Narayanan, A., Rezaali, M., Bunting, E., & Keellings, D. (2025). It’s getting hot in here: Spatial impact of humidity on heat wave severity in the United States. Science of the Total Environment, 963, 178397.
ABSTRACT: Heat waves pose significant risks to society, impacting human health, infrastructure, and the environment. However, on humid days, heat waves become more dangerous by raising the apparent temperature, or the temperature perceived by the human body. This study introduces an enhancement to the existing Heat Severity and Coverage Index (HSCI) that incorporates humidity (HSCIH) to provide a more robust assessment of heat wave severity relative to the human body. Our findings show that from 1980 to 2022, trends in humid heat severity have increased in every climate region of the Conterminous United States, while temperature-based heat severity shows variable rates of increase or decrease. Regional analysis shows that humid regions, particularly in the Eastern U.S., exhibit stronger upward trends in humid heat wave severity. Further, the incorporation of humidity into heat severity assessments significantly alters the ranking of historical heat wave events. Many heat waves that were previously ranked as moderate in severity are now recognized as more severe when humidity is considered. These results highlight the critical role of humidity in understanding the impact of extreme heat on human health and stress the need for comprehensive heat assessment metrics.