Know Your Clouds. Tim Harris

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      While a sky full of fair-weather cumulus has been likened to a flotilla of dinghies, a group of cumulus mediocris is more akin to an armada of galleons. These clouds are at least as tall as they are wide, sometimes with small turrets at the top. Each cloud forms within a column of warm, ascending air, and these currents are quite powerful; inside the cloud, air may rise at more than 17 ft per second (5 m/s). If present in the morning, these clouds indicate atmospheric instability and the likelihood of stormy weather later.

      Rows (“cloud streets”) of cumulus mediocris can sometimes be seen, lined up in the direction of the prevailing wind. These puffy clouds are often part of a chaotic skyscape with their cumulus cousins—smaller fair-weather cumulus and larger, towering cumulus—and often give warning of the approach of a cold front. In warm or mild conditions, cumulus mediocris produces only very light falls of rain, and this often evaporates before it reaches the ground. Viewed from a distance, these strips of evaporating rain are called virga. In winter, however––if their tops are cold enough—even these relatively small clouds can produce significant falls of snow over the Great Lakes of North America.