70 degrees with a chance of......

So the last few days I have been a little under the weather. And after having a couple people say that to me and using the phrase myself a couple of times I have wondered what it means? Well it means you are sick but how did it come to mean that? Technically we are always under the weather. Rain falls, snow falls, the sun beats down, all hail breaks loose and well then falls down. So I'm thinking we need to tweak the saying just a little bit, not much. Example: person 1 "hey how are you today?" person 2" hmm not that well, I AM the weather." person 1 "oh....okay." Person 1 would then say to person 3 "hey did you hear that, that guy over there thinks he IS the weather! is that guy sick or what?" Person 3 would have no choice but to agree seeing as anyone who would think that they are the weather would have to be considered sick. So this would get the point across.