Making Waves in the Sky off of Africa

Making Waves in the Sky off of Africa

On June 26, 2016, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image of cloud gravity waves off the coast of Angola and Namibia.

“I [regularly] look at this area on Worldview because you quite often have these gravity waves,” said Bastiaan Van Diedenhoven, a researcher for Columbia University and NASA's Goddard Institute for Space Studies interested in cloud formations. “On this day, there was so much going on—so many different waves from different directions—that they really started interfering.” A distinctive criss-cross pattern formed in unbroken stretches hundreds of kilometers long.

Similar to a boat’s wake, which forms as the water is pushed upward by the boat and pulled downward again by gravity, these clouds are formed by the rise and fall of colliding air columns.

Off of west Africa, dry air coming off the Namib desert—after being cooled by the night—moves out under the balmy, moist air over the ocean and bumps it upwards. As the humid air rises to a higher altitude, the moisture condenses into droplets, forming clouds. Gravity rolls these newly formed clouds into a wave-like shape. When moist air goes up, it cools, and then gravity pushes it down again. As it plummets toward the earth, the moist air is pushed up again by the dry air. Repeated again and again, this process creates gravity waves. Clouds occur at the upward wave motions, while they evaporate at the downward motions.

Such waves will often propagate in the morning and early afternoon, said Van Diedenhoven. During the course of the day, the clouds move out to sea and stretch out, as the dry air flowing off the land pushes the moist ocean air westward.

In this image, the clouds form several hundred kilometers off the coast, but at times when the air is less dry or westerly winds are less strong, clouds can be seen hugging the coast.

NASA Earth Observatory image by Jesse Allen, using data from the Land Atmosphere Near real-time Capability for EOS (LANCE). Caption by Pola Lem.

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