Where do all them ducks go

What: A few winters ago I spotted large numbers of mallards (200-400) flying westward towards the lake around 4:30pm or so, just around dusk. Despite Burlington Bay being frozen at the time, as it is now, the ducks were still flying that direction. My interest was piqued and I wondered where o where those ducks were all spending the night. I spent the next few nights out around the same time trying to spot them. Sure enough, around the same time, a large flock of ducks would fly westward over Centennial Woods.


Audio of the ducks. You can hear them flapping their wings as scores of them would leave and others arrive. Our presence seemed to make them jumpy.  

As with the crows (I have yet to follow the ring-billed gulls) I decided to track them down and see where they were roosting each night. A few days after seeing them for the first time, I fortuitously wound up spotting - or rather hearing - them while out on a run. As I was coming north on the bike path, I passed the water treatment center's clarifiers (those big circular tanks with exposed water) and heard some noises. I peeked over the fence and there they were!

Mallards landing in circulator

I hadn't been back in a while, and the recent postings on the Vermont Birds list host about ice closings on the lake inspired me to back last Thursday night with Jon. We were greeted with a raucous reception. Our estimate put the number of mallards in just one of the clarifiers well over 1000. In the photo above, there were probably about 20% of the original number of ducks.

The other question is where do they go during the day? I would assume that there are a number of small openings in rivers, industrial parks, and other spots where they have access to open water (eBird gives a good sense of this with birders' postings from around Vermont: Map)

Illusion of open water

What: When we first arrived at the Thrust Fault, looking north to Apple Tree Bay it appeared as though the bay hadn't frozen entirely on the other side of the bay. In the picture above you can notice the thin strip of darker area that's reflecting the houses and trees in it. I've often noticed this effect when looking across the lake at NY, where the thin strip of land closest to the lake often appears twice as thick, as though in reflection. I'd always kind of unthinkingly attributed this to simply being a reflection of the horizon in the water.

But when we climbed up to the top of the point and changed our perspective, it became quite clear that the bay was indeed completely frozen over and the "reflection" wasn't simply a reflection (the surface of the ice is super rough and not reflective. What was it then?

I had also noticed a thin rippling effect in the air while looking out across the lake. It reminded me of my childhood in Southern California. I loved watching those heat waves dance up off the pavement - the phenomenon of a distorted horizon when looking down a long road in the middle of the desert. As a child I remember seeing the ripples on the road, but I don't remember the reflective quality. I was more than surprised to see those with mid-day temperatures hovering around 0.

I remembered looking into this in regards to the lake a couple of summers ago, but had forgotten the details so wanted a good refresher. The cause of the effect is the same in the winter as in the summer, as the ripples are indeed caused by heat, but it's actually relative heat. The effect, known as an inferior mirage occur when the surface is much warmer than air directly above it. The gradient needs to be about 5degrees per meter (normal atmospheric change is about 1degree of cooling for each 100m you travel up in elevation). While the lake is frozen, it's not super thick ice in parts so the warm water temperature (about 35) heats up the air right above the ice while the ambient temperature was frigid (about 8 on that day). So sufficient temperature gradient to cause the optical illusion!

The refraction occurs only at shallow angles, so as soon as we changed our vantage by going up to the point (as in the photo above), the mirage disappeared and we were left wondering when the rest of the lake would freeze over already!!

Icicles abound!

What: Winter always makes me remember how I forget about the winter when it's summer. It's hard reconciling the stunted, gnarled growth of the silver maples and white cedars that grow along the edge while sitting on the shore, my feet gently washed by the warm waters lapping up under a lazy summer sun.

But being out during the winter, when spray whipped against the low hanging leaves and branches and supersatured "foggy" air right above the lake freezes into tight icicles, gives clue as to the real limiting factors for planst growing next to the edge. The sacrifice for all that sun is a 2" rind of ice that snaps branches and scouring icy winds that peel away bark.

Icicles were also abundant on the underside of overhangs. The icicles below had formed on a rock that was about 10' off the shore. I assume they formed as snow melted from above and froze as it dripped off the rock (and out of the sun). Supersaturated air condensed onto the little icicles forming the weird lichen-like flakes clinging to the sides (a secondary feature).

Also of note was the stalagtites on the underbelly of the overthrust. The chunky yellowish rock is the much older Dunham dolostone perched atop the scraggly charcoal-colored Iberville Shale. Both weather pretty easily, but the dolostone, a lightly metamorphosed and then aged limestone, is filled with lots of cracks. Because it's a limestone (made of calcium carbonates), it weathers rather easily under the acidity of rain (if you drop hydrochloric acid on a freshly cut surface it will fizz as the acid reacts with calcium carbonate and releases carbon dioxide). In this section, the deep groves in the rock have been exaggerated by acidity over time and the icicles are forming under one of these fissures.

Along the edge of the cliffs the fissures become breaking points where larger rocks fracture off the cliff and plunge into the water. A dramatic event, to be sure, as the more crumbly shale beneath erodes faster, so the dolostone plummets rather than roles to its resting place in the lake. The picture also shows pretty straight geometric lines. The fractures are older than the erosion by acidity, which just expedites the fracturing process. And in places more distant from the cliffes, the fissures form small to occasionally large caverns, called karst topography. Many of the caves in Vermont are located in these calcium-rich, or calcareous, bedrocks. 

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.