Wet bulb temperatures are rising
India broke several temperature records, recording its hottest March in more than a century and a new high of more than 49C in Delhi in May.
The UK, too, surpassed its own record by a whopping 1.6C, reaching more than 40C. Portugal reached 47C on the 21st of this month—the hottest July day on record—while several regions in France also recorded new highs.
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However, these headlines don’t take into account every factor when it comes to the effect that high temperatures have on humans, because humidity, which is more often than not factored into these figures, plays a massive role in how the average person experiences heat.
Taking humidity into account, scientists have coined the term “Wet-Bulb Temperature,” or WBT.
WBT combines dry air temperature (as you’d see on a thermometer) with humidity—essentially, it is a measure of heat-stress condition.
“The term comes from how it is measured,” says Kristina Dahl, a climate scientist at the Union of Concerned Scientists. “If you slide a wet cloth over the bulb of a thermometer, the evaporating water from the cloth will cool the thermometer down. This lower temperature is the WBT, which cannot go above the dry temperature. If the humidity in the surrounding air is high, however – meaning the air is already more saturated with waterless evaporation will occur, so the WBT will be closer to the dry temperature.”
“The [wet-bulb] temperature reading you get will actually change depending on how humid it is. That’s the real purpose; to measure how well we’ll be able to cool ourselves by sweating.”
The “threshold” or “critical” WBT for humans means the point at which a healthy person could survive for only six hours. This is generally considered to be 35C, approximately equivalent to an air temperature of 40C with a relative humidity of 75%. (At the UK’s 19th July peak temperature, relative humidity was approximately 25% and the wet-bulb temperature about 25C.)
Whilst the UK is considered to be at very low risk of wet-bulb extremes, there are many other countries that are in danger, especially in the global south. The region is facing larger consequences as a result of climate change despite producing far fewer emissions overall in comparison to their northern counterparts.
This is an increasingly important factor to consider as the fight against climate change becomes more and more urgent.
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