of visible particles of condensed water vapor. As in the formation of fog, each particle of cloud matter has condensed upon a dust mote. One cannot say why cloud matter floats in the air, apparently contrary to the laws of gravity. A theory that the cloud particle is repelled from the earth because it is charged with the same kind of electricity has been advanced; but it is not certain that this theory satisfies all conditions. That clouds form and disappear in accordance with the laws of temperature and dew-point is the fact that is important in weather science.
For convenience, cloud matter may be considered to be in a stage of condensation intermediate between vapor and liquid—a condition which may be brought about by several means:
- Local ascending currents, or updraughts, which are vertical or nearly vertical;
- Very slow obliquely ascending currents;
- The rapid chilling of the lower air by the radiation of earth warmth;
- The contact of high air layers which differ in temperature and humidity.
Any one of the foregoing conditions will produce cloud if the temperature falls below the dewpoint; nevertheless it is probable that cloud condensation is more complex in fact than the foregoing paragraphs indicate.
Classification.—Various schemes of cloud classification have appeared from time to time. Some of them have possessed great merit, but have been too complicated for practical use. More than a century ago, Luke Howard, of London, devised the classification upon which the scheme now in use was elaborated by the Cloud Committee of the International Meteorological Congress in 1891. The four fundamental forms are cirrus, cumulus, stratus, and nimbus.[1][2] Other forms are designated by the combination of the foregoing terms. Two
- ↑ From the Latin cirrus (pl. cirri), a curl or wisp; cumulus (pl. cumuli), a heap, or pile; stratus (pl. strati, rarely used), a layer; nimbus (pl. not used), a rain cloud. The adjective derivatives are: cirro-, cumulo-, and strato-. Other definitive adjectives are alto-, high, and fracto-, broken.
- ↑ Some observers still employ the abbreviations of cloud names employed when the Weather Bureau was a part of the Signal Corps: Cirrus, C; Cumulus, K; Stratus, S; Nimbus, N. These symbols are used in Army and Navy practice.
