When snowflakes trickle down from the sky, children and winter sports enthusiasts are happy. Snow is generally less popular with drivers. Snow transforms the landscape and appears peaceful, but this form of precipitation has a lot to offer. Snow crystals are the basis, and each one is unique.
Snow - more complex than you think
Snowflakes are a composite of several ice crystals. These in turn form in clouds at correspondingly low temperatures. If it is extremely cold, for example around -40 degrees, supercooled and previously liquid water can spontaneously freeze into an ice particle. However, this is a special case, as the temperatures are usually higher. Water molecules then attach themselves to the finest impurities, so-called condensation nuclei. If the temperatures are typically between -4 and -20 degrees, ice crystals begin to grow. The water molecule is known to consist of one oxygen and two hydrogen atoms, which are at a certain angle to each other. This structure turns it into a dipole, i.e. the charges are distributed differently.
Fig. 1: Model of the water molecule with the distribution of charges (negative for the oxygen atom, positive for the two hydrogen atoms); Source: Wiktionary
Without delving too deeply into the chemistry and physics of water, the snow crystal then begins to grow. More and more water molecules attach themselves to the crystal. This happens according to a certain pattern (namely "dentritic"). This means that a snow crystal does not grow evenly. Tips, corners and surfaces are formed. These structures influence the further course of growth, as more water molecules accumulate at the tips and edges than, for example, on the faces. The properties of the water molecule create a crystal lattice in which only 60° and 120° angles exist. This gives a snow crystal its typical hexagonal shape, i.e. hexagonal plates or columns. The exact shape and combination of these basic forms varies greatly - depending on the temperature and humidity conditions. The classic snow stars typically form at temperatures just below freezing. As they fall, they can become entangled with other crystals to form snowflakes. At temperatures above 0 degrees, liquid water also acts as a kind of glue and the flakes are then at their largest. If, on the other hand, it gets colder and colder, the shapes tend to become simpler, with more needles and platelets forming without further branching. Very cold air can no longer absorb or hold too much water vapor, so there is also less "building material" available. In heavy frost, snow crystals can also trickle out of seemingly almost clear air and glisten in the sun.
No two are the same!
Today and in the coming days it is cold enough for snow, and also cold enough for beautiful snow crystals. It is therefore an opportunity to take a closer look at the fine structures. This is best done on a darker surface and with a magnifying glass; modern cell phone cameras can also be very useful here. The American farmer Wilson Bentley (1865-1931) invented a method of photographing snow crystals at an early stage. From 1885 onwards, he photographed and cataloged over 5000 crystals, and each one was unique.
Fig. 2: Example of different snow crystals; Source: Wikipedia
In fact, it is virtually impossible that two identical snow crystals have ever fallen since it started snowing on earth (quite a long time ago). They may be similar, but there are always differences in the details. This is due to the large number of degrees of freedom (temperature, humidity, different isotopes of oxygen and hydrogen atoms) and possible combinations of the individual water molecules. The number of possible variations far exceeds the number of elementary particles in the observable universe! Faced with this huge number, one can of course be awestruck, but one can also simply enjoy the beauty and uniqueness of a snow crystal. Each one is transient and unique!
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