What "40% chance of rain" actually means
A "40% chance of rain" means there's a 40% chance that at least a little rain falls at your spot during that part of the forecast. It doesn't mean it will rain for 40% of the day, or over 40% of the area.
The chance is for one spot
The National Weather Service's chance of rain, called the probability of precipitation, is the chance that at least 0.01 inch of rain falls at a given point during the forecast period. "A point" is your spot on the map, roughly a few miles across. So 40% means: on days like this one, about 4 in 10 would bring at least some rain where you are. In winter the same number counts snow too.
It isn't the share of the day or of the area
Two common readings are wrong:
- "It'll rain 40% of the time." No. The number says nothing about how long it rains. A 40% day might bring one 20-minute shower, or nothing at all.
- "40% of the area will get rain." No. It's the chance at your spot, not the size of the wet area.
It doesn't tell you how much
The chance only says whether measurable rain, at least 0.01 inch, is likely. A 90% chance can mean a light sprinkle, and a 30% chance can mean one heavy thunderstorm that soaks the few places it hits. For how much, look at the rain amount in the forecast, and for storms, at watches and warnings.
Check which hours it covers
Most NWS forecasts give a chance for each 12-hour period: daytime, about 7 a.m. to 7 p.m., and overnight. A 60% chance "Tuesday" is for the whole of Tuesday's daytime, not every hour of it. Hour-by-hour forecasts give a separate chance for each hour, which is the better guide for planning an afternoon.
On Atmostorm, your town's or county's page shows one chance of rain or snow for each of the next three days. It covers the daytime and the evening: of the two, it shows the higher chance.
Using it
Our rule of thumb, not an NWS rule:
- Around 20%: rain is possible but unlikely at your spot. Fine for most plans.
- 30–50%: a real chance. Have a backup for anything that can't get wet.
- 60% and up: rain is more likely than not. Plan around it.
Common questions
Does 40% chance of rain mean it will rain 40% of the day?⌄
No. It's the chance that at least some rain falls at your spot during the forecast period. It says nothing about how long it rains.
Does 40% mean 40% of the area will get rain?⌄
No. It's the chance at a single spot, not the share of the area.
Does a high chance of rain mean heavy rain?⌄
No. A 90% chance only means measurable rain, 0.01 inch or more, is very likely. Check the rain amount for how much.
Why did it rain when the chance was only 20%?⌄
A 20% chance means rain falls about one time in five on days like that one. Sometimes you're the one.
Educational summary, not official guidance. Always confirm current watches and warnings at weather.gov and follow your local NWS office and emergency officials.