If you’ve ever gotten the impression that the percent chance of precipitation your weather man calls out in the nightly news might have been mostly crap, you would be partly wrong. Weather Wonk Ben Ayres gives us the details:

For all the boaters out there, ever wake up on a beautiful, warm August morning and say to yourself, “today’s a great day to go out on the boat”. You jump out of your bed to check the forecast and to your astonishment see there is an 80% chance of rain for the day. You look back out your window in bewilderment, pondering how forecasters could predict a “likely” chance of rain on such a gorgeous day. Although many may think forecasters are making these percentages up, there is actual science behind the probability of precipitation.

The chance of rain is actually referred to by meteorologists as Probability of Precipitation (POP). POP is defined as the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. Essentially, this means that POP is the percentage chance of a specific location receiving measurable precipitation for a specific time. Measurable precipitation is defined by the National Weather Service as 1/100 of an inch.

So how do forecasters come to a certain percentage of predicted precipitation? There is a fairly easy equation that forecasters abide by to find this. This equation is POP = C x A. “C” is the confidence that precipitation will occur somewhere in the forecast area and “A” is the percent of the area that will receive measurable precipitation. So, if there is full (100% or 1) confidence that there will be rain over 60% (.6) of the forecasted area, there is a 60% chance of rain. Strangely enough, forecasters are not magicians and are not always certain if there will be precipitation. Therefore, sometimes forecasters will only be 60% confident if there will be precipitation over 50% of the forecasted area. In this case forecasters will predict a 30% (.5 x .6 = .3 or 30%) chance of precipitation. Another way to look at POP is looking at days where weather conditions are similar to that specific day and deciphering how often precipitation will occur. For example, if an area has a 30% chance of precipitation that means that 3 out of 10 days where the weather is similar, there will be a measurable amount of precipitation somewhere in the area.

As one might expect, this method is hit or miss depending on location. Often times, people will take precautionary matters when precipitation prediction is fairly high, even though it may not be for their exact spot. For example, if forecasters are 100% certain measurable precipitation is coming but only for 50% of the forecasted area, a 50% forecast for precipitation will be issued. This can cause problems for people when planning activities outdoors, especially in the summer.

Understanding how forecasters predict precipitation is important for figuring out outdoor activities. After all, who likes to be left in the rain?

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