A rapidly rotating column of air (whirlwind) over a dry and dusty or shady area, carrying dust, leaves, and other light material picked up from the ground. When well developed it is known as a dust devil. Dust whirls typically form as the result of strong convection during sunny, hot, calm summer afternoons. This type is generally several meters in diameter at the base, narrowing for a short distance upward and then expanding again, like two cones apex to apex. Their height varies; normally it is only 30–100 m, but in hot desert country it may be as high as 1 km. Rotation may be either clockwise or counterclockwise. Dust whirls move erratically, from one patch of heated air to another, and generally slowly. In desert country it is not unusual for three or more desert devils to be visible at the same time. Another type of vigorous dust whirl occurs under the bases of cumulonimbus or cumulus clouds, almost always on or near a wind-shift line. These vortices often inflict little or no damage and are short- lived, but occasionally represent the first visible sign of a developing tornado. Another form of dust whirl, often seen at street corners, is merely an eddy caused by the meeting of winds blowing along two intersecting streets. Such whirls are small and very short-lived. Compare duststorm.