Exploring Kootenay Lake

The glory is a beautiful phenomenon that is only rarely noticed. Its rarity results from the requirement that your shadow must be cast down onto a cloud in the centre of the glory. Mind you, to see your shadow in the centre, you must be fairly close to the cloud, but that was normally the case when seen from a mountain. Thus we get names for it like the Brocken bow and spectre (19th century, Germany) and Buddha’s light (a couple of thousand years, China). The name, glory, is rather old and speaks to the assumed beatification of the person whose shadow appears surrounded by the colourful rings of light.

One of the easiest ways to see it these days is from an airplane, although the type of cloud, the wings, and the seat position often prevent a sighting. As such it is rarely noticed, plus the height of the plane above the cloud usually eliminates the shadow. It has been seen from mountains around Kootenay Lake, but flying in and out of the region provides the easiest observations.

The glory is often seen together with the cloudbow, so, there is an overlapping drop size that is common to both phenomena. Yet, it isn’t perfect. The cloudbow seen earlier lacked a glory (drops a bit too big?), and this glory lacks a cloudbow (drops too small?).

The glory seen on the stratocumulus when flying out from Kootenay Lake is a phenomenon of single scattering. Notice that it isn’t seen in gaps between the clouds where photon penetration has resulted from multiple scattering. The pattern is a series of concentric rings with reddish to the outside and bluish towards the centre. There is a faintly visible third ring. The aircraft is too high above the cloud for a shadow to be seen.

Last Sunday, I watched something over the Lake that I had not recalled seeing with such clarity before: two different mechanisms for droplet formation from the same source.

A decade, or so, ago, I wrote a piece about the two mechanisms for droplet condensation. There I criticized teachers for getting it wrong, by saying that cold air cannot hold as much water vapour as warm air. This was despite it having been known for two centuries what was actually happening. That essay is called condensation. Here, I will just accept that, and go with what is correct.

Condensation might occur when two parcels of sub-saturated water vapour mix. Or, it can occur when water vapour is cooled. 

Typically, clouds (or fog) near the surface result from vapour mixing — in which different forms of cloud result from different mixing circumstances. In this scene, the cloud (fog) at the Lake’s surface is the result of vapour mixing. It is known as steam fog and can result when quite cold air overlies much warmer water.

Typically, clouds well above the surface form as a result of vapour cooling — in which different forms of cloud result from different cooling circumstances. The convective towers of cloud that arise high over the Lake’s surface is the result from this water vapour cooling. 

This scene was a little uncommon. Usually, when cold air flows over warm water in the fall, the high winds cause the convective towers to warp up as steam devils. In this case, the cold air must have arrived overnight and it was now calm. This resulted in convective towers growing out of the steam fog.

A cloudbow is just a rainbow formed by the much smaller droplets found in a cloud or a fog. It is as huge as a rainbow in that it spans nearly a quarter of the sky. But, it is nearly white with only a touch of colour. 

Indeed in this case, it is the very small size of the fog drops that produces the lack of colour. As with the rainbow, light is refracted as it passes through each drop but diffraction causes the the light to spread out causing the different colours to overlap and mainly give a broad white bow.

The cloudbow in the picture has a variable intensity. As it formed in a thin fog, there are more droplets in the brighter upper portion than when looking down toward the water.

As this one formed over water (and was seen from a boat) there is an apparent reflection of the bow seen in the picture. 

I took many pictures of portions of the bow, but in the end I particularly liked the exceedingly wide angle picture taken by my son, (also named) Alistair Fraser, and so show his shot.