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Heat measurement by our drone team during heatwave

Published on
September 18, 2023

Using drone for study in urban climate

Published on
Month Day, Year

The dronemeasurements took IR pictures of a sub urban site in Ghent consisting of buildings, forests, separate trees, grass, dried mud, pavement with tiles and pavement with asphalt during a national heatwave in June 2023. The IR pictures were taken during the day and the night at June 14. The daytime pictures were taken at almost the hottest moment of the day around 3 pm, the sky was clear during and a long period before the measurements. The nighttime pictures were taken around 11 pm, after sunset, again with clear sky. These measurements were part of a bigger measurementcampaign of Ian Hellebosch (Ghent University, VITO) about extreme heat in cities. During these measurements, the locations were also monitored by weatherstations measuring the air temperature, relative humidity and radiation (in the form of globe temperature) in order to calculate heatstress. The amount of heatstress indicates how a human being experiences the heat. Air temperature and relative humidity was also necessary to calculate the surface temperature from the IR images. An emissivity of 0.95 is assumed for all objects.

First have a look at an example of a daytime picture. As you can see on the visual picture, it concerns an asphalt road with pedestrian crossings, parking places with a parked blue car, small pieces of grassfields, a small forest with trees partly giving shadow to the pavement. Also the weatherstations which measured the necessary air temperature and humidity to process the IR pictures is visible. The IR picture shows that the pavement with grey tiles or asphalt in the sun is around 60°C surface temperature. The white painting of a pedestrian crossing can cause a temperature decrease of 5°C compared to the black/grey asphalt. The temperature of the grass is similar or sometimes even higher than the asphalt when it is located into the sun. This can be caused by the lower thermal inertia of grass. Moreover, it was a very dry period so the grass was very dry. The trees have a much lower temperature of more or less 28°C. The pavement in the shadow of the trees is 20-25°C lower than the pavement in the sun. In this picture there is also a car and a person shown. The person has a temperature of 41.6°C, a value that is usually measured for the surface temperature of the head of a person in the sun. There is a blue car in the picture with a surface temperature of more than 80°C, this is in line with the expectations that a car can get very hot.

The nighttime picture shows that the surface temperature of the tiles and the asphalt already decreased by 30°C compared to the daytime situation, while it was only yet the beginning of the night. At this moment, the grass is around 20°C and has thus a lower surface temperature than the pavement. This is as expected with the lower thermal inertia of grass. This also explains why there is a so called urban heat island effect in which the nighttime temperatures in a city cool down slower than in a rural area. This is partly caused by the higher thermal inertia of the materials used in a city. The place where the car was parked, which is now gone, is also some degrees lower than the surrounding asphalt, the shadow of the car decreased the warming of this piece of asphalt.

These results have a potential to be useful in extreme heat adaptation measures concerning the type of pavement.