Snow denisty and crystal look changes over its lifetime
These changes are called snowmetamorphsm. As you see in the outfit photos, the snow is going away. Snow does not just melt. When a snow crystal hits the ground or the snow surface, it joins a process called snow metamorphism. In Interior Alaska in spring, the top of a snowpack is often near the triple point of water at least during the day. The triple point is the temperature and water vapor pressure condition at which the solid, liquid and gas phase of water all co-exist. Consequently, minimal temperature, water vapor, and pressure variations can quickly change the appearance of an ice crystal in the snow pack. The water in the snowpack is subject to gravity and percolates through the pores of the snow thereby transporting heat as it may be warmer than the snow crystals deeper in the snowpack. Here the water may refreeze thereby releasing heat which further warms the snow pack.
Gravity plays a role too
However, even at temperatures way below the freezing point, the ice crystals of a snowpack already change their form due to the load above, the diurnal temperature variations in the upper part of the snowpack, and the different water vapor pressure over concave and convex surfaces. Thus, the beautiful dendrites loose water vapor at their edges that then deposits on the crystal to form more hexagonal plate-type crystals. Concurrently, the snowpack becomes more compact and its density increases. The density of dendrites is less than 100kg per cubic meter, while that of clear ice is about 916kg per cubic meter. Later in springtime, the snowpack has turned into sphere of the size of needle pin heads. Their density is about 400 to 450kg per cubic meter.
Look of the day
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Photos: G. Kramm
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