alt Hacker News> Will it rain over forty percent of the city? Will it rain for forty percent of the day? Or does the speaker just not know and is too shy to say so?
Unrelated, but to answer his question, it's the probability that a random spot in the covered area will get at least a smidge of rain at some point in the covered time period. A 10% chance of rain can mean either that the forecaster is 100% sure that exactly 10% of the area will get rain, or it can mean that the forecaster sees a large rainstorm nearby which would cover the whole area if it arrived, but which only has a 10% chance of arriving. Or anything in between. If there's a rain cloud meandering over the city, covering 1% of the city at a time, and over the whole day it randomly walks over 10% of the city, that's also a 10% chance.
I had wondered about this. What you're saying makes sense, but, living in Portland, I had come up with my own explanation. When I see 10% here, I usually take it to mean something like "it will rain for six minutes every hour", or for 10% of the day.