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Rainfall amounts measured to hundredths of an inch–How is it possible to measure such small amounts?

Dear Tom,
I have always seen rainfall amounts measured to hundredths of an inch, most recently with Hurricane Florence in North Carolina and South Carolina. How is it possible to measure such small amounts?
— Jim Storey, Crown Point, Ind.
Dear Jim,
You are correct to question rainfall amounts “measured” to hundredths of an inch. It’s not possible. However, by using a trick of multiplication, it can be done. It’s done this way: Rain gauges have a collection area (the area into which rain falls) that is 10 times the area of a collection funnel (into which the rainwater drains). Therefore, water stands 10 times more deeply in the collection funnel than the depth of the rain that actually fell. A measuring stick graduated to tenths of an inch is then used to measure the depth of water in the collection funnel. Every tenth of an inch of water in the funnel equals 0.01 inch of rain.

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