Ever felt like you need a PhD in meteorology to decipher the daily weather forecast?
Like that “50% chance of rain” thing.
What does that mean, exactly?
Does it mean it’ll be raining 50% of the time today?
Or that there’s a 50% chance that it’ll rain at some point today?
Meteorologists call this number the probability of precipitation.
According to the National Weather Service, this percentage is supposed to tell you the
chance of precipitation occurring at any one spot in the area covered by the forecast.
They define it with a mathematical formula: the probability of precipitation equals C times A.
C is the meteorologist’s level of confidence that precipitation will occur somewhere in
the forecast area.
And A is percentage of the area that’s expected to get measurable precipitation, which is
defined as 0.25 of a millimeter in the United States.
That’s usually enough for a couple little puddles to form.
If your weather person is 100% confident that 25% of the forecast area is going to get rain,
that’s a 25% chance of rain.
But if they’re 50% confident that 50% of the area is going to get rain, that’s a
25% chance of rain, too.
This also means that the more you move around, the more likely you are to run into that part
of the map that has a chance of getting rain at some point!
If you’re still a little confused, don’t feel bad.
This is the official NWS definition of the probability of precipitation, but even meteorologists
admit that they aren’t consistent about it.
A 2015 survey of 188 meteorologists and broadcasters found that many people calculated probability
of precipitation in slightly different ways.
Different specialists had different ways of turning the outputs of their forecast models
into percentages.
They also varied in how they generalized from the chance of rain in one very specific spot
to a wider area.
And how they generalized a prediction for half a day to a more specific time.
What they all had in common, though, was a lot of confidence that their answer was the
right one.
Bottom line?
Predicting the future is hard.
And the probability of precipitation could be better defined by meteorologists, and better
understood by the public.
But if the forecast says 10% chance of rain, and it rains, your meteorologist wasn’t
wrong.
Don’t call them and complain.
With the current conditions, you just happened to draw the short straw.
Thanks to all of our Patreon patrons who keep these answers coming!
If you want to learn more about weather science, check out our video where I explain why rain
has a smell.