What a forty per cent chance of rain actually means
Almost nobody reads probability of precipitation the way it is meant. The number is not how much rain, and it is not how long.
What most people assume
That forty per cent means light rain, or rain for part of the day.
What the number is
It is the probability that measurable rain falls at a given point in the forecast area during the period. Forty per cent means: in four out of ten runs of this situation, the spot you are standing on gets wet.
It says nothing about how hard, and nothing about for how long. A forty per cent day can be a five-minute shower or an afternoon of it.
Why it feels wrong so often
Because people grade it as a yes or a no. Stay dry on a forty per cent day and the forecast 'was wrong'; get soaked and it 'was right'. Neither is true — the number was a bet, and bets lose six times in ten.
Over a season a well-calibrated forty per cent rains about forty per cent of the time, which is exactly the point and is invisible from any single day.
What it looks like
What to do
- Read it as odds, not as intensity.
- Under about thirty per cent, plan normally and carry a contingency.
- Above about sixty, plan for rain and be pleasantly surprised.
- Between the two, look at the hours rather than the day — that is where the answer usually is.