15.07.2025
Snow is a form of frozen water. It consists of many small ice crystals that form into snowflakes. Snow can be light and powdery or moist and heavy, depending on the circumstances under which it forms. Snow consists largely of air, which is why it is physically described as a “porous medium”. If you like, it is an airy ice foam.
Snow is an important part of the water cycle and has many effects on the environment. It covers the ground and influences the regional climate. Snow also has a major impact on the habitat of animals and plants, as it provides an insulating layer and can store water. When it melts in spring, it becomes a source of water.
Snow is also important for people, both as a means of recreation (skiing, snowboarding and sledging) and for transportation (e.g. ski lifts, ski-doo). However, snow can cause hazards such as avalanches, traffic problems and damage to infrastructure, especially when large amounts fall.
Overall, snow is a fascinating meteorological phenomenon that plays an important role in nature and everyday life.
The processes that lead to the formation of snow crystals are complex and depend on many factors, e.g. temperature, humidity, air pressure and the amount of water vapor in the air.
In general, the formation of a snow crystal begins when water vapor condenses on tiny particles in the air. These particles serve as nuclei or so-called “nucleation centers” for the condensation of water vapor. In order for the water vapor from the air to dock onto the particles at all, the air parcel in which the water vapor is located must be saturated. Saturated means that the air packet has reached a relative humidity of 100 percent. To prevent the snow crystal from melting immediately, the air temperature must be below freezing point. If sufficient water vapor accumulates around these nuclei under these conditions, small snow crystals made of ice form.
Temperature and the so-called supersaturation, i.e. the absolute proportion of water in the air, now determine what form the snow crystal takes.
The snow crystals can have different shapes and structures, depending on the ambient conditions – but one property is always the same: they have six corners. If the supersaturation is 0.1 g per cubic meter of air and the temperature is close to 0° C, small platelets tend to form (see also illustration and photo). If there is more water available in absolute terms, the platelets begin to form arms. The classic snow stars or dendrites are formed. These snow crystals grow continuously in a cloud. If they become too heavy, they fall downwards due to gravity. In the process, they are often whirled up by winds. As a result, they become entangled and get stuck, forming the snowflake. As this is even heavier than the individual dendrites, it then falls to the ground.
Snow consists of water. Water is a unique substance with different aggregate states and a special feature known as the anomaly of water.
The aggregate states of water are:
The anomaly of water refers to some unusual properties that water exhibits compared to other substances. Here are some examples:
These anomalies of water are closely linked to the unique structure and interactions of water molecules. The hydrogen bonds between the water molecules play a crucial role in the observed properties of water. Ice has a lower density than water because when it freezes, the water molecules are arranged in a lattice structure that requires more space and therefore takes up a larger volume for the same amount of water. The effect: the density decreases (see also next chapter). Anyone who has ever forgotten a drink in the freezer will be familiar with this effect. If you wait too long, you will end up with nothing but broken glass and frozen liquid in the freezer. The density anomaly of water is largely responsible for plants and animals being able to survive in a body of water in winter. In a lake, for example, the ice that forms floats on top, preventing the lake from cooling down too much. In addition, the remaining water in the lake remains liquid and can continue to provide the necessary habitat for living organisms.
Anomaly of water:
The degrading transformation describes the processes that transform a new snow crystal into small round grains via the so-called felt (snow).
The originally finely branched, relatively large fresh snow crystals transform into small round grains. The reason for this is the uneven distribution of water molecules in the fresh snow nests: at the tips, each water molecule has only a few neighbors that can hold it in the ice structure. At the depressions and indentations, however, there are many neighboring molecules that can hold the water molecule in place. Physically speaking, the water vapor pressure over convex shapes (ridges, peaks) is greater than over concave shapes (indentations, depressions). Over time, this pressure difference causes ice to sublimate at the peaks, migrate as water vapor to the concave areas of the new snow crystal and be deposited there again as ice.
Behind this process is nature’s universal desire to minimize surface energy. The geometric shape with the lowest surface energy is the sphere. As a result of the degrading transformation, the complex snow crystal becomes more and more spherical. The resulting strengthening of the bonds can be summarized under the term sintering. Sintering is therefore a consequence of the degrading metamorphosis.
This process begins immediately after the deposition of a new snow crystal. Because the pore space becomes smaller and the ice grains also become smaller, the volume decreases and the snow cover snowpack settles – a process that we can observe with the naked eye after new snowfall. The duration of this process depends on the temperature. Higher temperatures in the snow cover snowpack (approx. -5° C to 0° C) lead to a relatively rapid decomposition; at lower temperatures, this process is slower. High pressure (e.g. a load due to a lot of new fallen snow) additionally accelerates the degradation transformation. As mentioned at the beginning, the sintering process usually takes place over three grain forms (see table of grain forms): New fallen snow transforms into felt, which shrinks into small round grains as the degradative transformation continues.
