India heatwave: What is wet bulb temperature and how hot is too hot?

A blistering heatwave has gripped India affecting hundreds of millions of people across the north and the east of the country.

As temperatures have soared above 50 degrees Celsius, water and electricity supplies have become stressed, crops have started to take damage and bitumen on roads has melted.

The severe heatwave has left several hundred people dead, 18 of them on Thursday alone.

The heat stress has been made worse by high humidity, according to a report by the Centre for Science and Environment, a nonprofit based in Delhi.

The combination of extreme heat and high humidity has made it dangerous to people to undertake outdoor activities.

That is because high humidity levels increase the wet bulb temperature, which can be deadly beyond a point.

What is wet bulb temperature?

The temperature commonly reported in weather forecasts is the dry bulb temperature, which measures outdoor air temperature.

The wet bulb temperature, so called as it is measured by wrapping a wet cloth around a thermometer and seeing how much cooler it gets, indicates how much the body can be cooled through evaporation of sweat, according to Climate Check.

It explains that “humidity also factors into how humans cope with and experience heat as expressed by a measurement called wet bulb temperature”.

“Drier air can absorb water faster, so lower humidity means lower wet-bulb temperature,” explained Annie Preston, the head of data and risk visualization at Climate Check, “your body is able to stay cooler than the ambient temperature through evaporation.”

“At 100 per cent humidity, wet bulb and dry bulb temperatures are the same because the air cannot absorb more water, which means sweating can’t cool us down.”

The wet bulb globe temperature measures temperature as well as wind speed and solar radiation, offering a more comprehensive assessment of heat stress on the human body.

What is safe heat limit?

A 2010 study suggested that a wet bulb temperature of 35C was the maximum that the human body could endure as it could not cool by sweating beyond this point.

But recent research at Penn State University involving young, healthy participants has found this limit to be lower.

The researchers had the participants monitor their core temperatures while performing minimal activities in gradually increasing heat and humidity.

They found the critical environmental limit, where core temperature rises uncontrollably, was lower, with an ideal wet bulb temperature around 31.1C at humidity levels above 50 per cent.

“India is typically more humid than equivalently hot places like the Sahara. This means that sweating is less efficient, or not efficient at all,” Kieran Hunt, climate scientist at the University of Reading told Bloomberg.

A report by the Intergovernmental Panel on Climate Change in 2022 said if greenhouse gas emissions weren’t reduced, they would likely raise temperatures in the Ganga and Indus river basins beyond the critical wet bulb threshold.

Risk of wet bulb temperature for humans

High heat combined with humidity poses a greater risk to human health than high heat alone because sweat does not evaporate effectively, leading to increased body temperature, heat exhaustion and potentially fatal heatstroke.

Children, the elderly and people with chronic illnesses are particularly at risk.

Intense heat and high humidity can also make buildings less habitable and potentially damage materials.

What should you do if wet bulb temperature is deemed dangerous?

According to the US National Weather Service, “always follow the advice of your local officials before engaging in outdoor activities” since temperatures fluctuate by region and so does the body’s response to heat exposure.

The US Department of Labour’s occupational safety and health administration recommends several protective measures for outdoor workers.

“Acclimatise workers starting the first day working in the heat and after any extended absences, provide shade for outdoor work sites, schedule work earlier or later in the day, use work or rest schedules and limit strenuous work.”

In India, as temperatures continue to soar, experts said understanding the wet bulb temperature was crucial for preparing and protecting the population.

Is India ready?

“There is no national repository of heat action plans and very few HAPs are listed online. Further, it is unclear whether these HAPs are being updated periodically and whether this is based on evaluation data,” the Centre for Policy Research said in its report released in March.

In any case, the report noted, heat action plans, meant to address economically damaging and life-threatening heat waves, predominantly target dry extreme heat and overlook the risks posed by humid heat.