Mountains in the Sky

Mountains in the Sky

If you were standing outside in the Mid-Atlantic region on April 17, 2018, and looked up in the afternoon, you may have noticed long, linear rows of clouds overhead. The clouds looked pretty remarkable from above as well.

“Holy gravity waves” was how meteorologist Dakota Smith put it, when he tweeted an animation of satellite imagery that showed the wave clouds rippling through the atmosphere. (Gravity wave is a term used to describe waves generated in a fluid medium where the force of gravity or buoyancy tries to restore equilibrium.)

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this image of the wave clouds. Below the clouds, signs of spring washed through the region, with forests in the Piedmont of North Carolina and Virginia showing widespread greening even as the cooler mountain areas remained brown. In the large image, the abundance of farms in the coastal plain gives that region a yellower color.

Wave clouds form when air flows over a raised landform. In this case, the northwesterly winds of the jet stream passed over the Appalachians and made gravity waves on the lee (east) side of the mountains. When the air hit the edge of the mountains and began to pass over, it began to oscillate—much like the suspension of a car bounces after it goes over a speed bump.

There is a particular height in the atmosphere at which the air is saturated and clouds form—the lifting condensation level. Wave clouds form when the crests of the waves rise above that level, even as the troughs of the wave remain below it. The horizontal spacing of the waves offers a clue about the speed of the winds passing over the mountains. Higher wind speeds yield wave clouds with more space between each row.

“You need relatively strong winds to generate the gravity waves,” said Grant Gilmore, a meteorologist with WTSP, a television station in St. Petersburg, Florida. “The jet stream—even a jet streak—was almost directly over where these gravity waves formed on the 17th.”

NASA Earth Observatory image by Joshua Stevens, using MODIS data from LANCE/EOSDIS Rapid Response. Story by Adam Voiland.

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