A weather vane, wind vane, or weathercock is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building.
Although partly functional, weather vanes are generally decorative, often featuring the traditional cockerel design with letters indicating the points of the compass. Other common motifs include ships, arrows and horses. Not all weather vanes have pointers.
The word 'vane' comes from the Old English word fana meaning 'flag'.
The Tower of the Winds on the ancient Greek agora in Athens once bore on its roof a wind vane in the form of a bronze Triton holding a rod in his outstretched hand, rotating as the wind changed direction. Below this was a frieze adorned with the eight Greek wind deities. The eight-metre-high structure also featured sundials, and a water clock inside. It dates from around 50 BCE. 
Pope Gregory I said that the cock (rooster) "was the most suitable emblem of Christianity", being "the emblem of St Peter", a reference to Luke 22:34 in which Jesus predcits that Peter will deny him three times before the rooster crows. 
As a result of this, the cock gradually began to be used as a weather vane on church steeples, and in the 9th century Pope Nicholas I ordered the figure to be placed on every church steeple. 
One alternative theory about the origin of weathercocks on church steeples is that it was an emblem of the vigilance of the clergy calling the people to prayer. 
A few churches used weather vanes in the shape of the emblems of their patron saints. The City of London has two surviving examples. The weather vane of St Peter upon Cornhill is not in the shape of a rooster, but a key; while St Lawrence Jewry's weather vane is in the form of a gridiron.
Early weather vanes had very ornamental pointers, but modern wind vanes are usually simple arrows that dispense with the directionals because the instrument is connected to a remote reading station. An early example of this was installed in the Royal Navy's Admiralty building in London - the vane on the roof was mechanically linked to a large dial in the boardroom so senior officers were always aware of the wind direction when they met.
Modern aerovanes combine the directional vane with an anemometer (a device for measuring wind speed). Co-locating both instruments allows them to use the same axis (a vertical rod) and provides a coordinated readout.
World's largest weather vane
According to the Guinness World Records, the world's largest weather vane is a Tío Pepe sherry advertisement located in Jerez, Spain. The city of Montague, Michigan also claims to have the largest standard-design weather vane, being a ship and arrow which measures 48 feet tall, with an arrow 26 feet long.
A challenger for the title of world's largest weather vane is located in Whitehorse, Yukon. The weather vane is a retired Douglas DC-3 CF-CPY atop a swiveling support. Located at the Yukon Transportation Museum beside Whitehorse International Airport, the weather vane is used by pilots to determine wind direction, used as a landmark by tourists and enjoyed by locals. The weather vane only requires a 5 knot wind to rotate.
The term "weathervane" is also a slang word for a politician who has frequent changes of opinion. The National Assembly of Quebec has banned use of this slang term as a slur after its use by members of the legislature.
- Windsock, in aviation
- Apparent wind indicator, in sailing
- List of weather instruments
- Old Father Time, a famous weather vane at Lord's Cricket Ground, London
- Weather station
- Examples of weather vanes as sculptures
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Crépeau, Pierre; Portelance, Pauline (1990), Pointing at the wind : the weather-vane collection of the Canadian Museum of Civilization, Hull, Quebec: Canadian Museum of Civilization, p. 84, ISBN 9780660129044
Nesbitt, Ilse Buchert; Nesbitt, Alexander (1970), Weathercocks and weathercreatures; some examples of early American folk art from the collection of the Shelburne Museum, Vermont., Newport, RI: Third & Elm Press, OCLC 155708
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Media related to Weather vane at Wikimedia Commons