Advisory U S Department of Transportation Circular

    The taper or the fabric windsock from the throat to the trailing end must be designed to cause the windsock to fully extend when exposed to a wind of 15 knots (28 km/hr or 17 mph). 3.2.3 Fabric. Fabric for the windsock may be made of cotton, a synthetic material, or a blend of the two, and may be coated.

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