How to Read the Airport Windsock
The windsock, a timeless and colorful fixture at every airport, offers important information to pilots. It's much safer—and more efficient—for an airplane to take off and land into the wind and to avoid taking off and landing with a tailwind. In addition, all aircraft have a maximum certified crosswind component—a crosswind speed at which flying becomes hazardous. As such, it's important for pilots to have a quick and easy way to determine the wind speed and direction before taking off or landing—like the windsock.
Truth be told, except under the most unusual of circumstances, pilots have much more accurate—not to mention more technologically advanced—ways of finding out the wind speed and direction than looking at the windsock. For instance, air traffic control (ATC) routinely provides that information before clearing an aircraft for takeoff or landing.
Pilots can also call for weather reports through the Pilot's Automatic Telephone Weather Answering Service (PATWAS) or the Telephone Information Briefing Service (TIBS). And many airports constantly broadcast weather conditions over the Automatic Terminal Information Service (ATIS), Automated Surface Observing System (ASOS), or the Automated Weather Observing System (AWOS). The conditions will include the wind speed and direction as determined by an anemometer or other type of sensor located on the field—sometimes on the pole that supports the windsock.
Nonetheless, the windsock, also known as a wind cone, can provide essential information to pilots when technology fails or when landing at airports or airfields without ATC.
According to Federal Aviation Administration (FAA) specifications, windsocks may be solid orange, yellow, or white and should not have any lettering or logos. The ones that are the best indicators of wind speed, however, have alternating colors—such as orange and white—or have stripes at key points.
The FAA recommends either a length of eight feet and a throat diameter of 18 inches or a length of 12 feet and a throat diameter of three feet. The fabric must be water-repellant and colorfast.
The framework the windsock is attached to must be able to hold the throat of the fabric windsock fully open when there is no wind. And it must enable the windsock to pivot like a wind vane. The framework may include lighting for the windsock, or the windsock may be illuminated from within.
The windsock assembly must be able to operate correctly in a temperature range from -67 degrees Fahrenheit (-55 degrees Celsius) to 131 degrees Fahrenheit (55 degrees Celsius) and at a wind speed up to 75 knots (86 miles per hour).
Estimating Wind Speed
Windsocks are made to orient themselves against the wind when the wind speed reaches three knots (3.5 mph). At that wind speed, only the first segment of the windsock will be extended. If the windsock is extending to the northeast, the wind is coming from the southwest, or is southwesterly.
The second segment of the sock extends when the wind speed has reached six knots; the third segment, nine knots; and the fourth segment, 12 knots. At a wind speed of 15 knots (17 mph) or more, the windsock will be fully extended and pointing away from the direction the wind is originating from.
History of Windsocks
Many centuries ago, on an annual Boys' Day, the Japanese used koi-shaped paper or cloth tubes, called koinobori, that were mounted on bamboo poles and blew in the wind to celebrate fathers and their male offspring. The largest tube was usually black and represented the father. The oldest son's would often be colored red.
Starting around 150 AD, Romans used colorful windsock-like banners to identify different divisions of the military.
In the 19th century, sailing ships used wind-sails that were shaped like broad tubes or funnels to transport oxygen to lower levels of the ship. These wind-sails are believed to have been the inspiration for the modern windsock.