Weak layer formation
Weak layers in the snow cover are caused by pronounced temperature gradients. This means that the temperature difference between the snow crystals is relatively large in a relatively small area. Snow is not always equally "warm". In practice or when creating a profile, the snow temperature ranges somewhere down to -25°C and can - nonanetically - reach a maximum temperature of 0°C.
Crust formation
Once a layer of snow reaches the melting point, it naturally does not heat up any further, but the energy (= heat) supplied beyond this point is used to convert the phase transition from solid to liquid. This creates a water-snow mixture. In practical terms: the snow becomes moist. The higher the moisture content of the snow layer, the greater the proportion of water. This continues until the mixture reaches something like saturation, which is the case at the very latest with a "liquid water content" of 15 percent by volume in the snow cover. The water begins to look for vertical and horizontal paths within the layer, i.e. simply: to flow off.
When the water-snow mixture freezes again, it is referred to as a melt crust - which is no longer moist, but dry, as the water content has frozen back into ice. Melt crusts as well as melting forms occur in the form of so-called melt lumps, which are smaller or larger in diameter. The round circle for "melting forms" in snow profiles stands for the wet state, i.e. not frozen. The spectacle symbol of the melting crust stands for melting forms whose water content has refrozen and thus encrusted and turned to ice. You can still partially recognize old grain shapes with a low original water content, which is why there is still space in the glasses for the symbol of another grain shape.
