In 2019, meteoblue installed a dense network of temperature and precipitation sensors to monitor the climate within the city. The project started in Zurich and Basel and was extended to many more cities. The network in Basel is now used as a living lab and is continuously expanding. In July 2023, we installed 10 black globe temperature sensors in the city centre to assess the heat vulnerability on a small scale.
The black globe temperature sensors measure the mean radiant temperature that reflects the apparent temperature of human bodies, which is the perceived temperature that people experience, considering the influence of shading effects from trees or buildings.
As a first sample test area, two different locations were selected. One location (second screenshot) is a park where one sensor is installed under a tree, and another sensor is exposed to the sun. The second location (third screenshot) is a street in which one sensor is set up on the street's southern side (shaded from high buildings) and one sensor on the northern side of the street (fewer shading effects).
The first results indicate high differences between shaded (green line) and non-shaded (black line) locations and underline the importance of planting trees and ensuring enough shaded areas in cities. The mean radiant temperatures of the sensor on the sunny footwalk reached up to 49°C (2023-07-11). In contrast, the shaded footwalk on the other side of the street reached a maximum 36°C in the analysed time period.
The described differences in apparent temperatures on a small scale influence the level of thermal stress for individual locations. Thermal stress describes the effect on material or human bodies caused by temperatures. Scientists typically use 9 different levels to categorize thermal stress, as listed in the table (the second column shows the temperature ranges of each heat stress level).
The table demonstrates that on the sunny side of the Horburgstrasse, strong or extreme heat stress was measured in more than 20 % (70 hours) of the analysed period. This can be reduced by shading the street (e.g. planting trees), which is done on the other side of the street. There, strong and extreme heat stress was only measured in 3.2 % (10 hours) of the analysed time. Similar results were found for the Erlenmatt area (last 2 columns).
Since thermal stress directly impacts human comfort, it needs to be considered when planning adaptation measures in urban environments. These first results are now considered for the project's next steps.
