Rainbows, Halos, & Coronas2

Double the Pleasure

Often you can see a second rainbow above a brighter one. This is caused by extra reflection inside the raindrops. Because the secondary rainbow is formed from two reflections instead of one, it has a wider radius than the first rainbow and its colors are reversed, with red on the bottom and indigo on the top.
The secondary rainbow also isn't as bright as the first because it's a secondary reflection. A third or a fourth rainbow also can occur because light can be reflected more than two times within a raindrop. But we rarely see them because they are even fainter than a secondary rainbow.

A Few Oddities

Because rainbows are light and because light rays strike everyone's eyes a little differently, the rainbow you see will be a little different from the one someone else sees, even if he or she is standing right beside you. Someone else a short distance away or looking from a different angle may see a much different rainbow -- or no rainbow at all. Another oddity is that you might see a rainbow's reflection in a lake without seeing any rainbow above it in the sky! That's because you may be at the wrong angle to see the rainbow but are in the perfect spot to see its reflection in a lake.

How to see a halo

As any avid sky watcher knows, there are many other examples of color in the sky. Have you ever seen a single ring of color around the sun or moon? It is a halo, and it occurs more frequently than a rainbow. To see a halo, don't look directly into the sun. Instead, block the sun from your view with your hand, a car visor or other object so you can just see the clouds around it. Sunglasses also may help you see a halo -- but even with sunglasses, you'll need to block the sun from your eyes. Sometimes the halo is white. Sometimes you can see red and orange in the middle, with yellow and blue at the outer edges. A 22-degree halo is the most common. It is formed when light refracts -- or bends -- around the edges of long ice crystals at a 22-degree angle.

If a thin cloud's ice crystals are in the right position, you might see arcs just above or below the halo. The arcs form when light refracts inside long pencil-shaped ice crystals.

Flat ice crystals can produce an effect high above the halo called a circumzenithal arc -- or an upside-down rainbow.

Also when light refracts through flat horizontal ice crystals, you might see bright spots of light along the right and left sides of a halo. These bright spots are commonly called sun dogs; their scientific name is parhelia.

Coronas

They are similar to halos but are formed from a cloud's water drops rather than from ice crystals. Light, which shines through cloud droplets, curves around the circular drops and gets diffracted. This means it spreads out, creating an area of light larger than the sun or moon. This "crown" of milky light around the sun or moon is the corona. When all the cloud droplets are about the same size, the diffracted light can make the corona separate into colors. These colors may repeat themselves.