The snow is over the city again. Under normal circumstances, when the world is not chained by infection, and the snow falls in the winter months, it is an opportunity for a little joy. But, snowy joy, sooner or later, can be replaced by quiet grumbling, nervousness and then, as the days go by, open intolerance, and often justified anger at the local authorities and winter services.
What exactly is snow? What kind of a rare, although regular guest in our region, has come to us again in bad weather? What kind of science is behind the snow?
Snow is just water, water in a solid state. But snow, for example, is significantly different from sleet or icy rain because its crystallization takes place in the atmosphere, although its chemical formula is the same, H20, as in water or ice.
Raindrops, which originate from the condensation of water vapor in the clouds, can freeze and turn into sleet and ice due to the low temperature during the fall, but the icy particles formed near the ground are not snow because they differ from snowflakes in shape and structure.
Snowflakes are already formed in the clouds, where snow crystals are formed directly from water vapor. Snowflakes can be made of one or more fused crystals, while at higher temperatures they are formed from a large number of crystals and fall to the ground in snowballs.
Each snow crystal is actually an ice crystal, so its geometry is determined by the geometry of the water molecule, which is composed of one oxygen atom and two hydrogen atoms, spaced 105 degrees apart.
The growth of a snow crystal begins in a cloud of supersaturated water vapor by water molecules condensing around a tiny dust particle and forming a hexagonal prism to which new layers of molecules are added. At first, while the crystal is small in size, it grows slowly and the entire lattice retains the same shape.
As the snow crystal grows larger, its six corners become more and more spaced apart and more and more surrounded by supersaturated air, so that the corners begin to grow a shade faster than the rest of the crystal.
Due to this small difference in speed at the corners, the hexagonal crystal begins to branch into six arms. The surrounding atmospheric conditions are practically the same for all branches, so they grow in approximately the same way and at the same speed. That is why all snowflakes have six mutually identical arms.
However, the growth of the snow crystal and the final shape of the snowflake primarily depend on the concentration of moisture in the cloud and the air temperature.
In laboratory experiments with controlled conditions, it was found that at different temperatures, in the interval from -3 to -20 ° C, flakes are formed that differ drastically from each other. At -5 ° C, flakes with the longest arms usually form, while between -20 ° C and -15 ° C flakes form in the form of flat six-pointed crystals.
It is interesting that two exactly the same snowflakes will never fall from the clouds to the ground. The probability of two identical snowflakes forming is almost equal to zero, because each snowflake constantly survives dramatic changes in the environment, is carried on different sides of the clouds, suffers from different pressure and humidity.
But the uniqueness of the snowflakes is still a consequence of the molecular structure of the ice. Due to the existence of several isotopes of hydrogen and oxygen, one in every 5,000 molecules of water is different from the others. A small snow crystal contains several thousand million billion molecules of water, which means that it contains about a million billion molecules that are different from the others. They are randomly arranged on a crystal lattice, making each snow crystal unique.
If a million billion flakes were formed in each year, the probability that even during the period of 15 billion years, which is the age of the whole universe, two to one molecule of identical flakes would form is still practically zero.
The magic of snow is unquestionable. Snowflakes, each of them completely unique, descend to the ground in a chaotic flight and create a snow cover that, in the whiteness, in the innocence it associates, balances every landscape, temporarily beautifies the scenes of the dirty city and abolishes ugly localities. It causes silence.