Plastic snow

Sastrugi - the sharp features of a windswept snow surface bordering fractured plates of ice in middle of photo.

When I was younger, I had one word to refer to all that white stuff in winter: snow. As I got older and had more experiences with it, particularly through snowboarding, I realized that not all snow is created equal. Snow taxonomy has a history much older than winter ecologists, and much of the rich language found in cold climates to describe snow has been adapted by winter ecologists (like the Russian sastrugi - above, or Innuit qali - below).

Snow can be grouped into three categories:

1. Falling snow
2. Snow on the ground
3. Surface-generated ice features (like ice needles being pushed up from the ground on wet trails, hoar frost, icicles seeping out of cracks in bedrock)

Qali describes snow hanging from trees (like in the image above). Because our snow the other night fell in a frenzy of blustery winds, there wasn't a whole lot of qali left on the trees. Sintering is a natural process where snowflake crystals fuse together. Sintering can happen due to pressure (like stepping on the snow pack) or temperature (melt-freeze together). Because the snow crystals actually bond together, qali will act as a liquid moving in slow motion. Snakes, like in the image above, are common features in northern woods after a good snow storm. We observed a spot today where the sintered snow had bent down around the rock, creating a perfect overhang for a weasel to pop in and out of while hunting.

Sun halo

What: Over the next week I'll be posting photos from an wonderful wander I took out at Rock Point with Crow's Path intern extraordinaire, Brooke and her friend, Alana. I took the above photo after we watched a bald eagle fly around the point. We caught a glimpse of the eagle about an hour later soaring north over Apple Tree Point (look here for more recent sightings of bald eagles in Burlington).

Ecological notes: Ice crystals in the atmosphere cause all sorts of atmospheric disturbance, resulting in beautiful optical effects (like irridescent swaths carved across high altitude cirrus clouds). According to Storm Dunlap's The Weather Identification Handbook (yup, his name is Storm), these sun halos are quite common, occuring on about 1 out of 3 days in Britain and western Europe. Like all rainbow optical illusions, the phenomenon is created by the refraction (scattering of light) traveling through a medium, in this case ice crystals.

sun halo again here on Lone Rock

Conditions that favor appearance of sun halos:

• Thin veil of cirrostratus clouds (cirrostratus clouds are among the more common, but least noticed clouds. In part because they are very thin, and often nondescript, just giving the sky a general "milky" wash). 
• Incoming warm front (often associated with previous bullet) - where was our warm weather??
• Winter in continental regions (as opposed to polar regions) where tiny ice crystals drift through the air. 

Sun halos form as little ice, in the shape of hexagonal prisms like the ones below (click here for a chart of snow crystal classification and here for a chart relating crystal type to temperature/vapor supply), drift through the air. Under aerodynamic forces, they tend to fall like as leaves do, with their horizontal axis parallel to the ground, or rather, broader surfaces at the bottom, like the prism on the right. When light that passes through two faces joined at a 60/120o angle (as on the left), light is bent at a 22o angle off the straight line. Scattered light appears to us in the halo offset about a hand length's distance from the sun. When light passes through two faces joined at a 90o angle (as on the right) light is offset by 46o and forms a wide arc around the sun. Since the crystals tend to fall horizontally aligned, it is very rare to see a 46o halo. 

22o sun halos are not rare, yet they are rarely observed. The reason this common phenomenon goes unnoticed is that the conditions for creating this phenomenon are bright conditions where the clouds are mostly imperceptible, again, that milky white sky. This was particularly true last Thursday with the salt making the roads whiter and bright white snow reflecting all that sunlight making it hard to see much of anything (and indeed at times I regretted not bringing sunglasses).

I exagerrated the colors to bring out the optical illusion.

Where: Rock Point, Burlington, VT

Wet vs dry snow

I'll be posting over the next week about some of the snow/ice phenomena that I observed today while walking around Rock Point. But before I go into that, I wanted to say a bit about what I've been thinking about since last week's snow fall. The first snow fall (on the 16th) fell during the warmer spell. You may have noticed giant snowflakes, which were actually just conglomerates falling together. My suspicion was that the white orbs in the center of each flake (seen in the photo below) are melted snowflakes that fused together with flakes that hadn't yet melted as well as had begun again the process of snowflake formation.

