A long stretch of humid heat followed by a powerful thunderstorm is a familiar weather pattern in the tropics, but it’s also becoming more common in midlatitude regions such as the US Midwest. A recent study by two MIT scientists identifies a key atmospheric condition that determines how hot, humid, and stormy such a region can get: inversions, in which a layer of warm air settles over cooler air.
Inversions were already known to act as an atmospheric blanket that traps pollutants at ground level. Now Funing Li, a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), and Talia Tamarin-Brodsky, an assistant professor of EAPS, have found that they also trap heat and moisture at the surface. The more persistent an inversion, the more heat and humidity a region can accumulate, which can lead to more oppressive, longer-lasting humid heat waves. And when an inversion eventually weakens, intense thunderstorms and heavy rainfall can be the result.
In typical conditions, the atmosphere’s layers get colder with altitude, and a heat wave that warms the air at ground level will trigger convection: The warmer, lighter air will rise, prompting colder air to sink. When the warm air hits colder altitudes, it condenses into droplets that fall as rain, often cooling things down.
Li and Tamarin-Brodsky found that when warm or light air has settled over colder or heavier ground-level air, more heat and moisture are needed for a given “parcel” of air to build up enough energy to rise through that inversion layer. The upper limit on how hot and humid it can get depends on how stable the inversion is. If a blanket of warm air parks over a region for a long time without moving, it allows more moisture and heat to build up, which also makes the eventual storm more intense when it finally happens.
Inversions often form at night, when surfaces that warmed during the day radiate heat to space so that the air in contact with them becomes cooler and denser than the air above. Or they can form when a shallow layer of cool marine air moves inland and slides beneath warmer air over the land. In some cases, however, persistent inversions can form when air heated over sun-warmed mountains is carried over low-lying regions. In the US, Li says, “the Great Plains and the Midwest have had many inversions historically due to the Rocky Mountains.”
But global warming is likely to make the effect more pronounced. “Our analysis shows that the eastern and midwestern regions of the US and the eastern Asian regions may be new hot spots for humid heat,” he says.
“As the climate warms, theoretically the atmosphere will be able to hold more moisture,” says Tamarin-Brodsky.And because inversions will likely intensify, “new regions in the midlatitudes could experience moist heat waves that will cause stress that they weren’t used to before.”
She adds, “Our theory gives an understanding of the limit for humid heat and severe convection for these communities that will be future heat wave and thunderstorm hot spots.”
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