Why Does the Arrow Point Into the Wind? The Tail Does the Work

The device is mounted on a vertical pivot and is deliberately unbalanced in area, with a large flat surface on one side of the pivot and a small one on the other. Wind pushes on both surfaces, and the larger one receives more force, so it is pushed downwind until it lies directly behind the pivot, at which point the forces on either side balance and the device is stable. Any deviation from that position exposes more of the large surface to the wind and produces a correcting force, which is why the arrangement holds steady rather than wandering. The small end therefore points into the wind, which is the direction being reported.

Several details determine accuracy:

• •Balanced weight about the pivot, so gravity does not bias the position
• •Unbalanced area about the same pivot, which supplies the correcting force
• •A low friction bearing, so light winds move it
• •Enough mass to avoid swinging wildly in gusts, but not enough to lag
• •Mounting well clear of buildings, which deflect wind unpredictably
• •A fixed set of direction letters beneath, correctly aligned to true north

Wind direction is stated as the direction the wind comes from, so a westerly blows from the west towards the east, which confuses people constantly and follows directly from what matters. Where a wind comes from determines what it brings, since air arriving from over an ocean is moist, air from a continental interior is dry, air from the poles is cold and air from the tropics is warm. The origin is therefore the useful fact and the destination is not. Ocean currents are named the opposite way, by where they are going, which is equally logical for a different reason and which makes the two conventions a reliable source of confusion.

The figure most often mounted on a church is a cockerel and the reason given is specific. A ninth century papal instruction is traditionally credited with requiring the bird on church towers as a reminder of the denial of Peter before the cock crowed, and whether that instruction existed is disputed while the association is very old and very widespread. Other figures carry local meaning, including fish on churches by water, arrows and flags on secular buildings, ships in ports, and grasshoppers, dragons and heraldic beasts chosen by whoever paid. Several are notable objects in their own right, and the one on the Royal Exchange in London and the grasshopper of Gresham are recognised landmarks.

Automatic stations have replaced the visual device and the underlying principle is unchanged in most of them. A vane of the same shape turns a shaft connected to a sensor reporting its angle, which is logged continuously and averaged over stated periods. Ultrasonic instruments have no moving parts at all, measuring the time sound takes to cross between transducers in several directions, which differs according to the airflow, and deriving both direction and speed from those differences, which is more reliable in ice and in salt air. Aircraft and ships derive wind from the difference between their motion through the air and their motion over the ground. Radar and satellite methods infer it over large areas from the movement of cloud and from the roughness of the sea surface.

A shape with more area on one side of a vertical pivot is pushed until that area lies downwind, so the small end points into the wind and any deviation produces a correcting force. Balanced weight, unbalanced area and a low friction bearing are what make one accurate. Winds are named by where they come from because that determines their moisture and temperature, while ocean currents are named the opposite way.