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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


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