Rain Gauge, an instrument for measuring the amount of rain which falls upon a given area during a certain space of time. For approximate purposes a tub or bucket, with a thin-edged mouth, placed in a horizontal position for catching the rain, whose depth may afterward be measured by means of a graduated rod, may be employed, and if well constructed and used with care may fulfil most of the requirements of exactness. It is more common, however, to catch the fall in an accurately made funnel whence it flows into a holder, whose form is immaterial; it is subsequently measured either by weighing or by means of a tall graduated cylinder, which gives the average depth of the rainfall. The holder should have a capacity abundantly sufficient to receive all the rain that may be caught. It is difficult to employ a rain gauge to measure a snowfall unless the air is perfectly still, as the wind interferes with the reception of the snow; it is therefore generally preferable after a snow storm to take a cylindrical vessel of sufficient depth and with it cut out a section of the snow from some region which has an average depth of covering; the snow thus collected should be melted or dissolved in a known quantity of water and measured.
For special studies different forms of gauges are employed, in which the opening of the mouth may be horizontal, inclined, or vertical; rain gauges are also attached to wind vanes so as always to be turned toward the wind. Very great discretion is required in the selection of the site of the instrument; the standard position of the mouth of the gauge is 8 to 16 in. above the surface of a broad level lawn; gauges in the neighborhood of trees and buildings or on the tops of isolated buildings are not allowable except for the purpose of investigation. - For the numerous details in reference to this important subject, see the annual volumes of Symons on "British Rainfall".