The melting transformation process
Sunlight and warm temperatures (primarily in spring) heat up the snow surface, causing it to reach temperatures in the melting range. Water ingress from rain can also promote the melting process. Initially, the melting process only takes place on the snow surface - this is referred to as a low water input into the snow cover.
The crystals become increasingly rounder as a result of the melting process. The crystals often clump together and so-called clusters are formed. These clusters can grow to several millimetres in size in a very short time.
The capillary forces between the crystals increase due to the low water input, which initially only settles in the pore angles (indentations and contact points of the crystals). This leads to solidification within the snow layer. However, this only occurs as long as the moisture and the grain diameter are low. The result is cardboard snow, which slows you down when skiing, but is great for building snowmen.
As the snow becomes increasingly moist, the pore angles fill up with meltwater until it can no longer be held in place. It then runs deeper and deeper towards the ground into the snow cover. The snow crystals are covered by a layer of water due to the large amount of meltwater, which dissolves the crystals from one another. The binding of the crystals is lost, which in turn leads to a great loss of strength in the snow cover. Also known as rotten snow.
Changes in the amount of water can occur very quickly, especially in spring. Small differences in the amount of water are decisive in determining whether the wet snow is still stable or not. It is said that the snow cover remains stable up to around three percent liquid water by volume. If this value increases, a rapid loss of stability is the result.
On clear nights, the moist/wet snow surface freezes again, creating a melting hard cap and increasing the stability of the snow cover again. Repeated freezing and re-melting causes the melt shapes (grains) to become larger and larger, resulting in "skier's firn". Let's call it that, because "firn" is actually not correct.
The absolute final stage of the melting transformation is summer-solid snow. This is primarily found on glaciers. It can be recognized by its humpy and wavy surface with large melting grains. The difference between summer snow and slush snow (rotten snow) is that summer snow only contains a small amount of meltwater and solidifies again, which means that it no longer collapses. However, I won't go into this any further, because you can read all about summer-solid snow in an older rummage.
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