A person who is not a dedicated fan of a sports team. They only watch a game if it peaks their interest.
In most cases fair weather fans are bandwagoners who join in on the hype of a team or sporting event. Examples of this are: The Superbowl, The World Series, and Playoffs in general.
In other cases fair weather fans are people who just watch the sport or certain games when 'the weather is right'(when they feel like it). These fans are not characterized as bandwagoners because they have been fans of the sport/team before the hype and or big game (ex. Superbowl).
A Fair weather fan will usually tune in to watch big games such as: The Superbowl, The World Series, and Playoff games etc.
This is an example of a fair weather fan that is also a bandwagoner:
My brother: The World Series on tonight! I can't wait!
Me: Since when did you like baseball?
My brother: I don't, but its the World Series!
This is an example of a fair weather fan:
My friend: Celtics are on tonight.
Me: Since when do you watch basketball?
My friend: I watch it occasionally, plus its the playoffs.