What Does Percent Chance of Rain Mean in a Weather Forecast? (2024)

FAQs

What Does Percent Chance of Rain Mean in a Weather Forecast? ›

If the chance of rain is less than 50%, there's a greater likelihood of it staying dry than not. For example, a 20% chance of rain means there's an 80% chance of dry weather. On the other hand, an 80% chance of rain means there's only a 20% chance of dry weather.

Let's look at an example of what the probability does mean. If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.

1/100 (0.01) of an inch of rain – The first measurable amount of rainfall reported by The National Weather Service. This would not leave puddles on the ground and would slightly wet the surface. This might occur during a light shower for 2-5 minutes or even a drizzle for 2 hours.

You may have seen a video that went viral. that said that chance of rain. isn't actually the likelihood it's going to rain, but it's the percent of area of a city. that is going to get rain.

The percentage is called the Probability of Precipitation (PoP)Credit: Apple. When opening the iPhone weather app, users can find a number and percentage under a rain cloud on any upcoming days that are expecting to see some rain. On some days, it might read “80%,” while others say “40%.”

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.

Say the models show 50% confidence there will be rain over 50% of the area. That would be 0.5 X 0.5, which leaves us with 0.25, a 25% chance of rain. Bear with us for one more example: Say there's 100% confidence there will be rain, but it's only for 80% of the area. That's 1 X 0.8, which is an 80% chance of rain.

CHANCE OF RAIN 40 PERCENT. What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area? The "Probability of Precipitation" (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain.

That is 1/50th of an inch, or a half millimeter. It would lightly wet the sidewalks if all of it fell within a few minutes. You would not be soaking wet if you were outside during this downpour. If that 0.02″ fell over an hour, it might go unnoticed, though the sky may be overcast.

Per the USGS Rainfall Calculator, one inch of rainfall equals 6 gallons of water per square yard or 27,154 gallons of water per acre!

What does chance of rain 100% mean? ›

When you see a "100% chance of rain," it doesn't mean it's going to pour everywhere, all the time. Think of it like this: the forecasters are super confident that rain will fall somewhere within a certain area, but they're not sure exactly where or for how long.

The National Weather Service uses “PoP,” or probability of precipitation, when talking about the chance of precipitation in a specific forecast area. The PoP represents both confidence and area. It is defined as the probability that a select area will receive at least 0.01” of rain.

Closer look at percentages

Let's use 60% as the rain chance in the forecast. That means 60% of the entire area (Central Florida) could see measurable rain at some point in the day, while the other 40% won't. That goes beyond the city that you live in.

It's not chance of rain. The percentage means How much of your current area will see rain. If you're in NYC and it's says 99% then all of NYC will have rain today. If it instead says 10% then only a small part of NYC will see rain. It's more a measure of how big the rain clouds are.

For example, if we are 100% confident that it will rain in 30% of the city of Cardiff, then there is a 30% probability of rain. If we are 50% confident that 100% of the city of Cardiff will receive rain, then there is a 50% probability of rain.

The "Probability of Precipitation" (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain. So, in this example, there is a 40 percent probability for at least 0.01" of rain at the specific forecast point of interest!

If there are 100 days in which the forecast for rain is 10%, then it should rain on 10 of those days and not rain on the other 90. So if you hear a forecast of 10% chance of rain, and it rains, it doesn't mean that the forecast is wrong, it just happens to be one of those rainy days.

A chance of rain refers to a probability between 30% and 50%. A likely chance of rain would be between 60% and 70%. And with any probability of precipitation higher than 75%, the forecaster has the option of describing it as “periods of rain,” “occasional rain” or simply “rain,” Blier said.