Dear Tom,
What does a half-inch of rain mean?
–Pat Broz
Dear Pat,
When we say that one-half inch of rain has fallen, we mean a blanket of water 0.50 inches in depth would cover all horizontal surfaces if none of the rainwater had soaked in, run off, evaporated or been blocked by nearby or overhead obstructions such as vegetation, wires or buildings. Rainfall is measured in hundredths (0.01) of an inch, an increment too tiny to measure directly. Rain gauges therefore employ the principle of multiplication. In a standard gauge, rainwater falls into a circular collection area whose area is exactly 10 times that of an accumulation tube into which it drains. Each .01 inch of rainfall therefore stands 0.1 inch deep in the accumulation tube, and that water is measured with a ruler graduated in 10ths of an inch.
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
Weather Report is prepared by the WGN-TV Weather Center, where Tom Skilling is chief meteorologist. His forecasts can be seen Monday through Friday on WGN News at 11:30 a.m., 5:30 p.m. and 9 p.m.
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