So what happens to snow once it falls? After snow falls on the ground it can either remain unchanged, which is never permanent, or it undergoes metamorphosis. Metamorphosis can happen in a few ways:

1. constructive: like the formation of depth hoar - aka sugar snow, which forms the ball bearings on which avalanches often form. This occurs when there's a sharp contrast or gradient in temperature from the surface of the snow pack to the bottom. Temperature gradient creates a gradient in vapor pressure and water gets passed from warmer areas (high vapor pressure), near the ground, to colder areas (lower vapor pressure), near the surface
2. destructive: (the "melting" of a snowflake to a little knobby snowball on slightly reminescent of the flakes original shape). Water vapor is released from the points of a snow flakes and is passed to the pits of the flake (near the base). This is a very complex process and I suggest Jim Halfpenny's Winter: An Ecological Handbook
3. firnification: melt-freeze temperature swings or applying pressure (like stepping on snow or another snow storm putting weight on top of older snow) can fuse snow crystals together. The first process can create dangerously slick crusts on the surface of the snowpack, which coincidentally protects small tunneling mammals underneath; the second force is what gives quinzhees their solid, insulating structure.
4. wind: wind can break snow flakes under mechanical force into smaller pieces. 

Winter Storm Nemo time lapse

What: It didn't turn out nearly as well as I had hoped, but on Thursday night I set out my camera to capture a timelapse of the incoming winter storm Nemo. If you look at the pallet at the bottom of the screen you can see rise in snow as it accumulated. The winds knock off most of the snow from the trees by the second day.

Where: My backyard

Other notes: Meryl pointed out to me that winter storms only recently started being named storms. The Weather Channel, aka weather.com, published a press release explaining their new self-appointed role of winter storm namers. here's an excerpt:

"During the upcoming 2012-13 winter season The Weather Channel will name noteworthy winter storms. Our goal is to better communicate the threat and the timing of the significant impacts that accompany these events. The fact is, a storm with a name is easier to follow, which will mean fewer surprises and more preparation. 

Naming Winter Storms
Hurricanes and tropical storms have been given names since the 1940s. In the late 1800s, tropical systems near Australia were named as well. Weather systems, including winter storms, have been named in Europe since the 1950s.  Important dividends have resulted from attaching names to these storms:

• Naming a storm raises awareness.
• Attaching a name makes it much easier to follow a weather system’s progress.
• A storm with a name takes on a personality all its own, which adds to awareness.
• In today’s social media world, a name makes it much easier to reference in communication.
• A named storm is easier to remember and refer to in the future."

Wind crusts and all that snow

What: I went for a walk with Jon yesterday and while there was certainly a lot of snow on the ground, almost all of the snow in the trees had come down. I started thinking about all of the other ways I might observe wind. I was at Rock Point Friday night and by Saturday morning the winds had picked up and cleared out the thin layer of slush in Little Eagle Bay (what we call Mink Bay) and replaced it with stacks of ice plates. The winds picked up all day and by the afternoon they were stinging our noses. Because of the cold, the snow was "dry". But the wind had dried the snow right up, and sure enough it slid right off of railings, my car, and anything else it was perched unto.

The dryness of the snow meant that by late morning almost all the snow had come down off the deciduous trees and conifers that weren't sheltered by other conifers. I tried to capture the grace of snow clouds drifting whimsically through the woods but couldn't quite get it. I think it needed video.

Deep wells had formed at the base of most of the larger trees indicating sustained unidirectional winds that had carved gullies where eddy currents downwind of the trees. The trees all had gullies on the south sides, and sure enough our winds were out of the north.

Jon commented on the lack of scale with snow. I think topography is one of the easier things to confuse my sense of scale. Looking at ripples in the sandy basin of a river I can easily imagine running across sand dunes. Looking down at a snowy aster poking up through the snow I was immediately transported out onto the vast open tundra.

Ecological notes: Crusts on the top of snow form in three ways. 1) Sun crusts (aka spring crusts) form when the sun melts surface snow, which then refreezes. 2) Freezing rain: supercooled rain falls and freezes when it hits the snow surface, forming a solid crust (other surface water processes could be included here), and 3) wind crusts, which form as wind breaks up flakes and packs them together.

Red Squirrel Tracks

What: One of my favorite finds of the day on Saturday was a beautiful set of red squirrel tracks that had been preserved on top of the snow. Compressed snow melts slower than light fluffy snow, so you can often find tracks embossed, or raised in the snow as the surrounding snow melts away. Here, we found a whole set of red squirrel tracks that looked like little ice buttons sitting on the surface of the snow. Once we spotted these I started noticing similar tracks everywhere - particularly the raised ridges of cross-country ski tracks. Sorry for the high contrast photos, but it was about the best I could do with the lighting (I had to ramp up the contrast to make them more visible).

Here the squirrel is moving away from the camera. It's two smaller front feet land in the middle, slightly offset from one another. The larger rear feet land after the front feet pick up and on the outside, in line with each other. Red squirrels tend to land with their feet more offset than gray squirrels. I think I learned this from Jim Halfpenny's tracking book: animals that spend more time in trees (e.g. gray squirrels) land with their front feet in parallel (like they'd been while perching on a branch) and animals that spend more time on the ground (e.g. rabbits) land with their front paws far more offset.

Where: The Intervale