Exploring Kootenay Lake

Suncups are abutting bowl-shaped depressions in a snow surface. Typically they form high in the mountains during the spring when the wet-bulb temperature is below 0 °C. 

I have travelled through suncups three-metres across and a half-metre deep on the névé of Mount Garibaldi — progress was slow. Starting from dimpled snow, suncups grow slowly to the size of salad bowls, then to washbasins, wading pools and beyond. 

Imagine being a bug at the bottom of a small depression in a field of snow on a beautiful sunny day. You are warmed both by direct sunlight and by sunlight diffusely reflected off the sides of the depression. You will be warmer than another bug sitting on an adjacent snow ridge which only experiences direct sunlight. Consequently, the snow at the bottom of the depression will be slightly warmer than the snow at the ridge. This means that evaporation at the bottom of the dip is greater than on the ridge, and this causes the depression to deepen.

Growth of a suncup is slow and the process depends upon both low temperatures and copious sunlight. This is probably the reason that it is seen most commonly at colder altitudes where snow has persisted even as lengthening days give more sunlight. While I look for suncups at the valley bottom, I haven’t really expected to see them.

But, there they were: Salad-bowl sized depressions, albeit spread over a flat snow-covered roof (viewed from a higher floor). Something had accelerated the formation of suncups on this roof, and it didn’t take long to guess the cause: heat loss through the roof from the building below. Snow is a good insulator and the thicker the snow, the better it is. Consequently, the thinner snow at the bottom of a bowl will be warmer than the thicker snow at the edges. This additional warming enabled suncups to form in an unexpected location.

Suncups have formed on the flat roof of a building where heating from the building supplements the sunlight. Incidentally, this picture also contains an illusion. The scene actually shows narrow ridges and broad bowl-like valleys. Sometimes when I look at it, everything seems reversed with narrow valleys separating broad mounds of snow. The trick to seeing it correctly is to realize that sunlight is coming from the left as can be seen with the structures on the roof.

I am confused by success. 

Below is a picture I have sought for months, yet, I don’t understand why I was able to get it now. It is a simple illustration of the dispersion of gravity waves on water. 

On more than one occasion, this blog has treated the subject of water waves — what can I say, the blog’s theme is that of exploring a lake. Water waves possess many interesting behaviours, some of which I have treated over the years. Recent ones relevant to this posting are 23 cm/s (which illustrated that when a water disturbance moves at less than 23 cm/s, no waves are created) and making waves (which showed both gravity waves and ripples, and that it is the shortest ripples that travel fastest). 

The term, wave dispersion, describes the situation where waves of different lengths travel at different speeds. On water, both ripples and gravity waves are dispersive: for ripples, the shortest waves travel fastest; for gravity waves, the longest waves travel fastest.

To my mind, these are all really interesting features of the natural world around the Lake. But, while it was easy to illustrate the fact that for ripples (surface tension is the restoring force) it is the shortest waves that travel fastest, I found it really challenging to illustrate that for gravity waves (gravity is the restoring force) it is the longest waves that travel fastest. While the behaviour is undeniable, taking a single picture to illustrate the latter simply proved a challenge. Alas, often such scenes are really cluttered, such as the jumble of waves generated by the wind. A boat’s wake is also rather complex. How might I capture a simple illustration?

Then I saw it illustrated clearly in the waves made by the cable of a cable ferry.

Why the cable provided such simple waves is not obvious to me. However, the disturbance caused by a small cable rising out of the water is different than that of an object moving across the water. The simplicity of the situation merits more exploration.

As the (Harrop) cable ferry crosses the Lake, the (orange) cable rises out of the water ahead of the ferry off the picture to the lower left. This disturbs the water and creates gravity waves which are seen spreading towards the upper left. What the picture makes clear is that the longest waves have travelled farthest (towards the upper left) as a result of having travelled fastest. (The wake of the turbulent bow wave of the ferry itself is seen on the right.)