Rain-gauge. Rain-gauges measure the quantity of rain which falls, and are of various constructions. One of the best is that known as Symons' rain-gauge, consisting of a funnel-shaped receiving vessel, and a glass measure of much smaller diameter so as to allow as nice graduation as may be desired. In the fig. a is the funnel-shaped receiving vessel (shown in section in the fig.), 5 inches in diameter, with an upright, sharp-edged and strong rim, in order that it may retain its circularity; b, the vessel which receives the rain collected; and c, the graduated glass measure which measures the amount in tenths and hundredths of an inch. If desired it may be graduated to still greater nicety. Another excellent gauge is one which is a modification of Glaisher's gauge, brought into use by the Meteorological Office. The diameter of this gauge is 8 inches, and there is added to it a vertical cylinder on the top of the receiving funnel, to retain the snow as it falls and prevent its being blown out of the gauge, as is likely to happen when the funnel is shallow. Beckley's is perhaps the best continuously recording rain-gauge, and is now very generally superseding Osler's so long in use.
Since different amounts are collected at different heights above the ground, it is indispensable, if uniformity and comparability is to be attained, that the rims of rain-gauge be placed at one uniform height above the ground. The height now generally adopted is 12 inches. The points it is necessary to secure at the height adopted are (1) the prevention of in-splashing as the rain-drops strike the ground and are broken into minute droplets, a large number of which in their second descent fall into the gauge; (2) the prevention of out-splashing, which occurs when the funnel is very shallow; and (3) the protection of the gauge as much as possible from strong winds, which so seriously interfere with the amount collected, by placing it as near the ground as can be done consistent with the prevention of in-splashing. These conditions are best fulfilled by using the gauges named, and placing their rims 12 inches above the ground. It has been proved from carefully conducted experiments that rain-gauges with diameters varying from 3 inches upwards collect amounts, when in positions exactly similar, within about 3 per cent. of each other. Accordingly, gauges with diameters exceeding 8 inches insure no greater accuracy than smaller ones, being only really required in the case of continuously recording gauges to hold the self-registering apparatus. On the other hand, gauges of from 3 to 5 inches diameter, such as Fleming's and Jagga's, give good trustworthy results.
Special care should be taken that the gauge and its glass measure be kept clean; that it be firmly fixed and the rim kept in a horizontal position; and that it be made of a material which will stand exposure to the weather well. It should be placed in a situation as open as can be secured for it, particularly towards the directions from which the rain chiefly comes; and in each case it should be as distant from any neighbouring house, tree, or other object at least as far as the height of the object itself.