1902 Encyclopedia > Raingauge

Raingauge




RAINGAUGE (PLUVIOMETER, ____METER, UDO-METER). The value of the measurement of rainfall (see METEOROLOGY) has long been understood, although it is only within the last hundred years that trustworthy results have been obtained. Marriotte is claimed as the originator of the raingauge in 1677. The simplest form is an open vessel of uniform diameter exposed to, the rain, in which the depth of water collected during any interval of time may be measured. In order to reduce evaporation the mouth of the gauge is usually a funnel of the same diameter as the vessel; and some means, such as an external narrow glass tube graduated in inches and parts to show the height of the water inside, or a float bearing a graduated rod, or, in more delicate forms, a movable scale which may be set by a vernier to the surface of the water, is adopted to facilitate measurement. Raingauges on this plan cannot be very accurate; their one advantage is that the area of the collecting surface does not require to be known. The disadvantages are that so much water is required to wet the sides of the vessel as to make the instrument read too low when a side tube is used, and the tube is liable to be broken in frost; when a float and rod are employed, the rod projecting above the funnel catches rain and the resulting reading is too high. Almost all raingauges now used have a circular funnel of known diameter, which con-ducts the rain-water to a receiver, from which it may be poured into a special narrow measuring-glass so graduated that what would cover a space of the area of the funnel to the depth of 1 inch fills a portion of the glass large enough to be easily graduated into 100 parts. The funnel may have any diameter from 3 to 24 inches without introduc-ing a greater discrepancy than 1 or 2 per cent, of the amount of rain collected, but 5 and 8 inches are the diameters usually employed, and the measuring-glasses are graduated accordingly. Advantage is sometimes taken of the fact that for a funnel 4-697 inches in diameter 1 fluid ounce of water collected represents one-tenth of an inch of rain, since the area is 17'33 square inches, and a fluid ounce at 60° Fahr. contains 1 _ 733 cubic inches of water.





The best form of instrument, Glaisher's rain and snow gauge recommended by the Royal Meteorological Society, is a cylindrical copper vessel 8 inches in diameter and 18 inches high, in which the funnel is placed about halfway down (see fig. 1). The Scottish Meteorological Society largely employ Howard's raingauge, a plain glass bottle holding about half a gallon and provided with a long 5-inch copper funnel, which has a collar fitting over the neck of the bottle to prevent rain from being blown in laterally by the wind. In some forms the funnel leads to a long glass tube divided into inches, tenths, and hundredths of rainfall. Mr Symon's storm raingauge on this principle is intended to be read from a -p^ _[ 2 distance, and is only graduated into , . tenths of an inch. The water collected by a raingauge may be weighed instead of measured, but the latter measuring process being much more simple is always adopted, at a slight expense of accuracy, however, as the variation of volume with temperature is not taken into account. Precipitation and evaporation being complementary phenomena, an atmometer or evaporation-gauge ought, strictly speaking, to accompany each raingauge. But none of the instruments yet devised can be regarded as satisfactory, accordingly a number of devices have been introduced to calculate or to minimize the evaporation from raingauges. Dr Garnett in 1795 proposed to use two gauges of unequal size, and recently Prof. Michie Smith has introduced a simplification by making the area of one gauge exactly double that of the other. If the evaporation is the same from each, the difference between the readings of the two gauges gives the true rainfall in the smaller. If A be the area of the funnel in the smaller, 1A that of the funnel in the larger, V a certain volume of water placed in each gauge, E the evaporation, and R the inches of rainfall, then V+2AR -E-( V+ AR-E) = AR, however V and E may vary. The simplest and best method is to use a funnel terminating in a long straight tube, which reaches almost to the bottom of the receiving vessel. Gauges have been constructed for experimental purposes always to face the wind, and with openings capable of being lixed at any angle. For use at sea they may be swung on gimbals ; but when so employed the record must be sup- plemented by the readings of a hydrometer so as to detect and allow for any admixture with sea-spray. Self-registering and self-recording raingauges, as frequently used in meteorological observatories, are constructed on two leading types. In Hermann's "hyetometrograph," 1789, a fixed funnel conducts the rain into one of twelve glasses placed on the circumference of a horizontal wheel, which is turned by clockwork so that each glass remains under the funnel for one hour. In Stutter's more recent instrument the receiving funnel delivers into a smaller funnel, which has a sloping tube and is carried round by clockwork so as to remain for one hour over each of twenty-four fixed glasses arranged in a circle. The second kind of self-registering instrument produces a continuous record of rainfall, indicating the hour of commencement and close of each shower, the amount of rain that has fallen, and the rate at which it fell. In Beckley's "pluviograph " a pencil, attached to a vessel which sinks as it receives the rain, describes a curve on a sheet of paper fixed round a rotating cylinder; when full the receiver empties itself by means of a siphon and the pencil is carried rapidly upwards, describing a straight vertical lira The higher a raingauge is placed above the ground, or rather above a broad flat surface, the smaller is the rainfall registered, as the following figures indicate :—

== TABLE ==

When the mouth of the gauge is on or within a few inches of the ground the insplashing of raindrops increases the amount of water, llinute rain-spherules, which usually float in horizontal or oblique planes, are 'most numerous near the ground, where consequently they coalesce to form regular drops which fall into the funnel. The raindrops also increase slightly in size by condensing moisture as they fall. But the greatest effect is probably produced by wind, wdiich forms eddies round high and isolated objects, thus more or less interfering with the fall of rain into the gauge. It is obvious that all raingauges intended for comparison should be fixed at tho same height, and in Great Britain the standard distance of the mouth of the funnel from the ground or from a broad flat surface is one foot. The situation of a raingauge should be perfectly open, especially in the direction of the prevailing rainbringing winds. In measuring rain it is essential to see that the funnel is not indented or deformed in any way, and that the collecting vessel is inaccessible to air or rain except through the funnel. The temperature of rain as it falls should be observed whenever it is possible to do so. The amount of solid matter collected in the raingauge should be ascertained and recorded as bearing on Mr Aitken's theory of rain (see EVAPORATION), and it should be examined microscopically for volcanic and cosmic dust. (H. R. M.)






The above article was written by: H. R. Mill, B.Sc.



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