@eric no, no, that is the mistake I made yesterday with a post I quickly redrawn, before someone got to comment on it.
I argued that if during the whole day you had a period with 5%, 10%, 15%, 5% rain, that the chance of being dry the whole day would be 0.95 * 0.90 * 0.85 * 0.95 * 100%. = 69% and thus 31% chance of rain. But that didn’t match the value at all. I soon realized that the hourly chances aren’t independent. There could be a small chance for one rainy cloud to pass, but the exact hour wouldn’t be known. You cannot simply add up and divide.
@mrburns it could be that there is a 10% chance that it will rain sometime during the day, but if the exact hour isn’t know, the chance per hour is very small, and rounded down to 0.