Have you ever looked up and seen a huge ring of light, a halo, around the Sun or the Moon? You may have seen one around the Moon more often as we’re conditioned, and quite rightly, too, not to look towards the Sun. These rings of light are known as halos, and are optical effects caused by ice crystals suspended in the sky They are some of the simplest forms of optical effects we can see in the sky.
The halos all have the same width, 22-degrees of arc, because of the hexagonal shape of the ice crystals the light is being refracted through. I have seen some amazing photos from the northern hemisphere of halos and associated optical effects in clear-looking sky during very cold conditions, but here in Australia it rarely gets that cold. Most of the halos I’ve seen down here are caused by thin cirrus cloud, and I’ve even seen halos around the Sun in summer! Just because it’s hot near the surface doesn’t mean it’s not cold aloft.
Depending on the intensity of the display, you may also see sundogs, or tangent arcs. To learn more about what these are and how to find them I highly recommend visiting Les Cowley’s Atmospheric Optics page where he gives a simple but good explanation of all the types of effects that can be seen with ice crystals and how they are formed.
I have heard an “old wives tale” that if you see a halo around the Moon that means there is rain on the way, but I wouldn’t put much trust in that. It’s similar to the one I was told about kookaburras having a big communal laugh-fest indicating that the weather would change within three days; When you think about it the weather does usually change within three days whether there are kookaburras around or not.