What is Probability of Precipitation (PoP)?

PoP is the official term for the "chance of rain." It is defined as the probability that measurable precipitation (0.01 inch or more) occurs at a specific point during the forecast period. Forecasters compute it as Confidence × Area — how sure they are precipitation forms, times the fraction of the area expected to get wet.

Does a higher chance of rain mean heavier rain?

No. PoP says nothing about intensity or duration — a 100% chance can be a brief five-minute shower, while a 40% chance could be a downpour if it does hit. For how much rain to expect, look at a rainfall-amount (QPF) forecast instead.

Does 100% chance of rain mean it will definitely rain?

It means the forecaster is essentially certain that measurable rain (0.01″+) will fall at your location during the period. It still says nothing about how long or how hard — 100% can be a single passing shower.

What Does "Chance of Rain" Mean? — Probability of Precipitation

What Does "Chance of Rain" Mean? — Probability of PrecipitationThe chance of rain (Probability of Precipitation, or PoP) is the odds that at least 0.01″ of rain falls at any given point during the period. The Confidence × Area formula, and why it says nothing about how hard or how long it rains.The "chance of rain" — officially the Probability of Precipitation, or PoP — is the chance that at least 0.01 inch of measurable precipitation falls at any given point in the forecast area during the time period. So "30% chance of rain" means that at your specific spot there is a 30% probability that measurable rain falls during that window. It does NOT mean it will rain 30% of the time, or that 30% of the area will get rain.The formula behind itPoP = Confidence × Area. The forecaster multiplies how confident they are that rain forms somewhere (Confidence) by the fraction of the area expected to get wet (Area). Being 100% sure it rains over half the area gives 0.5 × 1.0 = 50% PoP. Being 50% sure it rains, but everywhere if it does, also gives 0.5 × 1.0 = 50% PoP — the same number from two very different situations."Measurable" means 0.01 inch or more. A few sprinkles that do not reach 0.01 inch technically do not count, which is why a "20% day" can still have a passing drizzle.Why high PoP is not intensityPoP says nothing about how hard it will rain or for how long — a 100% chance can be a five-minute shower. For amounts, look at a rainfall-total (QPF) forecast instead.How to use itTreat PoP as a planning confidence dial. Under about 20% is "unlikely, but keep an umbrella in mind." 40–60% is a genuine coin-flip — check the radar before heading out. 70% and above means plan around it. Because PoP is point-and-period specific, a "40% chance today" spread over 12 hours feels very different from 40% concentrated in a single afternoon hour.

Frequently asked questions

What does 30% chance of rain mean?: A 30% chance of rain means there is a 30% probability that at least 0.01 inch of measurable rain falls at any given point in the forecast area during the time period. It does not mean it will rain 30% of the time, or that 30% of the area will get rain.
What is Probability of Precipitation (PoP)?: PoP is the official term for the "chance of rain." It is defined as the probability that measurable precipitation (0.01 inch or more) occurs at a specific point during the forecast period. Forecasters compute it as Confidence × Area — how sure they are precipitation forms, times the fraction of the area expected to get wet.
Does a higher chance of rain mean heavier rain?: No. PoP says nothing about intensity or duration — a 100% chance can be a brief five-minute shower, while a 40% chance could be a downpour if it does hit. For how much rain to expect, look at a rainfall-amount (QPF) forecast instead.
Does 100% chance of rain mean it will definitely rain?: It means the forecaster is essentially certain that measurable rain (0.01″+) will fall at your location during the period. It still says nothing about how long or how hard — 100% can be a single passing shower.

Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.