The journey of water molecules from snow to ice is not always a linear one.

Imagine a stack of spheres. You drop all of the spheres haphazardly into a pile, and then shake them around a little bit until they settle into their lowest energy state-- that is, they achieve "perfect packing". Perfect rhombohedral packing, to be exact.



The story is much more complicated with snow, you see, because a snowflake is about the furthest shape you can get from a sphere.



As snow falls into a pile, the snowflakes on the bottom are put under pressure from the ones above. They are also insulated from the cold surrounding atmosphere by their mates. At their spindly edges, the snowflakes begin to buckle, break and deform, becoming more like spheres. They settle slightly due to gravity, eliminating large empty spaces. They also start to melt just slightly, especially at the pointed edges. They call it "cintering". Over time the snowflakes are cintered and deformed until they form a kind of snowflake lattice, where all of the snowflakes form the walls of cavities filled with air. Eventually these cavities break or shrink, putting greater and greater pressure on the air. Air is a gas and eminently compressible, so it shrinks and shrinks in volume along with its shrinking cavity. As the layers of snow build up above, the layers at the bottom are under so much pressure that the air pockets close.

Unable to escape, the air is forced to somehow become a part of the molecular lattice of the ice. One way to do this is to dissolve, sometimes splitting into component atoms and existing in the empty spaces between atoms in the ice lattice. The other way is to squeeze into cage-like molecular structures called "clathrates". Clathrates in this case are a collection of water molecules that form a cage around an empty space. All kinds of molecules can find their way here such as methane, sulfur species, or air.

Climate scientists often attempt to use the bubbles trapped in the ice as a measure of what the atmosphere was like when the bubbles were closed off from the environment way back when the ice was still made of snow. In this technique you are limited to snow that hasn't completely become ice yet (it is called "firn"). In order to get at the climate of times long long ago, climate scientists investigate the atmospheric gases trapped in the clathrate structures.

Both of these techniques are very clever, but the fact that air comes in and out of the firn quite a lot during its compaction into ice means that the resolution on the exact date of that atmospheric composition can sometimes be shockingly bad. After all, it can take 80-90 years for snow to become ice, and all that time the firn is interacting with the atmosphere and smearing out the atmospheric signal. In one case at the Russian base in Antarctica (Vostok), the date at which the atmosphere was trapped has an error bar of up to 490 years! The exactness of the ice dates is directly dependent on the accumulation rate of snow at the site. If new snowflakes and atmospheric bubbles are buried very quickly, they will turn to ice more quickly and the smearing effect will be less. If the accumulation rate is slow, the snow and firn will sit there circulating atmosphere in and out of pore space for hundreds of years, smearing out all of the climate signal during that time.

For more information, see the awesome book by Paterson, 1981 The Physics of Glaciers

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