A common WBGT limit activates unequally across Grand Slam tennis venues because dry heat suppresses the wet-bulb term

Wet-bulb globe temperature (WBGT) thresholds now govern heat safety in most major sports, yet the same numerical limits are applied across venues whose climates differ fundamentally. That humidity modulates WBGT, and that warming venues which dry out experience a buffered heat-stress rise, is established (Cvijanovic et al., 2023, 2026). Its consequence for fixed-venue competitions governed by explicit, divergent WBGT limits has not been examined. We assess whether a common threshold activates equally across the four tennis Grand Slam venues, each subject to formal WBGT-based heat policies. Using hourly surface observations (NOAA Integrated Surface Database, 2015–2025) and WBGT computed with an ISO 7243-consistent iterative Liljegren black-globe model—assigned to each venue from its nearest station rather than the open-area reanalysis grid used in prior sport-heat studies—we find that a single threshold activates unequally across venues. The Australian Open, the hottest venue in air temperature (twenty days ≥ 38 °C, peaking at 44.1 °C), never reaches the 32.2 °C suspension threshold and crosses the 30.1 °C cooling threshold on only two days, because its extreme heat is dry. The humid US Open, despite air temperatures more than 10 °C lower, comes closest to suspension and crosses the cooling threshold on six days. At matched air temperatures (33–37 °C), WBGT is 3.2 °C higher at the US Open than at the Australian Open (29.6 vs 26.4 °C; Mann–Whitney p < 0.001; bootstrap 95% CI 2.6–3.9 °C). A uniform WBGT limit therefore produces a systematic disparity in how often it is triggered across venues of opposite climate—under-registering dry extreme heat relative to humid heat. Whether this differential activation has implications for player protection depends on a physiological premise we discuss and do not test; as a measured fact, what we report is a systematic, venue-level disparity in how often the governance instrument is triggered, not a climatological observation alone. We quantify this venue-level disparity, show with a first-order projection that it widens unevenly under warming, and argue for venue-specific calibration or a combined temperature–WBGT criterion. Station observations capture synoptic conditions well but not the metre-scale heterogeneity of a playing surface; recent UAV-based low-altitude thermal remote sensing has begun to resolve such fine-scale variability in built environments (Zhong et al., 2026), a future complement to the station-based approach used here. To ensure this disparity is not an artefact of daily-maximum air temperature and daily-maximum WBGT occurring at different hours, we repeated the comparison co-temporally on hourly observations within the 33–37 °C air-temperature band. The

Result: is unchanged: co-temporal WBGT averages 25.8 °C at the dry Australian Open versus 29.0 °C at the humid US Open, a 3.2 °C difference (Mann–Whitney p < 10⁻22). A linear mixed-effects regression of hourly WBGT on venue, air temperature, their interaction and tournament year (year as a random effect) confirms a significant venue × temperature interaction, indicating that WBGT rises faster with air temperature at the humid venue; the disparity is robust to alternative temperature windows (Supplementary Material).