[1] Probabilities are often calculated by ensemble forecasting and represents the number of simulations that show rain occurred.
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, with the effect that a one-hour PoP for the same region and period would be similar: about 1 in 5.
[3] 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.
[8] 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.
[11] The UK's Met Office replaced descriptive terms, such as "likely", with percentage chance of precipitation in November 2011.
[14][15] The Met Office explained that the proposed alternative, "chance of rain", would not describe all the forms of precipitation included in the forecast.