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What Are Crepuscular Rays?

Crepuscular rays are an atmospheric phenomenon that gives the illusion of originating from a single point in the sky. The rays are produced when sun-lit air streams through gaps in clouds or between other objects, such as trees or tall buildings. The name originates from the time of day they usually appear, the crepuscular hours. Crepuscular hours are those around dawn and dusk and are the period when there is the highest contrast between light and dark.

Occurence
- Crepuscular rays most often occur when a dark objects shadow the sun's rays, contrasting with the reflection of the light off of airborne particles. Weather can also affect when such rays are seen, as snow, rain, and dust all scatter light well. The main types of crepuscular rays are those that shine through holes in low clouds, those that shine from behind a dark cloud, and those that shine from below the horizon. The rays are most frequently seen near sunrise and sunset.
Appearance
- Crepuscular rays are typically red or yellow, depending on when the rays pass through the atmosphere. At sunrise and sunset, the sun's rays pass through up to 40 times as much air as do rays when the sun is higher in the sky. The low angle causes the red and yellow coloring because air particles scatter red and yellow light much less than other colors.
Alternate Names
- Crepuscular rays have been referred to by many different names, from ancient times to modern. The ancient Greeks referred to the phenomenon as "sun drawing water," believing that the rays drew water into the sky. Among other terms for these rays are: sunbeams sunbursts, Jacob's Ladder, Stairways to Heaven, God's Rays, Buddha's Rays, Beams of Jesus and God's Fingers.
Anti-crepuscular Rays
- Anti-crepuscular rays form in the same way as crepuscular rays but appear at a point in the sky exactly opposite of the sun. They are in fact a continuation of the crepuscular rays across the atmosphere. Anti-crepuscular rays are as common as crepuscular rays but are not spotted as easily. When typical crepuscular rays occur, anti-crepuscular rays can be seen by turning around and finding them, like a reflection of the crepuscular rays, on the other side of the sky. Both crepuscular rays and anti-crepuscular rays appear to converge on the horizon, but in both cases the convergence is an illusion. The rays are essentially parallel, but your eye is following them to the vanishing point on the horizon. Parallel railroad tracks or vapor trails of aircraft flying side-by-side would give the same illusion.

Nature