Scientists from the University of Leeds and the UK Centre for Ecology and Hydrology have unveiled groundbreaking research that sheds new light on the complex mechanisms behind humid heatwaves in tropical and subtropical regions. Their findings, highly significant in the context of accelerating climate change, promise to enhance early warning systems for vulnerable populations exposed to these increasingly frequent and dangerous weather phenomena. Published in the prestigious journal Nature Communications, the study marks the first comprehensive analysis linking daily rainfall patterns to the occurrence and severity of extreme humid heat events.
Humid heatwaves represent a unique and insidious threat distinctly different from their dry heatwave counterparts. Unlike dry heatwaves, which have been the focus of extensive climatological research, humid heatwaves involve a combination of high temperatures and atmospheric moisture levels that critically impair the body’s natural cooling mechanism—evaporation of sweat. This physiological stress, also known as heat stress, can push core body temperatures beyond 3°C above normal, leading to cognitive impairment, convulsions, and in severe cases, fatal organ failure. Understanding the meteorological triggers behind these events is thus vital for public health and safety, especially in tropical zones where humidity is naturally elevated.
The core metric that defines the lethality of humid heatwaves is the wet-bulb temperature, which integrates temperature and humidity to quantify how effectively a human body can dissipate heat through sweating. A wet-bulb temperature of 35°C is widely considered a critical threshold beyond which human survival becomes impossible over prolonged exposure since the body can no longer shed excess heat. Alarmingly, several subtropical coastal locations have already recorded instances surpassing this threshold, underscoring the urgency of improved monitoring and forecasting methodologies.
Dr. Lawrence Jackson, the lead author and a Research Fellow at the School of Earth and Environment, emphasized the growing real-world implications of their research. “Climate change is not only increasing the frequency of these humid heat events but also intensifying their severity, especially in vulnerable tropical and subtropical populations,” he noted. His team’s innovative use of near real-time satellite data for soil moisture and rainfall measurements offers a promising avenue for transforming how early warnings are issued, potentially saving lives by enabling timely preparation and response.
The research methodology involved an exhaustive analysis of climate and weather data spanning over two decades, from 2001 to 2022. By carefully identifying occurrences of humid heatwaves and correlating them with daily variations in rainfall, the team could discern distinct patterns that influence the emergence of these heat extremes. Their satellite observations, which provided precise measurements of soil moisture and recent precipitation, allowed them to differentiate days following wetter or drier conditions, a factor hitherto underexplored in humid heatwave forecasting.
Geographically, humid heatwaves manifest widely across the global tropics and subtropics, affecting monsoon-driven regions such as West Africa, India, East China, and northern Australia. They also impact consistently humid areas including the Amazon rainforest, the southeastern United States, and the Congo Basin, as well as hot, arid coastal zones in the Middle East. This distribution reveals the ubiquity of humid heat hazards, highlighting the need for tailored regional risk assessments and mitigation strategies.
One of the study’s key insights lies in the differentiated role of preceding rainfall patterns depending on the moisture state of the land surface. In relatively drier regions, periods following enhanced rainfall actually increase the likelihood of subsequent humid heatwaves. This paradoxical effect occurs because rain elevates soil moisture, enriching atmospheric humidity and thereby exacerbating heat stress conditions. Contrastingly, in wetter regions, a lull in rainfall over several days tends to precede such events, as decreased precipitation allows the land surface to absorb more solar radiation, driving up air temperatures.
This nuanced understanding is critical for accurately predicting humid heatwave risk, as it underscores the interplay between soil moisture dynamics and atmospheric processes. Rainfall not only adds moisture to the environment but also influences cloud cover, which in turn modulates surface temperatures. Consequently, periods of suppressed rainfall can lead to increased solar heating, setting the stage for dangerous heat extremes even in naturally humid environments.
Professor Cathryn Birch, who led the research, highlighted the profound implications for human health and climate resilience. She explained, “Sweating is our primary defense against overheating, but humidity effectively shuts down this cooling mechanism. Even moderate air temperature increases in the tropics can amplify the intensity and frequency of these deadly heat episodes. This makes the development of precise, early warning systems more important than ever, alongside global efforts to reduce greenhouse gas emissions.”
Co-author Professor John Marsham pointed out future research opportunities emanating from this study. He suggested that extending the current analysis from daily to hourly timescales could unlock more granular predictions. Such advancement would further enhance the utility of early warnings, enabling affected communities, especially outdoor workers, to respond dynamically to rapidly evolving conditions and mitigate health risks more effectively.
This pioneering research establishes a critical foundation for the next generation of climate and health monitoring tools. By integrating satellite-derived soil moisture data with detailed rainfall variability analysis, scientists can now anticipate the initiation of humid heatwaves with greater precision and lead time than ever before. This capability is likely to transform local and regional response strategies, from urban planning to emergency healthcare readiness, potentially reducing morbidity and mortality linked to extreme heat events.
As climate change continues to alter precipitation patterns globally, the dynamism uncovered in this study signals that humid heatwaves will not only become more frequent but also more unpredictable without adaptive technologies. The authors’ call for improvements in early warning systems resonates with broader global climate policy objectives, emphasizing the urgent intersection between environmental science, public health, and disaster risk management.
In summary, the University of Leeds and UK Centre for Ecology and Hydrology research presents a vital leap forward in decoding the complex environmental triggers of humid heatwaves in tropical and subtropical regions. By revealing the pivotal role of recent rainfall and soil moisture status, the study opens the door for practical solutions to one of the most pervasive climate-related health threats in the 21st century. Stakeholders ranging from climate scientists to policymakers and vulnerable communities stand to benefit from these insights, which promise to save lives as the planet warms.
Subject of Research:
Not applicable
Article Title:
Daily rainfall variability controls humid heatwaves in the global tropics and subtropics
News Publication Date:
29-Apr-2025
Web References:
https://www.nature.com/articles/s41467-025-58694-6
References:
Jackson, L. et al. (2025). Daily rainfall variability controls humid heatwaves in the global tropics and subtropics. Nature Communications. DOI: 10.1038/s41467-025-58694-6
Keywords:
Climate systems, Rain, Environmental stresses, Ecosystems, Environmental issues, Human health
Tags: climate research and meteorologycomprehensive analysis of heat eventsearly warning systems for heatwavesextreme weather phenomenahigh humidity and human healthhumid heatwavesphysiological effects of humid heatpublic health risks of humidityrainfall patterns and heat stresssubtropical climate challengestropical climate change impactsvulnerability to extreme weather
