Rainfall mode unlocks deadly humid heat wave trigger

Scientists have found that the risk of dangerous humid heat waves depends largely on recent rainfall patterns – a discovery that could change early warning systems for some of the world’s most vulnerable populations. This groundbreaking study, published today Natural Communicationsreveals how daily rainfall changes affect extreme humid and heat events in tropical and subtropical areas.

An international team of research at the University of Leeds and the UK Centre for Ecology and Hydrology analyzed weather and climate data spanning twenty years to find key relationships: In drier environments, wet heat waves are more likely after rainfall or after rain, while in wet areas, these dangerous events are often less or no rain.

“With the currents of climate change, especially in the frequent and intense humid heat events in tropical and subtropical areas, the risks for vulnerable populations and outdoor workers are increasing,” said Lawrence Jackson, PhD, principal author and researcher at the School of Earth and Environment. “Our research provides a new understanding that highlights the potential of improving humid heat warning systems, using near-real-time satellite observations for soil moisture and rainfall.”

Unlike dry heat waves, wet heat presents unique dangerous conditions for human health. When the wet bulb temperature reaches 35°C, the body cannot effectively cool itself by sweating, which may lead to heat stress, organ failure and even death. Several subtropical coastal areas have experienced this dangerous 35°C threshold.

The researchers identified different mechanisms that drive these events. In dry areas, rainfall increases soil moisture, causing greater and higher humidity to evaporate the surface. In an already wet environment, reducing rainfall means fewer clouds, allowing more solar radiation to reach the ground and increasing temperature while maintaining higher humidity levels.

Professor Cathryn Birch, who led the study, explained: “The prospect of tropical humid heat is really worrying. Humans avoid overheating by sweating. Vaporing of sweat will cool your body, thus keeping you safe body temperature. Humidity will lower humidity. Humidity will make humid heat relatively safe at air temperatures.”

The study focused on regions around the world in tropical and subtropical regions where humid heat waves are widespread, including monsoon regions such as West Africa, India and China, wet areas such as the Amazon and the Congo Basin, and hot coastal areas in the Middle East. Using satellite observations, the researchers distinguished between wet and dry days to calculate how rainfall patterns affect the probability of heat waves.

“The tropics are naturally humid, and even the global temperature rise is obviously small, which can lead to a massive increase in dangerous humid heat,” Professor Birch added. “We need not only urgently cut greenhouse gas emissions, but also improve wet early warning systems.”

These findings are of great significance for developing more effective early warning systems. By monitoring rainfall patterns and soil moisture conditions through satellites, authorities may provide more accurate predictions of dangerous and wet thermal events, providing important preparation time for vulnerable communities.

“Our results are focused on the daily time frames of these heat waves. The obvious next step is to extend our analysis to hourly time scales, which may allow us to achieve almost real-time predictions with all the benefits that bring disadvantaged communities.”

As climate change increases the frequency and intensity of humid heat waves worldwide, this study represents an important step in understanding and mitigating these deadly events. By identifying rainfall as the primary driver, this study provides a practical way to develop improved early warning systems that can save countless lives in the most vulnerable areas of the world.