December 11, 2011
Recent nights have brought forth two stunning lunar haloes.
The one on the10th December was one of the most intense I have ever seen and it surrounded the full moon. Naturally, the one two days previously could not match it as the moon was two days less full.
In both cases a weather front was involved. On the 8th a front that brought severe gales and torrential rain had recently passed through and the Cirrostratus was trailing behind. On the 10th the Cirrostratus was a precursor of rain the following morning, even though the night had started intensely frosty.
Such 22 degree radius haloes are formed by moonlight, in this case, but more often sunlight, passing through hexagonal ice crystals in cold high Cirrus or Cirrostratus cloud. The tiny ice crystals are tumbling randomly in the cloud but there are enough of them with their axes roughly perpendicular to the direction of the light so that when light rays enter these prisms through a side face and leave through another tilted 60 degrees to the first they are refracted 22° or more. The little glints, all connected up, form a ring and that is why the halo starts at that distance from the sun or moon and is diffuse but bright beyond that radius with a darker hole within – albeit one with a bright light source! No sparkles from the refracted light are turned into the middle – there are just direct rays from the light source itself.
Two people see the glints from different but suitably orientated crystals to each other, so your halo was not the same as mine. What’s more, a halo only exists if something is there to see it. It is the result of a collection of light rays travelling in particular directions that can only be captured by a receiving lens into which the rays converge. That lens may be in an eye or in a camera – but if a lens is not present, neither is the halo…