Halos, Sundogs and Sun Pillars

Sundogs, light pillars, and other kinds of halos seen in the sky are atmospheric phenomena that occur when light is reflected or refracted by ice crystals in the atmosphere.

Content:
Causes
Sun & Moon Halos
Sundogs
Sun Pillars
Lens Artifacts

What Causes A Halo?

Halos is the name for a family of optical phenomena produced by light (typically from the Sun or Moon) interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spots in the sky. Many of these appear near the Sun or Moon.

To better understand how the atmosphere produces these bursts of color, we need to understand a little something about light.

Light is electromagnetic radiation with a wavelength that is visible to the eye. This is the light we see. Visible light is composed of all the colors of the rainbow.

The visible spectrum of light from the electromagnetic spectrum.
The visible spectrum of light from the electromagnetic spectrum.

We can see these colors by using a prism. The properties of a prism allow it to break visible light apart into its component colors; colors ranging from the darkest reds to the deepest violets, and everything in between.

A prism scattering white light (Britannica)
A prism scattering white light (Britannica)

In the atmosphere, under certain conditions, water drops and ice crystals can act as a prism, allowing us to see the various colors that make up visible light. It’s because of these properties that we get various atmospheric optical effects.

To add some understanding as to how the various atmospheric optical effects manifest themselves, it helps to understand the difference between refracted light and reflected light.

Reflection

Water reflecting the the sunset and clouds.
Water reflecting the the sunset and clouds.

Reflection of light occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface. The light is not separated into its component colors because it is not “bent” (or refracted), and all wavelengths are being reflected at equal angles.

Refraction

A beam of light being refracted by a glass block. (Wikipedia)
A beam of light being refracted by a glass block. (Wikipedia)

Refraction is the change, in direction, of a wave (in this case light) due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°.

So, when light is refracted inside an ice crystal or water droplet in the air, it’s broken into its component colors. This creates a rainbow effect.

Sun & Moon Halos

22° Sun Halo seen from Penal on March 24th, 2019. (Riza Samaroo)
22° Sun Halo seen from Penal on March 24th, 2019. (Riza Samaroo)

Sun Halos, and halos in general, are a sign of high, thin cirrus clouds drifting 6 kilometers (20,000 feet) or more above our heads. These clouds contain millions of tiny ice crystals. The halos you see are caused by both refraction, or splitting of light, and also by reflection, or glints of light from these ice crystals. The crystals have to be oriented and positioned just so with respect to your eye, for the halo to appear.

That’s why, like rainbows, halos around the sun – or moon – are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals making the halo of the person standing next to you.

22° Moon Halo seen from Princes Town on July 12th, 2019. (Freeda SaherFree)
22° Moon Halo seen from Princes Town on July 12th, 2019. (Freeda SaherFree)

The prism effect of light passing through these six-sided ice crystals also separates the light into its various color frequencies, making the halo look like a very pale rainbow, with red on the inside and blue on the outside. A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.

Sun Halo Diagram
The familiar separation of the colors of white light in a prism is a result of dispersion, the variation of the index of refraction of light with wavelength. Since hexagonal ice crystals make up a section of a 60° prism, they separate the colors of white light. (Hyperphysics)

There is an old weather saying: ring around the moon means rain soon. There’s truth to this saying because high cirrus clouds often come before showers.

Sundogs

Sundog caputred from Point Fortin, Trinidad by Richard Lakhan on June 6th, 2020
Sundog caputred from Point Fortin, Trinidad by Richard Lakhan on June 6th, 2020

Sundogs are colored spots of light that develop due to the refraction of light through ice crystals. They are located approximately 22 degrees either left, right, or both, from the sun, depending on where the ice crystals are present.

The colors usually go from red closest to the sun, out to blue on the outside of the sundog. Sundogs are also known as mock suns or parhelia, which means “with the sun”.

Sun Pillars

Nancy Ricigliano at Jones Beach, New York caught this sun pillar on March 3, 2019. (EarthSky)
Nancy Ricigliano at Jones Beach, New York caught this sun pillar on March 3, 2019. (EarthSky)

Sun Pillars appear as a shaft of light extending vertically above the sun, most often at sunrise or sundown. They develop as a result of ice crystals slowly falling through the air, reflecting the sun’s rays off of them. Look for sun pillars when the sun is low on the horizon, and cirrus clouds are present.

What is that dot in everyone’s photos?

It is simply the bright light of a relatively small object that reflects inside the glass of your camera lens. It often appears as a dot, and many times it will have a partial circle or oval with a glow around the dot. It might also be tinted blue or purple or even green!

Bright lights like the sun often leave a reflection and other artifacts inside a camera lens, like a dot. These are called lens artifacts or lens flares, and they happen in all photography. It’s more noticeable when any part of your lens is flat- like the outside of a camera phone lens, or the flat lens filter many photographers use on DSLRs and even film cameras.

A more technical explanation: The lens elements in a camera contains anti-reflection coatings (thin films). Although they are intended to remove all internal reflections, they are not perfect. Their effectiveness generally depends on the incident angle and wavelength. When you photograph an intense source of light, such as the sun, the little bit of light that is reflected is enough to show up in the photograph and the color is an indication of the wavelength dependence of the particular coating. (StackExchange)

Facebook Comments