Probability of precipitation

A probability of precipitation (POP) is a description of the likelihood of precipitation that is often published with weather forecasts. Generally it refers to the probability that at least some minimum quantity of precipitation will occur within a specified forecast period and location.

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 inches (0.25 mm) of precipitation will fall in a single spot, averaged over the forecast area.^[1] This can be expressed mathematically:

^[1]

• C = the confidence that precipitation will occur somewhere in the forecast area.
• A = 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 thus usually expresses a combination of degree of confidence and geographic coverage.

Note that 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.^[2]

Explanation

Suppose the forecast were for Maui, Hawaii. One "given" point is your house near the top of Mt. Haleakala, where it rains almost constantly. A forecast of 40% is obviously not accurate for that given point. So assume that Mt. Haleakala is 10% of the area of Maui and that the average chance of rain today for the mountain is 80%. And assume that the average chance of rain for the other 90% of the island is 35%. So for the entire island, the average chance of rain is (0.9 × 0.35) + (0.1 × 0.8) = 0.395 = 40%.

Clearly, Mt. Haleakala pulls up the average for Maui. And clearly, the smaller the area, the more meaningful and accurate "chance of rain" is.

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 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.^[3] The Weather Channel has an observed wet bias – the probability of precipitation is exaggerated in some cases.^[4]

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."^[5] The values are rounded to 10% increments, but are never rounded to 50%.^[6]

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.^[7]

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.^[8] The precise meaning of these terms varies.^[9]

The UK's Met Office replaced descriptive terms, such as "likely", with percentage chance of precipitation in November 2011.^[10]

Public understanding

Probability of precipitation may be widely misunderstood by the general public.^[11]

The Plain English Campaign objected to the Met Office's use of the phrase "probability of precipitation" in 2011.^[12][13] The Met Office explained that the proposed alternative, "chance of rain", would not describe all the forms of precipitation included in the forecast.^[14]

See also

• Quantitative precipitation forecast

References