HadISDH.extremes Part II: Exploring Humid Heat Extremes Using Wet Bulb Temperature Indices

Abstract

Heat events may be humid or dry. While several indices incorporate humidity, such combined indices obscure identification and exploration of heat events by their different humidity characteristics. The new HadISDH.extremes global gridded monitoring product uniquely provides a range of wet and dry bulb temperature extremes indices. Analysis of this new data product demonstrates its value as a tool for quantifying exposure to humid verses dry heat events. It also enables exploration into “stealth heat events”, where humidity is high, perhaps enough to affect productivity and health, while temperature remains moderate. Such events may not typically be identified as “heat events” by temperature-focused heat indices. Over 1973–2022, the peak magnitude of humid extremes (maximum daily wet bulb temperature over a month; TwX) for the global annual mean increased significantly at 0.13 ± 0.04°C (10 yr)−1, which is slightly slower than the global annual mean Tw increase of 0.22± 0.04°C (10 yr)−1. The frequency of moderate humid extreme events per year (90th percentile daily maxima wet bulb temperature exceedance; TwX90p) also increased significantly at 4.61 ± 1.07 d yr−1 (10 yr)−1. These rates were slower than for temperature extremes, TX and TX90p, which respectively increased significantly at 0.27 ± 0.04°C (10 yr)−1 and 5.53 ± 0.72 d yr−1 (10 yr)−1. Similarly, for the UK/Europe focus region, JJA-mean TwX increased significantly, again at a slower rate than for TX and mean Tw. HadISDH.extremes shows some evidence of “stealth heat events” occurring where humidity is high but temperature remains more moderate.