Probability of precipitation

A probability of precipitation (POP), (also expressed as: "chance of precipitation," "chance of rain") 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.

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:

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.

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.

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.

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Wikipedia