Scientists have discovered that the risk of dangerous humid heatwaves depends largely on recent rainfall patterns – a finding that could transform early warning systems for some of the world’s most vulnerable populations. This groundbreaking research, published today in Nature Communications, reveals how daily rainfall variability influences extreme humid heat events across tropical and subtropical regions.
The international research team from the University of Leeds and the UK Centre for Ecology and Hydrology analyzed weather and climate data spanning two decades to uncover a critical relationship: in drier environments, humid heatwaves are more likely during or immediately after rainfall, while in wetter regions, these dangerous events typically follow periods with little or no rain.
“With climate change driving more frequent and intense humid heat events, particularly in tropical and subtropical regions, the risks to vulnerable populations and outdoor workers are increasing,” said Dr. Lawrence Jackson, lead author and Research Fellow at the School of Earth and Environment. “The new understanding provided by our research highlights the potential for improved humid heat early warning systems, using near real-time satellite observations for soil moisture and rainfall.”
Unlike dry heatwaves, humid heat presents uniquely dangerous conditions for human health. When wet-bulb temperatures reach 35°C, the human body becomes unable to cool itself effectively through sweating, potentially leading to heat stress, organ failure, and even death. Several subtropical coastal locations have already experienced this dangerous 35°C threshold.
The researchers identified distinct mechanisms driving these events. In drier regions, rainfall increases soil moisture, allowing for greater surface evaporation and higher humidity. Conversely, in already humid environments, reduced rainfall means fewer clouds, allowing more solar radiation to reach the surface and raise temperatures while maintaining high humidity levels.
Professor Cathryn Birch, who led the study, explained: “The outlook for tropical humid heat is really concerning. Humans avoid overheating by sweating. Evaporation of the sweat cools your body, allowing you to maintain a safe body temperature. Humidity makes this less effective. Humid heatwaves can be lethal at air temperatures that for dry heat would be relatively safe.”
The study focused on regions across the global tropics and subtropics where humid heatwaves are widespread, including monsoon regions like West Africa, India, and East China, humid regions such as the Amazon and Congo basin, and hot coastal areas in the Middle East. Using satellite observations, researchers distinguished between wetter and drier days to calculate how rainfall patterns influenced heatwave probability.
“The tropics are naturally humid and even an apparently small increase in global temperatures leads to large increases in dangerous humid heat extremes,” Professor Birch added. “We not only need to urgently cut greenhouse gas emissions but also need improved early warning systems for humid heat.”
The findings have significant implications for developing more effective early warning systems. By monitoring rainfall patterns and soil moisture conditions via satellite, authorities could potentially provide more accurate predictions of dangerous humid heat events, giving vulnerable communities crucial time to prepare.
Professor John Marsham, co-author and Professor of Atmospheric Science, noted: “Our results focus on the daily timescale for these heatwaves. An obvious next step would be to extend our analysis to hourly time scales which might allow us to work towards near real-time predictions with all the benefits that would bring to vulnerable communities.”
With climate change projected to increase both the frequency and intensity of humid heatwaves globally, the research represents a critical step forward in understanding and potentially mitigating these deadly events. By identifying rainfall as a key driver, this study provides a practical pathway toward developing improved early warning systems that could save countless lives in the most vulnerable regions of the world.
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