With all this white stuff falling from the sky and blanketing the ground, and the moon and Venus shining so brightly in the sky, I’m reminded of an interesting concept I learned about in graduate school: albedo. Astronomers use the term albedo to quantify how much visible light a surface or object diffusely reflects.

Consider snow, for example — plenty of that around to illustrate the point. The albedo of freshly fallen snow is very nearly 1, meaning that a surface of fresh snow reflects nearly 100% of the visible light that falls on it.

Note that a smooth, snowy surface reflects light differently than, for example, a nice clean mirror. A snowy surface on a sunny day reflects incident light rays every which way, making the surface appear bright no matter how you look at it. A mirror reflects incident light rays so that each ray goes out at the same angle, relative to the mirror’s surface, as it went in. The term albedo is usually used to quantify diffuse reflection (or scattering) off a relatively rough surface, like a snow hill or a dirt road, rather than what’s called specular reflection off a smooth, shiny surface, like a mirror.

The albedo of snow is among the highest of all naturally occurring materials; snow is very white and bright, even on a mostly cloudy day. And speaking of clouds, when illuminated by the sun, they are very bright too.

It’s interesting, but off topic, to wonder why clouds and snow — both composed of water, which is transparent to visible light — have such a high albedo. And why the foamy head on a nicely drawn mug of beer is bright white, when the beer itself is yellowish and transparent. That’s for another column.

Astronomers have long measured the albedo of solar system objects to learn what the planets might be made of. If you’ve seen Venus lately, you won’t be surprised to learn that Venus has the highest albedo of any planet in the solar system. Venus reflects between 60% and 70% of the sunlight that falls on it. You might also be surprised, given the recent famously bright full moons, to hear that the moon’s albedo is very low. The moon reflects only about 10% of the sunlight that falls on it.

Imagine if the moon’s albedo was as high as Venus’. A full-moon night here on earth would be six times brighter.

We can also consider what earth must look like from other planets. Earth’s albedo is about 0.35, about half way between that of the moon and Venus. Viewed from Venus, the earth, which is about the same size as Venus, would appear only half as bright as Venus does from earth.

What’s Venus got that earth doesn’t? Earth is one-third covered by dark landmasses and two-thirds covered by water. And although the albedo of water can be very high, like when water is in the form of tiny droplets or frozen flakes of snow, the albedo of liquid water — of the oceans — is in fact very low. Liquid water is mostly transparent, so sunlight goes right into the oceans and is eventually absorbed. In fact, if it weren’t for the clouds that always partially cover earth’s surface, earth’s albedo would be much lower than it actually is.

Venus is totally cloud covered, and that’s why its albedo is so high, and why it is now so bright in the evening sky.

This column originally appeared in the Grand Haven Tribune on 16 January 2009.

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.