Grades 6–8 · Everyday Life
What does a chance of rain mean?
A chance of rain describes uncertainty about a specified event. In National Weather Service terms, precipitation probability concerns measurable precipitation at one point during a stated period. Measurable means at least 0.01 inch of liquid, or the water equivalent of frozen precipitation.
In the diagramFictional forecast examples40% chance of measurable precipitationAt one place, in a stated time window20 made-up forecastsWet caseDry caseEach square:one forecastcase8 wet out of 20 = 40%
A 40% probability concerns whether that event occurs. It does not say that rain lasts for 40% of the day or covers exactly 40% of a picture. The place, time window and precipitation threshold are part of the meaning of the forecast.
The illustration uses an invented record of twenty forecasts, all at 40%, with eight wet outcomes. Eight out of twenty is 40%. Each square represents one forecast and its outcome, not a patch of land or a part of an afternoon. The numbers were deliberately chosen to illustrate matching frequency.
A new forecast at 40% can have a wet or dry outcome. A small new group need not contain exactly 40% wet outcomes either. Calibration means assessing how forecast probabilities correspond to observed frequencies across many appropriately grouped forecasts. One dry afternoon cannot settle that question, and this invented grid does not measure an actual forecast service’s reliability.
About this explanation
Starting knowledge: Convert simple fractions to percentages and distinguish an individual case from a group of cases.
Sources
- National Weather Service — Forecast Terms: Precipitation Probability
- National Weather Service, Paducah — Precipitation Probability