A weather vane
, also called a wind vane
, is a movable device attached to an elevated object such as a roof for showing the direction of the wind. Very often these are in the shape of cockerels
and are called weather cocks
. Arrows are also popular, but a multitude of designs have been used.
Weather Vanes must be balanced so that half its weight is on either side of its axis, but also designed so that the momenta about the axis of the areas exposed to the wind are unequal
. This unequal momentum causes the vane to rotate to minimize the force of the wind on its surface. The design of the vane causes the end with the smallest momentum to turn into the wind, pointing to the source of the wind. Because winds are named from their source direction, the pointer enables the viewer to name the wind easily. Most simple weather vanes have directional markers beneath the pointer, aligned with the geographic directions. The pointer must be able to move freely on its axis.
Weather vanes, especially those with fanciful shapes, do not always show the real direction of a very gentle wind. This is because the figures do not achieve the design balance required in a weather vane: an unequal surface area but balanced in weight.
To obtain an accurate reading, the weather vane must be located well above the ground and away from buildings, trees, and other objects which interfere with the true wind direction. Changing wind direction can be meaningful when coordinated with other apparent sky conditions, enabling the user to make simple short range forecasts.
The ancient Tower of the Winds on the Roman agora in Athens featured atop a wind vane in the form of a bronze Triton holding a rod in his outstretched hand rotating to the wind blowing. Below, its frieze was adorned with the eight wind deities. The 8 m high structure also featured sundials and a water clock inside dates from around 50 BC.
The wind vane evolved from Triton to a weathercock as the Roman Empire converted to Christianity. Most churches have a weathercock on the tower or spire. The cock refers to the fall of St Peter and to intimate the necessity for watchfulness and humility. From the street level the size of many weathercocks is deceptive.
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. Also modern wind vanes are mounted with an anemometer, a device that measures wind speed. Co-locating both instruments allows them to use the same axis (a vertical rod). Weather stations of variable quality may be purchased, and these include wind vanes along with several other instruments with dials that can be read comfortably inside a home or office. Combining a propeller for wind speed and a tail for wind direction on the same axis is an aerovane, for accurate, precise measurements from a single instrument.
Another wind direction device is the windsock used at airports to show wind direction and strength. The wind fills the sock and makes it blow away from the prevailing wind. Strong winds make the sock point almost horizontally, while light airs allow the sock to hang limply. Because of its size, the windsock can often be seen from the air as well as the ground. Even the most technologically-advanced airports still use windsocks.
According to the Guinness World Records, the world's largest weather vane is located in Jerez, Spain. 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 atop a swiveling support. Located beside Whitehorse International Airport, the weather vane is used mainly by pilots to determine wind direction. The weather vane only requires a 5 km/hour 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.
The word 'vane' comes from the Anglo-Saxon word 'fane' meaning 'flag'.Originally the people used flags to show the direction of the wind.
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- American Folk Art from the Shelburne Museum in Vermont (Catalog of the) Albright-knox Art Gallery,. Buffalo, NY, 1965 pp.20, 23-28
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- Reaveley, Mabel E., Weather Vane Secrets, Westford, MA. 1984
- Whirligigs & Weather Vanes: Contemporary Sculpture Whirligigs & Weather Vanes: Contemporary Sculpture. Eugene OR: Visual Arts Resources 1994