“Sometimes when the wind is strong, it howls, and other times it doesn’t. I was wondering what makes the wind howl?”
Well, in some ways, wind can be like a swiftly flowing river. You know how whirlpools can form in river water due to rocks or other obstacles in the way. The same thing can happen with wind. Whirlpools can form when the wind passes an object — such as a chimney or building or tree — and the object disturbs the wind’s flow. Sometimes whirlpools form in wind without any objects in the way. There may simply be instabilities in the wind itself. In either case, these whirlpools are what cause the wind to howl.
Inside a whirlpool of wind, the air expands slightly — the air pressure gets lower — and this expansion of air creates a sort of pulse of pressure. When a series of pulses hits the ear, it produces the howling sound we hear. So you hear wind howl when there are whirling eddies in the wind.
These whirlpools set off a domino effect of vibrations — which travel through the air until they hit your ear drum — and make it vibrate. And that’s when you hear the wind howl.
The sound of the wind
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.