Exploring Kootenay Lake

Blossoms: This posting is just about white on trees resulting from water. Blossoms and seeds are another matter.

Following the reaction to my posting about a rime band, I thought it might be worthwhile to offer a guide to some sources of whiteness on trees.

Of course, the most obvious source of whiteness on trees is snow. When close at hand, snow is fairly distinctive (Jan. 1, 2012). However, when seen from a distance the assessment that the whiteness results from snow becomes less obvious — there are other possibilities.

Snow that falls on a mountainside will often fall right through the melting level and turn to rain below (Dec. 10, 2015). However all the snow does not melt at the same rate.

As the snow falls into air that has a temperature above 0 °C, the smallest crystals melt first, while the larger ones survive longer to fall farther. The result is that the bottom of the white is diffuse. The whiteness gradually tapers from extensive to nothing, without providing a sharp boundary in between (Dec. 13, 2015). So, the clue that we are seeing snow on the distant trees when the valley bottom is above 0 °C, is that the base of the white is tapered. 

What then is one to make of the sight of a sharp lower boundary? This obviously did not result from snow falling through the melting level. This is the result of rime. A cloud of supercooled water droplets (T < 0 °C) had been resting against the mountain. As the droplets collided with the branches, they froze to cover the trees with rime. The sharp base results from the transition of the droplets from above the melting level to those below it (Nov. 26, 2005).

The sharp lower boundary of the whiteness in this picture of rime probably did not result from the bottom of the cloud, but rather from the presence of the melting level. But, this rime shows a sharp upper boundary also, and it will have resulted from the cloud top. The result is a band of rime across the mountain (Nov. 29, 2013). 

Often when the temperature is low, there will be steam fog rising from the open water of local creeks. Then the trees alongside the creek can be covered with rime as the supercooled fog droplets collide and collect on them.

Now for something completely different. This is a picture of the sylvanshine, and although it looks to be a picture of snow-covered trees in the winter, it is actually retroreflection from dew-covered trees in the summertime when seen with a spotlight or the headlights of a car. Only a few species of trees can do this — those which have leaves (needles) that have a rather large contact angle with water. The effect is similar to that of retroreflective glass-beaded highway signs.

In this season, snow is seen that falls on the mountainside. Yet, all the white on distant trees need not be snow — sometimes what is seen is rime.

When snow falls from a cloud above the mountain, it spreads from the mountain top down to the melting level where it gradually diminishes. It thus presents a white mountaintop with a somewhat indistinct base.

What is one to make of a isolated white line across the mountainside? This is a result of rime from a shallow cloud. 

Normally a cloud is filled with liquid droplets even when the temperature is well below 0 °C: the droplets are supercooled. They remain liquid at sub-zero temperatures. But, when the drops collide with objects, such as trees, they instantly freeze to produce rime on those trees.

A thin cloud of super-cooled water droplets had rested against a mountainside. As the droplets collided with the trees, they froze to produce a band of rime across the slope.

As the cloud extended along the whole mountain range, so did the band of rime.

We have all seen them: star-like patterns of melt on the shallow ice of bays.

With any luck, this posting will be followed by one with more information on these so-called lake stars, for much of their formation remains a mystery to me.

Over the years of casually noting lake stars, I had just assumed that they were the result of the drainage of surface melt water back through a hole in the ice. Evidence is accumulating that this is backwards. Here is what seems to be the consensus of the few who have explored the patterns.

• a new thin (a few centimetres) layer of ice forms over stratified water
• atop the ice, it is important that a thin layer of snow has fallen
• a small hole forms in the ice (Why does it form?)
• while the lake water immediately below the ice is at 0 °C, below that, it is warmer
• capillary action causes some warmer water to wick upwards onto the snow
• this arrived water depresses the ice surface increasing the flow of water
• arrival of increasingly warm water from below increases the melting

The formation of the arms of the star remain a mystery for me. However, what I had not realized is that the flow of water is up, not down. The snow atop the ice is a necessary feature because it serves to wick the lake water onto the surface. The wicking is important as it can bring up the warmer water from below, whereas buoyancy would not do so. (Water is densest at 4 °C so the warmer denser water lies a bit below the surface.) It also remains unclear to me why the lake stars form at some places but not others. 

A common, but mysterious lake star forms in thin snow-covered ice on water.