CLOUDS, 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 ex panding, condensation takes place, and clouds are formed, appearing suspended at a certain level above the surface. Should the cooling continue, these glo bules 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 de position 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 ac cordingly.
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 compos ite forms—cumulo-stratus, cirro-stratus, cirrocumulus, and, lastly, nimbus, or rain-cloud. These varieties, and the con ditions accompanying them, may be brief ly described as follows: Cumulus—Con vex 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 de veloped 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 eleva tion. It forms dense clouds when mixed with smoke or dust. Cirrus—A lofty
cloud of loose or fibrous structure, feathery in appearance and of great va riety—said to have been seen at an alti tude of 10 miles. Its particles are sup posed to be frozen and crystallized, which give rise to halos, corona;, and other op tical appearances. Cirrus is said to be often the precursor of windy weather or changes. Their movement is generally different from that of the lower clouds. Cumulo-stratus—A modification of cumu lus; flat-topped, mushroom-shaped mass es; have a tendency to spread and over cast the sky, and indicate coming rainy weather. Cirro-stratus—A fibrous cirrus cloud in close horizontal arrangement. Sky mottled with these clouds is known as mackerel sky. Often precedes wind and rain. Cirro-cumutus—Small, round masses, disposed with more or less regu larity; usually a high level cloud, though below that of cirrus. They appear most frequently in dry and warm weather. Nimbus—A low cloud, from which rain is falling, the masses so blended together as to form no definite outline. Scud—A term referring to low, detached clouds, drifting rapidly before the wind.
Precipitation, or rain, is one of the most irregular of all meteorological phe nomena, there being places of habitual dryness throughout the year with but oc casional sprinklings. Tropical countries generally have a dry and a wet season, and there are other localities where rain may fall irregularly at all seasons. There are places where the annual rain-fall amounts to several hundred inches. Fifty inches per annum may be regarded as a moderate rain-fall, and below 20 inches a very light one, and generally insuffi cient for agricultural purposes unless it should fall in the right season.
The study of clouds and their move ments has, in recent years, been greatly extended through international agree ments and co-operation. The development of aeronautics has greatly advanced our knowledge regarding clouds and, on the other hand, has made more accurate and extensive knowledge regarding them a matter of practical necessity rather than theoretical interest.