What does it mean when there is a ring of clouds around the moon?

Question

Last night that was what was with the moon. I was just wondering if there was anything about the moon when it does that.

Answer 1

Explanation: Have you ever seen a halo around the Moon? This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similar Sun Halo may be visible during the day. The town in the foreground of the above picture is San Sebastian, Spain. The distant planet Jupiter appears by chance on the halo's upper right. Exactly how ice-crystals form in clouds remains under investigation.

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.
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. A few photos of solar halos using a Coolpix 995 digital camera. NOTICE: Never look at or photograph the sun directly.


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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.


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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.

Thanks to Jim Bell for the moon dog photo !!!
Visit Jim's Homepage for more great photographs.





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.

Thanks to Mats Mattsson for the moon corona photo !!!
Visit Mats Homepage for more great photographs.

AUSTRALIA Moon Corona
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 lunar halos and lunar coronas can appear together. This great photo of a moon corona was captured by Robert Studdert of Hurtsville Australia.

Thanks to Rob Studdert at Digital Image Studio for the moon corona photo !!!



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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.

View a few Sun Pillar examples.


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.

Thanks to Matt BenDaniel for the moon bow photo !!!
Visit Matt's Homepage for more great photographs.


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Light Phenomena Links...
Atmospheric Optics - A site with lots of good technical info and illustrations.

Light Pillars - An explanation of how light pillars are formed.

Moon Watch - A question and answer page about moon halos.

22 Degree Halo - This site explains how and why we see halos.


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The Question
(Submitted January 02, 1997)

Last night I looked at the Moon. The Moon was full and there was a huge ring around it. I have seen this type of thing before and I don't know what it is. I call it a Moon ring. I was wondering if you could tell me what it is called and how it happens. I think it has something to do with clouds. I am not a scientist but I do have an interest in astronomy. Please write me back.



The Answer
While our expertise is in high-energy astrophysics, we can point you in the right direction for the answer to your question. You are correct in thinking that it is an atmospheric effect. The ring around the Moon is caused by the reflection 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 always the same size.
Here is the title and author of a good reference book to physical phenomenon seen in our atmosphere (geared for HS senior or college freshman level):

Minnaert, M., The Nature of Light and Color in the Open Air, Dover Publications, New York, 1954.

Sincerely,
Andy Ptak and Michael Arida
for the Ask an Astrophysicist team

Answer 2

A ring around the moon (or sun, for that matter) as seen through thin clouds is called a halo. The halo is about 22.5 degrees in diameter and is caused by refraction and reflection of the moon's (or sun's) light as it travels through the water vapor clouds. In the daytime you can occasionally see what are termed "sun dogs", where there is a layer of cloud to either side of the sun, and within this layer is a rainbow colored bright area. If the clouds were continuous you would have a halo...it's just that the clouds are discontinuous. There are many similar and fantastic such atmospheric phenomena, and I urge you to get one of the Peterson's Guides to the Atmosphere, or similar guide. Most of these phenomena are explained there, and there are experiments, as well.

It used to be an "old wive's tale" that the number of stars you can see within the halo around the moon is the number of days before it rains. That certainly isn't true, but the halo does mean there is considerable upper level moisture, which can be indicative of wet weather ahead.

Answer 3

Religous veiwpoint: Start prayin' and atone for yo sins!
What reallly happened: The moon glows because sunlight hits it and bounces to us.Since it is basically sunlight, it bounces through those clouds because some clouds are transarent because they are made of water.The light shines throught the clouds like a lightly painted peice of plastic wrap. You can still see the moon clearly, it's just that the cloud are transparent.

Answer 4

I don t know whether anyone has noticed or not (or might be I am wrong), but this ring varies in diameter, Sometimes small and sometimes big. Why is it so (if it is!)