A "30% chance of rain" means there is a 30% probability that measurable precipitation (usually at least 0.01 inches) will occur at any point in the forecast area during the specified time period. In other words, there is about a one in three chance that rain will fall somewhere within the area covered by the forecast
. This does not mean that it will rain for 30% of the time, nor that 30% of the area will definitely get rain. Instead, meteorologists calculate this percentage as a product of their confidence that rain will occur and the portion of the area expected to receive rain. For example, if forecasters are 50% confident that rain will cover 60% of the area, the chance of rain would be 30% (0.5 x 0.6 = 0.3)
. The "30% chance" can also be interpreted as meaning that in 30 out of 100 similar weather situations, measurable rain has occurred historically
. In summary:
- It is the probability that measurable rain will occur somewhere in the forecast area during the time period.
- It reflects both the forecaster's confidence and the expected coverage of rain.
- It does not guarantee rain will fall on 30% of the area or for 30% of the time.
- It means there is about a 30% chance that any given location within the forecast area will experience rain
This explanation aligns with the official definitions used by the U.S. National Weather Service and meteorologists today.
