The “cooling limit temperature” is used to measure whether the human body can still cool itself enough. Days when this is no longer the case are increasing. A visual classification.
It was extremely rare in the first half of summer to be as hot as in 2022. In the last 158 years, a higher average temperature has only been measured four times in Zurich. And all of these years are relatively recent: 2003, 2015, 2017 and 2019. So 2022 continues the trend: the summer is getting hotter and hotter. And there are no signs that this could change in the future.
At the beginning of the week, the federal government also issued a heat warning. From then on, there was “considerable danger” in large parts of the lowlands. These warnings are calculated using a simple scheme: there must be a certain average temperature over a certain period of time during the day and night. For a “considerable risk of heat” this means: a daily average of 25 degrees Celsius or more for at least three days.
Hot is not the same as hot
However, how dangerous a heat wave is for humans does not only depend on the temperature alone (read here how to protect yourself from the heat). People have lived (survived) in the desert under extreme conditions since the beginning of the human era. It becomes particularly dangerous when the humidity rises in addition to the temperature. Because while dogs can cool themselves with panting, the human body cools itself mainly through evaporation.
To do this, however, it is dependent on sweat evaporating and thus being able to extract energy from the body in the form of heat. However, the higher the humidity, the less well this process works. This relationship between temperature and humidity describes the so-called cooling limit temperature. Its value can be used to determine whether the body can still cool itself. It is therefore also an important factor in the question of how global warming could affect the human organism.
Lower than normal temperature
The cooling limit temperature refers to the lowest temperature that can be reached through evaporation. It can be calculated, but it can also be measured. In order to measure it, to put it simply, the tip of a thermometer is moistened (the cooling limit temperature is actually given as the «wet bulb temperature»). If the liquid now evaporates, heat is withdrawn from the measuring tip and the temperature drops. Thus, the cooling limit temperature is always lower than the dry measured temperature at a humidity below 100 percent.
At 40 degrees Celsius and a humidity of 10 percent – values that occur in the Sahara – the cool limit temperature is around 19 degrees. A value at which the body (in the shade) is still able to cool itself. In comparison, the cooling limit temperature increases rapidly the more saturated the air is. At temperatures of 35 degrees and a humidity of 80 percent, it reaches dangerous values of 32 degrees.
Theoretical values exist as to when the cooling limit temperature becomes dangerous:
However, when it becomes really dangerous or even deadly is controversial. The reference values are based on models and estimates, because experiments on humans are problematic for understandable reasons. However, Pennsylvania State University did trials on healthy young adults this year. Dangerous overheating of the body (hyperthermia) is said to have started in the test persons at a cool limit temperature of 31 degrees (at 100 percent humidity).
Depending on the relationship between heat and humidity, the resistance of the subjects also changed. At very high temperatures – but low humidity – dangerous effects on the body have already been observed at a cool limit temperature of 25-28 degrees. The authors of the study suspect that sweat production can no longer be increased above a certain temperature.
Peak values are reached more often
What does that mean for the future? In Switzerland, the cool limit temperature remains relatively moderate, even if it should get significantly warmer. This is mainly due to the low humidity in this country. This drops to between 20 and 40 percent in summer when temperatures are high. This means that a healthy body can still regulate its core temperature, which is around 37 degrees Celsius, in the shade.
From a global perspective, values over 30 degrees have rarely been found. In the last 40 years, however, days with a cool limit temperature between 32 and 35 degrees have tripled. A research team has calculated that by 2080 around 3 billion people could be exposed to heat, which could be extremely hazardous to health. The eastern United States, northern India, eastern China and western Africa are particularly exposed to this “heat stress”.