The Detroit Post
Thursday, 28 October, 2021

How Do Weather Percentages Work

Carole Stephens
• Monday, 07 December, 2020
• 7 min read

Rather, the correct interpretation would be: there is a 30% chance that 0.01 inch (or more) of rain will fall somewhere (at anyone or multiple locations) within the forecast area. Sometimes a forecast won't mention the percent chance of precipitation outright, but instead, will use descriptive words to suggest it.

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Forecast TerminologyPoPPrecipitation's Areal Coverage--Less than 20%Drizzle, sprinkle (flurries)Slight chance20%IsolatedChance30-50%ScatteredLikely60-70%Numerous Notice that no descriptive words are listed for probabilities of precipitation of 80%, 90%, or 100% This is because when the chance of rain is this high, it's basically a given that precipitation will occur. Instead, you'll see words like periods of, occasional, or intermittent used, each conveying that precipitation is promised.

If yours doesn't, pay attention to whether the chance of rain is advertised in your daytime or nighttime forecast. If it's included in your overnight forecast (Tonight, Monday Night, etc.

Where “confidence” and “areal coverage” are both percentages in decimal form (that is 60% = 0.6). In the U.S. and Canada, the chance of precipitation values is always rounded to increments of 10%.

Those two simple words can ruin picnic plans or herald rescue for drought-stricken crops. “Storms and all the other interesting things that Earth’s atmosphere brings us have this big effect on our daily lives in a lot of ways.” But even though we tune in to local news stations or check apps to find out what the weather will bring, we don’t always trust the forecasts.

You’ve probably heard the joke: Meteorology is the only occupation where you can be wrong all the time and still get paid for it. In reality, weather forecasts have improved in leaps and bounds in just the past few decades.

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As air flows from one place to another, it carries its properties with it, changing the temperature, humidity and more. When regions of air with different temperatures and densities meet, the boundary is called a front.

Sometimes these cloudy clashes can cause rain, as the cooling warm air is forced to drop its water. As warm, moist air rises, it also cools, and its water condenses onto airborne particles such as dust.

Once a storm has formed, if there’s nowhere for it to get more moisture from the ground or the air, it will peter out as it lumbers along. Scientific weather observations began in the Renaissance, when barometers and thermometers were invented.

European scientists of old, like Galileo, used these instruments to take the types of measurements that would one day explain weather events. But early forecasts were limited and relied on persistence, or the assumption that a system’s past would dictate its future behavior.

Ground radar, weather balloons, aircraft, satellites, ocean buoys and more can provide three-dimensional observations that a model can use. This allows meteorologists to simulate what the atmosphere is currently doing and predict what will happen in the next few days or, for some models, hours.

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Weather models divide a region, say a single state or even the whole globe, into a set of boxes, or cells. Large boxes mean poor resolution, or the inability to tell what’s happening over small areas, but a broad picture of large-scale weather trends over long timelines.

These models are more expensive in terms of computing power, and only run to the one- or two-day mark to tell people whether it might storm in their local area. That’s why two models might spit out slightly different results, even with exactly the same starting observations.

“For a lot of the routine weather, the forecast models are so good now that there’s really not that much that the human forecasters are going to add,” says Schumacher, who is also an associate professor in the Department of Atmospheric Science at Colorado State University. “A forecaster might tweak what the computer tells you if they know their area really well, and they know that models struggle with a certain kind of weather situation,” says Henson.

According to National Oceanic and Atmospheric Administration, today’s five-day forecast is accurate about 90 percent of the time. “Twenty-four hours before a hurricane strikes land, we’ve already pretty much nailed down where it will go,” says Just.

These stats are more impressive when you consider how many meteorologists have improved the number of days out to which an accurate forecast can be made. You’ve probably heard that a butterfly flapping its wings in Hong Kong might cause the weather to change in New York.

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“ essentially said there’s just no way you can predict weather features beyond that time because those little butterfly wing flaps and countless other little things will add up to so many big changes, and there’s so much uncertainty beyond that range, that it’s just impossible to say anything,” he says. Just, whose work focuses on the theoretical limit of accuracy in weather forecasting, says we’ll never be able to predict thunderstorms more than a couple of hours in advance, regardless of how good observations become.

But we will never be able to measure everything about every point in the atmosphere all the time with ultimate precision, and our models will never be flawless. Understanding how people use forecasts and warnings allows meteorologists to provide information in the most useful way.

So the next time you see a low chance of rain in your forecast, check the full weather report before you leave the umbrella at home. NOAA is working with social scientists to develop forecasts that are more relevant and better targeted.

This is especially important because of how the internet has changed the way people obtain and share information, Kölsch says. For instance, when creating the official forecast, meteorologists account for uncertainties by running a model several times.

For meteorologists, then, the end goal is to make almost every day’s forecast an utterly forgettable one. In many countries, a single public weather service is typically the only source available for forecasts, warnings and alerts.


By contrast, the United States has strong public, private (commercial) and university-based weather observation and forecasting programs. Forecasters at the National Weather Service are all hands on deck during a major storm.

For long-term, large area predictions, the most popular U.S. model is the Global Forecast System, or GFS. The upgrade incorporates a new dynamical core, which is the model’s description of how the atmosphere behaves.

The new system, called GFS-FV3, is better at modeling moisture and clouds, allowing meteorologists to forecast storms with greater accuracy than ever before. Each company fills a different niche, providing different forecasts that focus on, say, surfing conditions, fire conditions or transportation concerns, based on specific observations and models that refine the broad public-sector data.

Explaining “Probability of Precipitation” Forecasts issued by the National Weather Service routinely include a “Pop” (probability of precipitation) statement, which is often expressed as the “chance of rain” or “chance of precipitation”.


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.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS AFTER MIDNIGHT. The “Probability of Precipitation” (Pop) simply describes the probability that the forecast grid/point in question will receive at least 0.01” of rain.

So, in this example, there is a 40 percent probability for at least 0.01” of rain at the specific forecast point of interest! Regardless of what industry you work in, precise and controlled results are a necessity.

On the same note, when relative humidity is lower, we can actually feel cooler than what the temperature really is, because the sweat evaporates, thus providing a cooling effect. By using a calculator, you can determine what the air will actually feel like based upon the temperature and the relative humidity.

It’s important to understand the desired humidity percentage that will work best within your environment. The average person will feel most comfortable when the relative humidity is around 45%.

As this is not always achievable indoors, commercial humidifiers are used as a way of creating the necessary humidity. The internal temperature of the room and the way the HVAC system works will impact the overall level of humidity.


In some industries, it is of the utmost importance that the humidity percentage is maintained at a specific level and is not faltering. This is why it is often important to have a customized solution that will allow for easy management and monitoring.

By learning more about humidity percentage and what it means, you can start to take greater control of your work environment. You may find that by establishing the right percentage, you can eliminate a significant amount of dust and airborne particles that circulate within your environment.

Contact Smart Fog Inc today to learn more about commercial humidification and how to find the right percentage of humidity for your work environment.

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