Windsock Wind Response Requirements
A windsock must respond accurately across the full range of wind speeds it is designed to indicate. Too stiff and it fails to show light winds; too heavy and it drags in the breeze and under-reads stronger gusts. The international specification exists precisely to define the performance envelope within which a compliant windsock must operate.
The specification: 3 knots to 15 knots
ICAO Annex 14, referenced by CAA CAP 168 for UK licensed aerodromes, sets the performance requirement clearly. A windsock must begin to extend and indicate direction at 3 knots (approximately 5.6 km/h or 3.5 mph) and must be flying fully extended and horizontal at 15 knots (approximately 28 km/h or 17 mph).
Between these two thresholds, the angle the sock makes with the mast gives a visual indication of wind speed. As a practical guide:
At around 3 to 6 knots the sock lifts at a shallow angle. At around 7 to 8 knots it flies at approximately 45 degrees. At 12 to 14 knots it approaches horizontal. At 15 knots and above it flies fully extended.
These are practical observation guides rather than calibrated measurements. A windsock is a visual indicator, not a precision anemometer, and it is designed to be read at a glance rather than interpreted against a scale.
What affects wind response
Fabric weight. A heavier fabric needs more wind energy to lift and extend, which raises the effective lower threshold. Piggotts200 at 200gsm lifts readily in light winds. Piggotts500 at 265gsm is designed for high-wind environments where the higher lift threshold is acceptable in exchange for greater durability.
Water absorption. Once the waterproof coating on a windsock fabric begins to degrade, the fabric absorbs moisture and flies significantly heavier than its dry specification. A waterlogged sock under-reads wind speed consistently, which on an aerodrome or COMAH site means the indicator is giving misleading information. This is one of the most common functional failures and one of the most overlooked.
Fabric damage. Holes or tears in the cone disrupt the pressure differential that keeps the sock inflated and extended. A damaged sock will collapse, twist or flutter inaccurately regardless of the wind speed.
Swivel condition. A seized or stiff swivel prevents the sock from tracking wind direction changes and can cause the sock to twist and bind, restricting its extension in certain wind directions. A free-running swivel is as important to accurate wind response as the fabric itself.
Siting. A windsock in turbulent air behind a building or structure does not see the free-stream wind. It sees the chaotic local conditions around the obstruction, which bear no consistent relationship to the actual wind speed. See our siting guide for guidance on positioning.
How to check wind response
On a day with a steady moderate breeze, compare the sock’s angle against a handheld anemometer reading taken in the open. If the sock appears barely lifted in a wind that the anemometer confirms is above 8 or 10 knots, the fabric has likely absorbed water or the sock is in turbulent air. If the sock does not extend fully in a confirmed 15-knot wind, the fabric is too heavy or waterlogged.
A sock that responds sluggishly or inconsistently should be inspected closely and replaced if water absorption or fabric damage is found. See our inspection checklist and 5 signs your windsock needs replacing for the detailed checks.
Browse our windsock range for replacement options matched to your site environment.
FAQ's
At what wind speed should a windsock fully extend?
ICAO Annex 14, referenced by CAA CAP 168, requires a compliant windsock to be fully extended and horizontal at 15 knots (28 km/h). It should begin to lift and indicate direction at 3 knots (5.6 km/h).
Why is my windsock not extending in the wind?
The most common causes are water absorption in degraded fabric, physical damage to the cone, a seized swivel preventing free rotation, or the sock being sited in turbulent air behind an obstruction. Inspect the fabric for stiffness or waterlogging and check the swivel rotates freely.
Does a heavier fabric windsock respond differently?
Yes. Heavier fabrics need slightly more wind to lift, which raises the effective lower response threshold. Piggotts500 at 265gsm is designed for high-wind and coastal sites where durability outweighs the need for light-breeze sensitivity. For sites where light wind indication is important, Piggotts200 at 200gsm is more responsive.
Can a windsock be used as a wind speed measurement tool?
Not as a precision instrument. A windsock gives a visual indication of approximate wind speed through its angle of extension, and the fully-extended position at 15 knots is the only point with a defined specification. For precise wind speed measurement, an anemometer is required alongside the windsock.