What does a '30% chance of rain' really mean? How meteorologists determine rain chances

Meteorologists often use an equation
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Posted at 12:03 PM, Jun 23, 2021
and last updated 2021-06-23 15:11:26-04

PHOENIX — When looking at rain chances in the forecast, you likely think it’s the probability that it will rain sometime during that forecast timeframe.

If there’s a 30% chance that it will rain, then there a 70% chance that it won’t rain, right?

Not exactly.

The official definition of the probability of precipitation by the National Weather Service is the chance of precipitation (rain, snow, etc.) occurring at any one spot in the area covered by the forecast.

One simple equation used among meteorologists is P = C x A, or the probability of precipitation equals the meteorologist’s confidence that it will rain, times the percentage of the area that is expected to get rainfall.

For example, if we’re 100% confident that 30% of the Valley will get rainfall, then there’s a 30% chance for rain. If we’re 50% confident that 100% of the Valley will get rain, then there’s a 50% chance of rainfall.

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Even with the official definition, many meteorologists have their own interpretation and method of determining the probability of precipitation.

Basically, if we say there’s a 50% chance of rain, it’s best you take your umbrella with you!