How white is snow?
About the concepts of snow reflectance
23th of July 2015
Adrian PETER, University of Berne, BE, Switzerland
Katherine POPYACK, Hartwick College, NY, USA
Elizabeth PERERA, DePaul University, IL, USA
In everyday life we experience different surfaces reflecting light. Whether it be by the sun shining into the sea during a nice boat ride or a skyscraper’s window reflecting the sun’s rays, we all know how it feels to be momentarily blinded by the sun. This is because the light from the sun travels to the earth whereupon it bounces off the surface and shoots directly into your eye (if the angle is correct).
A second observation we probably all make is that, compared to light colors, darker colors get warmer under the sun. That’s why in summer, wearing a white shirt rather than a black t-shirt in is often preferred, which scientifically is due to the relative amount of reflected sun rays to the absorbed sun rays. The darker a surface is, the more light it absorbs and the warmer it becomes.
So we’ve just stated two major processes. First, light is likely to be passed onwards by bouncing off a surface. Second, not all the incoming light will be reflected – a portion will be changed into heat and therefore absorbed by the surface. This all depends on the features of the surface.
This relation between incoming and outgoing light is referred to as reflectance. The higher the reflectance, the brighter the surface.
But why does this matter? Snow is white, right? Although most snow appears white to the human eye, there are in fact many different shades of white. Moreover, snow may also be covered by dust, soot, organisms or other debris. Individually, every single impurity reduces the mirroring ability of snow and leads to more surface melt. Additionally there are also light spectra we humans cannot see which also impacts the reflective properties of a surface. Such spectra are near infrared or ultraviolet.
But why do we care? By measuring the reflectance of snow we try to determine the impact of several parameters on the ability of snow to reflect light. Elizabeth wants to determine how much the grain size of snow crystals matters, Katherine is focusing on a particular species of reddish algae that is able to live in cold/harsh conditions, and Adrian is asking how much black carbon and dust are affecting the brightness of the snow. Although there are more surface properties that affect Icefield reflectivity, up here we are limited to only one instrument to measures spectral radiation (called a spectroradiometer). On a maritime glacier such as the Taku in Southeast Alaska, surface processes are playing a major role in mass loss.
JIRP students Katherine, Lara and Elizabeth (from left to right) taking reflectance measurements. Photo: Allen Pope
Here at Camp 10 on the Juneau Icefield, we have the opportunity to take reflectance measurements with our boots in the snow (in-situ). Our normal routine involves grabbing the scientific gear, our skis, some snacks, and skiing over to field locations to collect data. In order to take reflectance measurements we point the sensor above spots which have the specific features of interest, save the readings, and add notes manually (e.g. coordinates and description of what we measured and observed). Back in camp we process the data we’ve collected on the computer (over a hot cup of coffee) and compare it to our notes. Afterwards we analyze our processed data by comparing it to what we understand snow reflectance should look like. If we are able to find some patterns in the shape of the reflectance curve (e.g. same depression at same locations for every measurement of dirty snow) we may be able to link it to specific features and therefore gain an answer to our questions.
At the moment we are focusing on plotting and then comparing our first few measurements. Hopefully we will be able to provide you with answers in the next weeks. Stay tuned!
If you’re keen to know more, check out this video, too: