People during humid heat in Delhi in July 2023 (Photo by Sonu Mehta)
50°C (122°F) is a staggeringly high temperature.
Yet Avikal Somvanshi can just about live with 50°C heat in Delhi (where a dedicated heatstroke clinic opened in May amidst a brutal heatwave). What Somvanshi can’t handle is increased humidity, when “Delhi turns into a sauna.”
“You can’t really do anything,” he says. “You just have to sit in front of a fan and wait for it to pass.”
However, media attention peaks not with the highest humidity, in July and August, but with the highest temperatures, in May and June. This is a grave mistake, warns Somvanshi, who leads the Urban Lab at the Centre for Science and Environment (CSE), a Delhi-based research organization.
When people are somewhat acclimatized to very high temperatures already, “going to 50 is more of a psychological shock” than a physiological one, Somvanshi believes. But “the humidity shock to the body is very intense.”
Yet humidity remains under-researched and under-appreciated. This is a troubling gap, given that humid heat is more dangerous than dry heat.
Why Humidity Is So Dangerous
Around the world, heatwave messaging still often focuses on temperatures alone. It’s rare for a non-meteorologist to understand how to interpret other measures.
Two metrics that haven’t fully slipped from jargon into household terminology are the wet-bulb globe temperature (WBGT) and heat index. The heat index (sometimes called the “feels like” temperature) is a measure of both relative humidity (the amount of water vapor in the air, compared to the maximum possible at that temperature) and temperature. The WBGT also factors in wind speed and solar radiation. Compared to temperature alone, both the heat index and WBGT are more nuanced predictors of a human body’s experience of heat stress.
Like Fahrenheit or Celsius, these metrics use degrees, which can lead to confusion about interpreting the numbers. Another issue is that the WBGT is different from the wet-bulb temperature; the latter is a component of the former. The wet-bulb temperature essentially assesses the condition at which it’s no longer possible for people to cool themselves off through sweating. The proliferation of numbers again challenges the understanding and spread of actionable information.
It’s not clear whether one metric will prevail globally. “We will only be able to answer that when we have more health and mortality data from the tropical regions, which we currently largely lack,” says Ivana Cvijanovic, who researches climate and health at the Barcelona Institute for Global Health (ISGlobal). “Some studies suggest that there may not be one universal indicator”; this may depend on geography.
Evaporation is no longer possible at a wet-bulb temperature of 35°C (95°F), leading some to argue that this is the highest level of humid heat that humans can survive. But other studies suggest that the critical limit is much lower, at 31°C.
The Japanese government warns against all exercise at a WBGT level of 31°C, and issues heatstroke alerts at 33°C. “Even if the temperature is 40 but the humidity is very, very low, it doesn’t feel so hot,” says Kazutaka Oka, a researcher at Japan’s National Institute for Environmental Studies.
Overall, climate change, caused by human activities like extracting and burning fossil fuels, is likely to lead to more “moist heatwaves” globally, and to more frequent and lengthy heatwaves in India. Yet Indian heat action plans tend to focus on dry extreme heat.
Rising humidity affects human health not only directly, but also indirectly through the work that people’s lives depend on. Street vendors in Mumbai have reported that humidity risks spoiling the fruits, vegetables, and flowers they sell. In Delhi, a survey of street vendors found that 80% had fewer customers during heatwaves, and most of these workers could not take breaks during heatwaves. Intense heat creates a vicious cycle of lower income, fewer resources for healthcare, and worse health, driving a need to work more under punishing conditions.
Yet there’s still much that is unknown about just how badly heat is harming health in India. “There is no reliable data on heat-induced mortality that would help us understand what is really going on there,” according to Cvijanovic. “And I think this is a major missed opportunity for India, as it is highly likely that there is much higher loss of life than reported.”
India’s Humidity Is Rising, Yet Understudied
During a searing heatwave in May 2022, parts of India reached WBGTs of 33°C. Even in the cities whose temperatures have not risen since 2001, relative humidity has increased – by 8% in Delhi, and 10% in Hyderabad. “We are noticing that almost all the cities of India are getting more humid, irrespective of the climatic features,” says Somvanshi. “We know for sure that it’s rising, and it’s rising at quite a steady pace.” This is in contrast to other parts of the world, where relative humidity is actually decreasing.
