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Ask the Meteorologist: Chances for Rain

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ASK THE METEOROLOGIST

Explain what it means when there is a 20% or a 50% chance of precipitation.

Mary Ann McCoy, Grants Pass

When meteorologists are calling for a chance of rain, snow or whatever precipitation you’d like to choose …the percentage is referred to as the “probability of precipitation” or POP.

In simple words, is one is calling for a 40% chance of rain, it means that any given point in that region has a 40% chance of seeing rain.

However, to make it more complicated (I know, we always have to make it complicated), there is an equation that we “use” to arrive at this percentage. It can be seen below:

POP= (Confidence) x (Areal Coverage)

The two variables considered are the confidence the meteorologist has in the rain/snow, etc. And also, the areal coverage that they believe the rain/snow, etc. is going to cover. If they are calling for a 40% chance of rain they can arrive at this one of two ways:

40%= (100% Confident) x (40% Coverage)

or

40%= (40% Confident) x (100% Coverage)

Whether they believe the rain will cover only 40% of the area and are 100% confident of this, or they believe the rain will cover 100% of the area and are only 40% confident ..the same POP will be found. It gets even more complicated when percentages such as 30% & 50% are used.

Either way you look at it, when a meteorologist is saying there is a 40% chance of rain …it means that any location in that region has a 40% chance of seeing rain.

Meteorologist Alyssa Caroprese

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