Alan Stahler: Rare sunrise after Thanksgiving
December 6, 2017
This column is dedicated to my friends Dale and Diane, on whose ranch I enjoyed this sunrise — one among many.
Air is transparent — light goes right through it — but not without some tweaks.
If the air is foggy, light gets scattered — bounced around — by the fog droplets, making the view fuzzy.
Even in a clear sky, light is scattered by molecules of air. Blue is scattered most, all over the sky, turning the sky blue. Light from the rising or setting sun comes to us from the side, so it travels through more air, scattering, not just blue, but also green and yellow; the remaining sunlight is red.
Morning sunrise happens in a predictable sequence of events: The eastern sky grows light, silhouetting the clouds; the sky grows lighter still; sunlight paints the clouds red; the sky grows brighter; the clouds lose their color; the sun edges above the horizon.
The morning after Thanksgiving, however, the sunrise was different.
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Understanding the sunrise
When light moves from air into water, or from water into air, it bends (which is why a spoon in a half-glass of water looks bent).
Turn your back to the sun as a rain storm ends. Sunlight, coming from behind, enters the raindrops before you, hits the back of each drop, and reflects back toward your eye.
When light enters a raindrop from the air, it bends. When it leaves the drop, and re-enters the air, it bends again.
Different colors bend by different amounts: Blue bends more than green; green more than yellow; yellow more than orange; orange more than red. The colors spread out, creating a rainbow.
Next time you're flying, keep an eye out for a thick bank of clouds below. Find the shadow of the aircraft on the cloud tops. With luck, you'll see a halo of colors, encircling part of the plane's shadow. It's related to the rainbow, but it's not a rainbow. It's a glory.
Notice that the glory is centered on your location in the plane. Move around the plane, and the glory follows: it's centered on your eye.
Back on the ground, before dawn, draw a line from your eye to the eastern horizon. Extend that line into space. If the sun is below that line, you shouldn't be able to see it. But sunlight entering Earth's atmosphere from space — like light entering water from air — bends, down toward the ground. We see the sun some minutes before it's truly above the horizon.
Even before we see the sun, it paints the clouds red, against the backdrop of the brightening sky.
The sunrise on Friday after Thanksgiving followed the usual sequence: the sky brightens; clouds turn red; clouds lose their color; the sun appears over the horizon.
But very early that Friday, before the usual sequence, something else happened. With only a smidgen of sunlight lighting the eastern sky, the clouds over the foothills turned deep red — deep red against the dark, almost nighttime-black sky.
The dramatic tableau lasted some minutes, before the color faded from the clouds, and the background sky brightened. Then the clouds turned red again … now, against a more-usual, fairly bright, sky … and the usual sunrise sequence ensued.
Somewhere to our east, on that Friday morning — perhaps over the High Sierra, perhaps over Nevada — a lens formed in the air, a lens of cold, dense air.
The lens bent the predawn sunlight sharply downward, painting the clouds red against a still-dark sky. Mountain-dwellers call this dramatic skylight "alpenglow."
Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at email@example.com.
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