MadSci Network: Earth Sciences
Query:

Re: How is an inch of rainfall computed?

Date: Fri Aug 13 22:07:36 1999
Posted By: Dave Clark, Staff, Chemical and Environmental Technologies, Battelle
Area of science: Earth Sciences
ID: 931878984.Es
Message:

An inch of rain is just what it sounds like - one inch of water. If you had a sraight-sided cylinder, then there would be one inch of water in the bottom. Since a one-inch rainfall is a lot and pretty unusual, most of the time we are interested in amounts less than an inch. Rain gauges are usually designed with the collecting part at the top (where the rain enters) having a larger cross section than the straight-sided measuring part at the bottom. Picture a wide funnel feeding into a narrower cylinder.

As an example, consider a rain guage with a collecting area at the top of 2 square inches funnelling down to a measuring part of only one square inch. In this case, every one inch of water in the bottom would represent 1/2 inch collected at the top:

2 sq in top collection area x 1/2 in of rain = 1 sq in of measuring part area x 1 in of water in the bottom.

The ratio of the amount of rain to the height of the water in the bottom is the same as the ratio of the measuring part (bottom) area to the collection (top) area. Because the amount of water standing in the measuring part is related to the actual rain amount by this ratio, the calibration marks on the measuring part have to be "stretched" by the same ratio to convert from the height of water in the bottom to the actual rainfall collected by the larger collection (top) part.

The reason this is done is that it allows us to accurately measure smaller, more common amounts of rain. If we had to measure directly, we could never measure rainfalls like 0.05 inches. However, if our collection part area/measuring partarea ratio is 20:1, that 0.05 inches will result in one inch of water (0.05 inches x ratio of 20 = 1 inch) standing in the bottom measuring part - an easily measured amount.


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