English: Dramatic designs in chilly clouds covered the Bering Sea in February 2017. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured a true-color image of the phenomena on February 17.

The close, parallel rows of cloud are known as “cloud streets” and are formed by the interaction of cold, dry air over water. As wind drives across frigid expanses of land, such as Siberia and Alaska, it becomes very cold as well as quite dry. The waters of the Bering Sea are relatively warm and, of course, moist. When the air blows from the land over the water, the temperature and moisture differentials lead to the development of parallel cylinders of spinning air. On the upper edge of these cylinders of air, where the air is rising, small clouds form. As the air descends, the skies are clear. This clear/cloudy pattern, formed in parallel rows, gives the impression of streets.

The clouds begin over the sea ice, but they primarily hang over open ocean. The streets are neat and in tight rows closest to land, while further over the Bering Sea the pattern widens and begins to become more random. The rows of clouds are also not perfectly straight, but tend to curve. The strength and direction of the wind helps create these features: where the wind is strongest, nearest to shore, the clouds line up most neatly. The clouds align with the wind direction, so the direction of the streets gives strong clues to prevailing wind direction.