In less humid places far from the tropics, the humidity and thus the wet bulb temperature and danger will be much lower.
Spain’s heatwave in April with maximum temperatures of 38.8 degrees Celsius had WBGT values of “only” around 30 degrees Celsius, the 2022 heatwave in the United Kingdom, when temperatures exceeded 40 degrees Celsius, had a humidity of less than 20 per cent and WBGT values of around 32 degrees Celsius.
Two of the authors of this commentary were part of a team who recently used climate data to map heat stress around the world. The research highlighted regions most at risk of exceeding these thresholds, with literal hotspots including India and Pakistan, Southeast Asia, the Arabian peninsula, equatorial Africa, equatorial South America and Australia. In these regions, heat stress thresholds are exceeded with increased frequency with greater global warming.
In reality, most people are already vulnerable well below the survivability thresholds, which is why we can see large death tolls in significantly cooler heat waves.
Furthermore, these global analyses often do not capture some very localised extremes caused by microclimate processes. For example a certain neighbourhood in a city might trap heat more efficiently than its surroundings, or might be ventilated by a cool sea breeze, or be in the “rain shadow” of a local hill, making it less humid.
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