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.
