← What a Chance of Rain Means · Lesson 2 of 3
One number built from two
In the United States the chance of rain is worked out as the forecaster’s confidence that rain will fall somewhere in the area, multiplied by the share of the area it would cover. Half sure of rain over six tenths of the area gives 0.5 × 0.6 = 0.3, so 30%.
Certain of showers over three tenths of the area gives 1 × 0.3 = 0.3 as well. Both days print 30%, but one is "maybe a big rain" and the other is "surely some scattered showers". The single number hides how it splits.
A forecaster is 80% sure of rain, and if it comes it would cover half the area. What chance of rain does the forecast give?
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