Page:Handbook of Meteorology.djvu/95

Rh round which delicate “false cirrus” floats. At other times the edges themselves separate into a fringe of filaments similar to cirrus clouds. This last form is particularly common in spring showers.

The front of thunder-clouds of wide extent frequently presents the form of a large arc spread over a portion of a uniformly brighter sky. The difference between the ordinary cumulus cloud and the cumulo-nimbus is mainly one of depth and intensity of motion within its mass. If condensation is so intense that its water content reaches the ground, the cloud is cumulo-nimbus. But Humphreys points out the fact that, in arid regions, where the ground is very warm, a well-developed thunder-head sinks until the excessive warmth vaporizes and scatters it. The aborted cumulo-nimbus has been observed by the author. On the other hand, a torrential shower may fall for a few minutes from a tropical cumulus cloud—shallow as to extent and without the angry-appearing cauliflower head of the ordinary cumulo-nimbus.

The fibrous mantle that hovers over the top of the cumulo-nimbus is a cloud of snow flakes. The cloud itself is usually, but not always, a thunder-storm. The observer may disregard all theoretical matters and record it as a cumulo-nimbus if rain is falling from its base. The marvelous photograph of a thunder-storm obtained by Lieutenant W. F. Reed, Jr., U.S.N. (p. 106), surpasses any verbal description of a thunder-storm.

The strong updraught caused by forest fires and burning strawstacks has resulted in the formation of cumulus clouds that still later developed into typical cumulo-nimbus shower clouds. The eruption of Vesuvius in 1872 created a series of cumulo-nimbus clouds with mammoth cauliflower heads. Torrential rains fell on the leeward side of the cinder-cone during a considerable time.

Experience has taught the airman that the cumulo-nimbus cloud is an object to be avoided; its beautiful exterior hides a generous accumulation of holes and bumps. The turbulent cumulus cloud has been called “the most treacherous wild beast of the air.”

10. (St). — A uniform layer of cloud resembling a fog, but not resting on the ground. When this sheet is broken into irregular shreds by the wind, or by the summits of mountains, it may be distinguished by the name fracto-stratus (Fr-St).