Probability of precipitation

Definitions

U.S. National Weather Service

According to the U.S. National Weather Service (NWS), PoP is the probability of exceedance that more than 0.01 in of precipitation will fall in a single spot, averaged over the forecast area.

The NWS provides hourly forecasts. These forecasts include PoP over a given twelve-hour period (i.e. daytime vs night) which indicate a PoP if the given conditions at the start of the twelve-hour period hold for the next twelve-hours. They also include hour-to-hour forecasts, which is the PoP for that given hour. Because weather events hour to hour may be varyingly independent, hour-to-hour can be very similar to the current or twelve-hour PoP, or it can vary dramatically.

Other US forecasters

AccuWeather's definition is based on the probability at the forecast area's official rain gauge. There is also a probability of precipitation for every location in the United States for every minute for the next two hours. This is also known as a minute-cast. The Weather Channel's definition may include precipitation amounts below 0.01 inch (0.254 mm) and includes the chance of precipitation 3 hours before or after the forecast period. This latter change was described as less objective and more consumer-centric. The Weather Channel has an observed wet bias – the probability of precipitation is exaggerated in some cases.

Environment Canada

Environment Canada reports a chance of precipitation (COP) that is defined as "The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period." The values are rounded to 10% increments, but are never rounded to 50%.

UK Met Office

The UK's Met Office reports a PoP that is rounded to 5% and is based on a minimum threshold of 0.1 mm of precipitation.

Alternative expressions

The probability of precipitation can also be expressed using descriptive terms instead of numerical values. For instance, the NWS might describe a precipitation forecast with terms such as "slight chance" meaning 20% certainty and "scattered" meaning 30–50% areal coverage. The precise meaning of these terms varies.

The UK's Met Office replaced descriptive terms, such as "likely", with percentage chance of precipitation in November 2011.

Public understanding

Probability of precipitation may be widely misunderstood by the general public.

The Plain English Campaign objected to the Met Office's use of the phrase "probability of precipitation" in 2011. The Met Office explained that the proposed alternative, "chance of rain", would not describe all the forms of precipitation included in the forecast.

References

References

1. Enyedi, Angie. (27 August 2010). "What are PoPs?". National Weather Service, Jacksonville, FL.
2. (23 May 2024). "What does a 30% chance of rain mean?".
3. (21 August 2014). "How Do Daily Weather Forecasts Relate To Hourly Forecasts? It Depends.". Inside Science.
4. "Get your hourly weather forecast from the NWS".
5. Bialik, Carl. (2008-12-09). "Deciphering a 20% Chance of Rain". The Wall Street Journal.
6. Silver, Nate. (2012-09-07). "The Weatherman Is Not a Moron". The New York Times.
7. "Weather and Meteorology - Glossary: Chance of Precipitation (COP)". Environment Canada.
8. "Guide to Environment Canada's Public Forecasts: Chance of Precipitation". Environment Canada.
9. (2014-08-14). "The science of 'probability of precipitation'". Met Office.
10. "Forecast Terms". National Weather Service, Binghamton, NY Weather Forecast Office.
11. The NWS alone has published other sets of terms, such as those in [http://pajk.arh.noaa.gov/Articles/articles/survey/poptext.html NOAA Technical Memorandum NWS AR-44] and [http://www.nws.noaa.gov/directives/010/archive/pd01005003b.pdf NWS Instruction 10-503 (p. 18)].
12. (2011-11-10). "Chance of a shower? You decide, as Met Office launches new style weather forecasts". The Telegraph.
13. (February 2009). "Probability of Precipitation: Assessment and Enhancement of End-User Understanding". Bulletin of the American Meteorological Society.
14. "2011 Golden Bull Awards". Plain English Campaign.
15. (2011-12-09). "Plain English award for Met Office 'gobbledygook'". BBC News.
16. (2011-12-09). "A golden conundrum". Met Office.