I’ve always thought of a fair weather fan as someone who supported a team only when there successful and then, when they’re not, will not follow them or care about them.
A proper football supporter only wants the best for his club and when he sees that the club make empty promises, and keep repeating the same mistakes every season, they want to see a change.
There is of course a different kind of supporter, they are called Wenger fanatics.
What they do is defend everything he does without question and call anyone else who isn’t a Wenger fanatic, morons, idiots, cry baby’s whingers, spoilt brats, etc.
When their hero eventually leaves the club they go with him.