The weather has been clear and sunny but a few whisps of feathery cirrus cloud are appearing. In a few hours, the blue sky has been replaced by a milky-white shroud telling us that rain is on the way. In temperate latitudes, this is a good time to see halos formed by clouds of ice crystals high in the atmosphere where the temperature is always well down below -30 deg. C or so.

What do halos look like and how do we recognise them? Most people feel comfortable looking at the sky away from the sun and know that this is the place to look for a rainbow when the conditions feel right. The most prominent halos, however, appear quite close to the sun where it can be uncomfortably bright to look. The most common is probably the 22 degree halo: a whitish circle around the sun with an angular radius of 22 degrees (give way of estimating size). The inner edge is quite sharp and has a distinctly reddish tinge. The brightest halo phenomena are the closely-related parhelia, known commonly as 'sun dogs' or 'mock suns'. When the sun is low in the sky, these appear as bright regions on the 22 degree halo on either side of the sun on the horizontal line passing through it. They are more distinctly red on their inner edges and can appear bluish on their outer sides. Sometimes there is a white band passing through them and extending on around the line parallel to the horizon. This is known as the parhelic circle.

These particular phenomena are in fact quite common and can be seen, in temperate latitudes, at least once a week by a keen observer. There are, however, a great variety of other ice crystal halos and arcs that can be seen more rarely and which provide a rich source of information about the nature of ice crystals in the atmosphere.

In a sense, halos are like rainbows, glories and coronae in that they require the presence of a bright source of light, the Sun or the Moon, and water floating in the air. They differ, however, in the sense that the water has to be in the form of ice: not just the snowflakes that we are familiar with in the winter, but simple, regular crystals in the form of hexagonal plates and pencil-like rods (illustration). These simple crystals act as prisms that can reflect and refract light in very particular ways to produce the regularities and symmetries that we see in halo phenomena. The real scientific interest in halos - and the reason for their sometimes especially spectacular beauty and complexity - is that these crystals tend to float in the air in particular orientations. The flow of air past them as they fall in the Earth's gravity determines how and if they settle into stable attitudes. The practical consequence of this is that many halo forms depend on the altitude of the sun, i.e., the angle between the vertical and the direction of illumination. In what follows, we'll try to explain how this happens.