Exploring Kootenay Lake

We don’t have just one way of estimating the distance to an object; we have many. When objects are nearby, we use our stereo vision to judge spacial distribution. Beyond ten meters or so, we need other clues. If we know the characteristic size of something, such as a person, house, or tree, we judge distance by seeing how large it appears in the eye (its angular size): the smaller, the farther away. And if one object blocks another, that object is assumed to be closer to us.

Contrast decreases with distance through air

When we try to estimate the distance to objects of markedly variable size, such as mountains or clouds that might, or might not block one another, we use contrast. When seen through our atmosphere, the farther away an object is the lower its contrast: with distance, dark objects become lighter; with distant, light objects become darker. In the end with great distance, everything approaches the uniform brightness of the horizon sky.

Contrast works fairly well to estimate distances when one is looking at a solid object that doesn’t, itself, transmit light. But, how does one employ contrast to judge the distance to an inherently diaphanous object? What happens if one is looking at a veil of cirrus? The answer is: we usually get it wrong.

This morning, there was a veil of cirrus. Indeed, along with presenting a slightly streaky, milky sky, it produced a 22° halo. Then an airplane came by and left a well defined contrail. To our practiced eyes, the high contrast of the contrail makes it seem closer to us than the low contrast of the cirrus. Indeed, the contrail appears to block the view of the cirrus. Clearly, the contrail is closer and thus lower in the sky.

Yet, we would be wrong. The clue to the error is the shadow of the contrail on the cirrus. The cirrus has to be lower in the sky or the Sun would not be able to cast the shadow of the contrail onto the cirrus. The geometry of shadows tells all.

Alas, although an object’s contrast is usually a reliable measure of relative distance, our perceptions would miss this one. This diaphanous veil of cirrus is lower in the sky, and so closer to us, than is the contrail.

The elements raged: Thursday afternoon brought lightning, thunder, graupel, strong west winds, waves, and best of all, clapotis gaufré.

Clapotis is derived from the French, le clapotis, meaning standing waves. Gaufré means waffled. So, together it describes a waffled pattern of waves that don’t travel but stay in one spot. But, describing it, does not explain it.

A search for clapotis on the web produced a recent (2004) additional meaning: a knitted shawl displaying a wavy pattern.

Clapotis (by itself) arises when a uniform train of water waves approaches and is reflected by a hard vertical barrier (as distinct from an irregular one such as rocks or a sloping one such as a beach). As each crest of the reflected waves passes back through a crest of an incoming wave, a very large stationary crest is produced: it does not travel; it stands on a spot and the wave goes up and down. This is assuming the waves approach the reflecting barrier at right angles.

When waves approach and are reflected diagonally, the pattern becomes cross hatched to give clapotis gaufré. That is happening in the picture, below. A train of waves from the west pounds the ramp giving a waffle–like pattern of water towers.

Before becoming very high, the crests of a travelling water wave will break and give white caps (as can be seen beyond the dock). Normally, the crests of clapotis do not break, but merely produce a tower of water. In the picture, below, the crests of the highest waves are being torn apart by the wind.

A uniform train of waves approaches from the right, hits the ramp diagonally and gives a waffle pattern of water towers: clapotis gaufré. In addition there are diagonal streaks of rain cutting across the picture.

Today the cirrus contained columnar ice crystals, rather than plates. This meant no circumhorizontal arc, but other haloes compensated.

At noon, for a few weeks around the summer solstice I watch for the circumhorizontal arc. This is a really very colourful arc lying parallel to and just above the horizon. Actually, it is not all that rare when the Sun is high enough in the sky for it to form. The ideal solar elevation is 68°. At Nelson, the Sun reaches a maximum elevation of 64° at solar noon (about 12:50DST) on the summer solstice (June 21). That’s close, so for maybe an hour a day, for a week a year, the arc might be able to be seen from around here.

Today was sunny and there was cirrus—almost optimal. The only other needed ingredient in the recipe was that the cirrus had to contain the requisite prisms: plate–like ice crystals. Now, most ice crystals in the atmosphere come in two basic forms (with lots of variations), hexagonal columns and hexagonal plates. Think of the columns as wooden pencils scattered randomly on a table; think of the plates as hexagonal dinner plates, again scattered over the table. The formation of the circumhorizontal arc requires the plates; today the cirrus delivered columns.

However, the columnar ice crystals did give a nice display—albeit not the one I sought. The picture below shows three haloes: The inner circle around the Sun (which looks too large owing to overexposure) is the 22° halo. Surrounding it is an elliptical shape called the circumscribed halo; it is coincident with the 22° halo at the top (upper part of the picture) and bottom (out of the picture), but shows a small separation on the left and right. At this solar elevation these haloes are caused by the columns. The centre of each of these haloes is the Sun.

The Kootenay Lake website offers other pictures and more discussion of these haloes. To get a sense of just how lovely the circumhorizontal arc can be, see the posting about its low–sun brother, the circumzenithal arc.

A third halo appears, but is a bit subtle: the parhelic circle. This is a white arc that circles the sky at the elevation of the Sun. In this picture it is seen passing through the sun, and cutting across the other haloes at about 2 and 10 o’clock. The centre of this halo is the zenith, which is locate just above the 22° halo.

What can I say: it was a nice display, even if it wasn’t what I hoped to see.