This November I posted 30 finished pieces of writing on Everything2, on whatever I felt like writing about at the time. By doing so I completed the Iron Noder Challenge, which has been running every November since 2008. This was the first time I took part in earnest – making the effort to write and re-write for an hour or two almost every day, in order to average at least one post a day that I could be happy with. (more…)
I think most people don’t pay nearly enough attention to what they’re walking on, especially in cold weather. The richness of the patterns that ice forms is staggering, and provides an intriguing glimpse into the physical processes going on both at a molecular level and on a much larger scale. Some of the most fun shapes emerge when the temperature varies enough so that ice alternates with water, and flow patterns meet crystal dendrites.
I have two theories about the sort of sideways icicles we sometimes see. Either they come from ice that has cracked and water has seeped through and refrozen, or they are caused by fingers of ice crystal which get a head start on the rest of the puddle for some reason – most likely, some facet of the surface they’re growing on just happens to provide a perfect nucleation point, and the crystals grow out from there because there’s nowhere else for them to get a foothold. Even though this starts at the level of water molecules forming neat little piles too tiny for any microscope to pick apart, in the right conditions these minuscule fingers of crystal just grow bigger and bigger…
Some bubbles usually form in ice as it’s freezing. These are due to the presence of dissolved air in the water, which is no longer able to stay dissolved when it gets colder, so it migrates into pockets as the water freezes around it. Bubbles like these, trapped in the Antarctic ice core, tell us what the air on Earth has been like over hundreds of thousands of years, providing the strongest evidence that the temperature on Earth varies in proportion to the amount of carbon dioxide in its atmosphere. We know, for instance, that levels of carbon dioxide and methane are higher, and rising faster, than they have been in 800,000 years.
Larger bubbles also form under ice when it starts to melt from beneath, forming a space between the frozen layer and the water underneath. This process is dominated by the formation of liquid water, dripping and surface tension coming to the fore, so rather than the complex, angular crystals associated with freezing, we see the air forming in great bubbles and voluptuous curves.
The patterns formed by frost depend on a number of factors – the relative temperature of the air and the ground and how much they vary, the speed of the wind and the level of moisture, and so on. Another factor is the nature of the surface the frost forms on – sometimes frost closely follows the lines of the surface, and sometimes it forms much more quickly in some spots than others, where imperfections in a smooth surface get the crystallisation process started. The patterns formed can give us insight into hidden features of the surface below, the subtleties we see speaking of deeper subtleties beyond our perception…
a quickr pickr post
I have been fascinated by caustics for a long, long time. I still remember the first time I noticed them – a bright, ethereal form dancing in the shadow of my mother’s wine glass. I was entranced by the way the light moved when the wine swished in the glass, and disappointed when my usually all-knowing mother wasn’t able tell me anything much about them.
Many years later, a friend asked me if I happened to know anything about caustics; I had never heard of them, so she explained that she was talking about the shifting patterns of light made by rippling water, the curves of light you see at the bottom of mugs, and so on. Finally I had a name for these patterns that had enchanted me since my infancy; when I got home I looked up the word, wondering what these things might have to do with caustic soda or holocausts.
Most caustics are quite harmless, but if you have ever used a magnifying glass to focus the light of the sun into a tight point to make smoking holes in things, you have witnessed their potential destructive power; this is where they get their name. Archimedes is famously said to have used a giant parabolic mirror to set fire to Roman ships using reflected sunlight, during the siege of Syracuse in 212 BC. In modern times, the Olympic Torch is similarly lit by a large parabolic mirror focusing the sun’s rays on a single point.
Caustics can occur whenever light leaves a curved surface; most often that means they have been reflected or refracted. Refraction caustics, caused when light rays are bent by passing through something, tend to show less extreme distortion than reflection caustics, but often show subtle colour variations like light from a prism, because shorter wavelengths of light are refracted more than longer ones.
Either kind of caustic can hugely amplify tiny imperfections or very subtle curvature into striking patterns, the effect increasing with distance from the surface. For example, very few windows are truly flat, and it is common to see cross-like shapes or mottles reflected on the walls opposite, when the sun is low in the sky.
Strictly speaking a caustic is the entire envelope of light which leaves a curved surface; the patterns of illumination we usually see are just the intersection of that three-dimensional structure with another surface. Something of the 3D nature of caustics comes out when the distance to the illuminated surface varies, with some features getting washed out with distance while others become ever more prominent. See this short video clip for an example; there’s a much longer film, with music, linked here.
We’d notice very quickly if they weren’t there – simulating realistic caustics is an important issue in computer graphics mainly for this reason, and an otherwise convincing scene will seem oddly flat and unreal if it is missing caustics that should be there. Mostly, though, caustics are one of those kinds of things which quietly make life that much more pretty while they just sit in the background, beneath our threshold of conscious attention – but which often reveal truly striking beauty when we pay them a bit of attention.
Additional photos courtesy of Reciprocity, SEngstrom and suchstuff; see more in the Caustics pool on Flickr. You might also like to play about with my interactive caustics-simulation animation, Zoobie.
