Heat Health Height Dependency in an Urban Environment

Abstract The urban heat island effect is influenced by radiation from sidewalks and streets, which alters the apparent air temperature near the surface. Therefore, urban dwellers who are close to the ground (children, pets, etc.) should have higher heat exposure, increasing vulnerability. However, it is not well known how heat health risk varies in the near-surface atmosphere given a high surface radiative temperature. To investigate this problem, wet-bulb globe temperature (WBGT), air temperature, humidity, and wind speed were measured with a Kestrel 5400 at two levels, 0.5 and 1.5 m, in two nearby locations over 4 summer days and within 2 h (1300–1400, 1500–1600) and related to forward-looking infrared (FLIR) images of the underlying sidewalks in a hot neighborhood in Charleston, South Carolina. WBGT was consistently higher at 0.5 m than at 1.5 m, and this difference was larger than differences based on location or time of the day. Cumulative distribution functions between 0.5- and 1.5-m WBGT showed the largest differences at values well above the highest defined heat stress conditions of “black flag.” Air and dewpoint temperature differences between these heights were not significantly related to differences in WBGT, but wind speeds were. Infrared surface temperature appears to have little contemporaneous relationship with air temperature at 0.5 and 1.5 m. However, WBGT at both heights was significantly positively related to the maximum (and average) infrared temperature in the sidewalk images. The potential health impacts on vulnerable children and pets should motivate mitigation measures to reduce radiation coming off urban surfaces. Significance Statement The purpose of this study is to first determine whether there is a difference in experienced heat stress between a height of 0.5 and 1.5 m above urban sidewalks and second whether surface infrared temperatures are related to any differences noted. At two locations in the afternoon hours over 4 days, the 0.5-m heat stress was always greater than the 1.5-m heat stress, and these differences were most pronounced when the health danger was extreme. As the sidewalk became warmer, the heat stress at 0.5 and 1.5 m both increased equally. Our results have particular significance for children, pets, and anyone that spends time close to the ground in an urban environment.