HOW COME? When the wind whistles

Why does wind seem to whistle or make other weird sounds when it's blowing hard? asks a reader.

Telephone wires. The eaves of a house. Pine branches. When the wind wants to whistle -- or howl -- it needs instruments to play.

On a dark and stormy night, wind creates its spooky sound effects by rushing across obstacles in its way. As the wind speed rises and falls, so does the frequency of the sound produced. Depending on the rush of the wind, and the shape of what it's rushing over, wind may moan, scream, sing.

The sounds we hear come courtesy of sound waves, vibrations traveling through air (or another medium, like water). Air bunches up and spreads out again, in waves of varying pressure.

So when a door suddenly blows shut downstairs, it makes waves. Propagating through the air at about 1,100 feet per second, the sound waves quickly reach your eardrums. Startled, you go downstairs to close the windows.

When you do, you notice that the wind's whistling pitch changes as you pull the window shut. That's because the windy repertoire constantly shifts, as air streams across, under and through the world's cracks and crevices.

Imagine a high wind blowing through telephone wires. As it skims across the top and bottom of a wire, the wind becomes unstable. If wind were visible, you would see the thin, cylinder-shaped wire shedding swirling vortexes of air as the wind blows past it.

These eddies in the wind in turn set up patterns of changing pressure in the air ahead of the wire. When the sound waves created by the tumbling air reach our ears, we hear whistling. As the wind blows faster, the frequency of the sound gets higher. Wind really starts screaming through wires when its speed reaches at least 25 mph.

Just as with variously shaped musical instruments, differently shaped obstacles in a wind's path produce distinct sounds. Wind blowing through pine trees, their cylindrical needles setting air aswirl, has been compared to the sound of the sea.

Houses also create wind music, especially the eerie kind. Winds whine at small cracks or holes in a building. And a strong wind can howl as it blows down eaves and against sharp edges and corners. As air spills off the edges of a roof, some of the waves created reflect back into the streaming wind. This produces whirling vortexes of air, which strike and spin off the roof edge. The result is a complex, ever-shifting sound wave pattern, which we hear as howling.