windsock

A truncated cone of fabric, open at both ends, with a metal ring at its wider base attached by ropes or cables to the top of a mast. The windsock can rotate freely in the horizontal plane, driven by the wind. It carries several red and white stripes, arranged alternately. Depending on the wind's intensity, the portion of fabric that reaches the horizontal is greater or smaller, and the divisions between the stripes serve as a visual reference for estimating wind speed. When the stripe closest to the mast is horizontal and the rest are angled downward, a wind of 3 knots (5.5 km/h) is blowing. As the wind strengthens, the windsock gradually straightens until it becomes fully horizontal, which happens when the wind reaches 15 knots (28 km/h). This device is common at airfields and airports, next to the runways, and is also placed along stretches of motorways and highways, such as viaducts and narrow mountain passes, where the wind is channelled and tends to blow strongly.