Halos can be seen all the year round from the
tropics to the poles. Ice crystals in cirrus
clouds produce them. The clouds are 3
- 6 miles (5
to 10 km)
high and are always cold regardless of their location.
In very cold weather halos are also formed by crystals in air close
to ground level, called diamond dust.
Ice crystals behave like jewels. Sunlight passing between their faces is refracted
and reflected to send shafts of light
in particular directions. Halos are the collective glints of millions of crystals.
Regardless of their overall proportions, all ice crystals have identical interfacial
angles. It is this constancy which gives regular and predictable halos.
Why are the crystal angles always the same? The constancy comes from
a deeper order at a molecular scale. In ice, individual water molecules are linked
together and arranged in a regular lattice.
This submicroscopic order and symmetry gives us the halo forms and symmetries
of the skies.