Conventional knowledge has it that warm air rises while cold air sinks. But a study from the University of California, Davis, found that in the tropical atmosphere, cold air rises due to an overlooked effect—the lightness of water vapor. This effect helps to stabilize tropical climates and buffer some of the impacts of a warming climate.

The study, published today in the journal Science Advances, is among the first to show the profound implications water vapor buoyancy has on Earth's climate and energy balance.

"It's well-known that water vapor is an important greenhouse gas that warms the planet," said senior author Da Yang, an assistant professor of atmospheric sciences at UC Davis and a joint faculty scientist with Lawrence Berkeley National Laboratory. "But on the other hand, water vapor has a buoyancy effect which helps release the heat of the atmosphere to space and reduce the degree of warming. Without this lightness of water vapor, the climate warming would be even worse."

Humid air is lighter than dry air under the same temperature and pressure conditions. This is called the vapor buoyancy effect. This study discovered this effect allows cold, humid air to rise, forming clouds and thunderstorms in Earth's tropics. Meanwhile, warm, dry air sinks in clear skies. Earth's atmosphere then emits more energy to space than it otherwise would without vapor buoyancy.

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