So, you open your phone in the morning and see that the forecast calls for a 30% chance of rain. But what does that actually mean?
Does it mean that you have a 30% chance of rain in your neighborhood? Is it a 70% chance that it won’t rain in your neighborhood? Or does it mean that 30% of the area will definitely get rain?
It’s a question that puzzles people often, and one that we get a lot as meteorologists. So, let’s chat about what the “probability of precipitation” actually means.
Confidence AND Coverage
Put simply, probability of precipitation — or PoP for short — is a combination of the confidence and coverage of rain.
The National Weather Service describes it as, “the probability that the forecast point in question will receive at least 0.01″ of rain.”
Now, let’s get into the more complicated answer...
100% chance of math
When you break it down, meteorology is a science with a BUNCH of math. In fact, there’s a formula for the chance of rain, but it’s actually pretty simple:
PoP(%) = C x A
- C is the CONFIDENCE that it will actually rain. There are many ingredients necessary to make rain. When all of those factors are present, meteorologists are 100% confident that it will rain at some point in the day. When some ingredients for rain are in question, it lowers the confidence of precipitation.
- A is the AREA that has the potential to see rain. There are times when the weather pattern calls for all of a forecast region to see rain. However, there are also days when the weather pattern only favors certain areas or parts of a region for rain.
EXAMPLES
Let’s say that as meteorologists, we are...
- 100% sure it will rain in 30% of the San Antonio metro area. In this case C = 1 and A = 0.3. So, using the formula of PoP = C x A
- The Probability of Precipitation in this case is 1 x 0.3 = 0.3 or 30%
- 30% sure it will rain (C = 0.3) in 100% of the San Antonio metro area (A = 1)
- PoP in this case is 0.3 x 1.0= 0.3 or 30%
- 60% sure it will rain (C = 0.6) in only half of the San Antonio metro area (A = 0.5)
- PoP in this case is 0.6 x 0.5 = 0.3 or 30%
Making it simple
As you can see, a 30% chance of rain can mean many different things depending on the weather set-up and where you live. As meteorologists, it’s our responsibility to communicate the rain chance in a simple, straightforward way.
That’s why we choose to use words like “isolated,” “scattered,” etc. to explain the forecast. Here’s how that breaks down:
Stray (10%)
- Only one or two showers or storms are expected on the radar.
Isolated (20-30%)
- A few showers and storms will pepper the radar at some point.
Scattered (40-60%)
- Multiple downpours, potentially impacting travel and day-to-day life.
Widespread (70-100%)
- Numerous showers and storms. Practically a guarantee that you will see rain in your backyard.
Timing is everything
It’s important to keep in mind the chance of rain has nothing to do with time. If you see a 100% chance of rain in the forecast, that doesn’t necessarily mean it’s going to rain all day.
Another part of our job as meteorologists is to let you know which parts of the day will favor rain. We strive to do this every day as Your Weather Authority.
- Download KSAT’s weather app for hour-by-hour, customized forecasts in San Antonio, South Texas or wherever you are.
- Find the latest forecasts, radar and alerts on the KSAT Weather Authority page
- Read more from the meteorologists on the Whatever the Weather page