Sundogs, haloes, light pillars and hoar frost

Sundogs

A sundog — a parhelion, or mock sun — is one of a pair of bright coloured spots sitting roughly 22 degrees to either side of the sun, at the same height above the horizon. The light is refracted, meaning bent as it passes through ice crystals rather than bounced off them.

The crystals responsible are large hexagonal plates that fall with their flat faces horizontal, like leaves settling. Sunlight enters through a side face and leaves through another, and because all the plates are lying the same way, the bent light is concentrated into two spots beside the sun instead of a full circle around it.

The 22 degrees is not arbitrary. Light passing through the 60-degree wedge between two side faces of a hexagonal crystal bunches up near a bending angle of about 22 degrees, so that is where the glow is brightest. The figure is exact only when the sun sits on the horizon: as the sun climbs, the light takes more slanted paths through the plates and the sundogs slide outward, becoming hard to pick out at solar elevations much above 40 degrees.

Each sundog is red on the edge nearest the sun, running to green and blue on the outside, because ice bends each colour of light by a slightly different amount. They show up most often in winter in the middle latitudes, which is to say here.

The ring around the sun

The 22-degree halo — a complete circle around the sun, red on the inside and blue on the outside — uses the same 60-degree wedge of the same kind of ice, but a different population of crystals. Here they are small columns tumbling in every possible orientation.

That randomness is the whole difference. Plates that all lie flat send their light to two places; columns tumbling every which way send it to every angle around the sun at once, and you get a ring. It forms in a thin veil of cirrus, which is why a halo often means high cloud is moving in.

Light pillars

A pillar is the odd one out: it is made by reflection, not refraction. Sunlight bounces off the faces of slowly falling, nearly horizontal ice crystals, which behave like a stack of tiny mirrors hanging in the air, and the result is a vertical shaft of light standing above the sun. Sunrise and sunset are the usual times.

The same trick works on streetlights and rig flares on a cold night, which is why a town can appear to have beams rising out of it. Popular writing mixes pillars up with sundogs constantly; the distinction is simply light bouncing off ice versus light passing through it.

Diamond dust, or the sky doing this at eye level

Diamond dust is a fall of tiny ice crystals — needles, columns and plates, none of them branched like a snowflake — that forms in very cold air near the ground. It grows directly as ice out of the water vapour in the air rather than starting as a droplet and freezing.

Its importance for sky-watching is that it makes the same haloes and sundogs as cirrus does, except the ice is in the air around you instead of six kilometres up. On a still, bitterly cold morning the optics can be happening in the space between you and the neighbour's fence.

Hoar frost

Hoar frost is the interlocking crystal growth that turns branches, fence wire and every twig into something out of a jeweller's window. It forms when water vapour turns straight into ice on a surface, with no liquid water step in between.

The National Weather Service describes it forming on objects of small diameter freely exposed to the air, when air with a below-freezing dew point is cooled to saturation — the same process as dew forming on grass, except the object stays below freezing and the water arrives already as ice. Environment and Climate Change Canada's glossary describes the same mechanism far more briefly: it has no separate hoar frost entry, folding it into "frost" as a deposition of ice crystals on objects by direct sublimation of water vapour from the air. Both agencies call that vapour-to-ice step sublimation, while newer textbooks reserve that word for the opposite direction and call this one deposition, so you will meet both terms in good sources.