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 can-not 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 dia-meters 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.
"c5
The best form of instrument, Glaisher's rain and snow gauge
recommended by the Royal Meteoro-
logical 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 prin-
ciple is intended to be read from a -p^ _[ 2
distance, and is only graduated into , .
tenths of an inch. The water col- Fre- 1- Glaisher s ram and
lected by a raingauge may be weighed snow gauge,
instead of measured, but the latter FlG- 2.—Graduated measuring
process being much more simple is glass,
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 ac-
company 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 observa-
tories, 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 instru-
ment 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 :—
Height of funnel above ground in feet.. 0 1 10 20 40 200
Rain registered (average) 1'07 TOO 0'03 0'90 0 7 0 0'58
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 temper-
ature 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 micro-
scopically for volcanic and cosmic dust. (H. R. M.)