Humans need to keep their core temperature within certain limits. Too high (~40°C) and we die of hyperthermia; too low (~35°C) and we die of hypothermia. That is why when we exercise strenuously, we sweat. The evaporation of that sweat is the body’s way of attempting to cool itself. The effectiveness of that cooling depends how quickly that sweat evaporates. That depends mostly on how humid the air is, i.e. how much water vapour there is in it.
When we measure temperature with your average thermometer, we measure air temperature. That is the temperature given on the evening news. It is also known as the dry bulb temperature, as it is made with the bulb of the thermometer in the air (and out of the sun). However, when you want to measure the humidity of the air, you also use what is called a wet bulb thermometer. This is a thermometer which has its bulb covered completely with a wet cloth. Comparing the dry bulb temperature (the air temperature) with the wet bulb temperature will give you an indication of the humidity. At 100% humidity the temperatures are the same. The lower the humidity, the greater the difference between the two temperatures, with the wet bulb temperature always being less, as evaporation from the wet cloth cools the bulb1.
In Sydney and Melbourne, even during the hottest weather when the air temperature is getting up towards 40°C, the wet-bulb temperature usually peaks in the 20-25°C range. The highest wet bulb values commonly occurring anywhere are usually about 30-31°C, during the worst heat/humidity events in India, the Amazon, and a few other very humid places1. Like many people, I have experienced and survived daytime temperatures up to 45°C (I lived in Alice Springs for a while), but one can only do so when the humidity is quite low.
Humans’ upper physiological limit is at a wet-bulb temperature of 35°C with even lower values having serious health impacts. A normal internal human body temperature of 36.8° ± 0.5°C requires skin temperatures of around 35°C to maintain a gradient directing excess heat outward from the body’s core. Once the air temperature rises above this threshold, metabolic heat can only be shed via the evaporation of sweat, and at wet-bulb temperatures exceeding 35° C, this cooling mechanism loses its effectiveness completely, and the human body overheats.2
Climate models have predicted that the first 35°C wet bulb occurrences would happen by about 2050. However, a recent comprehensive evaluation of weather station data shows that some coastal subtropical locations have already reported occasional wet bulb temperatures of 35°C and that such extreme humid heat has more than doubled overall in frequency since 1979. There have also been records of sea surface temperatures (SST) in excess of 35°C reinforcing the validity of these dangerously high wet bulb temperatures. These most extreme wet bulb temperatures are highly localised and intermittent and because of this they are substantially underestimated in overall temperature analyses2.
The survey of weather station data has shown that there are many instances where wet bulb temperatures exceed 33°C and two stations that have already reported multiple daily maximum wet bulb temperatures above 35°C. These have mostly occurred only for 1-2 hours’ duration and are concentrated in South Asia, the coastal Middle East, and coastal southwestern North America, in close proximity to extraordinarily high SST and intense continental heat that together favour the occurrence of extreme wet bulb temperatures.2
While wet bulb temperatures like this are very rare in Australia, some over 33°C have been recorded around Exmouth Gulf in northwestern Western Australia and temperatures over 31°C have been recorded in the Tiwi Islands in the Northern Territory. Temperatures over 29°C occur all across northern Australia, on Cape Yorke peninsula, the coastal Northern Territory and all along the north coast of Western Australia. Wet bulb temperatures over 28°C can lead to heat stress from strong or prolonged exertion.2 The continuing increases in global average temperatures will further increase the frequency of extreme humid heat events.
As one of the authors of this survey, Radley Horton, said: “As with sea level rise and coastal flooding, we are already locked into large increases in the frequency and intensity of extreme humid heat events, and the risk is much larger than most people appreciate.”3