PNNL

Abstract

A central premise in urban climatology is that cities have higher heat stress than rural landscapes. This is primarily supported by satellite-derived skin temperature (Ts) observations since urban air temperature (Ta) measurements are rare. However, cities can be drier, which would reduce thermal discomfort. Here we examine the compensating effects of increased Ta and reduced relative humidity on wet-bulb temperature (Tw), a thermodynamic heat stress index, in ˜600 urban clusters using observations from ˜50,000 citizen weather stations during the July 2019 European heatwave. We demonstrate that daytime Tw differences between urban clusters and their surroundings (?Tw) are statistically insignificant and satellite-derived Ts, often lauded for its spatial continuity, is a poor proxy for intra-urban (and inter-urban) distribution of Tw (and ?Tw). Finally, we find that urban vegetation is much less effective for reducing Tw than for Ts. Our results provide empirical evidence challenging commonly held views on urban thermal comfort.