A weathervane, 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 weathervanes are in the shape of cockerels and are called weather cocks. Arrows are also popular, but a multitude of designs of weathervanes 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 areas exposed to the wind are unequal. This unequal area 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 area to turn into the wind, pointing to the source of the wind. Because winds are named from their source direction, the pointer of the weathervane 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.
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 weathervane: an unequal surface area but balanced in weight.