You look at your forecast. 30% chance of rain, that’s a bummer(unless you’re one of the rare people who like the rain). But what does this number really mean?
Many assume this number means that 30% of the given town would rain or it will rain for 30% of the day, but what it actually means is far more interesting and uses a clever calculation not many are aware of. When the forecast shows 30% it means that there is a 30% probability(I love that word) that at least 0.01 inches of rain will hit any given surface in the town over a certain time period(usually one,twelve,or twenty four hours). So now that we know what a 30% chance of rain means, how is it calculated?
The National Weather Service says the probability is calculated like this, where P=probability of rain, C=confidence of prediction, and A=percent area of town expected to receive rain:
P=C*A
I know its pretty surprising how simple that is, you were probably expecting some crazy formula, or applying Bayes theorem and finding the expected value, but no its just multiplying two values!
So for example, if P=30%(I don’t know why I keep using that number), you could have the following extremes:
1. You have small storms incoming on only a fraction of the area but the meteorologist is very confident that this event will occur
2. You may get a large storm flooding your entire town but the meteorologist is not super confident this will occur
These both will show the same number on your weather app but they are two very different types of storms and occurences.
So next time you look through the rain probability always think what kind of confidence or area of rainstorm you could be encountering!
DON'T BLAME THE MATH 