What does ‘chance of precipitation’ really mean? A meteorologist explains.
Table Of Content
Source: Popular Science
Category: Science
Originally Published: 2025-12-02
Curated: 2025-12-02 15:18
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Understanding the weather forecast can sometimes feel like reading tea leaves. Do I need a heavier coat? Should we move our plans inside because of rain? Will it be safe to drive?
With at least nine types of precipitation and constantly changing dynamics in the atmosphere, predicting the chance of precipitation can be a particular challenge for both civilians and meteorologists alike. But it’s not impossible.
What does ‘chance of precipitation’ really mean?
Precipitation is simply when water falls down to Earth’s surface as part of the natural water cycle. Precipitation can take several forms, including rain, snow, ice, and sleet and is forecast using satellite data and mathematical formulas.
When it comes to looking at the chance of precipitation in the forecast, it helps to first understand what chance of precipitation isn’t, says meteorologist Cyrena Arnold.
“If it says 30 percent, it doesn’t necessarily mean that it’s going to rain for 30 percent of the area,” Arnold tells Popular Science. “It also doesn’t have an indication of how heavy the rain will be. It’s not like 30 percent necessarily means light rain and 100 percent is heavy rain.”
There are several different types of forecasting models meteorologists use to predict weather, including the Global Forecast System (GFS), European Centre for Medium-Range Weather Forecasts (ECMWF), and more regional models including North American Mesoscale (NAM). They all use satellite data and mathematical formulas to track weather fronts and predict when and where precipitation might occur. Seeing a 30 percent chance of precipitation in your weather app means if meteorologists run the same weather forecasting model 10 times with some slightly different variables, precipitation will show up in three out of the 10 model runs.
“You also do have to remember that if it says, there’s a 30 percent chance of rain, that means there’s a 70 percent chance that it won’t,” Arnold explains.
The chance of precipitation also doesn’t predict how long the rain will last, how much will fall, or how intense it will be. For example, the forecast could say that there is a 100 percent chance of precipitation, but that could mean it will be in a squall line thunderstorm where it rains incredibly hard for only 15 minutes in a given area, or a steady drizzle that lasts all day.
“A percentage has absolutely zero indication of how long that rain is going to last, how severe is that rain,” says Arnold.
Does the type of precipitation impact the forecast?
Generally speaking, all forms of precipitation start out as snow because our atmosphere is so much colder than the ground.
“It doesn’t matter if it’s summer in New York City [rain] is starting [as] snow,” says Arnold.
Regardless of how high or low a cloud is, clouds are always colder than the ground temperature. “If your cloud bases are at 10,000 feet, the temperature will drop 50 degrees. So, if it’s 80 [degrees Fahrenheit] down here, it’s 30 up there.”
Those variations in atmospheric temperature make forecasting rain easier than forecasting sleet, freezing rain, or, in some cases, snow.
“If it’s cold up there and it’s cold down here, we know that it’s going to be snow,” says Arnold. “Where things get really interesting is in this middle area when you look at sleet and freezing rain.”
Sleet occurs when snowflakes partially melt as they fall through a shallow layer of warm air. Those slushy drops refreeze as they go through a deep layer of freezing air above the surface, and will eventually reach the ground as frozen rain drops that bounce.
Image: National Weather Service.
By comparison, freezing rain does not fall as an icy pellet, and freezes upon impact with a surface. It begins as snow, but when the water drop falls through a warmer and more shallow pocket of air, it melts and changes it from a solid to a liquid. The water drop will then expand and freeze if it hits a deeper and colder air pocket and if the ground’s temperature is below freezing. The result is an icy layer that is dangerous for drivers, pedestrians, and snow shovelers alike.
Forecasting these icy weather events “is incredibly difficult and incredibly nuanced, especially the freezing rain,” Arnold says.
With all of that interaction between warm air, cold air, deeper air pockets, and shallow air pockets, the freezing rain forecast can change on a dime.
“If you have one of these variables move slightly by a couple degrees, you have a completely different situation. It’s just a very volatile forecast,” says Arnold.
Trying to predict freezing rain is especially important since this kind of precipitation can be very dangerous, bringing down tree limbs, power lines, and increasing car accidents. Even a 0.01 inch of freezing rain and ice is enough for walking and driving to be unsafe.
“Sleet isn’t so bad because it’s not as catastrophic, and you just need a deeper pool of cold air at the surface to allow that drop to to be snowflake, melt into a water drop, and then freeze again,” says Arnold. “If it’s a deep cold, it will freeze again and fall with a little ice pellet.”
When is the weather forecast most accurate?
While we all would love to know exactly what the weather is going to be during our beach vacation, those 10-day forecasts are not always the best to follow. The closer you are to the timeframe, the better the information.
Arnold explains that one way to understand forecasting is to imagine that you are driving down a long dirt road that stretches for miles. You then notice a swirl of dust, so you know that there is something on the road, but don’t know if it’s a car, a truck, a horse, or something else entirely.
“As it gets closer, the picture gets clearer and clearer. You see that it’s blue, which you didn’t know before,” says Arnold. “As it gets closer, you can see if it’s a car or truck and eventually you can see the make and model of the vehicle.”
Unfortunately, there is no perfect threshold for knowing when the forecast will be the most accurate. The best guidance is that the closer we get, the better we know what the weather will look like.
What meteorologists do know farther in advance are temperature patterns. “There are very good forecasts one to two weeks out that help us understand if temperatures are going to be above or below normal,” says Arnold. “We’re very good at that.”
And remember, meteorology is very difficult. It combines extremely high-level math and physics that most of us can’t even compute with constantly shifting variables. So please be nice to your local meteorologist.
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