Page:Handbook of Meteorology.djvu/101

Rh Various other terms such as lenticulate, maculate, flocculent, and castellate, are used by observers. Any descriptive term which conveys a definite meaning is permissible in recording cloud observation.

Cloud Heights.—Bigelow’s measurements of cloud heights are somewhat greater than those determined by the International Cloud Committee, due to the fact that the measurements were made in a lower latitude. The table (p. 286), taken from Hahn’s Lehrbuch der Meteorologie, shows that the altitudes of the various cloud levels increase from polar to equatorial regions.

The level of each type of cloud is a level of maximum cloudiness; between cloud levels are levels of minimum cloudiness. The airman may find that neither Dr. Bigelow’s figures nor those of the International Committee apply to the locality in which his flights are made; but the altitudes of maximum and of minimum cloudiness for any locality are not far from the figures noted and are roughly proportioned. The airman will find also that the various cloud regions are thicker as one approaches equatorial latitudes. The lowest level of minimum cloudiness is that between “scud” cloud height and the base height of stratus clouds—from 300 feet to 1200 feet.

The thunder-head excepted, the lower clouds are shallow; but they vary greatly in depth. A mean of 10,000 feet (3000 meters) may be approximately correct for their depth, but it is unsafe as an estimated depth at any one time. The fact that the highest mountain peaks of the United States are snow-capped shows that precipitation occurs at an altitude of about 15,000 feet; and the fact that observers in mountain regions are frequently above storm clouds is evidence that the cloud blanket may be materially less than 10,000 feet in thickness.

The Distribution of Cloudiness.—In the Pacific Coast region of the United States, cloudiness is more or less seasonal. Practically all the lower clouds are prevalent during the winter months—that is, during the rainy season. During the summer the lower clouds may be absent for weeks at a time. From a