ETL 1110-2-540
30 Sep 96
Appendix C
Open Sites. In open areas, individual or small groups of
Raingage Location
isolated objects near a gage may set up serious eddy currents.
As a general rule, the height of such objects above the gage
should not exceed half their distance from the gage. Since it is
Raingage Exposure1
not always possible to select sites which provide adequate
protection from adverse wind effects, an open site away from
The exposure of a raingage is of primary importance in the
isolated objects may be the only location available.
accuracy of precipitation measurements. An ideal exposure
would eliminate all turbulence and eddy currents, near the gage,
Consistent Exposure. Good exposures are not always
that tend to carry away the precipitation. The loss of
permanent. The growth of vegetation, trees, and shrubbery,
precipitation in this manner tends to increase with wind speed.
and man-made alteration to the surroundings may change an
excellent exposure to an unsatisfactory exposure in a relatively
Protected Sites. When objects, which individually or in
short period of time.
small groups would constitute obstructions, are numerous and
are so extensive that the prevailing wind speed and, as a
Gage Level. For correct measurement of precipitation, the
consequence, the turbulence and eddy currents are reduced in
open end of the gage must be in the horizontal plane. This can
the vicinity of the gage, a more accurate catch is likely. The
be checked by laying a carpenter's level across the open top of
best exposures are often found in orchards, in a grove of trees,
the gage in two directions, with one crossing the other at right
bushes or shrubbery, or where fences and other objects acting
angles.
together serve as a wind break. As a general rule, in such
Wind Shields. Wind shields attached to the gage structure
areas, where the height of the objects and their distance from
the gage is generally uniform, their height above the gage
help to minimize loss in precipitation catch due to wind effects.
should not exceed about twice their distance from the gage.
1
National Weather Service. (1972). National Weather
Service Observing Handbook No.2, Substation
Observations, Silver Spring, MD.
C-1
Previous Page
