You may have once learned that “warm air rises, and cool air sinks.” But that doesn’t always hold true.
That’s because the buoyancy of air — its ability to rise — is dictated both by its temperature and by how much water vapor it contains. Dry air mostly contains the elements nitrogen and oxygen, assembled into different molecules. Water vapor is less dense than these heavy molecules; in humid air, water vapor takes up space that would normally be occupied by nitrogen and oxygen. Known as the “vapor buoyancy effect,” this phenomenon renders humid air lighter than dry air of the same temperature, pressure and volume.
On a grand scale, the vapor buoyancy effect helps direct the movement of air through the lowest region of the atmosphere, known as the troposphere, and particularly affects air over humid, tropical regions. Even in the tropics, some patches of air remain relatively dry compared to more humid air located to the East or West. Now, a new climate model suggests that this cycle of humid air rising and dry air sinking may somewhat buffer the effects of climate change in the tropics.
“Without this effect, the climate warming [in tropical regions] would be even worse,” said study author Da Yang, an assistant professor of atmospheric sciences at the University of California, Davis and a joint faculty scientist with Lawrence Berkeley National Laboratory. According to Yang’s model, which was published May 6 in the journal Science Advances, the vapor buoyancy effect amplifies the amount of thermal energy (heat) released into space from tropical regions, on the order of about 1 to 3 watts per square meter. The model suggests that, as tropical climates warm, the effect could increase exponentially, meaning that the region would let off more and more heat, even as temperatures rise.
That said, the vapor buoyancy effect in no way cancels out the effects of climate change, Yang said. But it may somewhat stabilize tropical climates while temperatures at the Earth’s poles climb at a comparatively faster rate, he said.
But how does that work, exactly?
Clouds and clear skies
The vapor buoyancy effect “has been known to meteorologists for a very long time,” at least a century, Olivier Pauluis, a professor of Mathematics and Atmosphere/Ocean Science at New York University who was not involved in the study, told Live Science in an email. Although familiar to many, the notion that warm air always rises and cool air always sinks is an “incorrect assumption,” he said.
“The correct ‘conventional wisdom’ goes back to Archimedes’ principle and is that light air rises, heavy air sinks.” However, moist air is lighter than dry air of the same temperature and pressure, Pauluis said.
While modern climate change models take this wisdom into account, Yang aimed to investigate how the humid air of the tropics influences overall warming in the region, more specifically.
The tropics lie within about 20 degrees of the equator, wrapped around the planet like a belt, according to National Geographic. In the tropics, global patterns of air circulation generate columns of humid air and columns of relatively dry air that sit alongside each other, extending skyward, Yang said. The same pattern of alternating air columns also manifests at smaller scales, but these localized pockets of air dissipate within a matter of days, while the larger scale ones remain stable over long periods of time and influence climate across the tropics, he said.
Clouds and thunderstorms form in the humid air, while clear skies largely span the dry regions, Yang said. Water vapor acts as a greenhouse gas, trapping thermal energy emitted from the oceans, land and lower regions of the atmosphere; therefore, little energy can escape into space from more humid regions of the tropics. “Most of the energy would be emitted from the clear-sky regions, not the clouds,” Yang said.
This is where the vapor buoyancy effect comes into play.
According to the team’s computer models, cool air imbued with water vapor rises upward, forming clouds and dropping rain as it goes. Meanwhile, relatively dry, warm air sinks in clear regions of the sky. As the tropical climate warms, more water heats up and transitions into its vaporous form, causing the air above to grow more and more humid. The subsequent change in buoyancy drives the humid air upward and drives ripples through the surrounding air; these ripples, known as atmospheric gravity waves, push heat out of the humid air and into the dry air nearby, Yang said.
Basically, the waves balance out the sudden increase in vapor buoyancy by reducing any additional buoyancy provided by heat, he said.
This cycle drives more and more heat into the dry air, which lets off thermal energy into the clear sky above, Yang said.
“In other words, [vapor buoyancy] will make the sinking dry air even warmer,” allowing more heat to be emitted from clear-sky regions, Yang said. “If we don’t have this vapor buoyancy effect, it would likely be the other way around,” meaning that the increasingly warm air would rise in humid regions where its heat would be trapped beneath clouds, he added.
The finding isn’t “necessarily groundbreaking,” as scientists have known about the vapor buoyancy effect for a long time, Pauluis said. However, it does highlight the need to take both temperature and relative humidity into account when modeling climate change, especially in tropical regions, he added.
Looking forward, Yang and his coauthors aim to develop large-scale models to test their theory. In the current study, they modeled small-scale systems of humid and dry air that remained stable through time, as a large-scale system would. To develop a full-scale model that captures activity across the tropics, at large, the team will require much more computing power. Additionally, Yang hopes to collect observational data from different tropical regions, to see how the team’s predictions hold up in the real world.
“We also want to know, how does vapor buoyancy impact clouds and winds on Earth?” he said.
“A central challenge in prediction of future climate change lies in correctly assessing the changes to low level clouds, which is where the [vapor buoyancy] effect is more significant,” Pauluis added.
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