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This year, the European Environment Agency published a report in which heat stress is defined as the most dangerous climate risk for the health sector. Society is expected to suffer from increasing heat stress events in the coming years. Moving towards impact-based forecasting, the Royal Netherlands Meteorological Institute (KNMI) has recently adopted the heat stress indicator Wet Bulb Globe Temperature (WBGT) as part of their Early Warning Centre. The WBGT presented here is computed analytically from air temperature, relative humidity, wind speed and global radiation using the theoretical Liljegren approximation. These WBGT values are compared with WBGT values as derived from black bulb (globe) and air temperature observations for a specific station in the Netherlands. We present a characterization of WBGT based on hourly observations made by Automatic Weather Stations (AWS) in the Netherlands and compare the safety levels as defined by the US Army, the world athletic organization and the Dutch fire brigade. By assessing the trends in WBGT over the past decades, we find a significant increase of hourly intervals where critical threshold values are exceeded for all five investigated stations. The safety levels of these organizations are based on the absolute values of WBGT. The effective WBGT (WBGTeff) also considers the effects of clothing and a person's activity. The total time in a day that the WBGTeff is above a certain threshold can therefore be used for warnings for specific work environments. Looking into a case study of a Dutch triathlon (25 June 2023) with a combination of hot weather conditions which resulted in a high amount of heat strokes and hospitalizations, we show spatial variations of WBGT for that day using detailed maps based on AWS and crowd-sourced data.
Besselaar et al. (Fri,) studied this question.