Crepuscular rays are sunbeams that are basically individual quantities of sunlight that poke through or around some object, usually a cloud. They’re made visible by dust or vapor that scatters light from them to our eyes, and by the contrast with the cloud’s shadow. Frequently, the rays project from a hole in cloud cover, appearing to spread out and widen.
They are also known as sun rays, cloud breaks, God’s rays, or the Fingers of God. They appear to radiate from a single point in the sky. They often occur when objects such as mountain peaks or clouds partially shadow the sun’s rays like a cloud cover.
The name comes from their frequent occurrences during crepuscular hours, the times between dawn and dusk, when the contrasts between light and dark are the most obvious. Various airborne compounds scatter the sunlight and make the rays visible, due to diffraction, reflection, and scattering.
Crepuscular rays are nearly parallel, but appear to diverge because of linear perspective. Actually, the rays are straight and nearly parallel to each other. Because they come from the Sun, which is very far away and much larger than the Earth, they appear to be a lot longer and bigger than they look. The bottom of a ray can be miles closer to us than the top, where it comes out of the cloud. Some rays make it all the way across the sky.
If we were to see the rays exactly sideways at a 90-degree angle, they would be parallel with each other, and slanted at whatever angle corresponds to the height of the Sun in the sky. But if we see the rays from any other angle, especially head on, they appear to fan out, and the rays seem to get wider and farther apart from each other the closer they are to us. In exactly the same way, if we look down train tracks, they appear to converge and finally disappear in the distance.
This illusion, known as perspective, is based on the fact that light travels off objects in straight lines at definite angles. The farther away an object is the smaller the angle of vision and the smaller the image it projects on the retina. As we look into the distance, the angle of vision gets smaller and smaller until it effectively reaches zero, the vanishing point.
There are three primary forms of crepuscular rays. The first consists of rays of light penetrating holes in low clouds, and are also called Jacob’s Ladders. The second appears as beams of light diverging from behind a cloud. The third type contains pale, pinkish or reddish rays that radiate from below the horizon, which are often mistaken for light pillars.
The rays of the second and third types may extend across the sky and appear to converge at the antisolar point, which is the point on the sky sphere directly opposite the sun. They are called anticrepuscular rays. Like crepuscular rays, they are parallel shafts of sunlight from holes in the clouds, and their apparently odd directions are a perspective effect.
Crepuscular and anticrepuscular rays behave in the same manner. Crepuscular rays are usually red or yellow in appearance because the atmosphere acts as a giant lens, which refracts low sunset rays into long curved paths that pass through up to 40 times as much air as rays from a high midday sun. Particles in the air scatter short wavelength blue and green rays much more strongly than longer wavelength yellow and red.
Crepuscular rays can also occasionally be viewed underwater, particularly in arctic areas appearing from ice shelfs or cracks in the ice.