Let it snow, let it snow, let it snow…It’s that time of year where I have holiday songs stuck in my head and here in Colorado I find myself surrounded by a foot of snow, which has obviously inspired this month’s article! For those of you in warmer climate, I guess you will just have to use your imaginations.
When you’re walking in a winter wonderland, you will notice that sunglasses are a definite necessity. A blanket of snow can be blinding since snow has a very high albedo, which is the measured reflectance of wavelengths off of a particular surface. The high reflectance of snow is not restricted to just one small section of the electromagnetic spectrum, its albedo can range between 80% and 90% in the visible and near infrared spectra.
As you can imagine, clouds have a very similar effect as they reflect wavelengths from the sun and other solar bodies back into space and through our atmosphere. There have been times in Boulder where the town is covered in snow while clouds hang in the air. On these days, as the sun disappears over the mountains, dusk stretches on for hours giving the town a purplish glow. This is caused by reflected light which is trapped in the area by the two white surfaces.
So then how can we tell the two surfaces apart in imagery if they look very similar? How can they be measured independently of one another? While snow and clouds reflect similarly in the visible and near-infrared, they start to diverge as you move further along the spectrum to the middle-infrared. Snow reflectance plummets at wavelengths greater than ~1.5-µm, while clouds continue to reflect at a very high rate. At 1.5-µm, snow appears almost ‘black’ and reflects less than 10% of incoming energy, while clouds are still very ‘white’ with a reflectance around 60%. This divergence in middle-infrared reflectance comes in handy in areas that are covered with snow, such as the arctic, where it is very difficult to differentiate between the two.
Spectral reflectance curves of snow and clouds (Graph Courtesy: Aligarh Muslim University, Department of Geology)
The world would look very different if the human eye could see wavelengths in the infrared region. So the next time you’re riding through a wonderland of snow or dreaming of a white Christmas, you might just see it in a different light. Happy New Year!