CHANCE OF RAIN - Making Sense of Precipitation Forecasts - Chance of rain — also known as the chance of precipitation and probability of precipitation (PoPs) — tells you the likelihood (expressed as a percentage) that a location within your forecast area will see measurable precipitation (at least 0.01 inch) during a specified time period. The correct interpretation would be: there is a 30 percent chance that 0.01 inch (or more) of rain will fall somewhere (at any one or multiple locations) within the forecast area.

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Chance of Rain

Making Sense of Precipitation Forecasts

by Tiffany Means

What's the chance of rain today?

It's a very simple question. And while its answer seems equally as simple, most of us misunderstand it without even realizing we do.

What "Chance of Rain" Does (and Doesn't) Mean

Chance of rain — also known as the chance of precipitation and probability of precipitation (PoPs) — tells you the likelihood (expressed as a percentage) that a location within your forecast area will see measurable precipitation (at least 0.01 inch) during a specified time period.

Let's say tomorrow's forecast says your city has a 30% chance of precipitation.

This does not mean...

·       There's a 30% chance it will rain and a 70% chance it won't

·       Three out of 10 times when the weather is similar, it will rain 

·       Precipitation will fall 30% of the day (or night)

·       Thirty percent of the forecast area will experience rain, snow, or storms

Rather, the correct interpretation would be: there is a 30 percent chance that 0.01 inch (or more) of rain will fall somewhere (at any one or multiple locations) within the forecast area.

PoP Adjectives

Sometimes a forecast won't mention the percent chance of precipitation outright, but instead, will use descriptive words to suggest it.

Whenever you see or hear them, here's how to know what percent that is:

Forecast Terminology	PoP	Precipitation's Areal Coverage
--	Less than 20%	Drizzle, sprinkle (flurries)
Slight chance	20%	Isolated
Chance	30-50%	Scattered
Likely	60-70%	Numerous

Notice that no descriptive words are listed for probabilities of precipitation of 80, 90, or 100 percent. 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.

You may also see the type of precipitation punctuated with a period — Rain. Snow. Showers and thunderstorms.

If we apply these expressions to our example of a 30% chance of rain, the forecast could read in any of the following ways (they all mean the same thing!):

A 30 percent chance of showers = A chance of showers = Scattered showers.

How Much Rain Will Accumulate?

Not only will your forecast tell you how likely your city is to see rain and how much of your city it will cover, it'll also let you know the volume of rain that will fall.

This intensity is indicated by the following terms:

Terminology	Rainfall Rate
Very light	< 0.01 inch per hour
Light	0.01 to 0.1 inch per hour
Moderate	0.1 to 0.3 inches per hour
Heavy	>0.3 inches per hour

How Long Will the Rain Last?

Most rain forecasts will specify a period of time when the rain can be expected (after 1 p.m., before 10 p.m., etc.).

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 daytime forecast (that is, This afternoon, Monday, etc.), look for it to occur sometime from 6 a.m. to 6 p.m. local time.

If it's included in your overnight forecast (Tonight, Monday Night, etc.), then expect it between 6 p.m. to 6 a.m. local time.

DIY Chance of Rain Forecast

Meteorologists arrive at precipitation forecasts by considering two things:

(1) how confident they are that precipitation will fall somewhere within the forecast area, and

(2) how much of the area will get measurable (at least 0.01 inch) rain or snow.

This relationship is expressed by the simple formula:

Chance of rain = Confidence x Areal coverage

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 are always rounded to increments of 10%.

The UK's Met Office rounds theirs to 5%.

Tiffany Means is a meteorologist, science writer, and avid cloud watcher/photographer.

Email Tiffany

Experience

Tiffany has been finding beauty skyward and sharing it with others since the age of 5. By twelve, she knew she wanted to pursue weather professionally—thanks in part to the release of the blockbuster film Twister. Since those days, Tiffany has interned with the domestic and international weather departments at CNN, written monthly climate reports for NOAA’s National Centers for Environmental Prediction, and participated in a number of science outreach events (such as the Science Olympiad Competition). She has personally experienced such weather greats as the Blizzard of 1993, and the floods of Hurricane Francis (2004) and Ivan (2004).

Education

Tiffany holds a bachelor’s degree in Atmospheric Science with a concentration in weather forecasting from the University of North Carolina at Asheville.

Tiffany is a proud member of the American Meteorological Society (AMS).

Tiffany Means

"Weather affects us all. We check it on a daily basis, and talk about it with complete strangers...but it is so much more than 5-day forecasts and small talk! Through my enthusiasm for and expertise in the weather sciences, I hope to spark your curiosity about our atmosphere, create an awareness that will keep you weather ready and safe, and strengthen your environmental responsibility to our atmosphere, water, and earth."

Contact Tiffany: Tiffany can be reached at aboutweatherexpert@gmail.com with questions, comments, reprint requests, or suggestions. You can also connect with her via the social links below.https://www.thoughtco.com/chance-of-rain-3444366