18.07.2022, Nordrhein-Westfalen, Bielefeld: ·Hitzewarnung" steht auf dem Display von einem Smartphone. Ab Dienstag (19.07.2022) sind Temperaturen bis 40 Grad möglich. Foto: Friso Gentsch/dpa +++ dpa-Bildfunk +++

As a result of the ongoing heat wave, so-called heat warnings are currently being issued in various regions of Germany.

This is intended to sensitize the population to special or extreme heat stress – also in order to protect people and risk groups who are particularly sensitive to heat.

By what standards are these heat warnings issued? Are there different warning levels? And based on which personal standards are the calculations made? We clarify the most important questions.

A so-called heat warning is issued by the German Weather Service (DWD) whenever risk groups could be particularly stressed due to high temperatures. Elderly people, the chronically ill, small children and pregnant women in particular should pay particular attention to their state of health during a prolonged heat wave.

Thanks to the heat warning, sensitive people can prepare for the high temperatures and adapt their behavior accordingly so that there are fewer heat-related complaints.

Health problems due to high temperatures should not be underestimated: According to an analysis by the Robert Koch Institute, 19,000 people in Germany died from the heat in the three summers from 2018 to 2020 alone.

An official heat warning is issued when a “strong heat load” is predicted for at least two consecutive days and “sufficient cooling of the living space at night is no longer guaranteed,” according to the DWD. Only when these two criteria apply does the German Weather Service issue a warning for risk groups at district level.

Cooling off at night is a prerequisite because, according to the DWD, a sleepless night the next day can be interpreted as an “additional burden” and is “harder to cope with” by people who are sensitive to heat.

In the case of a heat warning, the DWD differentiates between two different levels: One speaks of a “strong heat load” if the “perceived temperature is above about 32 degrees on two days in a row” and the nightly cooling is only very slight. There is a warning of “extreme heat stress” if the perceived temperature reaches 38 degrees in the early afternoon.

Incidentally, the German Weather Service not only issues different warning levels, but can also target certain groups of people in the warning text of the message.

When the perceived temperature is high, older people in particular can be addressed because they are more stressed due to the acclimatization possibilities. If the nocturnal cooling does not occur within cities, city dwellers can also be specifically addressed in the event of a heat warning.

Temperatures can be perceived differently. An outside temperature of 30 degrees is perceived as colder when sitting in the shade of a tree and a light wind is blowing with medium humidity. However, if you move around in high humidity in the blazing sun, you perceive an outside temperature of 30 degrees as warmer.

Getting used to the conditions every day also plays a role here. A resident of the polar region certainly feels 15 degrees differently than an American who grew up in Death Valley in the Mojave Desert with maximum temperatures of up to 53 degrees.

Factors such as body size, weight or even beard and hair growth in general can also play a role in the individual perception of warmth.

When calculating the perceived temperature, the German Weather Service relies on the so-called “Climate Michel Model”, which the federal authority developed itself.

This model is used to measure the thermal environment of a standard individual. With a constant walking speed of four kilometers per hour, the energy expenditure of this standard person is considered in the context of the atmospheric environment.

According to the DWD, the average “Michel”, who is used to calculate the perceived temperature, is male, 35 years old, 1.75 meters tall and weighs 75 kilograms.

According to a Statista study, there were around a million more women than men in Germany in 2021 – 42.17 million Germans were therefore female.

In addition, the age of the “standard Michel” is relatively low at 35 years. After all, according to Statista, the average age of the German population was 44.6 years at the end of 2020.

The DWD’s “Climate Michel Model” is used to calculate the perceived temperature. The perceived temperature, on the other hand, plays a not inconsiderable role when it comes to the targeted addressing of heat warnings to risk groups – including “elderly people” over the age of 35.