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Rainfall and moisture conditions in the soil may influence the onset of extreme humid heat in the global tropics and subtropics, including India, according to a new study published in Nature Communications.
These findings could help develop early warning systems for humid heat waves, which are defined as periods of elevated temperature combined with high humidity levels. The environmental conditions that bring about humid heat overwhelm the body’s ability to cool itself.
“Humid heat is a serious risk to human health, reducing the body’s ability to cool itself through sweating. The impact on humans will increase under climate change, particularly in tropical and sub-tropical ‘hot spots’, such as equatorial Africa and the Indian subcontinent, which are highly populated, and already very hot and humid,” Lawrence Jackson, a Research Fellow in the School of Earth and Environment at the University of Leeds and the lead author, told Down To Earth.
The study highlighted that humid heat extremes have intensified since 1979 and become more frequent.
Climate change is already driving more extreme humid heat events as seen in South and Southeast Asia in April 2023 and is expected to become more severe under future climate change.
Studies have shown when humans are exposed to a few hours of wet-bulb temperature (Twb) of 35 °C, they may lose their ability to shed heat effectively through sweating.
Twb is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. It combines temperature and humidity effects while dry bulb temperature measures ambient temperature.
Recent studies suggest that at Twb of around 31°C, health impacts can occur for young, healthy individuals. For vulnerable groups — the very young and the elderly — the value is likely to be much lower.
Still, the meteorological process driving humid heat waves at regional scale is not well-understood. “Whilst there is a growing body of research on dry-bulb temperature extremes, there is very limited understanding of the meteorological drivers of humid heat extremes,” the expert added.
So, the researchers analysed humid heat waves across the tropics and subtropics using weather and climate data from 2001 to 2022. They next identified heat wave events that occurred during this period and analysed how they were influenced by recent rainfall using satellite observations.
Their analysis showed that recent rainfall patterns along with soil moisture levels play a key role in triggering humid heat waves in tropical and subtropical regions.
Drier regions are likely to see humid heat waves following periods of enhanced rainfall while wetter regions tend to see a humid heat wave event following at least two days of suppressed rainfall.
In northwestern India, for example, the climate is arid. In arid regions, the seasonal climate is hot and dry, which means there is sufficient energy to support evaporation of surface moisture, but the levels of moisture are low. “In these regions, rainfall occurs immediately before or at the start of humid heat events. Rainfall provides moisture to the surface which, when evaporated, drives a strong increase in humidity,” the expert noted.
In humid regions during the monsoon season, soil moisture levels are relatively high, which means there is sufficient moisture to support evaporation. But deep cloud cover reduces sunlight at the surface, limiting surface evaporation, the expert added. But after the easing of rainfall, and the appearance of clearer skies, there is increased warming of the surface due to sunlight. This could be occurring in Northeast India, and perhaps in the peninsular part of the country.
These findings, according to the researchers, could help build early warning systems for humid heat waves. The strong and widespread connection between soil moisture, precipitation, and humid heat across the tropics, means satellites could be used for near real time observations of soil moisture and precipitation.
“When combined with projections and historical data from climate or weather models, we see a potential opportunity to improve short term forecasts of humid heat and, in turn, enhance the effectiveness of early warning systems,” Jackson added.
