Clouds

Collier's New Encyclopedia (1921)

formations owing their origin to aqueous vapor diffused in the atmosphere. The vapor is supplied from the evaporation of the sea and other water surfaces, under the influence of solar heat, and is diffused through the agency of winds. Air-under a given pressure and temperature-can absorb, or hold, only a certain amount of invisible vapor; when charged with this maximum amount it is said to be saturated. Should the temperature, under this condition, be lowered, as, for instance, when a current is ascending into colder regions, or expanding, condensation takes place, and clouds are formed, appearing suspended at a certain level above the surface. Should the cooling continue, these globules unite and are finely precipitated in the form of rain, and, with sufficiently reduced temperature, as snow, or perhaps hail or sleet. Mists and fogs are simply incipient states of clouds, and when in contact with cold bodies produce, by deposition of moisture, dew, and hoar-frost. The whole subject is comprised under the name hygrometry. The forms of clouds have been observed to depend greatly on altitude, and have been classified accordingly. The nomenclature proposed by Howard, at the beginning of the 19th century, is still generally adhered to, owing to its simplicity and appropriateness, though a few minor subdivisions have been added. He divides clouds into three primary modifications, named cumulus, stratus, and cirrus, with the intermediate composite forms-cumulo-stratus, cirro-stratus, cirro-cumulus, and, lastly, nimbus, or rain-cloud. These varieties, and the conditions accompanying them, may be briefly described as follows: Cumulus-Convex or conical masses, generally resting on a horizontal base of apparently dense structure, and of globular shape or rolls (so-called cotton bale); they form in the lower atmosphere under the influence of ascending heated air, and are most developed during the hottest part of the day; a fair-weather cloud. Stratus- Consists of continuous horizontal sheets, a fine-weather sign, appearing mostly during the night and at no great elevation. It forms dense clouds when mixed with smoke or dust. Cirrus-A lofty