Ask five people what "40% chance of rain" means and you'll get five answers: it'll rain 40% of the day, it'll rain over 40% of the area, 40% of forecasters think it'll rain. All wrong — and the real answer changes how you should plan.
The actual definition
The National Weather Service calls this number the Probability of Precipitation (PoP), and it means something precise: the chance that measurable rain (at least 0.01 inches) will fall at any given point in the forecast area during the forecast period. A 40% chance means that under these exact atmospheric conditions, a spot like yours gets wet 4 times out of 10. Six times out of 10, it stays completely dry.
Why hourly beats daily
Here's the planning breakthrough most people miss: a day with "60% chance of rain" often contains hours with a 10% chance and hours with an 80% chance. A daily percentage smears the whole day together. Summer thunderstorms in particular are creatures of the afternoon — a 60% storm day frequently offers a bone-dry morning. That's the entire reason Weather or Not checks your specific hours instead of handing you a day-long number: the question isn't whether it will rain today, it's whether it will rain on your 10am–1pm plans.
A practical decision scale
After enough hours staring at forecast data, most planners converge on rules like these. Under 20% during your window: proceed, don't think about it again. 20–35%: proceed, but know where the indoor fallback is. 40–55%: genuine coin flip — have a real plan B and check again a few hours out, because the number will sharpen as the event approaches. 60% and above: plan as if rain is coming and be pleasantly surprised otherwise. And any active thunderstorm or winter weather warning overrides every percentage — warnings mean the hazard is occurring or imminent, not probable.
Why the number changes as the day approaches
A forecast made five days out leans on large-scale computer models; the closer you get, the more the forecast incorporates real observed conditions — and the sharper it gets. This is why the smart pattern for any event is checking three times: a week out for the general shape, the day before for the go/no-go lean, and the morning of for the final hourly timing. Each check is genuinely more accurate than the last, and the last one is the only one that can see individual storm timing.