Name: Michael C.
Status: student
Age: 11
Location: N/A
Country: N/A
Date: 7/14/2004
Question:
I have a rain gauge. The wind was blowing in gusts up to
60miles an hr. It was raining hard for 2 hours. It appeared it had rained
about 4 inches. The rain gauge registered only 1.75 inches. Is a rain
gauge accurate when the wind is blowing hard?
Replies:
Michael,
Rain gauges do underestimate in winds and give
increasingly larger underestimates as the wind speed
increases.
Most "official" rain gauges have a circle of fins
around them (called an "Alter shield"), usually set
about 2 feet from the rain gauge; these fins help to
reduce the wind speed in the vicinity of the rain gauge
and counteract the loss of recorded precipitation caused
by the wind. However, the Alter shield becomes less
effective as the wind speed increases to even moderate
speeds.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
Dear Michael-
You are correct in assuming that strong winds can bias rainfall
measurements. The rain gages are calibrated using a known area of
collection, which is related to the diameter of the collection tube. Strong
winds effectively reduce the area of the collection tube by increasing the
"slant" (angle of rainfall from the vertical) of the rain. It is doubtful
that the difference would be as much as you suggested in your example,
though.
This effect is even more pronounced with snowfall. Most National Weather
Service official precipitation gages have wind deflectors installed to
minimize the effects of wind on precipitation measurements.
Wendell D. Bechtold, meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
