## Percent chance of rain

*October 15, 2018*

Google's weather service is ambiguously frustrating.

I'm going to be outside tomorrow from 1 pm until 6 pm. What is the chance it rains while I am outside? Google says 20% chance it rains at 1pm, 30% chance it rains at 3pm and 20% chance at at 5 pm. What does that mean for my question?

Do you kind of just average all of those and say there is a 23% chance it rains while I'm outside? Do you say each of those is the probability it rains in the given window, and each window is independent? Then the probability it rains is very very different. The probability it rains in at least one of those windows under this regime is 55%. The assumption that the window probabilities are independent seems definitely wrong, so what are the window-conditional probabilities? Everytime I see these window percentages, this goes through my head.