Any visible mass of water droplets, ice crystals, or a mixture of the two that is suspended in the air, usually at a considerable height. Clouds are usually created and sustained by upward-moving air currents. Meteorologists classify clouds primarily by their appearance. The 10 main cloud families are divided into three groups on the basis of altitude. High clouds, which are found at mean heights of 45,000–16,500 ft (13–5 km), are, from highest to lowest, cirrus, cirrocumulus, and cirrostratus. Middle clouds, at 23,000–6,500 ft (7–2 km), are altocumulus, altostratus, and nimbostratus. Low clouds, at 6,500–0 ft (2–0 km), are stratocumulus, stratus, cumulus, and cumulonimbus. A shallow layer of cloud at or near ground level is called fog.