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. 

More ice from the causeway

I was captivated by the way ice was flowing while out at the Causeway. I wanted each clip to run indefinitely, to be lulled by the lake's monotonous drone of wind and water. Nothing dramatic, just the water's quiet dramatization of winter's cold.

Cold Cold Colchester Causeway

What: The Jons, Christin, and I headed out to Colchester to walk the Causeway and see what it would like like with all the snow. I was away for most of the cold weather in December and early January and wanted to see what chance the lake might have of freezing over this year. According to NOAA, the water temperature up around Colchester Point is 34degrees, and since freeze overs don't usually occur until the end of January or beginning of February, I'd say it's looking close.

Jon Bond, our intrepid explorer, testing the resilience of the ice

The covering ice Niquette and Mallet's Bay was stunning. With low visibility it seemed like an endless artic desert. The ice was in stark contrast to the other side of the trail, which was almost entirely open. The Causeway, which forms the barrier between Lake Champlain and Niquette and Mallet's Bay, is an old railroad bed constructed 1899 to open up markets in the Great Lakes. The "fill" is actually large chunks of marble, which supports a few limestone loving trees, like basswood and white cedar. 

Above shows a the clear difference in habitat between the more exposed lake and the more sheltered bays. Wind and surface turbulence of the water will draw up relatively warmer water from the depths of the lake buffering the lake from freeze. Water movement can also prevent freezing. Lower winds combined with shallowers depths allowed for the bays to freeze of first. The tongue of unfrozen water on the right side of the above image corresponds to where a bridge creates a break in the buffer and has allowed that stretch to remain unfrozen.

With another polar vortex in full swing and hopefully another couple weeks of subfreezing temperatures, we just might get a freeze over and I just might get to run across the lake. I posted last year about lake freeze over events (it hasn't froze over at the widest part - 9.5 miles from Burlington Bay to Corlaer Bay - in 32 of the past 54 years; and 6 of the last 6 years - since I moved here - it hasn't frozen over).

In the distance there was an oddly geometric island (in the photo above, it's just to the left of the island, but in the distance beyond the ice-locked boat). Turns out to be a (see this photo for what it looks like on a very distant beautiful summer day). 

Beavers in winter

What: On our wander, I wanted to show Clay the new beaver pond. Clay and I have been exploring the various iterations of beaver ponds in Centennial for about 5 years now and he hadn't seen the new neighbors since they moved in sometime around the beginning of October.

Ecological notes: For the 4th year in a row, beavers adopted Centennial Woods in the middle of fall, which is a rather late start as far as beavers go. Typically beavers have their winter homes already staked out and built up by the end of summer. In a new area without previous beaver structures, they'll first dam up the river and live in a burrow in the bank while raising water levels high enough to where they can build a lodge in the newly formed pond. They won't start caching (storing for later) food to last them through the winter until sometime in the fall when the trees have stored more sugars in the bark. To be doing all of this in the late fall is quite the herculean effort.

Three years ago a family moved in after Hurricane Irene, made it through the winter and left in the spring. This repeated the past two years as well, and not to disappoint, beavers moved back in this fall around the beginning of October.

I've been away the last month and went to check in on them as soon as I got back. The pond is open thanks to the recent rains and warmer weather and their dam is active and in good shape. No lodge, so they must be nesting in a bank, but haven't found that spot yet.

Where: Centennial Woods, upstream for Fox Meadow by Centennial Field.

Butternut Bud says hello

Butternut twig

It's been a while since I've posted and I've wanted to get back in the habit for a while. My friend, Clay, was in town visiting and we took advantage of the delightful snow for a winter wander. I brought my camera along and took a few pictures of the beaver pond that I'll post in the next few days.