A hidden process may be making humid heat even worse than expected, new study finds

As the climate warms, studies continue to show how extreme heat can affect our health, raising the risk of heat-related illness. Scientists are also learning more about the atmospheric processes that trigger and intensify dangerous heat. A new study points to a hidden factor that could make hot, humid conditions even worse.

The unexpected role of dry air in making humid heat worse

Dry air may not sound like an ingredient for oppressive humidity. However, according to a new study in Geophysical Research Letters, dry air can indirectly allow hotter, more humid conditions to develop near the surface.

The main interaction takes place in the lower troposphere, the lowest part of the atmosphere. When a column of warm, moist air rises, some of the surrounding dry air mixes into it by a process known as entrainment. This causes the plume to weaken and enables more heat and moisture to accumulate close to the ground.

The buildup of moisture and heat raises wet-bulb temperatures. Wet-bulb temperature measures both heat and humidity, showing how hard it is for sweat to cool the body down.

Testing the effect of dry-air mixing

To see how much this process matters, University of St. Andrews researchers R. Jha and M.P. Byrne compared decades of atmospheric records with climate model simulations. They used atmospheric data from 1991 to 2020 and climate model simulations with different amounts of dry-air mixing.

The standard model produced a similar result, but the percentage dropped from 79% to 37.5% when dry-air mixing was removed.

Other processes can also contribute to this buildup near the surface. However, the large difference between the simulations suggests that dry-air mixing is an important part of the process.

Why the finding matters in a warmer climate

The researchers also tested a scenario with warmer sea-surface temperatures. With entrainment included, surface heat and moisture surpassed the traditional limit across 85% of the locations studied.

As the lower troposphere warms, it can hold more water vapor, meaning it takes more moisture for the air to become saturated. That can make the drying effect on rising plumes more pronounced, allowing additional heat and humidity to accumulate below.

The findings add another piece to the puzzle of future heat extremes. They also show why temperatures alone may not capture how dangerous a summer day can feel.