Articel Content
How do we deal with heat, Mr. Feigenwinter?
Text: Christian Feigenwinter
Summer days where the temperature gauge reads well over 30°C are no longer a rarity. What can we do to mitigate the consequences? Answers from Basel University atmospheric scientist Christian Feigenwinter.
In a meteorological context, “heat” can fundamentally be quantified via the air temperature. On a “hot day,” the maximum daytime temperature is at least 30°C. Since 2021, however, the MeteoSwiss heat warning concept has been based on the mean daily temperature, which also takes account of high nighttime temperatures. If the daily minimum temperature does not fall below 20°C, it is said to be a “tropical night.” These nights are much more common in cities than in surrounding rural areas, but the number of hot days is approximately the same.
Accordingly, the urban population experiences a greater heat load, particularly at night — for the “urban heat island” is a nighttime phenomenon. The Swiss CH2018 Climate Change Scenarios predict that, in the future, the Basel/Binningen measuring station will experience more hot days, more tropical nights, and more frequent and more extreme heat waves. With consistent climate protection efforts, the increase is much more moderate than in the business-as-usual scenario.
Even at pleasant temperatures, however, heat can still be uncomfortable. If you move from the shade into the sun, the air temperature will barely change in practice, but the heat stress can increase dramatically. The perception of heat is influenced not only by the temperature but also — to a considerable extent — by the radiation. If the atmospheric humidity is also high, it intensifies the heat stress even further and the weather is described as muggy.
Although a light breeze can bring some relief, the air in city streets is usually still on days such as these. Heat stress can also be reduced by wearing lightweight and light-colored clothing and a head covering. In the field of biometeorology, heat stress is quantified using thermal comfort indices — in other words, it can be calculated using the air temperature, humidity, mean radiant temperature, and wind speed.
This gives us a clue as to how heat stress can be relieved in urban environments: Shade reduces the amount of radiation; good ventilation provides cooling and increases “turbulence” and therefore intermixing of the air mass; and unsealing of the ground surface reduces heat storage in the urban fabric during the day, thereby lowering nighttime temperatures and increasing the storage of precipitation in the soil. This — as well as green infrastructure (vegetation) and blue infrastructure (water) in general — increases evaporation, which in turn leads to a reduction in the air temperature.
The Basel cantonal government’s “urban climate concept” of May 2023 essentially builds on the aforementioned principles with a view to improving the urban climate. Trees, in particular, act as natural air-conditioning systems — but it takes decades before trees can provide the required ecosystem services. This fact is often ignored by politicians.
Sources and further information (in German)
«Dolueg» Echtzeit-Messungen Atmosphärenwissenschaften, mit den Stationen Klingelbergstrasse, Aeschenplatz, Lange Erlen
Klimaszenarien für die Schweiz CH2018, National Centre for Climate Services NCCS
Klima-Analyse Basel (2019), Geoportal BS, Thema Atmosphäre, Luft, Klima + Raumplanung, Stadtklima Basel-Stadt
Stadtklimakonzept Basel-Stadt, Bau- und Verkehrsdepartement BS (2023)
Christian Feigenwinter researches and teaches in atmospheric sciences at the Department of Environmental Sciences. His research focuses on urban climatology, micrometeorology and CO2 emissions.
More articles in this issue of UNI NOVA (May 2024).