Moon Halo

22 Degree Halos

Content written by:
Heather Sickels
heather.sickels@gmail.com
March 17th, 2017

Have you seen a glowing halo around the sun or moon? It is possible to see this atmospheric phenomenon anytime of year, from anywhere on Earth. The halo is the eye picking up light interacting with ice crystals in high-level cirrus and cirrostratus clouds as they pass in front of the sun or moon. These clouds are characterized as thin and wispy strands of ice that form in the cold region of the atmosphere, typically at around 18,000-21,000 feet or more above the surface of the planet.

This high, icy atmospheric region explains how the halo effect can be seen even in the heat of summer or over steamy tropical regions. Halos are brightest when the sun or moon are lower on the horizon rather than higher in the sky. Halos can also be refracted through near surface ice crystals not just through high flying clouds. In very cold climates where ice crystals form in the air near ground level, known as diamond dust, halos, sundogs and other optical effects of light appear.

Did you notice that halos form a circular ring that lies at nearly the same distance from the sun or moon? The distance of the light of a halo averages about 22 degrees from the center of the sun or moon. This standard distance has to do with the way light from incoming sunlight changes angle by about 22 degrees as the sunlight exits the ice crystals. Sunlight hitting millions of plate-shaped hexagonal ice crystals causes this effect. There is no light refracted from angles less than 22 degrees due to the shape of the ice crystals. This is what gives the halo its dark center and what creates the sharp hit of light seen at the 22 degree point distance. The crystals have to be oriented in specific ways and have to be less than 20.5 micrometers across to create the effect. This may seem fairly specific to create the right conditions, but keep looking up in the sky you are bound to see an ice crystal halo!

Sundogs

Sundogs, also known as parhelia, are another brilliant optical effect associated with 22 degree halos. Sundogs are bright spots, sometimes called mock suns, seen as subtly colored patches of light to the right and left of the sun on or slightly outside a 22 degree halo circle. They are brightest and tallest when the sun in near the horizon at sunset or sunrise. Sundogs are caused by light interacting with hexagonal ice crystals just as with the light seen from a halo. Sundogs usually come in pairs, however sometimes only one sundog appears either to the left or right of the sun due to atmospheric conditions. Once the sun reaches about 40 degrees above the horizon sundogs are very difficult to see and fade from view.

The color of light in a halo and in sundogs shows red on the sharp, bright inside edge of the halo. The color changes to orange and yellow then bluish on the hazier outside edge of light. Strong sundogs give an amazing rainbow show, many halos and sundogs appear only as white light.

Moon Halos

The moon forms 22 degree halos and its dimmer, less colorful equivalent to sundogs are called paraselenae. These moon dogs, like sun dogs, form to the left and right of the moon along or near the halo. The best moon phase to see a moon halo is when the moon is full, however during other moon phases it is also possible.

When do halos appear?

In many myths and legends associated with halos around the sun and moon, they are viewed as a harbingers of rain. This is not necessarily true. In many cases the high sheets and isolated streaks of icy cirrus clouds means there is stable weather to come. However, a large number of approaching cirrus clouds can indicate a frontal system is moving in. In the tropics, a veil of cirrus clouds typically approaches as the visible leading edge of a hurricane. Sundogs and halos are caused by light passing through cirrus clouds and therefore do provide clues about the weather, they are however not reliable signs of wet or stormy weather to come.

You may be familiar with the saying: "If a tree falls in a forest and no one is around to hear it does it make a sound?". In regards to halos it would be reworded as: "If sundogs shine and no one is around to see them do they still shine?".

The answer is a confident no! Halos and sundogs are seen only by an eye -or a camera- that is looking from a specific angle at the right time. Someone standing only a few meters away, or on a hilltop away, would not see the halo the same or not see the halo at all. Someone standing at a different point would see the halo through a different set of ice crystals. A halo only exists if there is an eye to see it. Just as a rainbow can be viewed only from a particular angle and then disappears. This makes finding the pot of gold an elusive task.