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Rain Measurement Discrepancies
Name: Diane
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
I am a weather buff and keen gardener. Each day I check
the weather, especially precipitation, in the Metro Section of the
Washington Post. I've been puzzled for a long time to see huge
discrepancies between what I measure in the rain gauge compared with
the readings at Dulles International Airport, which is about 12
miles away. A couple of weeks ago we had two days of rain and I
measured 4 inches in one day and Dulles had 1.35. I know that
thunder storm activity is pretty local, but these storms were widespread.
Replies:
Diane,
There could be two reasons for the difference that you see.
First, the difference that you have already noted, that
rain amounts produced by convective storms can vary
greatly over short distances. This time of year,
precipitation is dominated by convective storms and
will vary greatly from place to place, even though the
storms are widespread throughout the area. Dulles is
12 miles away, which in the case of convective storms
is a long distance away for performing a useful rain
amount comparison.
Second, if you notice that you normally receive more rain
at your house than at Dulles on a consistent basis, part
of the cause may be the type of rain gage that you are
using. Inexpensive plastic rain gages can be inaccurate,
resulting in readings that are either too low or too high.
The location of the rain gage is also important; it needs to
be located in a fairly open area so that it is not affected
by wind breaks or obstacles. Most home rain gages also do not
have a wind barrier around them to slow the wind and thereby
provide a more accurate measurement, although lacking one
usually leads to an underestimation of rain (and snow
especially), not an overestimation.
David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012
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