Rainbows, Haloes, and Sun Dogs Above

Bright double rainbow.

Beauty in the sky is splashes of colors and arcs of light as rainbows, halos, and sundogs. How do they happen? When light passes from air into water, or from air into ice, it bends slightly and separates into seven colors. Red, orange, yellow, green, blue, indigo, and violet. Do you remember the acronym ROY G BIV? That will help you remember the seven colors of the spectrum. Refraction shows us the color spectrum and it is easy to see using a triangular shaped piece of glass or plastic called a prism. At times you will see the color spectrum in cut glass around your house such as light fixtures or crystal bowls.. You can even see it in a fish bowl when beams of sunlight bend to produce mini rainbows.

Bright double rainbow.

In nature, tiny raindrops refract light and then bounce it back to you. This is the cause for seeing colors. On a rainbow, red is the color on the outside. When the sun is low and bright we see a second rainbow or a double rainbow but the raindrops have to be just the right size. In the double rainbow the colors are reversed so that red is on the inside. You might also notice that the sky is much lighter on the inside of the main rainbow, and darker on the outside of the second rainbow.

Rainbow by David Williams

To find a rainbow look into the direction of your shadow when the rain is in front of you and the sun shines brightly behind you. The best times are close to sunset or shortly after sunrise. The lower the sun is, the more rainbow you could see. It will not always be one continuous arc but if you hold out your arms in the direction of the rain like you are going to give someone a hug just look around the circle that your hands would make to find the rainbow. Here’s an interactive tool that shows how the sun angle and your distance from rain control how much of a rainbow you see.

Rainbows also have a close cousin seen in fog. It forms in the exact same way and it’s called a fogbow.

Water droplets in cloud bank create a fogbow.

Fogbows are usually more white than colorful but you can sometimes see color in them. They are seen in the direction of your shadow so it’s more common when there is a bank of fog on one side of you and the sun shining brightly on the other side of you. If you can’t see your shadow then you probably won’t see a fogbow or a rainbow.

You cannot see a natural rainbow in the middle of the day because you must be between the sun and the rain. You can see rainbows in the middle of the day if you are standing next to a fountain or lawn sprinkler and then look down in the direction of your shadow.

There is something we see in the middle of the day and people often confuse it with a rainbow. It’s called a halo. It’s also seen at night when the moon is bright. A halo is a ring of color seen around the sun anytime and anyplace on Earth when there are Cirrus clouds. Cirrus clouds are made of ice crystals that act like prisms. While halos may appear white you can see the seven colors of the spectrum with red on the inside of the halo, and violet on the outside.

Haloes form in the ice crystals of cirrus clouds around the sun or moon.

At times, on either side of the halo, you might see brighter segments called parhelia. That means with the sun. Sometimes they are called mock suns. Most people just call them sundogs, probably because they follow the sun like a dog follows a person. Sundogs look like pieces of rainbows. Both halos and sundogs are very common but many people do not see them because they don’t take the time to stop and look up. Just look toward the sun whenever there are thin Cirrus clouds around it. If you extend your arm and stick out your pinkie and thumb, cover the sun with your thumb and your pinkie will trace the circle of the halo. Look at this variety of haloes.

circumzenith portion
Portion of circumzenith arc in cirrus clouds.

If the sun is low it is easier to spot sundogs on the halo. They will always be across from each other. Remember you can even see halos around a bright moon but you must have very thin Cirrus clouds overhead. Take the time to stop, look and enjoy. Halos and rainbows are not a distraction, they are refraction.

There is another phenomenon related to the Halo but you have to look almost straight up to see it. It is called a Circumzenith arc. It looks like an upside down rainbow but it only happens in thin Cirrus clouds when the sun is low.

Maybe you’ve seen random banding of colors way up high too in cirrus clouds. The odds are you saw iridescence. It’s caused by difraction of light when light in thin clouds passes around tiny water droplets or small ice crystals. It’s not unusual in cirrus clouds above thunderstorms.

Light passing around small water droplets or ice crystals can create iridescence.
sky optics
Cirrus clouds and sun produce rings, arcs, and other colors, as cousins to rainbows.
Water droplets in altocumulus clouds create a “corona” around the moon.

Find great images of rainbows, halos, coronas, glories, and more in our photo gallery and Facebook page or check NOAA’s website for more. See outstanding pictures of other fascinating sky optics at the Cloud Appreciation Society and Atmospheric Optics. If you spot something unusual, or just pretty in the sky, take a picture and share it with us!

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