What does a 40% chance of rain really mean?

A 40% chance of rain means that, on days with a forecast like this one, rain falls at your location about four times in ten. It says nothing about how long it will rain or how heavy it will be.

Three common misreadings

"It will rain for 40% of the day." The percentage says nothing about duration. A 90% chance can mean one short shower that is very likely to happen.

"40% of the area will get wet." The number applies to one point, your location, rather than a share of the map.

"Above 50% means take a coat." It depends on what is at stake. For a walk to the shops, 30% is fine. For a wedding marquee, 30% is a reason to have a plan B.

Why the number and the amount belong together

Two forecasts can show the same percentage and mean very different days:

| Chance | Amount | What it feels like | |---|---|---| | 80% | 0.4mm | Spots of drizzle, barely worth a hood | | 30% | 12mm | Probably dry, but soaking if it hits |

That is why our forecasts show both, and why our summaries count a day as wet using the two together.

Showers versus rain bands

A rain band is a large area of rain moving with a front. Forecasters can usually time it to within a few hours, so the percentage climbs high and the timing is reliable.

Showers pop up on their own. Models can tell that the air is right for them but not which street each one will hit. You get a medium percentage for hours on end, and your neighbour across town might stay dry while you get soaked. On these days, the rain radar is more useful than any forecast, because it shows where the showers actually are and how they are moving.

The short version

Use the percentage to judge how likely rain is, the amount to judge how much it matters, and the radar on the day. For why these numbers get less firm further ahead, see how far ahead you can trust a forecast.