A Chance of Rain: What Does This Really Mean?

You’ve probably weather forecasts say there’s an 80% “chance of rain” (or some other percentage). What does “chance of rain” really mean? Probably not what you think.

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.

If you heard an “80% chance of rain,” would you assume this mean you had an 80% chance of getting rained on? Many of us would.

Not true! Second guess. Does it mean that it will rain across 80% of the forecasted area? Not quite! That’s because weather forecasts give their “percentage chance” in probabilities.

In this example, there is an 80% chance that rain will fall somewhere within the forecasted area. Rain refers to 0.01 inch or more. 

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. 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, then 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 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 and this equals 0.216. Bingo: those are the odds that it will not rain during the entire three periods in question. Its 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, 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’re likely to get wet when the three periods in question have elapsed.

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