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Looking For Sunbeams

© Keith C. Heidorn

Here on Vancouver Island, the onset of autumn signals the end of generally clear, dry late-summer days and the start of the rainy season. Clouds will thicken and lower, becoming ever darker as their water burden increases, and soon rain will be commonplace and sunlight too rare.

One ray of sky glory common during these months is the brilliant displays of visible solar rays, often tinted red or orange in the twilight hours. They frequently break through during those last minutes of daylight and early minutes of twilight, vividly illuminating the base of an altocumulus or stratocumulus deck.

Solar rays. Every child learns to add them to drawings of the sun. We name them sunbeams and the Ropes of Maui; the Rays of Buddha and Jacob's Ladder; and more technically, crepuscular rays. To see them we need only the sun, something to cast a shadow, plus a little dust or other particles in the air to make them visible.

Sunbeams, as I will call them, appear as light or dark shafts that appear to radiate out from the sun. Although they can be seen anytime the sun is in the sky or just below the horizon, sunbeams are most commonly seen, and often their most beautiful, around sunrise and sunset. Indeed, their technical name crepuscular rays means "rays related to twilight."

Photo courtesy of NOAA/US Dept. of Commerce
To see sunbeams, we need a shadow to be cast. In the skyscape, clouds are the usual shadow makers, although an irregular mountain range may also paint sunbeams across the sky. If you walk or drive through a forest or a dense woodlot, sunbeams may appear filtering through the trees. A wall with windows or skylights invites sunbeams indoors, and in some cases, building designs have included sunbeams as part of the ambience. Whatever the caster of shadows, their critical role is to break up the sunlit sky into regions of light and dark.

Solar rays outside the direct beam, however, are not visible on their own. That requires the assistance of small particles -- atmospheric gases, dust, water droplets, snow or ice crystals -- to scatter or reflect light to our eyes. For example, dust particles can often be seen dancing in a sunbeam shining through a window by reflecting the light toward our eyes.

Generally, sunbeams appear brightest when a dark background, such as a black cloud, provides greatest contrast. A dark background may also bring out very vivid colours in crepuscular rays when sunlight in the red-yellow portion of the visible solar spectrum crosses dark clouds.