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When the ground has a thick layer of fresh, fluffy snow, sound waves are readily absorbed into the snow surface, dampening sound. However, time and weather conditions may change the snow surface. If the surface melts and refreezes, the snow becomes smooth and hard.
If the accumulated snow survives one melt season, it forms a denser, more compressed layer called firn. The snow and firn are further compressed by overlying snowfall, and the buried layers slowly grow together to form a thickened mass of ice.
In permafrost regions, however, the organic matter gets buried by sediment before it decays and becomes frozen into permafrost. Once frozen, the decay of organic matter stops, and the mercury remains trapped for thousands of years unless liberated by permafrost thaw.
As the old snow gets buried by more new snow, the older snow layers compact into firn, or névé, a granular mass of ice crystals. As the firn continues to be buried, compressed, and recrystallizes, the void spaces become smaller and the ice becomes less porous, eventually turning into glacier ice.
Oct 1, 2020 · Firn is usually defined as snow that is at least one year old and has therefore survived one melt season, without being transformed to glacier ice. Firn is transformed gradually to glacier ice as density increases with depth, as older snow is buried by newer snow falling on top of it.
When temperatures are warm and some melting occurs, the change from snow to glacial ice can be very rapid, as short as a single year. The ice, now about 10 percent air, is as dense as it will...
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Surface hoar can lead to avalanches. As you can see from the photographs (Figs. 7i.1-7i.4), surface hoar is very feathery and delicate. When it's on the snow suface, it presents no danger. Once it's buried by subsequent snowfall, however, the surface hoar acts as a weak layer within the snowpack.
