That would be the way to do it if the probability of rain in each hour were independent of the probability in every other hour. But in real life that's definitely untrue. For example, suppose there were a 25% chance of a storm hitting your area in one day, and if it did hit, it would rain all day long. Then the reported probability of rain might be about 25% in each hour, but that certainly doesn't mean that the chance of rain any time in the day is 1.75^{24} = 99.9%.
