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…
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