Photo: Circumzenithal arc
Winter is the perfect time to watch the skies – and not just at night. To be sure, the cooler drier air this time of year does make for clearer skies at night. But I’m talking about daytime sky watching, and in the winter this means atmospheric phenomena.
I’m something of an atmospheric phenomena geek. I actually have a “life list” of atmospheric effects I’ve witnessed, from double rainbows to sun pillars, and rarer phenomena like glories and circumzenithal arcs. Once you know what to look for and when to look for it, you start noticing things that others rarely see – and some of it is pretty amazing.
Atmospheric light shows happen all the time. Think of the colors you see at sunset, or the way sunlight creates those convergent rays when peeking through clouds (those are called crepuscular rays). Chances are, you’ve seen many of these effects and wondered what caused them. It generally comes down to one or more of the following properties of light: reflection, refraction, diffraction, and scattering.
People have written whole books about this topic and I’m not going to go too deeply into the physics of it here, but in case you’re interested I recommend Robert Greenler’s fine book Rainbows, Halos, and Glories. There are several great websites on this too, including http://www.weatherscapes.com/ .
Atmospheric phenomena can happen anytime of the year, day or night, but wintertime can be especially good for glimpsing ice crystal effects like halos, arcs, and sundogs. Here are a few of my favorites:
Ice crystal halos
Halos are large, white rings of light that partially or completely encircle the sun or moon. Haloes are caused by sunlight refracting through specific shapes of ice crystals in the upper atmosphere. When you see a halo, chances are it will be located 22 degrees away from the sun or moon in any direction. If you hold your fist at arm’s length and extend your thumb and pinky, the span between thumb and pinky will be just a bit more than 22 degrees. The 22 degree halo is very common and can be seen practically anytime there are high altitude cirrus clouds in the sky near the sun or moon.
Sundogs are very common and resemble bright spots of light about 22 degrees away from the sun on either side. Sometimes you’ll see concentrated rainbow colors in sundogs, with red on the side closest to the sun and blue on the outside. They occur most often when the sun is low in the sky, and are frequently associated with halos.
Sundogs form as sunlight is refracted by hexagonal plate-shaped ice crystals that have their flat faces horizontally oriented with respect to the viewer. The ice crystals act like prisms, bending sunlight to create the effect. A similar phenomenon can happen at night with the full or nearly full moon, called a moondog.
As its name suggests, this delicate arc of rainbow-colored light appears to partially encircle the zenith – the overhead point in the sky. It is one of the most overlooked atmospheric phenomena, mainly because one must look very high in the daytime sky to see it.
The same milky white cirrus clouds associated with halos and sundogs often create circumzenithal arcs. The sun must be less than 32 degrees in elevation to see them. I always make a habit of looking up when I go outside, covering the sun with my hand to prevent glare and looking high overhead for this beautiful arc.
Keep Looking Up!
All of these phenomena can be seen year-round in the daytime sky if conditions are right. Hopefully I’ve enticed you to keep your eye on the sky so that you, too, can enjoy nature’s light show.