Also called a wind cone, a windsock is a device for measuring wind speed; it takes its name from its roughly sock-like shape. It is primarily used in aviation, where the speed of the air flow over one's control surfaces is more important than ground speed.
A windsock is typically a frustum-shaped sleeve or tapered cylinder of fabric open at both ends. The wide end has two grommets which slide onto a vertical pole. The pole is placed vertically into the ground, and is long enough so that in no wind, the sock's loose end does not touch the ground. There may be a cylindrical stiffener at the wide end which is always horizontal.
A windsock is often calibrated (and the loose end, with the tapered throat, weighted down) so that the local maximum winds make the sock stand horizontally off the pole. In intermediate winds, the sock hangs flaccidly at various angles, indicating a less stiff breeze. A calibration chart can be broken down into many divisions--the windsock is not just a binary device. Because the windsock is free to swing around the pole, it gives azimuth as well as magnitude for the local wind. Several windsocks can be placed at the corners of an airfield to give the observer an idea of the local vector field; this is especially useful for slower-moving aircraft like dirigibles.
FAA approved windsocks are uniformly International Orange or white in color, may be illuminated at night, and come in two sizes. Size 2 is for primary use, and is twelve feet in length with a thirty-six inch mouth; Size 1 is for supplemental use only, and is eight feet in length with an eighteen inch mouth. If either is intended for night use, it must be illuminated from below and also carry a red obstruction light.