What in the world does it mean when one of us weather people tells you there is a 20% chance of rain in Teton Valley today? It probably means 2 out of ten of you will think we got it wrong because we said there was only a slight 20% chance and you got soaked anyway. The other 8 out of 10 will be sure we got it wrong because we said there was a 20% chance and you didn’t see any rain at all. Some say a 20% chance of rain means a 100% chance of confusion, and rightfully so. Unless you’re a professional gambler, a mathematician, a meteorologist, or some other shady profession, you might not have much use for percentages and chance.
Let’s start with what a 20% chance of rain doesn’t mean. It doesn’t mean a dang thing without including an area you’re talking about (Teton Valley, for example) and a time frame (today, for example). Sometimes those parts are left out or just assumed that you understand, but without a clear understanding of a place and a time, a percent chance of rain is just some numbers.
Those two factors, the time frame and the area, are both key to understanding what meteorologists mean by the percent chance of rain. A 20% chance of rain today in Teton Valley doesn’t mean it will be raining on you 20% of the time today; 20% of 24 hours would mean almost 5 hours of rain! Neither does it mean that it will rain all day on 20% of Teton Valley. What it does mean is a combination of both of those things, that there is a 20% chance that someone somewhere in the valley sometime today will see a measurable amount of rain. We could also say there’s an 80% chance that nobody anywhere in the valley will see rain at anytime today. That would probably be your glass half full meteorologist’s way of saying it.
If we were 100% sure that there was going to be rain somewhere in Teton Valley today, but we didn’t think it would cover the whole valley, just half of it, we’d say there’s a 50% chance of rain today. If we were only 80% certain that there was going to be rain, and if it happened we thought it would only cover a fourth of the valley, we’d say there’s a 20% chance of rain. So the chance is really a combination of both our confidence in the forecast and the amount of acreage we expect it to cover. A mathematician might say it as an equation, Chance = Confidence x Coverage. A gambler might say to roll the dice twice, once for the rain and once for how much it will cover. A meteorologist might just say there’s a slight chance of showers today, because to a meteorologist, a 20% chance is a slight chance. (Not so to a poisonous snake handler.)
So, when rain is falling on your head, there is a 100% chance of rain for you. But if it’s only falling on 20% of the valley and you just happen to be in the lucky 20% of us getting it, the chance of rain for the valley is still just 20%. That gives your genial Teton Valley weatherman plenty of wiggle room to claim a correct forecast even if you’re dressed in a dry slicker or a soaked Hawaiian flowery shirt because of it. Please don’t shoot the weatherman.