A type of mutual fund that has a tendency to perform well during a bull market. In other words, it is a mutual fund that generally outperforms the market when the market is doing well, and underperforms the market when the market is doing poorly.

Fair weather funds are very active during bull markets. For example, mutual funds that focused on technology companies in the early stages of the tech bubble in the 1990s were very successful. To determine whether a prospective mutual fund is a fair weather fund, simply compare the fund's relative returns to the market index during both bear and bull markets.