As the name implies, atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down.
On its descent into the low point of the wave (the trough), the air can sometimes touch the surface of an ocean, forming an impression on the water.
If a satellite happens to capture an image of such an event as the sun’s reflection is also hitting the water, the waves become clearly visible.
Such an event happened off the coast of northwestern Australia at midday on October 27, 2010. The image to the right was captured by NASA’s MODIS sensor orbiting aboard the agency's Terra satellite.
The darker areas of the ripples are the crests of the waves, while the lighter areas between them are the troughs.
As often happens, clouds formed along the crests of the waves where the air had been lifted by buoyancy and cooled during the ascent to the point of condensation. This can clearly be seen in some of the northernmost wave crests over the eastern Indian Ocean.
A variety of influences can cause such gravity waves in the air, and their subsequent impressions on bodies of water beneath. 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.
Besides their interesting appearances, gravity waves are vital in transferring energy, momentum and various gases between different layers of the atmosphere. This influences the winds, turbulence, temperature and the chemical makeup of the atmosphere.