All images, except where noted, credit Wiebke Salzmann CC-BY-SA. Click images for larger versions.
Cloud of the Day – Corona
Here is a meteorological phenomenon that is often misnamed “halo.” A corona is similar to a halo in that they both form rings around the Sun and Moon. The Sun’s corona (the one formed in Earth’s atmosphere, not the one around the actual Sun) is hard to see because the Sun is so bright. A corona is a more subtle effect and needs the more muted light of the Moon to really show itself.
While haloes result from the light being refracted by ice crystals high in the atmosphere, coronae are caused by the diffraction of light scattered by particles – water droplets, ice crystals, dust motes, etc – in the lower atmosphere. A corona can also form on a foggy window pane. Haloes have fixed dimensions, calculable from the known refractive index of ice. Coronae come in various sizes due to the variability in the size of the light-scattering particles. Smaller droplets make larger coronae. In addition to the light scattered from the surface of the particle, small contributions to the corona are made by light that reflects directly off the droplet, or passes through it.
A classic corona consists of a bright aureole in the center, with one or more colorful rings around it. For the sharpest coronae, the droplets must be all close to the same size, so the interference pattern in the light can be well defined. It is constructive and destructive interference among the scattered light waves, where they add to make bright regions and subtract to make dark regions, that make the alternating rings of bright and dark.