Sintering and the degrading transformation have different effects on the formation of avalanches (see article on the formation of avalanches). During the transition from a fresh snow crystal to a felt crystal, for example, there is a short-term loss of strength, as the degradative transformation destroys the interlocking of the crystal branches, but new bridges that could compensate for the loss of strength have not yet been formed. In practice, this is manifested by the loose snow avalanche points that often occur immediately after a fresh snow event. As a result of the degrading transformation, the new fallen snow also becomes bound and board-like and, in combination with a weak layer, could lead to a slab avalanche. In principle, however, the degradative transformation is favourable for the stability of very porous layers (e.g. weak layers), as the bonds in the crystal structure are strengthened and stabilized.
During kinetic metamorphism faceting, the snow crystal grows. The driving force behind the build-up metamorphosis of snow is the temperature gradient in the snow cover snowpack, the main ingredient being water vapor. The temperature gradient describes the temperature difference between the snow surface and the ground in relation to the snow depth. The greater the gradient, the faster and stronger the kinetic metamorphism faceting takes place. As the snow cover snowpack insulates very well, a constant temperature of close to 0 °C is established in the layers close to the ground over the course of the winter. At the snow surface, the snow temperature varies greatly due to the energy exchange with the atmosphere and can reach very low temperatures. The greater the temperature difference in the snow cover snowpack and the lower the snow depth, the greater the gradient.
The warmer pore air at the bottom contains more water vapour than the colder layers above, which causes the water vapour to rise towards the colder layers and recrystallize as ice on the underside of the colder crystals (deposition). Facets and edges form, the grain grows and slowly becomes an angular crystal and finally a cup crystal or deep frost (floating snow). This mainly forms on the ground, but can also occur in higher intermediate layers. The decisive factor is always a large temperature difference over a short distance. If this difference exists in layers close to the surface, angular crystals can also form there, especially on shady slopes during long cold periods.
Another crystal form of kinetic metamorphism faceting is the surface hoar frost. The formation process for surface hoar frost is the same as for deep frost (deposition). However, the water vapor in the formation of surface hoar frost comes from the ambient air that passes over the snow cover snowpack.
The kinetic metamorphism faceting is so important because it produces weak layers. Due to their size alone, cup shaped crystals are a brittle material. Due to the large, angular and angular shapes of the crystals, only a few contact points can form between them. The ice structure loses strength. In addition, the large structures now have an even stronger leverage effect on each additional bond, which ultimately leads to better fracture propagation. In other words, if a single element breaks in a layer of deep ice, this weakens the ice structure disproportionately.
However, kinetic metamorphism faceting does not always reduce the stability of a snow cover snowpack. If, for example, the entire snow cover undergoes a massive kinetic metamorphism faceting due to a long period of cold weather, there is no longer any bonded snow that could slide off as a snow slab. The snow cover snowpack now consists only of loose, unbound snow crystals. In this case, the spread of fractures is greatly reduced and it is not uncommon to experience the finest powder snow in very safe conditions. The situation only becomes critical again when the next snowfall or drifting snow arrives and the loose layers become a weak layer covered in snow.
As soon as the snow temperature rises to 0 °C, the crystals begin to melt at their corners and edges. They take on a rounded shape and move closer together. Conglomerates (clusters) of several grains can often be seen under a magnifying glass (see table of grain shapes). The individual roundish snow crystals grow strongly and can become several millimetres in size within a short time. The resulting water initially only occupies the indentations and crystal contacts of the clusters. This nesting of small amounts of water in the pore angles increases the capillary forces between the crystals and increases the strength. As long as the moisture content of the snow and the grain diameters are low, consolidation occurs. The snow sticks together and is perfect for long snowball fights or building a beautiful snowman.
As the melt progresses, the pores increasingly fill with meltwater. The water can no longer be held in the corners of the pores and flows away under the force of gravity. The snow crystals are now almost completely covered by a skin of water and separated from each other. The disappearance of the grain bonds results in a great loss of strength (rotten snow). Small differences in the amount of water determine whether the wet snow is stable or not. However, changes in the amount of water can happen very quickly. Typically, the snow remains stable up to approx. 3 percent liquid water by volume. If this threshold value is exceeded, the snow cover snowpack loses its stability very quickly.
If a moist or wet layer of snow freezes, it forms a stable crust of melted snow. The snow becomes very firm. The repeated alternation of melting and refreezing creates coarse-grained corn snow, which skiers outside Switzerland refer to somewhat casually as “firn”.
