Weather Question for Huth

[EEL]

Last night the moon had a hazy look to it and the ring around it was large and thick. which made me wonder, which leads to this question. Does the diameter and thickness of a ring around the moon provide any clues regarding the timing and intensity of the precipitation about to occur? Same question regarding the nature of the haziness immediately around the moon?

Thanks for any input.

For those wondering if they should attend John's weather class, just be warned you will thereafter always be looking and thinking about what you see in the sky. Of course you might will also start to be a rather good weather forecaster too.

Ed Lawson

[JohnHuth]

Lessee, here. The halo around the moon (or sun) is caused by hexagonal-shaped ice crystals in the upper atmosphere. These are typically associated with cirro-stratus clouds, which can be quite thin, and then thicken up as a front progresses.

The most common halo is caused by the bending of light as it passes through the ice crystals. For a standard hexagonal crystal, and the index of refraction of ice, this angle turns out to be twenty-two degrees. You can also get a forty-six degree angle halo when the light passes a different way through the crystal, but the twenty-two degree halo is the most common. You can also get something of a "prism effect" that can separate out reds and such, if the ice crystals are large enough.

I've heard the weather adage "the bigger the ring, the bigger the wet", but I can't honestly say that I understand where that might come from, since the angle is always twenty-two degrees. The perception of the size of the ring, however, might change, depending on what you're looking at in the foreground.

Next time you see one, check out the angle, and see if it isn't 22 degrees (angle from the sun/moon to the rim of the halo).

The thickness of the ring itself reflects both a lack of perfect hexagonal shapes in the crystals and random orientations - so if you're getting thicker rings, it would imply thicker clouds.

In any case, the ring and frostiness is caused by cirro-stratus clouds, and is one of the standard indicators of an approaching low-pressure system.

Here's a fun fact: If you look at a moon or sun halo when either are close to the horizon, you'll sometimes see an increase in the amount of light on the horizontal parts of the halos. This is because the ice crystals in the atmosphere tend to be oriented with the flat side horizontal, exposing more of the hexagonal parts to light passing through them- this give an increase in the amount of refracted light.

Does that make sense?

[EEL]

You can also get something of a "prism effect" that can separate out reds and such, if the ice crystals are large enough.

The perception of the size of the ring, however, might change, depending on what you're looking at in the foreground.

Next time you see one, check out the angle, and see if it isn't 22 degrees (angle from the sun/moon to the rim of the halo).

The thickness of the ring itself reflects both a lack of perfect hexagonal shapes in the crystals and random orientations - so if you're getting thicker rings, it would imply thicker clouds.

Does that make sense?

Your responses always make more sense than my ability to fully internalize them. <G>

I seem to recall seeing a slight tinge of color on some rings at other times.

The radius of the ring was about 22 degrees and there were tree limbs about so that all makes sense. Thicker rings/thicker clouds/more precipitation? A nugget to test.

Ed Lawson

[Phil Allen]

Lessee, here. The halo around the moon (or sun) is caused by hexagonal-shaped ice crystals in the upper atmosphere. These are typically associated with cirro-stratus clouds, which can be quite thin, and then thicken up as a front progresses.

The most common halo is caused by the bending of light as it passes through the ice crystals. For a standard hexagonal crystal, and the index of refraction of ice, this angle turns out to be twenty-two degrees. You can also get a forty-six degree angle halo when the light passes a different way through the crystal, but the twenty-two degree halo is the most common. You can also get something of a "prism effect" that can separate out reds and such, if the ice crystals are large enough.

I've heard the weather adage "the bigger the ring, the bigger the wet", but I can't honestly say that I understand where that might come from, since the angle is always twenty-two degrees. The perception of the size of the ring, however, might change, depending on what you're looking at in the foreground.

Next time you see one, check out the angle, and see if it isn't 22 degrees (angle from the sun/moon to the rim of the halo).

The thickness of the ring itself reflects both a lack of perfect hexagonal shapes in the crystals and random orientations - so if you're getting thicker rings, it would imply thicker clouds.

In any case, the ring and frostiness is caused by cirro-stratus clouds, and is one of the standard indicators of an approaching low-pressure system.

Here's a fun fact: If you look at a moon or sun halo when either are close to the horizon, you'll sometimes see an increase in the amount of light on the horizontal parts of the halos. This is because the ice crystals in the atmosphere tend to be oriented with the flat side horizontal, exposing more of the hexagonal parts to light passing through them- this give an increase in the amount of refracted light.

Does that make sense?

Hi John-

The 22 degree makes sense to me for sundogs/moondogs (saw one the other night at the full moon before the recent storm), but wouldn't the "ring" be due to multiple scattering events and therefore related to the concentration of scatterers (sp)? I think this is what happens with imaging point sources in turbid media for microscopy, the turbidity increases the apparent size of the point source, as different photons take different routes to the detector, which makes them look like they come from different places.

Turning over the physics geek hat and returning to my role as a biologist/skier/kayaker now.

Phil

[JohnHuth]

Hi John-

The 22 degree makes sense to me for sundogs/moondogs (saw one the other night at the full moon before the recent storm), but wouldn't the "ring" be due to multiple scattering events and therefore related to the concentration of scatterers (sp)? I think this is what happens with imaging point sources in turbid media for microscopy, the turbidity increases the apparent size of the point source, as different photons take different routes to the detector, which makes them look like they come from different places.

Turning over the physics geek hat and returning to my role as a biologist/skier/kayaker now.

Phil

Good point. Off the top of my head, I'd think the following - any haziness can be related to some degree of scattering. If the halo has color separation, that implies only one scatter, however, since multiple scatters would produce only white. If there are too many scatters, it would wipe out the halo altogether - basically would be a opaque cloud at that point. So, a broad ring could be one more scatter after the refraction, but not many more - and then I'd expect that ring to be white with no color separation (that's why the sky is blue, the sunset is red and clouds are white - differences in the number of scatters before light reaches our eyes).

There are also different kinds of scattering, depending on the size of the object doing the scattering - in Rayleigh scattering, the scatterer is a lot smaller than the wavelength of light (why the sky is blue), in Mie scattering, the scatterer is about the same size as the wavelength (and produces some very freaky effects if you've ever seen this), and then there's scattering where the object is much larger than the wavelength of light. The ice crystals are typically in the last category.

A couple of other thoughts - the sun and moon aren't point-like sources of light, so I wouldn't expect a line-like halo in any case.

If you're interested in this kind of thing, a good book to check out is "Light and Color in the Outdoors" by M. G. J. Minnaert. I was befuddled by strange images when paddling out of Jonesport a couple of years back, and ran to this book to confirm that I was seeing a phenomenon called a "Fata Morgana".

[JohnHuth]

This is a test - I wrote a little piece about estimating angular distances with hand. It's attached. There's no guarantee that the formatting works, so I'm trying to post it here.

If it's helpful, great. If not, I'll try again!

Estimating_distances_and_angles_with_your_hands.doc