(WGHP) – Whether you’ve watched the FOX8 weather team give a forecast with the chance for rain or you’ve checked your weather app to see what the rain chance is throughout the day, you may have wondered what meteorologists mean whey they give a percentage for the chance of rain.
Eric from Greensboro asks, “What does a 30% chance of rain mean? Does it mean something different for an hourly forecast versus a daily forecast?”
If the forecast calls for a 30% chance for rain, it does not mean that 30% of the forecast area will see rain and it does not mean that it’ll rain for 30% of the day.
According to the National Weather Service, the probability of precipitation is the probability of 0.01 inches or more of precipitation at a location within the forecast area in a specified time period.
So, let’s look at how forecasters come up with the probability of precipitation.
The probability of precipitation, also known as the chance of rain, is determined by two factors: the coverage of rain in the viewing area and the confidence that measurable rain will fall within the viewing area.
Forecasters can use a mathematical equation to determine the chance for rain. It looks like this:
Chance for rain = (Forecasters confidence that rain will form or move into the area) x (Coverage of rain within the viewing area)
Chance for rain = (Percent Confidence) x (Coverage in decimal form)
Using the equation, here’s how the FOX8 weather team may determine the chance for rain on a particular day.
If we’re 100% certain that rain will develop in the Piedmont Triad but it’s only expected to cover 30% of the forecast area, then the forecast would read “a 30% chance of rain.”
This is because our confidence is 100% and it’s multiplied by coverage in decimal form, 0.30, which equals 30%.
Alternatively, if we’re 60% certain that rain will develop or move into some area of the Piedmont Triad but it’ll only cover 50% of the forecast area, then the forecast would again read “a 30% chance of rain.”
This is because our confidence is 60% and it’s multiplied by coverage in decimal form, 0.50, which equals 30%.
If confidence is 60% (0.60) and coverage is 50% (0.50)
Chance of rain = (0.60) x (0.50) = 0.30 = 30% chance of rain
Something else to keep in mind when it comes to the chance for rain is that it does not factor in how heavy the rainfall will be.
So, even if there’s “only” a 30% chance for rain, it’s still possible for some to see over an inch of rainfall. It’s also possible that some people only see a tenth of an inch of rain on a day where the chance of rain is 100%.
Sometimes, to make the forecast easier to understand, we’ll use words like isolated or scattered showers.
When we say “isolated showers,” we’re referring to a 20% or 30% chance for rain.
“Scattered” showers or thunderstorms typically refer to a chance of rain between 40% and 60%.
Anything above 70% is typically a chance for “widespread” showers or storms.
If you or your child have a weather-related question, let us know using the form on our “Ask Alex!” page. Meteorologist Alex Schneider joined the FOX8 Max Weather Team in August 2022. She has a bachelor of science in atmospheric science and meteorology from the University of North Carolina at Asheville.