Weather 2026-05-21 4 min read
What Does 40% Chance of Rain Actually Mean?
Probability of precipitation is not the share of the day it rains — here's what the number really encodes.
Probability of precipitation is confidence multiplied by coverage. Formally, PoP = C × A, where C is the forecaster's confidence that rain occurs somewhere in the area and A is the fraction of the area expected to get wet.
Same number, different weather 40% can mean the forecaster is 100% sure showers will hit 40% of the region, or 40% sure a system covers the whole region. Both print as 40%.
What it does not mean It is not 40% of the hours and not 40% of the intensity. It also refers to a measurable amount — usually 0.2 mm or more — so a light drizzle may not count.
Practical thresholds Under 20%: ignore it. 20-50%: carry a compact umbrella, plan an indoor fallback. Over 60%: assume you'll get wet at some point. Over 80%: reschedule outdoor work.
Hourly PoP is more useful than daily. A day showing 60% is often three hours at 80% and the rest dry.
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