Icy HazesThe classification of dry and wet hazes causing Rayleigh, Mie and diffuse scattering holds only while temperatures are warm enough. When hazes freeze during cold winter temperatures, scattering is almost always much more complicated.Strictly speaking this has little to do with sunsets as such - scattering on ice would also happen when the sun is higher in the sky. However, usually temperatures need to be really cold (with the air below -15 degrees Centigrade), and unless you're in Antarctica that means winter in northern latitudes. At that time, the sun is however never high in the sky, so most of the short day is characterized by a low light setting and sunset colors.
Why ice is complicatedWhen liquid water is in the air, surface tension makes it form round (spherical) droplets. When these droplets grow large and fall as precipitation, they can be somewhat distorted by air drag, but by and large the range of forms they can assume is rather limited.That is not the case for ice crystals - they can occur in a myriad of forms, from needles to prisms or columns, hexagonal plates or star-shaps and also the complex dendrites better known as snowflakes, in addition to irregular forms that are generated by thawing and re-freezing. Into what form the water freezes is primarily determined by temperature and relative humidity. Unlike a spherical droplet, such an ice crystal has often a preferred orientation to tumble through the air, the small aerodynamical resistance sees to that (it helps that really cold air usually occurs in high pressure weather situations in which the air is often very still). For instance, flat, hexagonal plates tend to be oriented with their area parallel to Earth's surface. Now, the reflection or refraction profile of a single crystal is also not simple in any way - some directions have high reflectivity, others do not. Combine the fact that the scatterers in the haze have a preferred orientation with the information that the scattering also has strongly preferred directions, and you start to understand why scattering in icy hazes can have really peculiar properties - so-called halo phenomen. Here is a fairly common combination - the 22 deg ring combined with sundogs - bright spots at the side of the real sun disc.
Probably, this is caused by ice frozen into hexagonal columns.
Sunset halo phenomenaThe scattering on ice crystals does not change the light color, but if the color of the illumination has been changed before by Rayleigh scattering in the atmosphere, the effects can be quite striking. In particular, colors can appear in rather unexpected ways before a differently-colored background, as the precise scattering pattern for the icy haze isn't readily apparent to the viewer.Consider this colorful light pillar (caused by flat hexagonal ice sheets).
Or for instance, here, the Sun generated a 'last greeting' from below the horizon which contrasts strikingly with the darker clouds.
This can give rise to a false sunset when only the uppermost part of the light pillar is visible and resembles the appearance of a hazy Sun disc while the actual disc is already below the horizon.
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