The impacts of summer heat extremes are mediated by humidity. Increases in temperatures due to human-caused climate change are generally expected to increase specific humidity; however, it remains unclear how humidity extremes may change, especially in climatologically dry (low-humidity) regions. Here we show that specific humidity on dry days during summer (defined here as July–September) has decreased over the past seven decades in the United States Southwest, and that the greatest decreases co-occur with the hottest temperatures. Hot, dry summers have anomalously low evapotranspiration that is linked to low summer soil moisture. The recent decrease in summer soil moisture is explained by declines in June soil moisture, whereas the interannual variability is controlled by summer precipitation. Future projections of hot, dry days in the Southwest are uncertain due to the large spread in the Coupled Model Intercomparison Project phase 6 (CMIP6) trends in soil moisture and precipitation through 2100.

夏季极端高温的影响是由湿度调节的。由于人为气候变化导致的温度升高通常会增加比湿度；然而，目前尚不清楚极端湿度如何变化，尤其是在气候干燥（低湿度）地区。在这里，我们展示了在过去的七年中，美国西南部夏季干燥日（此处定义为 7 月至 9 月）的特定湿度有所下降，并且最大的下降与最热的温度同时发生。炎热干燥的夏季蒸发量异常低，这与夏季土壤湿度低有关。最近夏季土壤水分的减少可以用六月土壤水分的减少来解释，而年际变化则受夏季降水的控制。未来的热点预测，