Explainer1 min read
What a “30% chance of rain” really means
Rain probability is one of the most misunderstood numbers in a forecast. Here is what it says, what it does not, and how to plan around it.
What the number says
Probability of precipitation is an estimate of how likely it is that measurable rain falls at a specific place during a specific period. It comes from running many simulations of the atmosphere: if three out of ten simulations produce rain at your location, you get 30%.
It does not mean it will rain for 30% of the day, or that 30% of the city will get wet. It means the chance of any rain at your location is about three in ten.
What it does not say
- How hard it rains: 80% can be light drizzle all day, while 40% can be a heavy downpour.
- How long it lasts: a ten-minute shower and a three-hour rain are very different.
- When it happens: a daily value covers the whole day, an hourly value only that hour.
How to use it
- Under 20%: you can usually leave the umbrella at home.
- 30 to 50%: carry a light umbrella or jacket and check the radar before heading out.
- Over 70%: expect rain and plan indoor activities.
Check the hours for your city
Every city page has an hour-by-hour forecast, for example for Toronto, Dublin or Chicago.
