Page:Handbook of Meteorology.djvu/234



All moisture condensed from the air—rain, snow or hail—is classed as “precipitation” and is measured in terms of rain. It represents the depth of water which would accumulate on a level surface without loss by run-off, percolation or evaporation. For convenience in measurement the water of precipitation—rain, or melted snow—is caught in vessels of special construction called rain gauges. These are of various forms, but they have practically the same principle—the exhibition of a depth of rain expressed in inches and hundredths.

The standard Weather Bureau rain gauge is a cylindrical barrel of galvanized iron 26 inches in height over all. The receiver is a funnel with an upright collar of bronze 2 inches high. The edge of the collar is beveled so as to present a sharp cut-water to the rain. The receiver delivers the water to the measuring tube; a sleeve at the lower end of the funnel holds the mouth of the measuring tube in place and prevents the loss of water. The brass measuring tube, accurately calibrated, rests in a seat at the bottom of the barrel.

The receiver is exactly 8 inches in diameter; the section of the measuring tube is exactly one-tenth the area of the receiver. Its diameter is 2.53 inches and it is 20 inches high. An inch of rain therefore measures 10 inches in the tube, and the latter holds 2 inches of rain. Any excess beyond 2 inches overflows into the larger vessel; it is poured into the emptied measuring tube and added to the amount. The measuring stick is graduated to measure inches, tenths, and hundredths, and the depth of the rainfall is indicated by the length wetted when the stick is inserted in the measuring tube. Care must be used to keep the surface of the measuring stick free from grease. If it fails