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Cloud Streets Across Caspian Sea

Cloud Streets Across Caspian Sea

The Caspian Sea and the surrounding landscape were swathed in winter white on January 10, 2008, when this image was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. A thin layer of snow covered the ground in Russia and Kazakhstan; the Volga River and its distributaries were white ribbons; and a rim of solid ice clung to the shoreline. The northern part of the sea is more likely to freeze than the southern for several reasons: it is shallower, it experiences a more continental climate (colder winter), and it is less salty.

South of the loosely packed ice in the northern part of the sea, parallel rows of clouds line up along the north-south axis of the wind. These “streets” of clouds form when cold air blows over the warmer, moister air that sits over the water. When the air over the water is stable, the arriving wind can generate evenly spaced cylinders of spinning air—horizontal roll vortices (view a drawing of the circulation of air in horizontal roll vortices). These vortices line up with the direction of the prevailing winds like spinning straws. Clouds form above the “upstroke” part of the spin, where air is rising. Above the “downstroke” side of the spin, where air is sinking, the skies are clear.

You can download a 250-meter-resolution KMZ file of the cloud streets over the Caspian Sea suitable for use with Google Earth.

NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team, Goddard Space Flight Center. Caption by Rebecca Lindsey.