CERTAIN things ought not to work, but do like the internal combustion engine, a piano, democracy, and the jumbo aeroplane. To these, perhaps, we could add the ultrasonic anemometer, an instrument which uses state of the art technology to measure the speed and direction of the wind.
The device is 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. Are you with me so far?
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 eight or nine 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.
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 airflow in the vicinity. It is then an easy matter to calculate the speed and direction of the airflow in the horizontal which of course we call the wind.
The ultrasonic anemometer has many advantages over traditional instruments. It has no moving parts, so requires a minimum of maintenance and can operate effectively in severe conditions. It can calculate the speed of movement of the air in the vertical, which is useful for many applications in meteorology. And the reading, having been compiled by a computer in the first place, are easy to assimilate for electronic storage and manipulation.