Although a weathervane does not show you the precise weather, you can interpret the weather from the direction of the wind. In the Northern Hemisphere wind coming from the North usually brings colder...
It tells the direction of the wind in a prticular area, and so helps predict which weather is moving where.
A weather vane is a flat piece of metal, wood, etc. that can rotate in the wind to show from which way the wind blows.
A weather vane is an instrument for measuring wind direction
The weather vane indicated the direction of the wind was from the North.