Probability of precipitation
A probability of precipitation (POP), also referred to as chance of precipitation or chance of rain, is a measure of the probability that at least some minimum quantity of precipitation will occur within a specified forecast period and location. It is often published with weather forecasts.
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 inches (0.25 mm) of precipitation will fall in a single spot, averaged over the forecast area. This can be expressed mathematically as
where C is the confidence that any form of precipitation (e.g., snow or rain) will occur somewhere in the forecast area and A is the percent of the area that will receive measurable precipitation, if it occurs at all. For instance, if there is a 100% probability of rain covering one half of a city, and a 0% probability of rain on the other half of the city, the POP for the city would be 50%. A 50% chance of a rainstorm covering the entire city would also lead to a PoP of 50%.
The PoP measure is meaningless unless it is associated with a period of time. NWS forecasts commonly use PoP defined over 12-hour periods (PoP12), though 6-hour periods (PoP6) and other measures are also published. A "daytime" PoP12 means from 6 am to 6 pm.
The NWS also provides hourly forecasts. The hourly PoP can be similar to the daily PoP and vary little, or it can vary dramatically. For some events such as thunderstorms, the hourly probabilities are statistically independent: the probability of a thunderstorm occurring in a given hour is not related to the probability in some other hour. In that case, there is a simple mathematical relationship between probabilities over shorter and longer periods. For example, if the probability of a thunderstorm each hour is 0.3, then the probability of there not being a thunderstorm is 1-0.3 = 0.7. Over three hours, the probability of no thunderstorm is , which is approximately 0.34. The probability of there being a thunderstorm is 
An example of an event where the hourly PoPs are not independent is a hurricane. In that case, there may be a 1 in 5 chance of the hurricane hitting a given stretch of coast, but if it does arrive there will be rain for several hours.
Other US forecasters
AccuWeather's definition is based on the probability at the forecast area's official rain gauge. 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 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 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.
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.
- "Explaining 'Probability of Precipitation'". National Weather Service, Peachtree City, GA Weather Forecast Office. Retrieved 27 November 2018.
- Enyedi, Angie (27 August 2010). "What are PoPs?". National Weather Service, Jacksonville, FL. Archived from the original on 10 June 2015. Retrieved 27 November 2018.
- National Weather Service. "Get your hourly weather forecast from the NWS". Weather.gov. Retrieved 4 December 2018.
- Gorski, Chris (21 August 2014). "How Do Daily Weather Forecasts Relate To Hourly Forecasts? It Depends". Inside Science. Retrieved 4 December 2018.
- Bialik, Carl (2008-12-09). "Deciphering a 20% Chance of Rain". The Wall Street Journal.
- Silver, Nate (2012-09-07). "The Weatherman Is Not a Moron". The New York Times.
- "Weather and Meteorology - Glossary: Chance of Precipitation (COP)". Environment Canada. Retrieved 2015-06-07.
- "Guide to Environment Canada's Public Forecasts: Chance of Precipitation". Environment Canada. Retrieved 2015-06-07.
- "The science of 'probability of precipitation'". Met Office. 2014-08-14.
- "Forecast Terms". National Weather Service, Binghamton, NY Weather Forecast Office. Retrieved 2015-06-07.
- The NWS alone has published other sets of terms, such as those in NOAA Technical Memorandum NWS AR-44 and NWS Instruction 10-503 (p. 18).
- "Chance of a shower? You decide, as Met Office launches new style weather forecasts". The Telegraph. 2011-11-10.
- Joslyn, Susan; Nadav-Greenberg, Limor; Nichols, Rebecca M. (February 2009). "Probability of Precipitation: Assessment and Enhancement of End-User Understanding". Bulletin of the American Meteorological Society. 90 (2): 185–193. Bibcode:2009BAMS...90..185J. doi:10.1175/2008BAMS2509.1.
- "2011 Golden Bull Awards". Plain English Campaign. Retrieved 2015-06-07.
- "Plain English award for Met Office 'gobbledygook'". BBC News. 2011-12-09. Retrieved 2015-06-07.
- "A golden conundrum". Met Office. 2011-12-09.