Anti Crepuscular Rays
When the sun is hidden by a thick cloud, there sometimes appear dark, fan-shaped rays emanating from the cloud. These are simply the shadow of the cloud. The shadow rays are only visible when the atmosphere contains lots of small water droplets or dust particles. The part of the sky hit by sunlight scatters the light and thus stands out from the shadow of the cloud. All rays seem to converge in the sun. Their fan-shaped spreading is caused by a perspective effect. Actually all rays are parallel. But just as rails seem to converge in the distance, the sun rays also seem to converge in one point. A contrary effect appears when the sun shines through a small gap in the clouds. Then a pyramid of light appears standing out especially well from dark clouds. Also in this case the light rays are actually parallel.
When the sun is very low near the horizon or has just sat, the light
and shadow rays can extend over the whole sky (crepuscular rays) and converge
at the antisolar point (anticrepuscular rays).
© AKM e.V.