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Dear Tom,
When a forecast calls for a 20 percent chance of rain, does it mean it will rain 20 percent of the time or that there is an 80 percent chance it will stay dry?
—Martin Cobb
Dear Martin,
A forecast of a 20 percent chance of rain means the forecaster believes there is a one in five chance (20 percent) that measurable rain will occur in the time period covered by the forecast. In weather forecasts, “today” refers to the 12-hour period from 6am through 6pm; “tonight” refers to 6pm through 6am of the following morning; “this afternoon” means noon through 6pm; “remainder of the night” refers to the period ending at 6am the following morning. Measurable precipitation is defined as at least 0.01 inch of precipitation (water content) in the forecast period.
