Wave Clouds off West Africa
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Wave Clouds off West Africa

October 15, 2007

The cloud patterns seen in this image, acquired by the MODIS on the Terra satellite on October 9, 2007, resemble ship waves or "Kelvin ship waves", which are the V-shaped wakes left by moving objects, such as ships or even ducks. The pattern is not coincidental; wind behaves like a fluid, so when it encounters an obstacle, it must move around it, leaving behind a wake or a visible wave pattern. As the air crested a wave, it cooled and clouds formed. Then, as the air sank into the trough, the air warmed, and clouds did not form. This pattern repeated itself, with clouds appearing at the peak of every wave. In this case, the obstacle is an island. As the wind flows past the island, it is swept around and over it leaving a wake similar to that of a ship-- hence the name "ship-wave-shaped" clouds.

In this case, the wave clouds are being caused by the Cape Verde Islands, off the coast of West Africa in the Atlantic Ocean. Sometimes, an island's highest point will cause the wind-blown dust that encounters it to flow around it in swirls and eddies, called von Karmon vortices. In this image, you can see one not well formed vortice coming off of Sao Nicolau Island.

Near the top of the image you can see some dust blowing from Western Sahara and Mauritania. You can see a dust storm from this region a few days prior.

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