Behind the Forecast: Gravity Waves

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Science Behind the Forecast: Gravity Waves (5/16)

LOUISVILLE, Ky. (WAVE) - The National Weather Service defines a gravity wave as

“A wave created by the action of gravity on density variations in the stratified atmosphere. A generic classification for lee waves, mountains waves, and many other waves that form in the atmosphere.”

So what does that mean in English?

Think of it this way. If you drop a pebble in a calm pond or lake, ripples will emanate from the point where it makes contact with the water. The same thing happens in our atmosphere, just think of air as a fluid. For a gravity wave, the pebble could be a cold front, strong thunderstorms, wind blowing over a mountain or even solar radiation. The calm pond is a stable atmosphere (think calm weather conditions). A stable atmosphere will have cooler air settled beneath warm air.

For a gravity wave to form something must displace the air vertically, for example, air rising in a strong thunderstorm. The air must rise into stable air for a gravity wave to occur; this forms the crest of the wave. If the displaced air rises into already unstable air it will continue to rocket upwards. However, stable air wants to maintain its stability so air pushed up into it will want to sink back down in order to regain equilibrium. The sinking air forms the trough of the wave. Just like the wave in the pond, the air will rise and fall propagating away from its initiation point until it’s intensity decreases and equilibrium is regained.

The wave can grow when there is a layer of stable air anywhere between 5,000 to 12,000 ft above the surface. This acts as a channel that allows the wave to propagate horizontally along the surface.

Gravity wave over Tama, Iowa, on May 7, 2006
Gravity wave over Tama, Iowa, on May 7, 2006 (Source: Iowa Environmental Mesonet Webcam)

In a gravity wave, clear skies can be seen in regions of sinking air with clouds in areas with rising air. This is why a gravity wave may manifest as parallel streets of clouds. Thunderstorms can produce gravity waves as they rocket into the tropopause which is an area of very stable air in the atmosphere. This force will create gravity waves inside the clouds that are trying to push into the tropopause.

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Gravity waves can stretch for thousands of miles and last from a few minutes to a day.

Gravity waves are usually accompanied by pressure rises and fall (measured in millibars) as well as strong winds. The greater the pressure change, the stronger the winds.

Late on the night of Friday February 3, 2006 into the wee hours of February 4th, a gravity wave raced towards Charleston South Carolina. It initiated from a group of severe thunderstorms that were ravaging northeast Florida and southeast Georgia. As the gravity wave got closer, the atmospheric pressure dropped around 5 millibars in less than an hour, rising almost as quickly as the wave passed by. It caused wind gusts more than 75 mph. That’s a weak hurricane! Dozens of sailboats were damaged and one person was seriously injured.

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