This structure on a hill top near Burnley, England known as the Ringing Singing Tree. When the wind blows the pipes emit a sound depending on the direction of the wind: Dave Whalley

The howling wind –  How does this happen? Are sounds made by the wind primarily due to the air itself vibrating or to solid objects vibrating when the air blows against them?

1. What we call sound are in fact just pressure waves propagating through the air. Our ears and brain detect and convert these pressure waves to electric signals which we interprete as being sound. The frequency of these waves determines the altitude of the tone, while the amplitude defines the volume of the sound.

When waves (eg pressure waves) pass through openings that have about the same magnitude (or less) than the wavelength of the incident wave, diffraction will occur. Once passed through the opening, diffraction will “generate” a phase difference between waves starting from the top and bottom of the opening.

2. Sound waves are by definition a succession of compressions and rarefactions. The alteration of the wind gives a pulse. When you have a succession of several pulses in a high enough frequency then you hear a pitch. There must be at least 20 pulses per second to hear a pitch.

When whirlpools are created by wind flowing around objects, the frequency at which these swirls or eddies are generated is related to the wind speed and to how wide the object is.

Sometimes the wind howls even though there are no structures around to create whirlpools in the wind. This just happens because of the instabilities and the breakdown of the even flow of the air itself without having something necessarily in its way.

“Aeolian” means a tone that is produced by (or sounds like it is produced by) the wind. An Aeolian harp is used to describe those mixed tones you hear from wind impinging on telegraph wires, or long open pipes.