The Life History of a Snowflake
Once upon a time, say last November, a snowflake fell in Alaska. It did not land at low elevations near the Pacific, where it would have melted and returned to the ocean within a few days, but in one of the colder parts of the state, where it was comfortably below freezing. There the snowflake existed through the beginning of this month.
It was a dendritic snowflake when it landed, with long feathery arms that had grown rapidly in the wet cloud where it had been born. Once it was on the ground, with no cloud to keep it growing, the sharp points on the arms began to sublimate, going directly from solid ice to water vapor. Within hours the fine detail of the snowflake's shape was lost, and within days it became an amorphous blob of ice.
More snow fell, and the weather became colder. While the snow at the top of the pack chilled to air temperature, the ground near which our snowflake lay remained only a few degrees below freezing. The temperature contrast caused water to sublimate from the warmer crystals near the base of the snow and condense on the colder ones a little higher in the snowpack. These crystals in turn lost water to those higher yet. Many of the snowflakes near the bottom of the pack disappeared entirely, but our snowflake was well placed to intercept some of the water leaving the crystals below it, so it grew. And it changed: it had now become a cup-shaped crystal of depth hoar.
Our particular snowflake had fallen into a valley in the Interior, and was more than once threatened by burrowing mammals. Depth hoar is very light and easy to tunnel through, and being near the comparatively warm ground, it is a far safer and more pleasant winter environment for small animals than is the top of the snow. Red-backed voles, shrews, meadow voles and least weasels passed and repassed throughout the long winter nights. North of the Brooks Range, lemmings would also have been active.
Similar snowflakes in the mountains of southeastern and southcentral Alaska faced -- and posed -- more dramatic threats. The same weak layer of depth hoar that made a safe winter home for voles in the Interior formed a slippery, failure-prone layer at the base of the snowpack on upland slopes. In place after place the increasingly heavy upper part of the pack, held to the slopes only by the weakening layer of depth hoar, broke loose and roared into the valleys below as avalanches.
Finally the sun returned north, and temperatures rose. The sun shone on the snow and penetrated it, warming twigs and leaves that were dark enough to absorb its rays. The invisible thermal infrared radiation from the sky warmed the snow surface. The temperature inside the snowpack rose to near thawing during the day, and a little actual melting took place. Only the surface cooled much at night. In this uniform temperature, the edges of the depth hoar cups evaporated, and their centers filled. Our snowflake, once part of a delicate lattice of cups as much as a couple of centimeters (about an inch) across, became one of a pile of angular ice grains the size of coarse sand or fine gravel. Its neighbors next to twigs, leaves, grass blades or tree trunks often evaporated entirely, leaving wells in the snow.
As the sun's warmth continues to increase, more liquid water will continue to develop in the upper part of the snowpack and percolate downward. At night the temperatures are still dropping below zero Celsius, and the freezing water will cement the ice gravel into an ever more solid mass, impenetrable by further meltwater. Channels will develop over, under, and through the cemented snow. In low areas small ponds will appear, at first made of saturated slush, and later of water that develops a skim of ice in the increasingly brief nights. South slopes show rapidly enlarging patches of bare ground, and soon only isolated patches of snow and ice will remain.
Our snowflake will soon be gone, but the water from which it was made will return to the air and the oceans. Next winter, or a thousand winters from now, some of that same water may again make part of a snowflake that falls on Alaska.
