Weather » The "Front" Page

What Does a 30% Chance of Rain Really Mean??

By Justin Culligan
July 22, 2010 Updated Jul 22, 2009 at 7:49 AM EST

You see it all the time: a forecast calling for "partly sunny skies with a 30% chance of showers." But what does the 30% value actually mean? To some, it could mean it will rain throughout approximately 30% of the viewing area. To others, it could mean it will rain during approximately 30% of the entire day. But neither of these theories are correct!

A 30% chance of rain simply means that on any day with the exact same atmospheric setup, it will rain approximately 30% of the time. In other words...for every ten days that are expected to have a 30% chance of rain, it will only rain on three of them. So there ya have it!

Obviously the same explanation is valid for a 40%, 50%, or any % chance of rain.

To submit a comment on this article, your email address is required. We respect your privacy and your email will not be visible to others nor will it be added to any email lists.