Heat waves heat up cities even more

Urban areas are most affected by heat waves because these regions exhibit an effect known as urban heat island (UHI), which is characterized by higher air temperatures compared to rural environments. Reasons for urban heat islands are dark surfaces, e.g. of roofs and streets, which lead to a higher absorption of solar energy, a lack of evaporative cooling by vegetation, little open space and thus a lack of ventilation and night cooling.

Urban heat islands are known from more than 400 cities around the world, with "excess" temperatures of up to seven degrees, especially at night. Heat waves not only contribute to general malaise and sleepless nights, but can also cause serious illness, exhaustion, heat stroke and even heat-related deaths due to their effects on the human cardiovascular system and respiratory system.

800-1000 additional deaths due to heat

Switzerland experienced pronounced heat waves in 2003 and 2015 - the years with the two hottest summers in more than 150 years. According to a 2016 study by researchers at the Swiss Tropical and Public Health Institute in Basel, these summers were estimated to have increased mortality by 6.9 % and 5.9 %, with 960 and 804 additional deaths, respectively. Overall, approximately 70,000 additional deaths were reported across Europe in the summer of 2003.

Heat waves can be simulated using weather prediction models, but their impact on cities is still not fully understood. That's why Jan Carmeliet, a professor of building physics at ETH Zurich, and Dominik Brunner, an atmospheric scientist at Empa, combined the forecast models with the effects of buildings and roads to better predict local air temperatures in cities. The resulting "heat map" of Zurich shows heat island details with a resolution of up to 250 meters:

UHI effects are most pronounced at night because of the storage of heat by building materials during the day and its release during the night. Gianluca Mussetti, a PhD student at ETH Zurich and Empa, investigated the characteristics of the recent heat wave in Zurich from June 20-24, 2017. During the night of June 21-22, he observed a UHI intensity of nearly six degrees - 1.5 degrees above the UHI intensity of the 2015 heat wave. In addition, Mussetti noted temperature differences between the coolest and warmest locations within the urban heat island of up to three degrees on those days. It is well known that heat waves manifest themselves mainly in local "hotspots" of the respective cities. In Zurich, particularly high nighttime temperatures are seen in the densely built-up city center, with relatively cooler temperatures for areas near the lake, along the Limmat River and on the slopes of the Züriberg, where cool air flows down from higher elevations during the night. In particular, the researchers observed a relationship between urban ventilation and air temperatures: A lack of inner-city air circulation leads to less heat dissipation - and thus to a higher UHI intensity.

Wanted: strategies to alleviate urban heat islands.

With an imminent increase in heat waves in the future, urban dwellers all over the world are at serious risk in terms of their well-being and health. Therefore, studying urban heat islands and developing strategies to mitigate them is becoming increasingly important for many affected countries and cities. However, to be effective, measures to address climate change and strategies to mitigate the UHI effect must be implemented and applied over a longer period of time.

Through future studies, researchers at ETH Zurich and Empa want to gain an even better understanding of the causes of urban heat islands and local hotspots in cities. They also want to develop countermeasures, such as increased use of urban water during heat waves, temporary cooling measures such as intelligent shading or cooling systems under streets and sidewalks.