I recently finished reading The Cloudspotter’s Guide (see review), and have concluded that it is one of my all-time favourite books. I have made skywatching a hobby for as long as I can remember, but the book has raised my awareness of the skies above us to new heights. As it happens, this has coincided with some absolutely pheomenal skywatching weather in Edinburgh, warm seas and sunshine feeding vast Cumulus clouds, weather fronts dropping stunning Cirrus squirls from the heavens, fascinating layers on layers of different clouds.
I’ll return to the book later; for now I just wanted to share the results of just two day’s skywatching…
The first of those days found me sitting in Holyrood Park one fine, sunny-cloudy afternoon after work, with my camera and a copy of The Cloudspotter’s Guide, with nothing to do but read about, watch and record the sky. The weather was as good as perfect for it – strange ice clouds high above, brooding storm clouds just far enough away not to alarm, and enough sunshine to keep us warm and illuminate the early-autumnal haze with lovely crepuscular rays.
With the help of the Guide I was able to identify this unusual net-like formation with some confidence as a Cirrocumulus lacunosus undulatus – that is, a collection of high, icy cloudlets forming a layer punctuated by holes – lacunas, if you like. Granted, that doesn’t tell us much of any real use, but still, it’s always nice to be able to put a name to something that’s been puzzling you.
The banks of Cumulus congestus dwarfed the Salisbury Crags, which in turn dwarfed the people climbing them. I knew that it would rain on us sooner or later, but I had time to capture a series of pictures to turn into a highly amateur time-lapse film, so that I could watch those beautiful convection cells in action later. There’s a small version of this here, but if your computer can take it I recommend the full 2-megabyte version. I could have saved myself a lot of work later if I’d had a tripod with me – and a timer would help – but I didn’t have anything fancy to hand, so I just balanced the camera on my knees and took a picture every few seconds for a couple of minutes – compressed here into a couple of seconds.
A few weeks later the skies around Edinburgh were dominated by vast, looming, ever-growing Cumulus congestus and Cumulonimbus: puffy, dramatic and often deceptively solid-looking rain clouds, pouring down their loads even as they burgeon with freshly condensed droplets, up-wellings of warm, moist air racing to refresh them before they rain themselves out. I was staggered to end up avoiding the rains entirely, though it can’t have been more than a mile or two away at any point in the afternoon.
That day’s convection clouds came accompanied by a smattering of Cirrus clouds streaming out of a subtle Cirrostratus, showing that the air was moist right up to the highest reaches of the troposphere – to the tropopause, where the weather stops. Their ice crystals refracted the sunlight in a stunning range of displays; I have been watching out for such things for years, but had never seen such a range of ice halo phenomena in one day.
There were striking sun dogs (also known as parhelia, or mock suns) – the most obvious of the halo phenomena, these appear more than once a week in northern Europe, but most people still fail to notice them. They are created by horizontally-aligned plate-like ice crystals; the sunlight passes through one side of these transparent hexagons and out of another, making a bright, coloured patch of sky 22Â° or so away from the sun – about the distance from thumb to little figure of an outstretched hand at arm’s length. The colours are not always obvious, but when they are you can see that the sun dog is reddish towards the sun, and bluish on the other side – sometimes with a long, not entirely un-doglike tail.
The exact same kind of ice crystals produce the circumzenith arc, a ‘sky smile’ in vivid rainbow colours, going part-way around the top of the sky, above the sun. As on that day, these are therefore likely to be seen at the same time – although they are not seen as often as sun dogs, partly because they only appear when the sun is quite low in the sky. In their case, the bright colours – which at their best can out-do any common-or-garden rainbow – are the result of light entering the top of the ice-plates and leaving through the sides.
Accompanying these, less spectacular but undeniably still pretty, I was unsurprised by the appearance of a 22Â° halo, the most common halo effect produced by Cirrostratus; these are made by columnar ice crystals, like tiny pencils, which are randomly oriented, and they appear almost one day in three. When these crystals have smooth, flat ends (which they rarely do) they can also produce a 46Â° halo, much fainter and much larger than their cousin. When the columnar crystals are roughly horizontal, they can also produce a tangent arc, somewhat resembling a giant dove made of pure light. If my judgement is on, I was privileged to see both that afternoon.
The salt bin opposite my flat provides me with a suprising amount of intrigue. Somewhere down the line, it filled up with water enough to become distended – or became distended enough to fill with water – so now it sits there and forever grins invitingly, like some kind of fat plastic crocodile.
It’s permanently full up with water now – intensely saline water, of course, which does some pretty interesting things when it’s stagnant… when someone dumped an old paperback in there, for example, it quickly became encrusted with those characteristically square salt crystals, like the ones you can buy at fancy delicatessens (‘fleur de sel‘)… although not so appetising.
Then, most recently, a combination of wear and tear with hot, hot sun and heavy rains have led the bin to start cracking at the sides, sweating its saline drips in waves to leave a story of the weather inscribed on its sides.
I suppose this would be a good place to write about the way crystals derive their shapes from the way their component molecules stack together, or about the echoes of geological forms in small-scale emergences like this.
…maybe some other time.