1.2. Heat stress

(© Photo: iStockphoto_ SVproduction )

Human heat stress can be estimated with formulas implying several atmospheric variables and data from special sensors (Figure 7).

Figure 7. Grey globe and black globe installed in an urban parc (Palais U) to estimate heat stress in Strasbourg during the heatwave of July 2015. Kastendeuch et al. (2016)

Heat stress increases in highly artificial environments during day and night. During the heatwave of July 2015, the heat stress was at a maximum level in Strasbourg (Figure 8a and b). During the day, the city center was the place that suffered the most from the heat (black curve). Even at night, thermal stress remained at a moderate level due to the heat island phenomenon (particularly during the two last night). In comparison, heat stress systematically disappears at night in the urban park. However, the green sites are not systematically cooler than the urban areas during the day, it is even rather the opposite for this urban park in the morning (red curve). This happens because the trees are widely spaced and don't offer sufficient shadows, while the streets in the city center are still in the shade of the buildings.

Figure 8. Air temperatures in a urban park (Palais U) during heat wave of July 2015 (Strasbourg). Kastendeuch et al. (2016)
Figure 8. Thermal comfort index (UTCI) calculated with grey globes placed in the city center (Kleber) and in an urban park (Palais U). Measurements made during the heatwave of July 2015 in Strasbourg. Kastendeuch et al. (2016)