Crepuscular Rays

The literal meaning of the word "crepuscular" is " . . . relating to twilight" (Compact Oxford English Dictionary).  It refers to the in-between time betwixt "diurnal" and "nocturnal" and is used in biology to describe animals, especially insects, which are most active at twilight.  The phrase "crepuscular ray" is used because atmospheric conditions favorable for seeing these rays are most likely in the early morning or late afternoon.  Most commonly, however, the rays are seen when the sun is still above the horizon, so in that sense the term is a bit of a misnomer.  Actual twilight occurs before sunrise and after sunset.

Since these rays are produced by Mie scattering, they are colorless and most intense when the atmosphere is laden with aerosols, e.g., haze, dust, fog, etc., insufficient in number to completely block the sun's rays.  The rays become visibly prominent only when contrast is produced in their intensity by shadowing.  Most commonly the shadows are produced by clouds, but easily seen crepuscular rays are sometimes produced by shadowing mountains (especially in Provo just before sunrise), vegetation, or even buildings.  Sunbeams passing through a dusty room may even produce indoor crepuscular rays.  Sometimes the aerosols producing indoor crepuscular rays are so close to the observer as to be individually visible.

Because there are always some particulates in the atmosphere, crepuscular rays are always present when the sun is shining, but without the shadowing mentioned above, they are not seen because of lack of contrast.  They are much less likely to be prominent when the sun is high in the sky because the bright illumination of nearly everything at mid-day washes out the contrast which makes them easily visible. 

Under favorable conditions moonlight, especially if the moon is full or nearly so, can produce easily seen crepuscular rays.