Crepuscular rays are rays that occur during the crepuscule, which is a fine old word for “twilight”. They’re the rays of brightness and shadow that seem to fan outwards and upwards from the setting or rising sun when it is masked by cloud. What’s happening is that the shadow of the clouds is being projected across the sky above your head. Dust and moisture in the air up there is being illuminated, or cast into shadow, and we see bright and dark streaks across the sky as a result. So we only get crepuscular rays if there’s something in the air to be illuminated—on a dry day with little dust or smog, there’s no hope of a spectacular display of rays like the one above.
One striking thing about the image above is that there is a noticeable dark shadow framing the cumulus cloud. That implies that there is an illuminated surface somewhere above the visible cloud. There’s actually a thin layer of higher stratus cloud, on to which the shadow of the cumulus is being projected. The illuminated stratus also accounts for the beautiful golden yellow hue of the sky. Just a couple of minutes later, the lengthening shadow on the stratus is more evident:
In the daytime, a place to look for clouds casting shadows on other clouds is around the tops of towering cumulonimbus. Sometimes the rising tops of these clouds push upwards through a layer of cirrus, and the sun will project the shadow of the crown of the cumulonimbus downwards on to the thin layer of cirrus.
Crespuscular rays become less evident as they fan out from the sun. My diagram shows what’s going on:
The individual bright rays are delineated by cloud shadow. When you look towards the sun, you’re looking diagonally through each shadow zone. The long sightline within the shadow makes it appear noticeably darker than the background sky. But as your gaze sweeps upwards, away from the sun, you begin to look through the shadows at right angles. The shorter sightline makes them progressively less evident, and its unusual to see crepuscular rays extend right overhead. But look what happens behind you in the diagram—your sightline is diagonal again and, although the shadow will have become more diffuse as light scatters into it from the surrounding air, there’s a possibility it may return to visibility behind you. So whenever you see crepuscular rays, you should turn around and check the sky in the opposite direction.
You may also see clouds in the sky behind you generating their own visible shadows. Just as perspective makes the crepuscular rays seem to radiate outwards from the sun in front of you, it makes these anticrepuscular rays behind you appear to converge on a point directly opposite the sun:
The same phenomenon that produces crepuscular rays is also responsible for the appearance of sunbeams shining downwards through the clouds:
Perspective again makes these beams appear to radiate from a central point, centred on the sun above the clouds.
Since these don’t happen at twilight, they shouldn’t really be called crepuscular rays, though they often are. The common word sunbeam seems as good as any; solar rays is a catch-all term that includes crepuscular rays; computer graphics artists call them god rays; and Marcel Minnaert, in his marvellous book Light and Color in the Outdoors, introduced me to the old expression “the sun drawing water”, from the old belief that water evaporated along the sunbeams.