Exploring Kootenay Lake

Yesterday, there was a halo around the Sun; this morning, the rain (or snow) began. These events were related.

The earliest sign of an approaching mid-latitude storm is often a canopy of cirrus—a veil of ice crystals. While simple hexagonal ice crystals will produce a 22° halo, there are many forms of ice crystals and the requisite shape for halo production might not be present. Further complicating the relationship between haloes and storms is the fact that other types of weather can also produce a halo. Alas, it is neither true that a storm is always presaged by a halo, nor that a halo is only associated with a storm.

Yet, a storm is preceded by a halo sufficiently often that the link between the two has become a part of weather lore. 

Yesterday’s 22° halo presaged today’s rain.

Uncinus is Latin for hooked and when applied to cirrus—itself Latin for a lock of hair—it nicely characterizes the cloud, below: formally, it is cirrus uncinus. A familiar name for these clouds is fallstreaks, for they are streaks of ice crystals falling from a cloud, one which was originally made of water drops. 

The reason ice crystals fall from a parent cloud of water drops was discussed last October in a posting called fallstreaks. In the picture shown at that time, the crystals fell, more or less, straight down because the wind through which the crystals fell did not change much with height. Under circumstances when they don’t fall straight down, they often take a parabolic path, a shape that looks like a hook, thus, the name, uncinus. Why does cirrus so often assume a parabolic shape?

Curiously, the reason for the parabolic shape of uncinus is quite easy to account for using simple calculus, but is quite difficult to explain using simple English. This is just one of many situations where natural language falls short. Natural language often presents a view through a glass, darkly, under conditions that mathematics shows things face to face. Despite language limitations, here is an inevitably flawed attempt.

The ice crystals are all falling at roughly the same terminal velocity, but they fall though a wind that changes with height at a constant rate. Such a wind variation is common. One might expect that the streaks would take the same shape as this linear wind profile and so appear as a sloping straight line. This does not happen.

The problem is that each crystal falls through a succession of different winds and so its path is the cumulative effect of an acceleration from each level above it. The result is a streak with a parabolic shape. Of course, to the mathematically inclined, this is trivial: the integral of a linear function is a quadratic. (Another example of this behaviour is the parabolic path of a ballistic projectile, where the constant acceleration is not applied by a wind changing uniformly with height, but by gravity).

Often, the wind in the atmosphere changes at a constant rate with height. When ice crystals fall through such an atmosphere, the streaks are parabolic in shape. They look like a hooked lock of hair: cirrus uncinus.

Frost flowers (aka, ice flowers) seen five years ago at this time of year.

I last saw frost flowers bloom along the shore of Kootenay Lake five years ago. Since then, watching for these delights has been rather like searching for a rarely blooming wild orchid.

Frost flowers bloom under a rather narrow range of conditions, live for a few hours, and are gone. Abruptly last Saturday morning, beds of frost flowers sprang up along the lakeshore.

Most types of frost result when water vapour condenses onto a cold surface and so they form a more-or-less continuous blanket (see surface hoar). Contrastingly, frost flowers result from vapour mixing just above a surface that is (relatively) warm, and they bloom as discrete blossoms.

On Saturday, the air temperature was about -6C, but near the shore, portions of the beach were wet from sun-warmed seepage. Such conditions will produce steam fog if the humidity is high enough. However, ice crystals can form at a lower humidity than that needed for water drops, and on Saturday, there were only frost flowers.

The sparse distribution of frost flowers is reminiscent of wild flowers in a meadow. While, normal frost blankets a cold surface, frost flowers are discrete blossoms on a (relatively) warm surface.

Some botanical flowers are heliotropic: they face the Sun. Frost flowers are anemotropic: they face into the flow of air—or more to the point, the flow of vapour. These frost flowers point upslope (away from the Lake) and into a barely detectible katabatic drift of air (flowing towards the Lake) bringing the vapour that feeds flower growth.

Regular frost covers the surface with a blanket of crystals because the condensation is directly onto that surface. Frost flowers form slightly above a warm moist surface in a thin layer where warm and cold vapour mix. The frost must form onto objects that stick up into that layer and in this picture, those objects seem to be tiny twigs.

This sinuous flower formed along what seems to be a stolon. 

Will it take another five years before the frost flowers of Kootenay Lake bloom again?