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. 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. 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 0.7 × 0.7 × 0.7 {displaystyle 0.7 imes 0.7 imes 0.7} , which is approximately 0.34. The probability of there being a thunderstorm is 1 − 0.34 = 0.66. {displaystyle 1-0.34=0.66.} 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. 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%. 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.