Dear Tom,
Rainfall totals are recorded in hundredths of an inch, but that’s too small to be “measured,” so how is it actually done?
Georgie Tatum, New Orleans, Louisiana

Dear Georgie,
It can be done by using a trick of multiplication. The collection area of a rain gauge is a funnel that is ten times the area of the tube into which the rainwater drains. Rainwater therefore stands ten times more deeply in the tube than the depth of the rain that actually fell.  A measuring stick with graduations to tenths of an inch is then used to determine the depth of rainwater in the tube. Every one tenth of an inch of water in the tube equals 0.01 inch of rain. This simple but accurate multiplication technique is used in rain gauges around the world wherever rain totals are recorded in hundredths of an inch.