What Does Percentage Chance of Rain Really Mean?

The percentage chance of rain is actually a formula: PoP = C x A, where C is the confidence that precipitation will occur somewhere in the forecast area, and A is the percentage of that area that will receive measurable rain if it occurs. So when a meteorologist says there's a 30% chance of rain, they might be saying they're 100% sure that only 30% of your area will get rain, or that they're only 30% confident it will rain across the whole area. Either way, the number you see is the product of those two values.

Say the weather service says there's a 40% chance of rain today. That doesn't mean it will rain for 40% of the day, or that you have a 40% chance of getting wet if you go outside — it means there's a 40% chance that measurable rain (typically at least 0.01 inches) will fall in any given spot in the forecast area during the forecast period.

If you're in the area that gets hit with a pop-up thunderstorm during a 60% chance of rain forecast, you might say "it definitely rained!" while a friend across town under clear skies scoffs that the forecast was wrong. Actually, the forecast may have been spot on — it rained over 60% of the area, just not all at once. It's also worth knowing that PoP typically refers to a specific time period, like a certain block of hours, not the whole day.

If the percentage is 10% or lower, you're probably safe to leave the umbrella at home. At 20%-40%, you might get a shower, especially in the late afternoon — be prepared, but it's not a guarantee. At 50%-70%, there's a good chance of scattered rain, so consider changing outdoor plans. At 80% or higher, it's likely to rain somewhere nearby, if not right over you — grab the raincoat! These percentages are also more accurate closer to the actual forecast time, since weather predictions keep evolving.