The record-union., 1899-05-06
MEASURING THE RAINFALL.
FACTS REGARDING LOCATION
OF GAUGES.
Why Some Vary From Others, and
How to Avoid the Causes
Thereof.

The following instructions in refer
ence to rain gauge exposure has just
been received by Observer Barwick OF
the local Weather Bureau Office from
the Chief Of the Weather Bureau,
Washington, D. c., and as it will be
of interest to the many persons
throughout the State who take rain
fall measurement, the "Record-Union'
publishes them in full:
THE exposure of gauges is a very
important matter. The wind is the
most serious disturbing causes in col
Letting precipitation. In blowing against
the gauge, the eddies of wind formed at
it's top and about the mouth carry
away rain, and especially snow, SO that
too little is caught. Snow is often
blown out of a deep gauge after once
lodging therein.
Rain gauges in slightly different po-
sitions, if badly exposed, catch very
different amounts of rainfall. Within a
few yards of each other two gauges
may show a difference of 20 per cent.
in the rainfall in heavy rainstorm.
The stronger the wind, the greater the
difference is apt to be. In a high To-
cation eddies of wind produced by
walls of buildings divert rain that
would otherwise fall in the gauge. A
gauge near the edge of the roof, on
the windward side of a building, shows
less rainfall than one in the center of
the roof
The vertical ascending current along
the side Of the wall extends slightly
above the level of the roof, and part
Of the rain is carried away from the
gauge. In the center of a large, fiat
roof, at least sixty feet square, the
rainfall collected by a gauge does not
differ materially from what is collected
at the level of the ground. A gauge
on a plane with a fence three feet
high around it at a distance of three
feet will collect 6 per cent. more rain
than without the fence. These differ
ences are due entirely to wind cur
rents.
Observers will, therefore, take par
ticular care in selecting a place for the
location of the gauge, as the value of
the records is sometimes greatly i'm
paired by improper exposure. 1t is
scarcely necessary to say that every
precaution should be taken to protect
gauges from the interference Of ami
mails and unauthorized persons. Sc
lect, if possible, q position in some
open lot, unobstructed by large trees.
buildings or fences. Low bushes and
fences, or walls that break the force
of the wind in the vicinity of the
gauge are however, beneficial, if at a
distance not less than the hight of the
object. Such a place, in general, af-
fords the best exposure. Gauges should
be exposed upon roofs of buildings
only when better exposures are not
available, and when so located the mid
die portion of fiat, unobstructed roof
generally gives the best results.
Snowfall is preferably measured as
depth Of water rather than by the
thickness of layer it forms on the
ground. When it cannot be measured
by melting, it is customary to take
one-tenth the measured depth of the
snowfall on a level, open place as the
water equivalent of the snowfall The
relation between the depth Of snow and
depth Of melted snow is very different
in different cases, depending on the
wetneSS of the snow. The equivalent
depth of water in some cases is as
great as one-seventh of the depth of
snow, and in others may be as small
as one thirtyfourth. 1t is always best
to reduce snow to a liquid condition
for measurement, and the simplest way
of doing this is to add to it known
volume of water sufficient to reduce
it to a state of SluSh'