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What Do 'Chance of Rain' Percentages Mean in Weather Forecasts?

"The probability of precipitation forecast is one of the most least understood elements of the weather forecast," National Weather Service says.

Published Aug 8, 2023

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For years, social media users have been lecturing one another about what percentages mean in weather forecasts. "Y'all know on the weather app the percentage isn't a "chance" of rain. It means it will rain in 50% of the city," one social media user claimed.

(Twitter user @glassofchas)

Numerous videos have gone viral on TikTok on that topic, claiming that the percentages in weather forecasts do not mean the probability of precipitation, but rather how much of the area would be affected by rain.

One viral video claimed that the 30% in weather app means that there is a 100% chance of rain in 30% of the area. A tweet with a similar tone read:

Why am I 30 years old and I just now learned that the percentage for the precipitation on a weather forecast is not the CHANCE of it raining/snowing but the percentage of the AREA actually getting rain/snow/etc? Like 50% of my town will get snow, not 50% on whether it happens

"I just found out that we've all been reading the weather app entirely wrong," another viral video lamented, summarizing all the misconceptions on the subject.

On the other hand, a meteorologist named Mike Collier claimed in a TikTok video that the percentages in weather apps simply mean the chance that rain will occur.

The National Weather Service (NWS) agrees. Contrary to the claims of social media users, the "chance of rain" expressed as a percentage in a weather forecast refers to the likelihood of precipitation occurring in a particular location over a certain period of time. 

According to the NWS, the probability of precipitation (PoP) is one of the most misunderstood elements of weather forecasts. The PoP, often expressed as the "chance of rain" or "chance of precipitation," is defined as:

[...] the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01 inches. Unless specified otherwise, the time period is normally 12 hours. 

In simple words, the PoP describes how likely it is for a measurable amount of rain to occur in any given point in the forecast area. It does not indicate the percentage of an area to be affected by rain, as some social media users have claimed. The NWS article continued:

Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be
covered by precipitation at given time in the given forecast area or (2) that you will be seeing
precipitation 40% of the time in the given forecast area for the given forecast time period. 

And again:

If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time. 

Sources

Joslyn, Susan, et al. "Probability of Precipitation: Assessment and Enhancement of End-User Understanding." Bulletin of the American Meteorological Society, vol. 90, no. 2, Feb. 2009, pp. 185–94. journals.ametsoc.orghttps://doi.org/10.1175/2008BAMS2509.1.

Stewart, Alan E., et al. "Through the Eyes of the Experts: Meteorologists' Perceptions of the Probability of Precipitation." Weather and Forecasting, vol. 31, no. 1, Feb. 2016, pp. 5–17. journals.ametsoc.orghttps://doi.org/10.1175/WAF-D-15-0058.1.

US Department of Commerce, NOAA. About the NWShttps://www.weather.gov/about/. Accessed 8 Aug. 2023. 

FAQ - What Is the Meaning of PoPhttps://www.weather.gov/ffc/pop. Accessed 8 Aug. 2023. 

Forecast Termshttps://www.weather.gov/bgm/forecast_terms. Accessed 8 Aug. 2023. 

What Does Probability of Precipitation Mean? https://www.weather.gov/lmk/pops. Accessed 8 Aug. 2023.

Aleksandra Wrona is a reporting fellow for Snopes, based in the Warsaw area.

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