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A Chance of Rain: What Does This Really Mean?

How to Calculate a Chance of Rain

Bob Berman
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When the local weather station (or your weather app) says that there is an “80% chance of rain,” do you assume that this means you have an 80% chance of getting rained on? Not true. Bob Berman explains what a “chance of rain” really means!

If you’re a sky watcher like yours truly, the first step in observing the sky is having decent weather. We probably all check the forecast from time to time. But relatively few of us know how to make sense of it.

Many of us would assume that an ”80% chance of rain’ means you have an 80% chance of getting rained on. This sounds logical but is incorrect.

Guess again. Does it mean there’s a chance of rain across 80% of the forecasted area? Nope. That’s because weather forecasts give their “percentage chance” in probabilities.

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The answer? An “80% chance of rain” means an 80% chance that rain will fall somewhere within the forecasted area. Rain refers to 0.01 inch or more. 

Confused? If you want to understand this further, read on. 

How to Calculate a Chance of Rain

So let’s say the forecast goes like this: Sunday, 40% chance of rain; Sunday night, 40% chance of rain; Monday, 40% chance of rain.

Now, maybe you’re a gardener and really want it to rain (consider catching the rain by using rain barrels in your garden!). Or, perhaps you have a stargazing session or outdoor event planned and desperately want it NOT to rain.

The question is simple: If the Weather Service’s forecasts are accurate, what are the chances that it will rain sometime during the period?

What do you think? With a 40% chance tomorrow, tomorrow night, and then again the next day, what’s the likelihood we’ll get rain any time during that entire time span?

Here’s how you calculate it. Stick with me for a few minutes. This is fun, as long as you don’t totally hate math.

  1. First, you determine the odds that it won’t rain.  In this case, it’s 60% for each of the three periods in question.  
  2. So, you grab your calculator and punch in 0.6 x 0.6 x 0.6, which equals 0.216. Bingo: those are the odds that it will not rain during the entire three periods in question. It is 21.6%.  
  3. Finally, you subtract this from one to get the chance that it will rain: 78.4%. Roughly speaking, there’s an 80% chance it will rain. Amazing, right?

Let’s do another for practice.  Say the Weather Service predicts only a 30% chance of rain or snow today, and again tonight and again tomorrow.  

  1. First, you determine the odds that it won’t rain.  In this case, it’s 70% for each of the three periods in question.  
  2. Now you multiply 0.7 x 0.7 x 0.7, which equals 0.343, which means a 34% percent chance of no precipitation falling.  
  3. Thus, there is a 66% chance that it will rain or snow during the period.

If this sounds illogical and you’re unconvinced, consider that 30% is roughly a one-in-three chance.  Pretend you had three marbles in a bag, one red and two green.  You blindly reach in and pull out a marble. What are the odds you’ll get the red one?  It’s one in three, right?

But now, pretend you get three chances. If you first pull out a green one, you throw it back and get another try, and then a third try. Doesn’t logic tell you that with three chances, you’re likely to succeed? Similarly, with three chances for rain, even if it’s just a one-in-three probability each time, you’ll likely get wet when the three periods in question have elapsed.

Just thought we’d get this out of the way.

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About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman

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Comments

Tom Chase (not verified)

1 month 1 week ago

From NH and spending time in Alabama between Pensacola FL & mobile AL, have weather forecasts in both areas. A saying about rain potential in the north is When the cows are laying down, it’s going to rain”. That has proven true in many cases. In both the north and south weather forecasts are frustrating. They seem to be highly emotional for potential disasters, floods (gully washers), to droughts. The average rainfall in Mobile is over 72 inches making it having the most rain in the country as reported locally. Pensacola FL is a close second, 50 miles apart. Not Oregon. Noted potential Hurricane forecast tracks utilizing about 15 models, all being different. Have to wait it out and see where they are going and take precautions. Many Hurricane escape routes marked. I’ve escaped Katrina.

The sensationalized forecasts are a “wait and see” proposition. Being in between Mobile and Pensacola the storm tracks usually traverse from New Orleans North East to TN & NC and then turn north to New England. I’m in a pocket here escaping most storms. But rain does affect the area. The soil is mostly sand, so it filters into it and is gone, not accumulate very long. Storms come in from the Gulf and drains back into it. Hurricane Sally was a slow mover, dumped 30 inches here. We survived.

I had an acquaintance that flew on the Hurricane tracker planes into the eye. The videos of that is amazing. They flew out of southern FL. Hurricane season is June 1st to December 1st. But some hurricanes happened after Dec 1, they didn’t believe the norms.

Trust the cows!

Rod Kinkade (not verified)

8 months ago

Here in Tucson they say 60/70/60% chance of rain today/tonight/tomorrow afternoon respectively all the time during out monsoon season. And it does not rain. It is as if predicting rain increases their ratings or advertising revenue or something. As the math in the article shows, it is VERY unlikely that it won't rain. I have watched them be totally wrong, day after day for over a decade. The break it down town by town, city by city. I ONLY pay attention to where I live, Tucson. Oh it rains in Bisbee, Nogales, Sierra Vista, Green Valley, Oro Valley, Safford, Lukeville, Benson, Ajo and Willcox. But as I watch the precipitation radar, it dissipates, or goes AROUND Tucson. After years and years, the local news weather meteorologists still can't get it through their heads that there is a heat dome over Tucson that diverts rain unless there is so much moisture in the air it condenses like water on a glass of lemonade.

After it DOES'T rain over and over again, nary a "oops, we were wrong again" statement.

A ROCKWELL (not verified)

2 years 5 months ago

Interesting; I'd always wondered why forecast says Rain; then we get light showers; (a bumbershoot is an umbrella; I had to look it up):

James (not verified)

2 years 5 months ago

I just carry a bumbershoot