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Weather: What is a ‘gravity wave’?

This atmospheric phenomenon can rattle aircraft for miles around

Atmospheric gravity waves may form when winds blow across mountains. 
1. The wind blowing over a mountain pushes up a bubble of air.
3. The cooling of the air as it rises makes the air denser; the air’s ascent slows and it eventually begins sinking.
5. With the bubble of air now warmer and less dense than the surrounding air, the air begins rising again.
P&E March
2. Momentum carries the bubble up, opposing the pull of gravity, after it cools below the temperature of the surrounding air.
4. As it sinks, gravity accelerates the downward motion. The air warms as it sinks.
6. The up-and-down motion continues, slowly dying as the wind carries the wave along.

Flight instructor Gregory Bean described a strange weather encounter in an April 8, 2004, “Never Again” column on AOPA Online.

Bean says he “noticed no significant wind” on an April afternoon as he was preparing to take off from Burlington, Vermont. His weather briefing didn’t hint of any weather to worry about. Nevertheless, as he was preparing for takeoff, the airport’s automated terminal information service reported “winds from 140 degrees at 20 knots gusting to 35 knots with an altimeter setting of 29.74—pressure falling rapidly.” Pressure continued to drop; during his takeoff roll the controller said, “Altimeter 29.65!”

After takeoff, “The turbulence was as rough as I had ever encountered. My hand flailed wildly as I made the frequency change to Departure, and my eyeballs were rattling around as I tried to focus on my instruments,” Bean said. During the climb, “my altimeter wound up through 3,600 feet. The vertical speed indicator was pegged up.” After telling the controller he was coming back to land, “reading my instruments was hard enough, let alone reading a checklist! Thankfully my landing was uneventful.”

Bean had encountered a gravity wave. Gravity waves form on the boundary of fluids with different densities. Probably the most familiar gravity waves are those on top of a body of water. As wind blows against the water, it pushes some of it up. As the water rises, gravity pulls it down. Eventually gravity wins, but wind continues pushing the water, creating more waves.

In atmospheric gravity waves, the density differences are caused by different temperatures. Fortunately, atmospheric gravity waves don’t travel as far as ocean waves before they die out. Nevertheless, they can travel far from the disturbance that created them. For example, wind flowing over the Rocky Mountains sometimes creates gravity waves that are felt as turbulence by airliners high above Kansas.

Jack Williams

Jack Williams is an instrument-rated private pilot and author of The AMS Weather Book: The Ultimate Guide to America’s Weather.

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