The record-union., 1899-04-17

aiS utIlseu Utmost wholly 7lTOlu siOoi
exposures, and this must naturally be
the case whenever an observing station
Us located in the midst Of large city.
Moreover, the exigency of the service
soften require changes from one build
ing to another, thus breaking the con-
tinuity of the environment of the gauge
and adding an element of probable I'M
certainty to the recorded results. Ac-
ceptfng the general principle that the
depth of rain as determined by roof
gauges falls short of the true amount,
1t is next of Importance to ascertain
the actual amount of the deficiency.
COMPARISON OF. ROOF AND
GROUND EXPOSURES.
fortunately, for purposes of com
parfson between roof and ground
gauges, independent rainfall observa-
lions have been made at a few points in
rather close proximity to Weather Blu
reau gauges. AT ST. Louis, Mo-, a, series
OF rainfall measurements have been
maze at Forest Park, almost four miles
due west of the Weather Bureau office
Inc rain gauge at the park is exposed
IN an open space. about seventy-fve
Feet from the nearest object. The top
OF the receiver of the gauge is four feet
absve the ground. The Weather Blu
reau office in the city is domiciled di-
rectly under the dome of the city post
ONCE and custom house. The rain
gauge is placed on the roof. about TWEEN
ty 1eet back from the edge and 100 feet
above the street. 1t is fairly. typical
OF the best roof exposures in other cit
les. A comparison of the record of
1,n
tons for the five years 1991-1S95 show
that the catch of the Weather Bureau
gauge in winter is slightly greater than
mat at the park. In the warmer
months, particularly May and June, the
park gauge catches the greater amount
On the average of the year the excess of
the park above the Weather Bureau is
about inches or 5 per cent.
'Fhe rain gauge at the Philadelphia
office of the Weather Bureau is situat-
ed ON the roof Of the Postoffice build
ing, Io6 feet above ground L. NJ Dey
the official in charge of that station
has made a set of comparative readings
between the Postoffice building gauge
and ground gauge at a point three
miles east. The ground gauge, on the
average of six years observations.
catches three inches g per cent? per
annum more than the roof gauge. Other
comparative readings in the vicinity of
Philadelphia give substantially the same
result. Mr.. Dey, who has given the
matter some study, is of the opinion
that the deficiency in the catch Of the
roof gauge is due to the counter wind
currents caused by the dome shaped
roof of the building.
In New York City a comparison be-
tween the rainfall registers made at the
Weather Bureau office on lower Broad
way, at an elevation of about $50 feet
above the street, and in Central Park. at
an elevation CT s xty-three feet above the
surface of the ground, for a period of
twenty years, shows the catch of the
Weather Bureau gauge to be 2.11
inches per annum @ per cent? greater
on the average than the Central Park
gauge
In general, however it does not seem
possible to avoid the conclusion that
the absolute amount of precipitation as
registered by Weather Bureau gauges
TAILS short of the true amount by GUAM
titles varying from ) to 10 per cent.
OF the annual fall. Heretofore. in deal
ing with the statistical aspect of the
subject, it has been sufficient to accent
the available registers OF the various
geographic districts, whether of ten or
twenty years duration, as representing
the true average precipitation
The inquiry, What constitutes q true
normal?' is Of consider abe eeonc m C i'm
portance, in view of the fact that the
average length of continuous rainfall
registers in some States is less than ten
years. The extensive western move
ment of settlers within the transmissis-
slppI section some years ago was in
fuenced in great measure by several
successive years of abundant rainfall.
There was an undercurrent of opinion
moreover, that the increased rainfall
was in some measure due to the per-
mament occupation of the major por-
tons of Eastern Kansas, Nebraska and
adjacent States by an agricultural pop
ulation. 1t was argued, and with some
snow of reason, that the breaking I'D
of the prairie sod and the cultivation Of
the soil over such a large extent of
territory must increase local evapora-
ton, and consequently the rainfall. 1t
VIII be shown however, that years of
fat and lean rainfall do not alternate
OF consecutive years of heavy rains can
not safely be accepted as indicating
permanent increase in rainfall
A true normal may be defined as
one which will not be materially al
tered, however long the observations
may be continued. In order to reach
conclusion as regards the least num-
Der of consecutive years that will sat-
lsfy such condition, several of the
longest continuous registers of the
United States Were examined The
rainfall of eighty-three consecutive
years has been measured at New Bed
ford, Mass. At ST. Louis, Mo-, the
combined registers of DR.. George Engle-
man and the Weather Bureau cover a.
period of sixty years. Registers have
also been maintained for many years at
Philadelphia, Pa, Cincinnati, O.; Fort
Leavenworth, Kans., and at. other
places.
Possible error of short-period rainfall
registers-In order to obtain the ex-
IRENE variation or possible error of a
ten-year period, the average of the first
ten years of each register 0-1O) was
computed; then, dropping the first year
the average of the second period of ten
years GID was computed Proceed
ing IN like manner, seventy-four sep
arate combinations of ten year periods
We're obtained for the New Bedford rec
ORD. Had observations been made dur
ing the ten years 1SS4-1S93 only, the
annual average, if computed from that
period would have been 504 inches,
whereas the true annual average. as de-
termined from the eighty-three years.