What causes a moon halo?

“I saw the moon . . . and way out away from it there was what seemed to be a perfect circle of light. What might have caused that moon halo?”

A ring around the moon

Well, we get a lot of reports of these sorts of observations — halos around both the sun and the moon. They form when ice crystals in high cirrus clouds cause light to be refracted or “bent.” Because moonlight isn’t very bright, moon halos are mostly colorless — but you might notice more red on the inside and more blue on the outside of the halo.

moon hallo

These colors are more noticeable in halos around the sun. If you do see a halo around the moon or sun, notice that the inner edge is sharp, while the outer edge is more diffuse. Also, notice that the sky surrounding the halo is darker than the rest of the sky.

A halo around the moon often comes before a storm. There’s even an old weather saying to describe this phenomenon. It’s “ring around the moon means rain soon.” It happens because the ice crystals that help create halos come in cirrus clouds, which often come before rain. So if you see a halo around the moon, be on the lookout for rain a day or so later.

Moon Light Effects – Moon Rings, Moon Dogs And Other Moon Light Phenomena…

The moon can produce interesting optical effects when conditions are right. The most common of which are moon rings, moon bows, which are similar to rainbows, moon dogs and moon pillars. A rainbow is produced when sunlight is refracted through water droplets – A similar effect is produced when moon light refracts through ice crystals. Below are a few photographs and examples about this interesting phenomena. Thanks to everyone that helped me put together this simple explanation of moon light effects.

Ring around the moon

A Ring Around The Moon

The ring around the Moon is caused by the refraction of Moonlight (which of course is reflected sunlight) from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon ring is almost always the same size.

Less typical are the halos that may be produced by different angles in the crystals. They can create halos with an angle of 46 degrees.

Moon Ring Weather Folklore

Folklore has it that a ring around the moon signifies bad weather is coming, and in many cases this may be true. So how can rings around the moon be a predictor of weather to come? The ice crystals that cover the halo signify high altitude, thin cirrus clouds that normally precede a warm front by one or two days. Typically, a warm front will be associated with a low pressure system which is commonly referred to as a storm.

It is believed that the number of stars within a moon halo indicate the number days before bad weather will arrive. Give it a try the next time you observe a moon halo.

Rings Around The Sun

The same phenomena that causes lunar halos can also be observed around the sun. NOTICE: Never look at or photograph the sun directly.


Anatomy of a Moon Halo

The ring that appears around the moon arises from light passing through six-sided ice crystals high in the atmosphere. These ice crystals refract, or bend, light in the same manner that a camera lens bends light. The ring has a diameter of 22° , and sometimes, if you are lucky, it is also possible to detect a second ring, 44° diameter. Thin high cirrus clouds lofting at 20,000 feet or more contain tiny ice crystals that originate from the freezing of super cooled water droplets. These crystals behave like jewels refracting and reflecting in different directions.

Cloud crystals are varieties of hexagonal prisms, (6 sides) and range in shapes from long columns to thin plate-like shapes that have different face sizes.

Moon Dogs

Moon dogs are the paler version of sun dogs: bursts of light often in reds and blues that appear on both sides of the moon. Both phenomena are the work of almost invisible clouds that reside in the atmosphere where commercial airliners cruise, at about 30,000 feet. The clouds are composed largely of ice crystals, known as diamond dust. The official name for a moon dog is a paraselene if seen at 22 degrees. If the image is at 90, 120 or 140 degrees then it’s known as a parantiselene.

Moon Corona

Another interesting effect caused by moon light is the corona. Just like lunar halos, coronas are produced by high thin clouds. But unlike halos coronas are very small in size. A typical corona is only couple degrees in diameter and closely fringes the moon. Although not as intense as a solar rainbow, coronas may appear in several colors. On rare occasions moon halos and moon coronas can appear together. 

Moon Pillars

Moon pillars can be seen when the moon is rising or setting near the horizon. They are pale shafts of light that extend out either above or below the moon. Pillars appear in the sky when ice crystals reflect light forward from a strong light source such as the moon. Those crystals with plate or column shapes provide an excellent surface from which the light may reflect toward the viewer’s eyes. Because the light rays forming pillars are reflected, and not refracted, they take on the colour of the incident light.

Moon Bows

A night time rainbow is sometimes called a moon bow because it is a reflection of the light of the moon. The same thing during the day is called a rainbow. It works the same way as its daytime counterpart, it just uses moonlight instead of sunlight. Any rainbow comes from light hitting drops of rain and going into the drop and being reflected off of the inside edge. As the light leaves the drop on it’s return journey the other side of the drop acts like a prism and separates it into the colors of the rainbow. Daytime rainbows work great because of the intensity of the sun light, but with moon light it makes something shaped like a rainbow with just a hint color. Moon bows occur in the side of the sky opposite the moon.

Resources about Moon Light Phenomena:

  • Atmospheric Optics   – A site with lots of good technical info and illustrations.
  • Light Pillars   – An explanation of how light pillars are formed.
  • 22 Degree Halo   – This site explains how and why we see halos.

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