Forecasting
Probability of Precipitation Explained: What a 40 Percent Chance of Rain Means
What PoP means in weather forecasts, why it is not simply how much of the day it will rain, and how to use rain chances correctly.
Quick answer: Probability of precipitation is the chance that measurable precipitation will occur at a point during the forecast period.
The common misunderstanding
A 40 percent chance does not mean it will rain for 40 percent of the day. It also does not necessarily mean 40 percent of the area will get rain in every forecast system.
Think of it as a probability signal for measurable precipitation at your location or forecast point.
Coverage and confidence
Rain chances combine uncertainty about whether precipitation will form with uncertainty about where it will fall.
A scattered thunderstorm day may have a moderate chance even if many hours stay dry.
Planning use
Use PoP with timing, radar, cloud cover, and event sensitivity.
A 30 percent storm chance matters more for an outdoor wedding than for a normal commute.