What does a 30% chance of rain mean?
(WGHP) – Whether you’ve watched the FOX8 weather team provide a rain chance forecast or checked your weather app to see the probability of rain throughout the day, you may have wondered what meteorologists mean by the whey they give relative to the probability of rain.
Eric from Greensboro asks: “What does a 30% chance of rain mean? Does that mean anything different for an hourly forecast versus a daily forecast?”
If the forecast says there is a 30% chance of rain, that means there is a 30% chance of rain. no This means that 30% of the forecast area will see rain no This means that it will rain for 30% of the day.
According to the National Weather Service, precipitation probability is the probability of 0.01 inch or more of rain falling at a location within the forecast area in a given time period.
So, let’s look at how forecasters arrive at the probability of rain.
The probability of precipitation, also known as the chance of rainfall, is determined by two factors: rain coverage in the viewing area and confidence that measurable rain will fall within the viewing area.
Forecasters can use a mathematical equation to determine the chance of rain. It looks like this:
Chance of rainfall = (forecasters’ confidence that rain will form or move to the area) x (rainfall coverage within the viewing area)
Chance of rain = (percentile confidence) x (decimal coverage)
Using the equation, here’s how the FOX8 weather team determines the chance of rain on a given day.
If we are 100% certain that rain will develop in the Piedmont Triad but are expected to cover only 30% of the forecast area, the forecast will be “30% chance of rain.”
This is because our confidence is 100% and it is multiplied by the coverage in decimal form, 0.30, which equals 30%.
Alternatively, if we are 60% certain that rain will develop or move into some area of the Piedmont Triad but will cover only 50% of the forecast area, the forecast will again read “30% chance of rain.”
This is because our confidence is 60% and it is multiplied by the 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
Another thing to keep in mind when it comes to the chance of rain is that it does not take into account how heavy the rain will be.
So, even if there is “only” a 30% chance of rain, some could still see an inch of rain. It’s also possible for some people to see only a tenth of an inch of rain on a day when there is a 100% chance of rain.
Sometimes, to make the forecast easier to understand, we will use words like isolated or scattered showers.
When we say “scattered showers,” we are referring to a 20% or 30% chance of rain.
“Scattered” showers or thunderstorms usually indicate a 40% to 60% chance of rain.
Anything above 70% is usually a chance of “extensive” rain or storms.
If you or your child has a weather question, let us know using the form at “Ask Alex!” page. Meteorologist Alex Schneider joined the FOX8 Max Weather team in August 2022. She holds a Bachelor of Science in Atmospheric Science and Meteorology from the University of North Carolina at Asheville.