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Rh 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:







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. Other forms are designated by the combination of the foregoing terms. Two