weather vane; weathercock
A device, often shaped like a rooster, that shows wind direction. The literal, physical object.
A weather vane stands on the roof.
fence-sitter; opportunist; turncoat
Figurative use for a person who changes their opinion or allegiance according to the prevailing situation, like a weather vane turning with the wind.
He's a fence-sitter, so he quickly agrees with whichever opinion is popular.