India’s rising humidity extends to night-time conditions. In general, the lingering effect of heat at night – when people are unable to control their responses to heat, and when their bodies desperately need a break from the heat – is a stark warning of a recent CSE report, Anatomy of an Inferno: Decoding Urban Heat Stress in Indian Cities. “Especially at the night time, there’s a lot of humidity added to the city from human activities,” Somvanshi says. While it’s well known that air conditioners pump heat outdoors, raising temperatures, what’s mentioned less often is that water dripping from air conditioners can also increase humidity.
To take one example, the coastal mega-city of Chennai was already notoriously humid (though it’s far from the most humid part of the country). But relative humidity and the heat index increased even further, by 5%, between the two time periods CSE analyzed: 2001–10 and 2014–23. Between 1990 and 2020, Chennai’s annual average relative humidity was 69% – a bit more than Atlanta, a bit less than Miami.
The heat index figures in the CSE report were calculated using the formula of the U.S. National Oceanic and Atmospheric Administration (NOAA). The Indian Meteorological Department (IMD) does not yet have its own official formula for calculating the heat index – a sign that it has been slow to adapt to the urgency of rising heat. The agency issues heat alerts based on dry temperatures during the daytime, not humidity or night-time temperatures. (The IMD did not respond to a request for comments.)
Despite the importance of humidity to heat stress, many people remain focused on temperature alone when making comparisons. “Delhi captures people’s imagination and all the headlines, that it’s 49, 50°,” Somvanshi notes. “But Delhi’s 50° is not as uncomfortable or unbearable as a 40° of Chennai, with 70 or 80% humidity.” At this point the heat index would be well beyond the heat index considered extremely dangerous, where heatstroke is highly likely. (This threshold is 54°C/130°F.)
In these conditions, certain tools no longer work to beat the heat. Evaporative coolers (sometimes called air coolers or swamp coolers) cool air through the evaporation of water. These aren’t efficient beyond a certain humidity level.
Technological improvements designed in India – like air conditioners with lower greenhouse gas emissions and efficient dehumidification – can help, but they can only form part of the overall package of solutions. Organizations are working on measures to bring down the costs of such appliances, but they’re still beyond the reach of most. And, of course, “We don’t have dehumidifiers for external space,” Somvanshi points out.
Compared to air conditioners, fans are relatively affordable. Fans can be enormously helpful in humid conditions, helping to clear off the moisture from bodies, and allowing sweat to evaporate. Yet some residents of Chennai also report that fans are of little use during peak humidity. Blackouts and electricity costs are also a concern for people of limited means. India is a major market for solar-powered fans, yet Somvanshi says that their popularity is limited by their slower speed compared to conventional fans.
A worker in Kolkata takes a break next to a fan. (Photo by Avijit Ghosh/SOPA Images)
Moreover, cooling devices are unhelpful to those who can’t seek refuge indoors. Many outdoor workers can’t stop working during the hottest part of the day. As Chennai delivery rider Ramesh told the Indian news publication Citizen Matters, he has to work during the peak lunch period, including 12–3pm. He believes that delivery companies should provide extra compensation in the summer. “They don’t view us as human beings at all,” he said.
Humidity affects not just the dangers of heat, but also the optimal responses. For people afflicted by heatstroke during humid heat, giving them lots of liquid would be less effective than cooling down their bodies, Somvanshi says. The threat of dehydration is greater in dry heat.
Beyond an immediate emergency, rising humidity along with rising temperatures call for changes to the built environment. There’s a need for architectural design that promotes air flow, as some ancient designs have done in coastal and humid regions. These have to be highly tailored to the location, as “design considerations for dry heat and humidity are very different,” Somvanshi says.
This extends to green spaces. While urban greenery is helpful in moderating daytime temperatures in dry heat (and brings a number of health and social benefits besides), it doesn’t reduce humidity.
Similarly, intensive irrigation in India cools the land and the air, but increases humidity and thus heat stress. Switching to more efficient (drip) irrigation can both save water and reduce humidity.
The world is watching the breathtaking acceleration of heat in India. “We are on the frontline of this attack,” Somvanshi notes. He believes that there needs to be multidirectional learning between different countries that have different levels of experience and resources to deal with this threat.
What’s clear is that around the world, new responses are needed. As Somvanshi says, “We cannot just now replicate the traditional wisdom; the climate has changed too much.”
