What is the point of trying to pin a percentage on the possibility of rain? Why not simply indicate whether it will rain or not? Better yet, the amount of expected rain.
This is what I check, and it turns out it is correct 100% of the time. The rest of you run around in all-weather-gear based on a 30% chance of rain. You do realise this means there is a 70% chance that it will not rain?
We all know predicting the weather is a difficult thing. There is no logical reason for making it even more difficult by trying to attach a percentage to it.
Sometimes the convention is wrong.