resilience strategies.

The monsoon has arrived, yet many parts of India are experiencing a dangerous overlap - high humidity from incoming rains combined with heat trapped in our cities. This is precisely when wet-bulb conditions spike. It is no longer a matter of weather. It is a matter of survival.

A response to a crisis of this scale must include everyone: government officials, scientists, businesses, civil society, and most importantly, the communities themselves. Heat cannot be tackled from air-conditioned offices alone. It needs local solutions, culturally acceptable tools, and the leadership of people on the ground.

This is not just an Indian problem. Cities across the world - from Europe to Africa - are waking up to the deadly mix of heat and humidity. But the Global South, especially India, will face this crisis first and most intensely. The good news is: we are not helpless. We already have the tools, the ideas, and the people to lead the change. What we need is urgency - and a willingness to listen to those who live with this crisis every day.

If we treat heat the way we treat any other disaster - with preparation, planning, and people-led response - we can reduce its impact. Wet-bulb temperatures may be new to public conversation, but their effects are already here. Our job now is to act before they become the new normal.

Dr Manu Gupta , Co-founder & Head, SEEDS. Views are personal.

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