Halos are an ice
-based atmospheric effect
formed by the interaction of sun
- and moonlight
with ice crystals
randomly scattered in the high atmosphere
. The main and most frequent halos are seen when these crystals are 6 km-12 km high in the stratosphere
. The shapes of ice crystals and their apparent distance from the sun
cause the particular halo effect seen by the viewer. The shape of the ice crystals is governed by the molecular structure
and the temperature
in which the crystal forms.
Needle shaped crystals that form between -4 to -6° C create the most often seen halos.
- 22º halo: The most common halo around the sun and the moon, this is also called the small ring.
- 46º halo: Much rarer and less dim than the small ring, the 46º large ring occurs only about 2% of the times halos are reported. When the sun is between 15º and 27º, large rings can be confused with the supralateral arc effect.
- circumscribed halo: Fairly common adjunct to the 22º halo that changes shape with the elevation of the sun.
Sector plate crystals (imagine two faceted cones connected at their bases with a thick faceted band around the middle) that form between -10 to -12° C make many of the rarer halos.
- 9° halo: Easily mistaken for a corona, the 9° is only seen in conjunction with other halos.
- 18° halo
- 24º halo: This halo is hard to detect if it is in proximity to a 22º. Indications include unusually thick 22º halos, the presence of a double sequence of faint rainbow colors in a 22º, or a double red ring at the sharply defined inner edge of a 22º halo. It is the 24º halo that houses parhelia, upper tangent arcs, and lateral bows.
How do you know which one you’re seeing?
Unless you have the angular equivalent of perfect pitch, it’s useful to know how to tell degrees in the sky so you know which of the halos you’re seeing. One trick is to hold out your hand at arms length with your fingers spread out. The distance between your thumb and pinky finger is about 20º. Your closed fist subtends about 10º. Use these as guides for estimation.
Photographing and viewing halos
Looking at the sun is dangerous. If you want to see if there is a halo around the sun, hold your eyes so that the sun is in your peripheral vision, and move your hand over the sun. You’ll know when the light dims. Then you can safely look at the area of the sky around the sun. When photographing halos, position your camera so that something, e.g. a lamppost, a mountain, or a blimp, completely covers the sun.