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. The weather vane 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 cocks, 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.
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