Clouds over the Sahara Desert

(Click on image for larger view.)

Although rain rarely falls in the extremely arid Sahara Desert, the skies are not always sunny. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured this spectacular image of clouds across southern Algeria at 10:35 UTC (11:35 a.m. local time) on February 12, 2012.

The annual precipitation in the Sahara Desert is less than 25 millimeters (0.9 inches), and in the eastern part of the desert total precipitation may be less than 5 millimeters (0.2 inches) each year. Precipitation, although rare, can fall in any season, and February tends to be the rainiest season in the Sahara.

More common than rainfall, fog sometimes forms over the desert, especially when warm, relatively moist air crosses a cooler, arid landmass. Although temperatures in the Sahara in the daytime can be quite hot, even in winter, temperatures often plunge in the evening. Under these conditions, fog or low stratus clouds can form. From space, such low clouds can look like a thin, white sheet covering a swath of land.

On the date this image was captured, Tropical Cyclone Giovanni swirled off the coast of Mozambique, to the southeast, and the strong storm was pushing warm, moist air northwestward. Meanwhile, to the northwest of the Sahara, a Siberian anticyclone was bringing bitter cold air to Europe and causing snow to fall in northwestern Algeria. The town of Tebessa, just to the north of this image, reported light snowfall beginning at 3:00 a.m. on February 12.

Credit: NASA/GSFC/Jeff Schmaltz/MODIS Land Rapid Response Team