Why you need to worry about the ‘wet-bulb temperature’

Several temperature records were broken by India. A new high of more than 49C was recorded in Delhi in May, which was the warmest March in more than a century.

The UK set a new record for hottest day of the year with a temperature of more than 40C. On the 21st of this month, Portugal reached 47C, the hottest July day on record.

The debate about how we can protect people from rising temperatures has been revived by the current heatwaves. When it comes to the impact of high temperatures on humans, the headline figures don't give the whole story because humidity plays a big part in how we experience heat.

Recent research shows that we may be nearing the threshold values for human survivability of temperature and humidity for short periods in some places of the world, and that this threshold may be far lower than previously thought.

What does wet-bulb temperature mean?

The wet-bulb temperature is a measure of heat stress on humans.

The term is derived from how it is measured If you put a wet cloth over the bulb of the thermometer, the water in the cloth will evaporate and cool it down. The WBT can't go higher than the dry temperature. The WBT will be closer to the dry temperature if the surrounding air is humid.

The temperature on the wet-bulb will change depending on how humid it is. The purpose is to measure how well we will be able to sweat.

Solar radiation and wind speed are other factors that can affect a person's temperature. W Larry Kenney, a professor at Penn State University, says that WBT is an important measure of indoor environments.

When do wet-bulb temperatures get dangerous?

The point at which a healthy person could survive for only six hours is known as the threshold. An air temperature of 40C with a relative humidity of 75% is considered to be 35C. The UK had a peak temperature of 25C on July 19 and the relative humidity was 25%.

Humans usually regulate their internal body temperature by sweating, but above the wet-bulb temperature, we can't cool down this way. If we can't escape the heat, our body's core can rise beyond the survivable range and organs can fail.

The 35C value was derived from a 2010 study. According to research co-authored by Kenney, the real threshold our bodies can tolerate could be much lower. The critical wet-bulb temperature is closer to 31.5C according to his data.

If the new finding is true, we are in a new ball game when it comes to extreme heat. The number of people exposed to potentially deadly combinations of heat and humidity across the world would be vastly higher.

It's important to remember that heat can be dangerous for people far below the threshold.

Where could the wet-bulb threshold be passed?

The UK is a relatively low-risk area for wet-bulb extremes due to its location. It is my opinion that a wet-bulb temperature of 35C would not be possible in the United Kingdom. The Met Office didn't expect the temperature to be 40 degrees.

There is a larger chance of passing the WBT threshold in other places. Extremes are likely to approach and exceed 35C in the region around the Arabian Gulf if greenhouse gas emissions are not reined in.

Some coastal areas have already experienced WBTs of 35C for a short time.

Colin Raymond said that previous studies projected that this would happen several decades from now. The areas affected by these events will grow in relation to global warming.

Between 1979 and 2017: the number of times that a WBT of 30C was reached more than doubled In Pakistan, India, Saudi Arabia, Mexico and Australia there were at least a dozen instances of a 31C WBT.

One important question is how the climate crisis affects temperature. The maximum WBT in the tropics will rise by 1C for each 1C of average warming, according to a study. The paper said that limiting global heating to 1.5C above the pre-industrial era would prevent most of the tropics from reaching the survival limit of 35C.

The climate crisis is making heat waves worse more quickly than any other type of weather. It made the India and Pakistan heatwave 30 times more likely, according to scientists. The question of whether the most impactful heatwaves could have occurred in a pre-industrial climate is no longer relevant.

The question of what we can do about the hot weather is becoming more important as it affects more people. As the world sees the deadly mix of high humidity and high temperature more and more often, this could lead to some places becoming too hot to live in, opening up the need for migration pathways to allow millions of people to get away from their home areas.