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