Sound and the wind

There is music in the woods if you care to listen for it

There is music in the woods if you care to listen for it. As the wind passes around a myriad of twigs and branches, the resulting eddies produce notes that vary widely in their pitch and volume. The resulting medley ranges from the friendly rustle of the leaves on a summer's day to the often plaintive murmur of a great oak; from the sibilant sigh of a single conifer to the loud wailing dirge of a pine forest. All these sounds have one thing in common: they are rooted in the wind.

But conversely, sound can also be used by meteorologists to measure the strength and direction of the wind. The acoustic anemometer is a device based on six small units, each of which is capable of acting as either a transmitter or receiver of high-frequency sound waves - sound waves inaudible to the human ear.

The units are fixed to six metal arms a foot or so in length extending from a central supporting mast, and deployed in such a way as to comprise three pairs. The straight line between the two units of each pair defines a path along which pulses of sound are destined to travel: the whole ensemble is arranged so that the three paths, while not necessarily at right angles to each other, are at least orientated in three widely separated directions in the three dimensions.

When the equipment is switched on, a pulse of sound is sent several times each second from a transmitter to its receiving twin. A computer calculates how long it takes to make the journey - very accurately, even though the distance involved is only 8 or 9 inches. Then, almost instantaneously, transmitter and receiver change their respective roles, a pulse of sound shoots back in the opposite direction, and is similarly timed.

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Since air is the propagating medium for sound waves, any difference between the two times measured by the equipment must be due to a movement of air from transmitter to receiver, or vice-versa. Moreover, since we know the speed of sound for a given temperature, the time difference provides sufficient information for the computer to calculate the rate of movement of the air along this path.

Meanwhile, a similar operation has been carried out along the other two sound paths, and with the three sets of data the computer can assemble a complete three-dimensional picture of the air-flow in the vicinity. It is then an easy matter to calculate the speed and direction of the air-flow in the horizontal - which, of course, we call the wind.