Dear Tom,
My “rain gauge” is a decorative pitcher that sits out on my deck. After the big storm on Sept. 11, I poured 47 ounces of rain from that pitcher into a standard 8-ounce measuring cup. What is the conversion formula to change that into inches of rain?
Kate Schneider
Dear Kate,
Such a formula does not exist because weather observers make direct measurements of accumulated rainwater in specially-designed rain gauges. They do not determine rainfall by weighing the water and converting that weight to a depth.
Rainfall is reported in hundredths (.01) of an inch, a depth too tiny to be determined with a ruler. A rain gauge works by gathering rain in a collecting area that is ten times the area of a tube into which it accumulates. Each tenth inch of water in that tube represents 0.01 inch of actual rain.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
