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. 

The Clouds and the Lake

What: I've now paddled the Salmon Hole to downtown Burlington loop about 7 times, this last time with my sister, Meryl, and our friend Carlos. It's amazing that I can walk my boat three quarters of a mile, paddle, 16 miles, and then walk 2 miles home. With all the rain we had the other night the river level was considerably higher and I had a blast running the rapids at Salmon Hole. The wind was also much stronger. The following day (Wednesday) we had fair weather, with a nice blanket of cumulus clouds settled over Vermont and New York. With a different perspective of being on the lake I was struck by the noticeable absence of clouds over the water.

Panorama showing absence of clouds over Lake Champlain

Ecological notes: My theory is that we had lots of rain on Monday, followed by warm temperatures. Already when the rain was falling it was evaporating and there was a beautiful mist carpeting the understory of Centennial Woods. As the warm moist air rises it reaches cooler air. When enough water vapor cools enough degrees (the birds and bees of making a cloud) it can reach what's called the dew point. Dew point is the point just beyond 100% relative humidity where water vapor condenses out onto a solid substrate, like dust or grass or your tent. An easy example is a cold glass of water on a warm humid day. As the air comes in contact with the glass it cools rapidly. Cold air can "hold" less water vapor, so the air in contact with the glass reaches the dew point and little water droplets form on the glass (but not on your much warmer skin). 

In the atmosphere, as that warm air rises, the water vapor can condense and will appear to us as a cloud. So the greater the difference in temperature between the ground and the sky, the lower the elevation at which clouds should form, because they'll reach that dew point much sooner.

We started paddling on the lake around 9:45am and I guessed the clouds were around 3000'. Once we rounded Apple Tree Point we had a great view of Camel's Hump and a little more than the summit (4081') was covered, so the clouds were probably closer to about 3500'. About an hour later it was considerably warmer outside, and more warm air had already risen into the atmosphere, thus, the air around Camel's Hump had warmed as well. This should have raised the dew point and the elevation at which clouds were forming. Sure enough, by about 11:15am, the top of Camel's Hump was exposed, indicating that the clouds were at about 4100'.

So why no clouds over the widest parts of the lake? The temperature of the lake is more stable and cooler, so the temperature gradient between water and air is far less. Because there is a lower temperature gradient, the air rises at a much slower rate, allowing for water vapor to reach equilibrium rather than condensing out.

Where: Lake Champlain & the mighty Winooski River

Other notes: The sunlight on Tuesday was stunning - those soft sunset golds. The winds were kicking up the leaves of the silver maples that line the Winooski River. I could never understand why they were called silver maples until I saw this a few years ago. The undersides have a much paler color and when the winds dance them around exposing their bellies the whole tree takes on this ethereal silvery sheen. Someone must have named it in the sunset after a good summer storm.