Weather vane

A weather vane is a device with a freely rotating pointer used to indicate the direction of the wind. Weather vanes served as simple devices for showing the direction of the wind and its speed for centuries. They were a critical tool for agriculture, travel and shipping. They serve a largely decorative function today. They have been replaced by specialized weather instruments. 
To work correctly a weather vane should be at the highest point of a building. It needs to be as far away as possible from other things that might interrupt the wind. The simplest form is a horizontal arrow or other form freely rotating on a vertical rod. When the wind blows, the arrow shows the direction and speed. The earliest known weather was made by the astronomer Andronicus in 48 BC. It sat on top of the Tower of the Winds in Athens. It was between and long and was the shape of a head and torso of a man with the tail of a fish.
The word 'vane' comes from the Old English "fana banner". This is related to the Old High German "fano" (cloth); from the Latin "pannus" (cloth or rag).