If lately you find yourself daydreaming about seeing rainbows in the April showers of spring, which, I’m sorry to say are still some ways off, you may want go with the flow of winter a bit and take time to look for some remarkable rainbow-like apparitions that are common this time of year: sundogs and sun pillars.
Sundogs are bright knots of light that sometimes appear on both sides of the sun when the sun is high in a cold sky, lightly draped by high, thin clouds. A sun pillar is a brilliant, vertical shaft of light that sometimes appears at sunrise or sunset, looking much like a powerful searchlight shining into the darker heavens above the horizon.
January through February is the best time of year to see sundogs and sun pillars (in the Northern hemisphere). Look for them anytime the winter sky is not quite clear, especially just after or before a very light snowfall.
If you’ve never seen a sundog or sun pillar and want to get a better idea of what to look for, do some searching on the Internet, or, for a quick fix, visit www.astrophys-assist.com and follow the “Weather Window” link.
Rainbows spring to life when sunlight shines just the right way through a mist of tiny water droplets. In somewhat the same way, sundogs and pillars form when sunlight shines through tiny ice crystals fluttering to the ground from high above earth’s surface.
Sundogs and sun pillars aren’t random. Sundogs always appear 22 degrees horizontally to either side of the sun when the sun is near the horizon; sun pillars are always vertical (but most of the time point upwards).
Now you might ask: how can a fine fog of fluttering ice crystals glint sunlight in such particular ways?
The answer becomes clear when you consider that tiny ice crystals, like snowflakes, are not randomly shaped. Ice crystals that form from water vapor high in the atmosphere where the humidity is relatively low end up regularly shaped like tiny six-sided platelets.
The six-fold – or hexagonal – symmetry of crystalline ice is perhaps nowhere more evident than in micrographs of individual snowflakes. Snowflakes are shaped in complex, unique ways, and although it is often said that no two snowflakes are alike, snowflakes are most often six-sided in some way. Check out SnowCrystals.com.
So, atmospheric ice crystals are not randomly shaped. But in order to turn every-which-way rays of sunlight into obedient sundogs and pillars, the crystals must also not be randomly oriented. What aligns them as they drift around in the atmosphere? You can answer this question for yourself by dropping a light-weight paper plate from high above your head. Watch how it falls to the ground, plate-side up, rocking gently back and forth.
High ice crystals drift slowly to the ground like tiny paper plates, rocking gently this way and that, but with their shiny surfaces mostly facing up and down.
A sun pillar appears above the sun near sunrise or sunset when sunlight shines through a thin cloud of falling ice crystals: rays of sunlight that would have passed overhead instead are reflected downward to your eyes by the shiny bottom surfaces of ice crystals acting like miniscule air-born mirrors.
Sundogs are produced in a similar way: they are formed by rays of sunlight that would have passed to your left and right but instead were refracted to your eyes from a particular spot in the sky by ice crystals this time acting like so many hexagonal, Lilliputian lenses. A sight to see.
Sun pillars, sundogs and a wide variety of other optical phenomena in the atmosphere are explained in great detail at www.atoptics.co.uk.
Embrace the winter! Get outside, even though it’s cold, and look for sundogs and sun pillars – winter’s rainbows. That is if it ever stops snowing.
