Weather 101: What's that circle around the sun or moon? - Tucson News Now

Weather 101: What's that circle around the sun or moon?

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Have you ever seen a ring around the moon or sun? I'm not talking about just a fuzziness around the sun or moon. I mean a discernable ring.

Cirrostratus clouds, like the one in figure 1 below, are made up of tiny ice crystals. That sets the stage for a moon halo or sun halo, as figure 2 shows.

A cirrostratus cloud

Figure 1: Cirrostratus clouds are layers of tiny ice crystals that reside 3-5 miles high.

A sun halo

Figure 2: A sun halo (source for picture is atopics.co.uk).

So, how does the atmosphere create this optical illusion?

It's all about the shape of the ice particles.

Millions of tiny ice crystals, shaped like 6 sided plates, oriented in all possible directions, each bend the sun or moon's light through them. It's the bending of the light that does the magic.

Halo explainer

Figure 3: The 6-sided shape, or hexagon, represents an ice crystal in the cirrostratus clouds. The white arrow represents the sun's light. The red line represents the sun's light after being bent by the prism effect of the ice crystal.

The hexagon-shaped ice crystal bends the sun's light approximately 22° from its straight-line path.

Here's the really cool part. The illustration in figure 3 shows how the light is bent by the ice crystal. This is as if you measured the degree of bending with a protractor (seen in Figure 4 below).

Protractor

Figure 4: For reference, here is an image of a standard protractor, as used in geometry classes.

That 22° angle can also be measured in another way! The halo is 22° out from the center of the sun or moon (USE EYE PROTECTION GUIDELINES BELOW).  A handy method for measuring distance in the sky is by using an outstretched arm. In the following figure, Figure 5, an outstretched arm with the thumb and pinky finger at maximum separation is approximately 20° (pretty close to the 22° that the halo is from the center of the sun or moon in our view!)

Measuring approximately 20° using an outstretched arm

Figure 5: Measuring distance between objects may be done with approximation by using an outstretched arm. With the thumb and pinky separated by stretching your hand, the distance between them is about 22°.)

So, the next time you see the fuzzy appearance of cirrostratus clouds, look for a halo around the sun or moon! Measure it, and you'll find that it's a 22° halo!

When looking for halos always shield both eyes from the sun. Never stare close to it even for a moment. Preferably, hide the sun behind the edge of a building or post. Take care when photographing halos if the unshielded sun is in the field of view. It is dangerous to look at the sun through some camera viewfinders, especially SLRs - do not take risks!
 
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