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.
Sunbeams appear brightest when there is great contrast.
A beam's contrast also depends on the density of scattering particles, the scattering angle of the sun, and the line-of-sight distance through the light ray. The highest degree of contrast occurs when the rays are viewed toward or away from the sun rather than when viewed at right angles to the beam, such as when they pass overhead.
The pattern recognition functions of our brain may also cause us to perceive "dark rays." These are usually seen when scattered clouds intercepting portions of the sun's radiance cause us to interpret their shadows as "rays" rather than the bright sky patches between the shadows. The accompanying figure showing alternate bands of light and dark illustrate this point. Look closely at the bands, are they light on dark, or dark on light? Your perception may jump between both visualizations.
Another illusion arising from sunbeams is the perception that they converge on the solar disk. In fact, light rays emanating from the sun are parallel when they strike the Earth. That sunbeams appear to converge on the sun is due to perspective the apparent convergence of parallel lines at some distant vanishing point. This is the same visual effect that causes railroad tracks or telephone lines along both sides of a highway to appear to converge in the far distance.
The most beautiful and striking visualizations of sunbeams are the crepuscular rays seen when the sun is near the horizon. Crepuscular rays are usually red, orange, and yellow in colour because blue light is effectively scattered out by air molecules and very small particles in the sky.
Photo courtesy of NOAA/US Dept. of Commerce
Although we see crepuscular rays diverge from the sun outward toward us, if we turn around, we may see them converging toward the eastern sky. These portions of the rays that converge on the anti-solar point of the sky are called . Anti-crepuscular rays when visible appear as a pastel pinkish glow against the darkening blue sky.
Now that dawn comes at a more civilized hour, I have two daily chances to marvel at the beauty of crepuscular rays. Often they tinge the underbelly of a long grey cloud with a reddish orange hue called salmon, an appropriate image for those of us on the Pacific coast.
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