A weathervane, also called a windvane, 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.
The weathervane 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 weathervane to rotate to minimize the force of the wind on its surface. The design of the weathervane 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 weathervanes have directional markers beneath the pointer, aligned with the geographic directions. The pointer must be able to move freely on its axis.
Weathervanes, 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 weathervane: an unequal surface area but balanced in weight.
To obtain an accurate reading, the weathervanes 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.
From : Wikipedia
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