Dear Tom,
Snow is measured in tenths of an inch, but how can they measure rain, which is reported in hundredths of an inch?
— Scott Burgess, Chicago
Dear Scott,
Measuring snow depth is straightforward: Use a ruler graduated in tenths of an inch and find a representative snow depth. Rain is a different matter because one-hundredth of an inch of water is too thin a depth to be measured directly.
A standard rain gauge utilizes the principle of multiplication. Rainwater drains by a funnel from a collection area (typically a circular area eight inches in diameter) into an accumulation tube whose area is only one-tenth as great.
Every hundredth of an inch of rain from the collection area stands 10 times as deep, or one-tenth of an inch deep, in the accumulation tube. A ruler graduated in tenths of an inch is used to measure the depth of the water.
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