Sometimes all you have to do is walk outside and look up to see something amazing to see in the sky. Earlier this week I happened to be driving in the right direction when a ragged cumulus cloud covered the sun, creating a fan of beautiful sunbeams and shadows across the western sky. I stopped the car and to the bemusement of a few nearby cows took a few photos of the scene. Those sweeping beams of dark and light are called crepuscular rays. Crepuscular (cree-PUSS-cu-ler) comes from the Latin word “crepusculum” meaning twilight. They can occur anytime but we’re more likely to notice them when the sun is low in the sky especially just before sunset or after sunrise. Puffy, textured clouds make the best displays.
If you look closely at the photo you can see that where a bulb of cloud is thick enough to block the sunlight, it casts a shadow. Bright beams are places where sunlight finds holes or openings in the cloud it can shine through. Together, they create a fan of alternating bright and dark rays crowning the western sky in heavenly glory.
You need one more ingredient for crepuscular creation — something has to scatter the light to make the rays visible. Airborne dust, aerosols (smoke, salt, organics from vegetation) provide a visible contrast between shadowed and illuminated parts of the sky.
Did you know this stunning sight is based on a well-known optical illusion? The rays are actually parallel to one another and only appear to converge into a crown the way the rails of a train track narrow to a point in the distance. If we could look down on the clouds from high above — the way the astronauts see them from orbit — the rays would appear neatly parallel to one another.
Sometimes around sunset or sunrise when atmospheric conditions are right, you can see crepuscular rays beamed by clouds massed along or just below the horizon reach all the way to the opposite end of the sky called the antisolar point. There they appear to converge (the illusion again) to form a curious bundle of dark beams that look like a gigantic sunless sunburst. Crepuscular rays seen opposite the sun (or moon) are called anticrepuscular rays. They’re much rarer than the crepuscular variety and well worth looking for around sunset and sunrise.
Since we’re often outdoors in summer it’s one of the best time to see both types of rays. Puffy clouds are also more common in summer than at other seasons. And don’t forget the moon. Around the time of full under the right clouds it can put on a great show of its own.