What does ‘chance of precipitation’ really mean? A meteorologist explains.
Understanding the weather forecast can sometimes feel like reading tea leaves. Do I need a thicker coat? Should we move our projects indoors because of the rain? Will it be safe to drive?
With at least nine types of precipitation and ever-changing atmospheric dynamics, predicting precipitation risks can be a particular challenge for civilians and meteorologists alike. But it’s not impossible.
What does “risk of precipitation” really mean?
Precipitation simply occurs when water falls on the Earth’s surface as part of the natural water cycle. Precipitation can take many forms, including rain, snow, ice and sleet, and is forecast using satellite data and mathematical formulas.
When it comes to considering precipitation chances in forecasts, it helps to first understand what the chances of precipitation are. isn’t itsays meteorologist Cyrena Arnold.
“If it says 30 percent, that doesn’t necessarily mean it’s going to rain on 30 percent of the area,” Arnold says. Popular science. “It also doesn’t give any indication of the intensity of the rain. It’s not like 30 percent necessarily means light rain and 100 percent means heavy rain.”
There are several types of forecast models that meteorologists use to predict the weather, including the Global Forecast System (GFS), the European Center for Medium-Range Weather Forecasts (ECMWF), and more regional models, including the North American Mesoscale Models (NAM). They all use satellite data and mathematical formulas to track weather fronts and predict when and where precipitation might occur. Seeing a 30% chance of precipitation in your weather app means that if meteorologists run the same weather forecast model 10 times with slightly different variables, precipitation will appear in three of the 10 runs of the model.
“You also have to remember that if the message says there is a 30 percent chance of rain, that means there is a 70 percent chance of no rain,” says Arnold.
The chance of precipitation also cannot predict how long rain will last, how much it will fall, or how intense it will be. For example, the forecast might say that there is a 100% chance of precipitation, but that might mean that it will be 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 no indication of how long the rain will last, or how severe it will be,” says Arnold.
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Does the type of precipitation impact the forecast?
Generally speaking, all forms of precipitation start as snow, because our atmosphere is much colder than the ground.
“It doesn’t matter if it’s summer in New York [rain] begin [as] snow,” says Arnold.
No matter how high or low a cloud is, clouds are always colder than the ground temperature. “If your cloud base is at 10,000 feet, the temperature will drop 50 degrees. So if it’s 80 [degrees Fahrenheit] down here, it’s 30 o’clock up there.
These 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 down here, we know there’s going to be snow,” Arnold says. “Where things get really interesting is in that middle zone when you look at the sleet and freezing rain. »
Sleet occurs when snowflakes partially melt as they fall through a shallow layer of warm air. These melting snowdrops refreeze as they pass through a deep layer of frigid air above the surface and will eventually reach the ground as bouncing frozen raindrops.
In comparison, freezing rain does not fall as icy pellets and freezes when it hits a surface. It starts out as snow, but when the drop of water falls through a warmer, shallower 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, colder air pocket and the ground temperature is below freezing. The result is a layer of ice dangerous to drivers, pedestrians and snow shovelers.
Predicting these frigid weather events “is incredibly difficult and incredibly nuanced, especially freezing rain,” Arnold says.
With all this interplay between warm air, cold air, deeper air pockets, and shallow air pockets, freezing rain forecasts can change on a dime.
“If any of these variables move slightly by a few degrees, you’re in a completely different situation. It’s just a very volatile forecast,” says Arnold.
Trying to predict freezing rain is especially important because this type of precipitation can be very dangerous, bringing down tree branches, power lines, and increasing car accidents. Even 0.01 inch of freezing rain and ice is enough to make walking and driving dangerous.
“Sleet isn’t so bad because it’s not as catastrophic, and all it takes is a deeper pool of cold air at the surface to allow that drop to turn into a snowflake, melt into a drop of water, and then freeze again,” Arnold says. “If it gets very cold, it will freeze again and fall with a little ice pellet.”
When are weather forecasts most accurate?
While we’d all like to know exactly what the weather will be like on our beach vacation, those 10-day forecasts aren’t always the best to follow. The closer you get to the deadline, the better the information.
Arnold explains that one way to understand the forecast is to imagine that you are driving on a long dirt road that goes on for miles. You then notice a swirl of dust, so you know there is something on the road, but you don’t know if it’s a car, a truck, a horse, or something else.
“As you get closer, the image becomes clearer and clearer. You see that it’s blue, which you didn’t know before,” says Arnold. “As you get closer, you can see if it’s a car or a truck and possibly see the make and model of the vehicle.”
Unfortunately, there is no perfect threshold for knowing when forecasts will be most accurate. The best advice is that the closer we get, the better we know what the weather will be like.
What meteorologists know more in advance are temperature patterns. “There are very good forecasts in one to two weeks that help us understand whether temperatures are going to be above or below normal,” Arnold says. “We are very good at this.”
And remember, meteorology is very difficult. It combines very high level math and physics that most of us can’t even calculate with constantly changing variables. So please be kind to your local meteorologist.
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