It used to be thought that pigs could see the wind, a talent that would be of inestimable use to meteorologists. Yet the notion is not as far-fetched as it might seem. Modern science allows a weatherman to be - in this respect at any rate - a pig, as it were, who scans the horizon with a telescope. The equipment that makes this possible is the Doppler radar.
All of us have heard an ambulance from time to time, and are familiar with the phenomenon whereby the siren of the moving vehicle changes pitch, depending on whether it is coming towards us, or departing. The effect was first explained in 1842 by an Austrian physicist called Christian Johann Doppler, who used the analogy of a ship upon a wavy sea - a sea where the waves are smooth and regular, and the distance from one crest to another always constant.
If the ship is stationary, the waves will break upon the bow, say, once per second. If, however, the ship moves into the waves they will break more frequently, and to an observer aboard it seems as if the frequency of waves is higher. The apparent frequency would drop, on the other hand, if the ship were to travel in the same direction as the waves.
The high-pitched whine of an approaching ambulance occurs because the acoustic source is following close upon the heels of every sound-wave. Each wave emanates from a point a little further forward than the last, and this reduces the distance between successive crests. This shorter wavelength gives a higher frequency when the sound eventually impinges on a listener's ear - and a higher frequency produces a more high-pitched note. This "Doppler effect" applies to waves of any kind when they emanate from a moving source, or alternatively, if they strike a moving target.
Now the electromagnetic waves from weather radar are reflected by any drops of water in their path, and are captured on their return by the "receiving" part of the equipment. This identifies the areas of rain. But when a radar beam bounces against a moving drop of water, the motion of the drop, in Doppler fashion, slightly alters the frequency of the reflected waves. The magnitude of this "frequency shift" can be measured and used to calculate the horizontal rate of movement of the water drop, and this, of course, in turn, is a measure of the local wind. In this way, meteorologists obtain a picture of the wind patterns that exist within a cloud, which is of considerable help in forecasting the future development of a thunderstorm.