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Recording the daily duration of sunshine is a part of the work of all Weather Bureau stations; it is carried on also at agricultural experiment stations, at university laboratories, and at many aviation fields. The objective information may be different in the various cases; the methods of measurement are usually the same.

Sunshine measurements are not required of cooperative observers; their reports, however, include an estimate of cloudiness; less than one-third of cloudiness, clear; one-third to two-thirds, partly cloudy; and two-thirds or more, cloudy. A day with a sky full of broken clouds is necessarily recorded as cloudy; nevertheless the registered sunshine may be almost continuous. At times the cloud film of a sky completely overcast may be so thin that a sunshine recorder of any sort will register a considerable part of the day. On the other hand, a light dust haze may interfere materially with registration, although to the sense of sight, the light seems normal. The amount of sunshine, therefore, cannot be reckoned from a record of cloudiness. Even with recording instruments having the best possible adjustments, the record of sunshine for a given period is an approximate only; there is no such refinement in sunshine measurements as exists in measurements of pressure or of temperature.

Sunshine Recording Instruments.—The various sunshine recording instruments may be reduced to three types: the burning-glass type, such as the Campbell-Stokes recorder; the photographic type, such as the Jordan recorder, for many years used in the United States Weather Bureau; and the thermoelectric type; of which the Marvin recorder is practically the only one.