Constructive Metamorphosis
Constructive metamorphosis is caused by the upward movement of water vapor within the snow pack. Warmer temperatures are found closer to the ground because it receives heat from the core of the earth. Snow has a low thermal conductivity so this heat is retained creating a temperature gradient between the air underneath the snow pack and the air above it. Warmer air holds more water vapor. Through the process of sublimation the newly formed water vapor travels vertically by way of diffusion from a higher concentration (next to the ground) to a lower concentration (near the snow pack surface) by traveling through the air spaces between ice crystals. When the water vapor reaches the top of the snow pack it is subjected to much colder air causing it to condense and refreeze, forming ice crystals at the top of the snow pack that can be seen as the layer of crust on top of the snow.
Read more about this topic: Subnivean Climate
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