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Name: Diane
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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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