Page:Handbook of Meteorology.djvu/242

 depression in which a standard Weather Bureau gauge is placed so that its mouth is 10 or 12 inches higher than ground surface. It is surrounded by a rim of earth in the form of a ring about 6 feet in diameter. A pit and ring of concrete with a movable cover of wire mesh, coarse enough to permit rain to enter without obstruction and fine enough to keep leaves out makes an ideal position for a rain gauge in an open and fairly level country.

The Ferguson gauge designed for isolated stations by S. P. Ferguson, of the United States Weather Bureau, totalizes a year’s rainfall month by month, or in such measured proportions as may be desired. A film of oil in each of the thirteen receivers prevents loss by evaporation.

The Measurement of Snow.—A reasonably accurate measurement of snowfall is desirable in regions of plentiful rainfall; it is imperative in regions where the irrigation of crops or a knowledge of possible floods, or of droughts is a prerequisite.

In moderately level regions of gentle drainage the measurement of the precipitation derived from snow requires the care and judgment that comes only with experience. In mountainous regions it requires judgment, patience and a lot of hard work.

On the prairies of Indiana, for instance, the measurements made at the Weather Bureau stations give a pretty accurate total of precipitation. If the aggregate error amounted to 12 inches of snow, or even 2 inches of rain, however, the result would not be materially harmful. In California, however, the floods of the Sacramento and San Joaquin River valleys are largely predetermined by the snowfall in the mountain slopes to the eastward; and in many arid regions the crop production which depends on irrigation must be foretold mainly by the total of snowfall.

In level regions where the snow is not disturbed by the wind the measurement is not difficult. The observer uses his measuring stick in a dozen or more places within a radius of 300 feet. Usually the mean depth will become apparent without computation.

To find the equivalent in terms of rainfall requires “puttering and patience.” A very convenient way is to cut a section in the snow with the inverted barrel of the standard rain gauge, thrusting a dust pan or a piece of sheet iron under the mouth