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Chance of Rain

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Definition: A chance of rain or chance of precipitation are terms used to describe the likelihood of a precipitation event. Technically, the percentages refer to the likelihood of an area receiving a measurable amount of precipitation over a particular time period.

There is often great confusion among individuals when a statement is issued by the National Weather Service about a chance of rain. In easy terms, a measurable amount of rain usually refers to 1/100th of an inch (0.01 inches). If an area has a 60% chance of rain, that means that in 6 out of 10 cases where the weather is similar, there will be a measurable amount of precipitation somewhere in the area.

The statements issued by the National Weather Service are known as Probability of Precipitation reports, or PoP reports. An even more advanced precipitation prediction is referred to as a Probabilistic Quantitative Precipitation Forecasts (PQPF). These specialized calculations indicate the likelihood of a precipitation event that is equal to or exceeds a higher rainfall amount.

Studies have indicated that people in other countries are more likely to misunderstand the definition. In some European countries, a 60% chance of rain was even interpreted to mean that it would rain over 60% of the area.

Common Precipitation Terms

  • Slight chance = 20% chance
  • Chance = 30 to 50% chance
  • Likely = 60 to 70% chance
A Probability of Precipitation report is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measurable precipitation.

Sources
Peachtree City NWS Forecast office Explaining "Probability of Precipitation"
PubMed.gov How the Public Understands Probabilistic Weather Forecasts
NOAA.gov Technical Memorandum Probability of Precipitation Forecasts and Verbal Description

Also Known As: Probability of Precipitation (PoP), Percent chance of rain, Probabilistic quantitative precipitation forecasts (PQPF), precipitation forecast, quantitative precipitation forecast (QPF)
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