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Gravity Wave Mystery: Earth Image of the Week November 18, 2011
Satellite image of Great Lakes.
The waves in the clouds were moving in almost the opposite direction of the wind flow pushing the clouds to the south.
Some meteorologists were stumped on Monday, November 14, by an unusual pattern seen rippling through the low-level clouds blanketing the Pacific Ocean west of Baja California.

National Weather Service meteorologist Ken Walters, of the Phoenix forecast office, noted that the ripples were going “upstream,” away from the apparent wind flow.

A small center of low-pressure to the southeast of the ripples was causing the clouds to move southward while the ripples were propagating to the northeast.

Unsure of the cause, Walters wrote to the experts at the University of Wisconsin-Madison’s Cooperative Institute for Meteorological Satellite Studies (CIMSS).

They confirmed Walters’ belief that the ripples were internal gravity waves being conducted through the cloud layer.

A temperature inversion responsible for the maritime low cloudiness over the area also created an ideal medium for the waves to travel through, according to CIMSS.

Experts there pointed to several inversions at various levels of the atmosphere, which were detected in the nearest available balloon sounding above San Diego.

Just as the impact of a stone can create ripples across a pond or lake, influences on the air above can create similar waves through the atmosphere. Both phenomena are known as gravity, or buoyancy, waves. An internal gravity wave is one that passed through the air or water rather than on the surface or between two fluids of different dinsities.

As the name implies, atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down. The dark stripes in the cloud bands are where the descending phase of the waves caused the clouds to briefly disperse.

A variety of influences can cause such gravity waves in the air. Wind flowing over mountain ranges, violent thunderstorms and sudden warming of the air by solar radiation are among the forces that can cause them. Depending on their wavelength, their periods can range from a few minutes to days.

The waves off Baja California on Monday lasted for several hours, and were accompanied by shorter wavelength counterparts faintly visible in the far bottom of the image to the upper right. They are especially noticeable in the wider, enlarged view. There was no immediate comment on what might have caused either or both sets.

Besides their interesting appearances, gravity waves are vital in transferring energy, momentum and various gases between different layers of the atmosphere. This influences the wind, turbulence, temperature and chemical makeup of the atmosphere.

Images Source: CIMSS University of Wisconsin-Madison