Page:Handbook of Meteorology.djvu/99

Rh

Unusual Cloud Forms.—Various cloud forms that are not readily classified are noted by every observer:

Billow clouds, or windrow clouds, are the same as the undulatory clouds noted in a previous paragraph. The name is derived from their wave-like form. Sometimes they are at cirrus height, and should be classed as cirrus clouds. For the greater part they form at lower altitudes. They are due to cross-winds in plane contact, the two differing in temperature and humidity.

Crest clouds frequently gather about the summits of snow-clad peaks. They are frequently observable shrouding the summits of Mounts Hood and Rainier. On even a grander scale they envelop the summits of Popocatepetl and Ixtaccihuatl, during periods of still air. When a moist wind blows against snow-clad peaks, a stream of condensed moisture flows from the leeward side, forming a banner cloud. The so-called “smoking” of Mounts Hood and Rainier is a cloud banner of this sort.

Mammillated, or mammato-cumulus clouds, are globular projections from the under side of thunder-heads. They usually accompany thunder-storms, hailstorms and tornadoes. A similar waviness, very strong in character, is sometimes observable in the bands of cirro-stratus clouds near to the horizon.

Scarf clouds are the feathery wisps that sometimes form at the summits of cumulus clouds, especially those of the storm type. They seem to increase in size as the turbulence within the cumulus cloud increases, and sometimes appear like a coverlet over its top.