The Aeolian Harp

Sometimes, for no apparent reason, a powerline thickened by frost or a clothesline laden with snow will start to bounce. The shaking line looks errie, but it has an understandable cause. It's the same thing that causes bare wires to hum in a high wind: the aeolian harp effect.

The aeolian harp is the Greek answer to wind chimes. It's a stringed instrument played only by wind (or, as a Greek of the classical era might say, it's intended to be plucked only by the fingers of Aeolus, the god of winds). Nowadays, as the Random House dictionary definition starts. an aeolian harp is a box equipped with a number of strings of equal length, tuned in unison and sounded by wind.

The aeolian harp creates sound for the same reason the frosted line bounces. The cause lies in the phenomenon known as vortex shedding. A vortex (or curl) forms in flowing air under certain conditions. Normally, vortices are invisible in transparent air, but smoke makes them easy to see. Aerodynamic engineers use colored smoke bombs to check air flow, but the moving end of a lit cigarette can also illustrate vortex formation, as it leaves a visible curly smoke trail behind.

Regular vortices form when the flow speed of the air multiplied by the diameter of the cylinder past which it flows falls within a certain narrow range. When the air is moving more slowly, the flow is said to be laminar---layered---and vortices do not form. When the air moves faster the vortices become irregular and turbulent.

Every time the flowing air forms a vortex behind a horizontal wire or rope, the vortex gives an up or down push as it moves away downstream. The frequency with which the wire is pushed is the speed of the wind divided by the size of the vortex or wire. For a large-diameter wire, say one thickened by snow, the frequency is about once a second. An observer would see the line bounce that often. For a thin wire, the frequency is high enough so a bystander would hear the wire hum as it vibrates.

Proper wind speed is not the only necessary condition for the aeolian harp effect. The frequency of vortex shedding must match the natural frequency of the suspended wire or rope. If you pluck the string of a harp, it sounds a definite note. If the wind were to play the harp, the frequency of vortex shedding must match the frequency of the note produced by the harp string. Because it is necessary to have this coincidence, the aeolian harp effect is only occasionally observed.

Finally, you might wonder why snow-covered wires are seen to bounce only on apparently windless days. The reason lies in the necessity for exactly the right air speed for vortex shedding to occur. With a wire thickened by snow, the right conditions for regular vortex formation arise only when the wind is blowing at about a tenth of a mile an hour. That is too small a breeze to feel. For a small-gauge wire, however, vortex formation demands a stiff breeze. That's why nearly everyone knows that wind makes wires hum, but not many people realize that wind also makes snowy wires bounce.