Precipitation probability
Precipitation & snow probability of precipitation precipitation chance PoP
Probability of precipitation is the chance of precipitation at a particular place during a particular period. It does not describe how long or how heavily it will rain.
What 40 per cent actually means
With a 40 per cent forecast, precipitation would be expected at the stated location in roughly four out of ten comparable forecast situations. It refers to an amount at or above the provider’s specified threshold. Snow can count too.
What it does not mean
- It does not necessarily rain for 40 per cent of the day.
- It does not necessarily rain over 40 per cent of the city.
- The amount is not 40 per cent of some normal rainfall total.
A dry outcome does not by itself disprove a 40 per cent forecast. Dry weather belongs to the other six cases out of ten. One day cannot show whether a set of probability forecasts is well calibrated overall.
Place and time are part of the forecast
Forty per cent for a whole day tells you less about the trip home at 16:00 than an hourly forecast. Do not simply add hourly probabilities: the events may be related.
What else your umbrella decision needs
Probability, amount and intensity answer different questions. A short downpour and prolonged drizzle can have the same probability. For outdoor plans, also consider the timing and the radar.