April 9th: Now that we have arrived the work begins. We spent the day checking out the snow, putting our ground-based LiDAR together and testing it, and dealing with some lingering logistical problems (like making sure ten barrels of aviation fuel arrive before the plane does).
Checking out the snow is my favorite activity, and being the expedition leader, I got assigned this task. I have been doing snow pits for 30 years. A snow pit is a hole you dig in the snow (neatly and with very vertical walls and square corners if you were trained by my mentor) in which you can look at the snow layers (Fig. 1).
The layers are a stratigraphic record of the winter’s events- - -wind events, snowfalls, and thaws. The whole story of the winter is there if you know what to look for, but it takes a trained eye. The layers in a snow pit are white on white ….the clues are nuanced and subtle . . . no bright color contrasts here, unlike a soil pit or geologic rock section. Still, with practice, anyone can learn to see these layers and understand how they came to be.
To help interpret the snow pit, we use a variety of tricks to delineate and accentuate the layers. For example, using brushes, we whisk the pit face (Fig. 1).
This sweeps away material from the weaker layers, making the harder wind slabs stand out in bolder relief. Another way is to back cut the pit, allowing sunlight to shine through the snow, essentially the same effect as if we were looking at a section of the snow on a light table (Fig. 2). Both the brushing and the light tend to highlight the grain differences between layers, and these differences arise from differences in snow metamorphism. Surprisingly little of the layer texture is due to the nature of the initial snowfall. A third, newer method to expose the layers, is to photograph the pit face using a near-infrared camera, which is very sensitive to variations in light reflection due to grain size differences. Unfortunately, this method requires post-processing, so it does not have the immediacy of the other methods.
There are dozens of types of snow layers in nature (not to be confused with snowflakes, of which there are many as well): new snow layers, recent snow layers, fine-grained layers, melt clusters, ice layers, grauple. And there is hoar: surface hoar and depth hoar. It turns out these latter are both very common in the Arctic, and in a moment I will explain why, but first a disclaimer: this type of hoar is quite different from the other type, more commonly found in gangster movies etc.
The word “hoar” is related to the word “hoary” meaning gray, old, and venerable. Apparently, the similarity between frost feathers and an old man’s beard led to the use of the word in snow science. Frost feathers are ornate, though usually flat or fern-like, frost crystals that condense on surfaces during cold still periods (Fig. 3).
Surface hoar is similar to frost feathers, but it forms on the surface of snow pack on cold, clear nights. Depth hoar is equally ornate, but is more three-dimensional (Fig. 4), and it forms throughout the winter at the bottom of the snow pack (hence the “depth” part).
The reason why the Arctic snow pack has such a high percentage of depth hoar and frequent surface hoar formation is simply because it is cold. It will come as no surprise that this is a cold place in winter, and even when spring is arriving (as it is now), it can still be extremely cold at night. Cold air creates temperature gradients. The snow surface will be perhaps -30°C, while the base of the snow will be -10°C.
Heat moves from warm to cold, and moisture follows the same gradient, so moisture in the form of water molecules are constantly moving upward from the relatively warm ground surface below the snow through the porous snow pack, condensing on the lower sides of crystals, causing them to grow (and have razor-sharp edges), and sublimating from the tops of the grains, making the tops rounded. In the case of the surface hoar, on a cold, clear night the snow surface cools by long wave radiation, and soon is the coldest surface around.
Any moisture migrating upward from below, or downward from above, condenses out as surface hoar. All three types of crystals are ornate, with sharp edges and well-defined facets because they grow in very moist environments. The supply of moisture for growth is not the limiting factor: instead the crystal kinetics control the growth (a good topic for another blog. The end result is beautiful crystals in all there cases.
These are the hoars….surface hoar, depth hoar, hoar frost. While they can be found in many snowy locations, they can be found in their prime in the Arctic.
The other common type of snow layer in the Arctic is the wind slab, and it too can be amazing. In a recent Guest Blog post I wrote about blizzards and how snow is moved by the wind. The depositional results of blowing snow are dunes, wind slabs, sastrugi, and barchans.
Many people have seen a barchan without realizing what it is: a moving dune of snow or sand, with horns or wings that point downwind. This winter I was flying back from Western Alaska when I saw a barchan field (Fig. 5) march down a hill into a small creek.
Each barchan was about 10 m long. I mention these because today, when we dumped the webcam at our meteorological tower, we found we had been lucky enough to catch the march of the barchan across the field of view (Fig. 6 film clip).
Barchans seem to form early in a combined snow and wind storm when the snow is easily transported. Later, the barchans snow will sinter (bond) and stabilize. Then if the wind comes up sastrugi, a beautiful, sculpted erosional form of snow will form (Fig. 7).
Neither sastrugi nor barchans are easy to recognize in a snow pit. Most common in a pit are wind slabs….hard, well bonded layers of snow……sometimes so well bonded they can only be dug with a sharp-pointed steel spade (Fig. 8).
We are not the only ones in the Arctic thinking about wind slabs. Large animals like this muskox (Fig. 9) and caribou have to work down through these hard slabs to get at their food.
A hard wind slab can make that effort too taxing, so they are excellent at finding where the slabs are thin or non-existent…..better even than Arctic snow scientists at doing so. But more on this process (called cratering) tomorrow.