Definitions

U.S. National Weather Service

According to the U.S.

National Weather Service The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...

(NWS), PoP is the

probability of exceedance The frequency of exceedance, sometimes called the annual rate of exceedance, is the frequency with which a random process exceeds some critical value. Typically, the critical value is far from the mean. It is usually defined in terms of the number ...

that more than of precipitation will fall in a single spot, averaged over the forecast area. 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. 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 AccuWeather Inc. is an American media company that provides commercial weather forecasting services worldwide. AccuWeather was founded in 1962 by Joel N. Myers, then a Pennsylvania State University graduate student working on a master's degree ...

's definition is based on the probability at the forecast area's official

rain gauge A rain gauge (also known as udometer, pluvia metior, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a predefined area, over a period o ...

. There is also a probability of precipitation for every location in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

for every minute for the next two hours. This is also known as a minute-cast.

The Weather Channel The Weather Channel (TWC) is an American pay television channel owned by Weather Group, LLC, a subsidiary of Allen Media Group. The channel's headquarters are in Atlanta, Georgia. Launched on May 2, 1982, the channel broadcasts weather foreca ...

'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 Wet bias is the phenomenon whereby some weather forecasters report an overestimated and exaggerated probability of precipitation to increase the usefulness and actionability of their forecast. The Weather Channel has been empirically shown, and ha ...

– the probability of precipitation is exaggerated in some cases.

Environment Canada

Environment Canada Environment and Climate Change Canada (ECCC; french: Environnement et Changement climatique Canada),Environment and Climate Change Canada is the applied title under the Federal Identity Program; the legal title is Department of the Environment ...

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 The Meteorological Office, abbreviated as the Met Office, is the United Kingdom's national weather service. It is an executive agency and trading fund of the Department for Business, Energy and Industrial Strategy and is led by CEO Penelop ...

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

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 The Plain English Campaign (PEC) is a commercial editing and training firm based in the United Kingdom. Founded in 1979 by Chrissie Maher, the company is a world leader in plain-language advocacy, working to persuade organisations in the UK and ...

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

{{Authority control Numerical climate and weather models Climate and weather statistics Weather forecasting