Humid air doesn’t just make heat feel worse. It also keeps Earth’s surface from cooling at night.

New research led by Nkosi Muse, a postdoctoral fellow at the Harvard University Center for the Environment, finds that humid nights are a distinct danger to human health, even when daytime temperatures are not extreme.  

We knew that increased air moisture reduces the efficiency of sweating as a mechanism to cool our bodies, but most previous studies looked at the health impacts during the day.

The research, published this month in Climatic Change, helps fill a gap by focusing on nighttime conditions in a chronically muggy place. It also explores how the urban heat island effect (the tendency for urban areas to be warmer than nearby rural areas) can behave differently in chronically humid environments.

Using southern Florida as a test case, Muse and colleagues at the University of Miami found that higher specific humidity is linked to warmer nighttime land surface temperatures and a smaller day-to-night swing. That keeps air temperatures and the heat index higher overnight.

Muse says the research “emphasizes the need for equitable heat reduction strategies, especially in coastal and tropical urban heat islands characterized by high levels of humidity, which absorb large amounts of solar radiation throughout the course of the day.”

“Without heat adaptation and mitigation strategies, thermal discomfort will remain increased in these regions and pose a greater threat to human health, especially as the atmosphere gets warmer and moister” with climate change, he added.