Ice halos

What they are

High, thin cloud such as cirrostratus is made of ice crystals a fraction of a millimetre across. Most of them are small hexagonal prisms: flat plates or long columns. Light that enters one face of a crystal and leaves through another is bent by a fixed amount, and millions of crystals together send that light to fixed places in the sky. That is a halo.

Low ice fog near the ground, called diamond dust, makes the brightest and most varied displays, because its crystals are often large and very regular.

Which way the light goes

The two ends of a hexagonal crystal are its basal faces, numbered 1 and 2; the six sides are numbered 3 to 8. A ray path lists the faces a ray passes through:

• 3-5: in through one side, out through the side two along, a 60° prism. It makes the 22° halo, the sun dogs and the tangent arcs.
• 1-3, 3-1 and 3-2: between an end and a side, a 90° prism. They make the 46° halo, the circumzenithal and circumhorizontal arcs and the lateral arcs.
• Paths with reflections inside the crystal make the fainter arcs, such as the helic, subhelic and Wegener arcs.

How the crystals fall

Which halos appear depends on how the crystals are oriented as they drift down:

• Any way at all: complete rings, the 22° and the 46° halo.
• Plates lying flat: sun dogs, the circumzenithal and circumhorizontal arcs, the subsun and the 120° parhelia.
• Columns lying flat: tangent arcs, the supralateral and infralateral arcs, the Tricker and Wegener arcs.
• Columns with two sides also level (the Parry orientation): Parry arcs, helic and subanthelic arcs.
• Crystals with pyramid ends: odd radius halos, at 9°, 18°, 20°, 23°, 24° and 35° from the sun.

The height of the sun matters too: the circumzenithal arc needs the sun below 32°, the circumhorizontal arc needs it above 58°, and the sun dogs move further out from the 22° halo as the sun climbs.

More to read and see

• Atmospheric Optics: halos, rainbows and glories explained, with many photographs
• The Halo Vault: reports and analyses of rare halo displays
• Taivaanvahti: the Finnish sky observation service, with many of the best displays ever photographed
• Ice crystal halos: the halo blog of the Finnish astronomical association Ursa
• Arbeitskreis Meteore: the German network of halo and meteor observers
• Halo simulators: Lumice, HaloRay, HaloSim
• The camera that inspired this one: HaloCam at LMU Munich (Forster, Seefeldner, Wiegner and Mayer, 2017)