What does a 30% chance of rain mean?

Dear Tom,
What does a 30% chance of rain mean?
— Daisy White
Dear Daisy,
A 30 percent chance of rain means there is a 30% chance of measurable rain (0.01 inch or more) in the time period to which it applies, such as “today” (6 a.m. to 6 p.m.), “tonight” (6 p.m. to 6 a.m. of the next day) or “this afternoon” (noon to 6 p.m.). More generally, a 30% chance of precipitation means there is, in the opinion of the forecaster, a 30% chance of measurable precipitation in the time period to which it applies ( ”measurable precipitation” is 0.01 inch of precipitation or more).
Thirty percent is equivalent to three chances in 10 or to 30 chances out of 100. A 30% chance of precipitation also means there is a 70% chance that measurable precipitation will not occur in the time period to which it applies because 100% minus 30% equals 70%.

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