For many days, the German weather service has recently warned of heat. But our data analysis shows that notification practices have changed in recent years. That’s what DWD says about it.
Since 2011, the German Weather Service (DWD) has issued more heat warnings for Baden-Württemberg than the national average. This shows the data analysis of a collaboration between Correctiv Lokal, Zeit Online and our newspaper.
On the one hand, it’s not surprising, because it gets warmer more often in the southwest than anywhere else. But our paper’s data analysis shows that notification practices are not really decisive. For example, the ratio between hot days with at least 30 degrees and heat warnings has changed significantly from year to year in the period considered. In 2015, there were significantly more heat warnings in Baden-Württemberg than in 2018, even though the two summers were similarly warm.
The following graphic shows the average number of heat warnings and hot days in urban and rural areas of Baden-Württemberg since 2011:
Warning practices have clearly changed over time. In 2013 there were more heat warnings than hot days in Baden-Württemberg, since 2018 it has been the other way around. The notification practice has not been turned fundamentally on its head, explains Andreas Matzarakis. “But it has been refined in recent years,” says the 62-year-old, who is responsible for climate and environment at DWD. This results in the meanwhile changed ratio between heat warnings and hot days.
When is the warning?
DWD heat warnings don’t follow rigid rules, Matzarakis explains. And: “We issue the warnings for today and tomorrow because the situation can change quickly in many cases,” he says. Normally, it must have felt 32 degrees in the early afternoon for at least two consecutive days for a heat warning to be issued. That, and possibly an accumulation of several hot days in a row, explains why there have been fewer warnings than hot days in recent years. In addition, they will not exaggerate with the number of warnings so that they can still have an effect, says DWD.
According to Andreas Matzarakis, the temperature at night also plays a role. If interior spaces can no longer be cooled sufficiently, sleep is no longer restful and the burden on people increases. From what feels like 38 degrees in the early afternoon, DWD then talks about extreme heat stress.
Because humans adapt to such environmental factors, the season may also play a role. So it may be that the threshold is higher in midsummer, because the heat is part of everyday life, says Matzarakis. In spring or early summer, on the other hand, even lower temperatures can be perceived as heat, because the body has not yet had time to adapt.
This is where many caveats come in
In Baden-Württemberg, heat warnings are not issued with the same frequency everywhere, as data since 2011 shows. In the districts of Heidenheim and Schwarzwald-Baar, heat warnings are issued as often as the national average, namely nine times a year. In Stuttgart, there were an average of 13 heat alerts per year during the same period, a total of 142. This is almost half the national average. Heat warnings are issued even more frequently in the Baden part of the country along the Rhine.
Since 2017, DWD has paid special attention to cities in its warnings. They heat more than rural areas. The following map shows where, compared to the national average, heat warnings are particularly common:
Heat can lead to heat stress, especially in older people, people with pre-existing conditions and young children. For these population groups, temperatures of around 36 degrees can also be an extreme strain. This is explicitly pointed out in the warnings from DWD.
On the other hand, not every hot day automatically means severe or extreme stress for people. Therefore, DWD only warns in cases that pose a potential health hazard. On such days, the recommendation is to drink a lot, stay in cool rooms and avoid physical exertion.
This research, based on data from the German weather service, is part of a collaboration between ZEIT ONLINE, Stuttgarter Zeitung and Stuttgarter Nachrichten as well as CORRECTIV.Lokal. The network conducts research on various topics and provides long-term reports on the climate crisis at correctiv.org/climate. More information about the heat in Germany: zeit.de/Wärmetote
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