An isolated thunderstorm associated with Hurricane Gustav after it made landfall in Louisiana in September 2008. Image Credit: Andrew Miller via Flickr

Checking the weather forecast can sometimes feel like you’re reading a statistics textbook. When it comes to the amount of clouds in the sky or the chance of rain or snow you’ll see that day, meteorologists convey that information to you in the form of percentages. Dealing with endless numbers makes forecasts more confusing than they have to be, so some weather forecasts will use terms like “scattered” and “isolated” to easily convey weather information in a way that makes sense.

Say that there’s a 20 percent chance of thunderstorms this afternoon. This is called the probability of precipitation, and the way they come up with it can get pretty confusing. That 20 percent chance doesn’t mean that rain will cover 20 percent of the area or that it will rain 20 percent of the day—it just means that there’s a 20 percent chance that rain will fall somewhere in the area. If the forecast is accurate, you’re more likely than not to escape any thunderstorms that day, but there will be a few towns that get a good soaking. If it rains on you, you were just in the lucky spot that happened to see a thunderstorm that afternoon. We’re using thunderstorms in this example, but it applies to all types of precipitation, whether it’s light rain showers, heavy thunderstorms, or a frozen mess like snow or sleet in the dead of winter.

Partly cloudy skies over the southeastern United States on August 11, 2016, with isolated thunderstorms over northern Georgia and scattered showers over eastern South Carolina. Image Credit: NASA/NOAA

Rather than throwing a bunch of numbers at you, some forecasts will opt to say there’s an “isolated chance” of thunderstorms that day. These terms are linked to the probability of precipitation. The National Weather Service will call for an isolated chance of storms if there’s a 10 to 20 percent chance of thunderstorms that day, meaning any storms that do form will likely be few and far between. A 30 to 50 percent chance of precipitation is called a chance for scattered storms—you’ve likely experienced this if it’s storming at work but not at home (or vice versa). Once you get above 50 percent, the odds that you’ll see rain that day are decent, so the forecast will say “likely rain/snow” or just call for rain or snow without qualifying the statement.

Another portion of forecasts that can get confusing is the difference between a day with partly cloudy skies and a day with mostly sunny skies. The terms they use to talk about clouds and sun are similarly linked to percentages, but this time it’s the percentage of the sky that’s expected to be covered by clouds. If your location has 50 percent cloud coverage, there’s just as much clear sky as there are clouds at your location.

When writing a forecast, the National Weather Service (and most other meteorologists) will call it “mostly sunny” when the sky is just 10 to 30 percent covered with clouds. The term “partly cloudy” refers to a sky that’s just under half-covered by clouds, and “partly sunny” refers to a sky that’s just over half-covered with clouds. A sky that’s more than three-quarters obscured is considered mostly cloudy. Much like a high chance for rain or snow, they’ll just call the day “cloudy” without qualifiers if the sky is more than 90 percent obscured.

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