ANYONE who has Polaroid sunglasses has what lawyers might call "constructive knowledge" of the polarisation of light: they are aware of it without necessarily knowing that they are.
The oscillations that go to make up the wavelike motion of a ray of ordinary light take place in a plane perpendicular to the beam itself. As a simple illustration, one might imagine a horizontal beam of light pointing north; the oscillations which, if they impinge upon the eye, are sensed as light, take place, as it were, both up and down and east and west, or in any orientation in between the two.
But if light becomes horizontally polarised, as happens partially, for example, on reflection at a horizontal surface. any vertical oscillations are damped out, and only those in the horizontal plane survive. Polaroid lenses, therefore, are designed so that they are transparent only to vertical oscillations, and so reflections from, say, the surface of a road - a primary cause of glare - are filtered out. Researchers are currently experimenting with ways of harnessing this phenomenon of polarisation to make weather radar more effective.
The microwaves used in weather radar are a form of electromagnetic radiation like radio waves and visible light, but they have the useful property of being able to penetrate cloud and fog with little or no attenuation; they are only reflected by the larger obstacles presented by an area of precipitation. The waves are transmitted by the radar in short pulses, and after each pulse the equipment "listens" for the small fraction of the energy that is scattered back in its direction by drops of rain, or hail, or snowflakes. The trouble is, traditional radar cannot easily tell us which is which.
It has now been discovered that microwaves reflected from raindrops tend to be horizontally polarised. This is because a falling raindrop is neither spherical as one might think, nor teardrop shaped as cartoonists like to draw it, but flattened rather like a bunburger. Microwaves scattered from these, in effect almost horizontal discs of water, therefore tend to be horizontally polarised. Hailstones, on the other hand, are nearly spherical, so the reflected radiation shows no bias in its polarisation. But snowflakes spin and sway as they tumble through the air; and apparently have the ability to transform horizontally polarised transmitted microwaves into vertically polarised reflected waves.
All you need, it seems, to distinguish hail from rain, and both from snow, is radar equipment capable of transmitting and receiving microwaves with a recognisable polarisation. Perhaps the next generation of radar will be able to do just that.