From AMS Glossary
(Also called shadow bands.) Literally "twilight rays," these alternating dark and light bands (shadows and light scattered from sunbeams, respectively) seem to diverge fanlike from the sun's position during twilight.
This apparent divergence of parallel sunlight is an artifact of linear perspective. Crepuscular rays may appear as 1) shadows cast across the purple light by high, distant cloud tops or 2) shadows next to light scattered from sunbeams by haze in the lower atmosphere. Sunbeams seen during the day are sometimes called crepuscular rays, even though they are observed outside twilight.