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Velo cloud is a name now commonly given to fog drifting in from the sea and hovering over a coast a few hundred feet from the ground. It is of frequent occurrence along the coast of southern California during summer months, and is occasional along the Atlantic coast. The velo is an example of advection condensation. Perhaps, strictly speaking, it should be classed as cloud rather than as fog; nevertheless it is advective condensation. Inasmuch as the term “high fog” is sometimes popularly used to denote a very thick fog meteorologists have generally adopted the term “velo.” The velo is rarely more than 1000 feet high.

Cooperative observers are not required to report information concerning cloudiness, except the extent of cloud-covered sky during the daylight period. At the regular Weather Bureau stations the character, movement and height of clouds are recorded and at some stations nephoscopes are provided. With the aid of these instruments, the velocity of the clouds, and therefore that of the upper winds, may be determined.

The photogrammeter is one of the most practical instruments for measuring cloud heights. It consists of a pair of cameras mounted in the same manner as a surveyor’s transit. Two instruments set at different positions are employed. The sensitive plates are ruled with intersecting horizontal and vertical lines. By the aid of these, the photographs of the cloud indicate its comparative position, and from this both its altitude and its velocity may be determined. Air navigation now demands definite knowledge of wind at different elevations, and this knowledge is best obtained by a study of the clouds.

Formation of Clouds.—A cloud consists of an aggregation