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Rh camp in the Sierra Nevada Mountains overlooking the Sacramento Valley, clouds formed by dust, smoke and fog are sometimes the only ones visible for a considerable period.

Along the Atlantic Coast and the Mississippi Valley a cloudless sky for more than one or two days is unusual; for a period of three days it is very rare. During the moist periods of midsummer a very thin cloud veil may prevail for a week or more at a time. The cloud veil, technically a “haze,” is too thin to obscure the sun visibly, but it is dense enough to prevent radiation to a considerable extent. During the period when the cloud veil prevails, the night temperature is from 5 to 10 degrees higher than at other times, and the amount of insolation is almost always lowered.

Arizona, southern California and southern Nevada constitute the region of minimum cloudiness in the United States. In this region cloudless skies may persist for a month or more. The observer who watches the hygrometer closely will acquire not a little useful information on clouds and their relation to atmospheric moisture. No part of meteorology is more fascinating than the study of clouds, and none is more important in forecasting weather changes.