Bhattarai et al. (2026) Rainfall relief or humidity havoc? The boon and curse of precipitation during heatwaves

Identification

• Journal: Natural Hazards
• Year: 2026
• Date: 2026-02-17
• Authors: Saurav Bhattarai, Cenlin He, Sanjib Sharma, Saurav Kumar, Rocky Talchabhadel
• DOI: 10.1007/s11069-025-07742-x

Research Groups

• Department of Civil and Environmental Engineering, Jackson State University, USA
• Research Applications Laboratory, NSF National Center for Atmospheric Research (NCAR), USA
• Department of Civil and Environmental Engineering, Howard University, USA
• School of Sustainable Engineering and the Built Environment, Arizona State University, USA

Short Summary

This study investigates the complex interaction between heat, humidity, and precipitation during heatwaves across the contiguous United States from 1980 to 2020. It reveals that while precipitation can terminate heatwaves, it often leads to increased humidity, paradoxically exacerbating heat stress, particularly in vulnerable regions.

Objective

• To analyze the dual-effect dynamics of precipitation during heatwaves across the contiguous United States (CONUS), offering novel insights into the complex interplay between temperature, humidity, and precipitation during these extreme events.
• To explore the interactions of atmospheric conditions that accompany heatwaves across the CONUS, focusing on the interplay between heat and moisture, and to reveal evolving spatial patterns and the dual role of precipitation during heatwaves.

Study Configuration

• Spatial Scale: Contiguous United States (CONUS), analyzed at the county level using 1-kilometer grid data.
• Temporal Scale: 1980–2020, divided into two periods: pre-2000 (1980–1999) and post-2000 (2000–2020).

Methodology and Data

• Models used: None explicitly for simulation; empirical formulas were used for calculating saturation vapor pressure, relative humidity, and the heat index (HI).
• Data sources:
  • DAYMET dataset (version 4): High-resolution (1-kilometer) daily climate data for maximum daily temperature (Tmax), daily precipitation (Prcp), and vapor pressure (used to calculate relative humidity).
  • Centers for Disease Control and Prevention and Agency for Toxic Substances and Disease Registry (CDC/ATSDR) Social Vulnerability Index (SVI) scores: County-level data for correlation analysis with heatwave duration changes.

Main Results

• Heatwave duration increased by 10.8% post-2000, translating to approximately 2.3 additional heat stress days annually per county, with the most pronounced intensification in the Southwest, Gulf Coast, and Eastern US.
• Humidity levels during heatwaves rose by 4.7% post-2000, particularly in the Southeast and coastal areas, compounding heat stress.
• The frequency of precipitation events that terminate heatwaves increased by 18.7% post-2000, especially in the East and Gulf regions.
• Despite providing cooling relief, precipitation events terminating heatwaves often lead to heightened humidity, exacerbating physiological heat stress due to land-atmosphere coupling and enhanced evapotranspiration.
• A significant positive correlation was found between social vulnerability and increases in heatwave duration (Spearman’s ρ = 0.259, p < 1 × 10−40).
• A significant positive correlation was observed between heatwave duration and humidity levels during heatwave events (Spearman’s ρ = 0.409, p < 1 × 10−120).
• The magnitude of precipitation terminating heatwaves remained relatively stable (8.7–13.0 millimeters), indicating that changes in frequency rather than intensity drive the observed dynamics.
• Observed humidity trends contradict some current climate model predictions, suggesting gaps in understanding evapotranspiration-humidity coupling.

Contributions

• Provides the first county-scale analysis of precipitation’s dual role (relief vs. humidity increase) during heatwaves across the contiguous United States.
• Quantifies the feedback between humidity and precipitation at policy-relevant spatial scales.
• Demonstrates systematic spatial heterogeneity in land-atmosphere coupling during heatwaves.
• Reveals that precipitation's cooling relief may be offset by moisture loading, which exacerbates physiological heat stress, offering a more comprehensive understanding of heatwave risks.

Funding

• Hydrological Impacts Computing, Outreach, and Resiliency Partnership (HICORPS) Project
• U.S. Army Engineer Research and Development Center (ERDC)
• WOOLPERT-Taylor Engineering

Citation

@article{Bhattarai2026Rainfall,
  author = {Bhattarai, Saurav and He, Cenlin and Sharma, Sanjib and Kumar, Saurav and Talchabhadel, Rocky},
  title = {Rainfall relief or humidity havoc? The boon and curse of precipitation during heatwaves},
  journal = {Natural Hazards},
  year = {2026},
  doi = {10.1007/s11069-025-07742-x},
  url = {https://doi.org/10.1007/s11069-025-07742-x}
}