Puffy cumulus clouds are common in summer. They look like sheep grazing in the blue sky. Sometimes cumulus grow into towering cumulus congestus clouds that resemble the form and texture of cauliflower. If they continue their vertical development the clouds can reach the cumulonimbus stage and produce rain, hail and lightning.

On a recent walk, I happened to look up in the west at a beautiful, burgeoning batch of congestus. The clouds grew and grew until they blocked out the sun. This got me wondering whether a thunderstorm would erupt before I made it back home. But those concerns melted away when sunbeams burst across the cloudscape like a scene from the end the world. Or even the beginning of the world. I stopped dead in my tracks.

In another scene from the same afternoon, the deep blue shadows cast by the growing clouds contrast sharply with the bright beam of sunlight (center) passing between them. (Bob King)
In another scene from the same afternoon, the deep blue shadows cast by the growing clouds contrast sharply with the bright beam of sunlight (center) passing between them. (Bob King)

Sunbeams and moonbeams are also called crepuscular rays, after crepusculum, the Latin word for twilight. They can occur anytime the sun or moon is up but are more obvious around sunset or sunrise, when they stream from between the clouds in glorious fans of deific light.

Haze, dust and humidity can enhance the appearance of crepuscular rays. Notice how the rays appear to converge at the sun. This is an optical illusion discussed below. (Bob King)
Haze, dust and humidity can enhance the appearance of crepuscular rays. Notice how the rays appear to converge at the sun. This is an optical illusion discussed below. (Bob King)

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If you look closely the next time crepuscular rays appear, you'll see an alternating pattern of dark beams and bright rays. The dark shafts are shadows cast by the clouds across the bright sky. The rays are gaps between the clouds where the sun shines through. Dust and moisture in the air make them visible the same way fog shows the light streaming from your headlights. When there's extra dust or haze, crepuscular rays are even more dramatic.

Anticrepuscular rays converge at the eastern horizon directly opposite the setting sun. Typically faint, they're still spectacular. (Kazezow Kumano / CC BY-SA 3.0)
Anticrepuscular rays converge at the eastern horizon directly opposite the setting sun. Typically faint, they're still spectacular. (Kazezow Kumano / CC BY-SA 3.0)

Sometimes you can trace crepuscular rays near the setting or rising sun all the way to the opposite end of the sky, where they appear to converge at the eastern or western horizon. That point is called the anti-solar point, and rays that "focus" there are called anticrepuscular rays.

They're caused by clouds massed at or just below the horizon. Often, these clouds are invisible to the observer, but the familiar dappled pattern of light and dark gives them away. Sunbeams glow pink from the reddened sun and contrast spectacularly with the deep blue of the shadow beams.

From the International Space Station it's obvious that the shadows cast by clouds are parallel. They only appear to radiate from the sun or moon, the same way railroad tracks converge in the distance. (NASA)
From the International Space Station it's obvious that the shadows cast by clouds are parallel. They only appear to radiate from the sun or moon, the same way railroad tracks converge in the distance. (NASA)

Sunbeams always appear to converge or radiate from the sun, but they're actually parallel to one another. We see the same optical illusion when looking down a train track. The two rails are parallel to each other but appear to converge in the distance. If we could fly far above the clouds and look down, crepuscular rays would appear perfectly parallel.

Crepuscular rays, seen here just after sundown in Ezeiza, Argentina, are caused by clouds below the horizon we can't see. Like the daytime version, they're a mix of shadows and sunbeams. (Piqui Diaz)
Crepuscular rays, seen here just after sundown in Ezeiza, Argentina, are caused by clouds below the horizon we can't see. Like the daytime version, they're a mix of shadows and sunbeams. (Piqui Diaz)

Since we're often outside in summer, sunbeams and moonbeams are easy to watch for, experience and record with a cell phone. They're often visible from parking lots, freeways and the beach. As for my shadow adventure, despite threatening skies, I made it home safely.

Happy beaming.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.