Observations and Estimates of Wet Bulb Globe Temperature in Varied Microclimates

Abstract

Wet bulb globe temperature (WBGT) is used to assess environmental heat stress and accounts for the influences of air temperature, humidity, wind speed, and radiation on heat stress. Measurements of WBGT are highly sensitive to slight changes in environmental conditions and can vary several degrees Celsius across small distances (10s to 100s of meters). Compared to observations with an ISO-compliant WBGT meter, this work assesses the accuracy of WBGT measurements made with a popular handheld meter (the Kestrel 5400 Heat Stress Tracker) and WBGT estimates. Measurements were made during the summers of 2019-2021 in a variety of suburban and urban environments in North Carolina, including three high school campuses. WBGT can be estimated from standard weather station variables, and many of these stations report cloud cover in lieu of solar radiation. Therefore, this work also evaluates the accuracy of clear-sky radiation estimates and adjustments to those estimates based on cloud cover. WBGT estimated with the method from Liljegren et al. (2008) from a weather station were on average 0.2°C warmer than observed WBGT, while the Kestrel 5400 WBGT was 0.7°C warmer. Large variations in WBGT were observed across surfaces and shade conditions, with differences of 0.9°C (0.3–1.4°C) between a tennis court and a neighboring grass field. The method for estimating clear-sky radiation in Ryan & Stolzenbach (1972) was most accurate and the clear-sky radiation modified by percentage cloud cover was found to be within 75 w/m2 of observations on average.