Snowflakes are beautiful on snowy days. How is it formed?

Snowflakes are formed by condensation of water vapor in the air. In general, water vapor will condense into water before freezing, but snowflakes are directly condensed by water vapor (people also call this process "condensation"). When the temperature drops and the water vapor in the air is supersaturated, it will start to crystallize on the crystal nucleus (such as dust), and then gradually grow up to form snowflakes. According to the knowledge of thermodynamics, the partial pressure of water vapor in wet air (air containing water vapor) is directly proportional to its content, and the highest pressure that can be reached at this temperature is the saturation pressure, when the content of water vapor reaches the maximum, so people often call it "saturated wet air". Moreover, the knowledge of thermodynamics also tells us that in this saturated state, the three existing forms of water, or solid (ice), liquid (water) and gas (vapor), are in equilibrium, and different phases can be transformed into each other. Therefore, when the temperature drops, the water vapor in the originally saturated air will be "excessively" saturated and condensed on the "core" such as dust.

Depending on the temperature, water droplets or ice crystals will be produced (sometimes both exist at the same time). The cloud we see is a collection of water droplets or ice crystals left in the atmosphere. If it is basically a water drop, it is called "water cloud"; If it is mainly ice crystals, it is called "ice cloud". There are two main shapes of small ice crystals in the "embryo" of snowflakes in the cloud. One is hexagonal and slender, called "columnar crystal"; Among them, some columnar crystals have sharp ends and are called "needle crystals". The other is hexagonal flake, which is called "flake crystal".

So far, about 20,000 different snowflake patterns have been known, but they are basically hexagonal and developed from thin sheets. Why hexagon? This is related to the crystallization habit of water. Ice frozen by natural water and snow condensed by water vapor in the atmosphere belong to hexagonal system. As we all know, water molecules are composed of two hydrogen atoms and one oxygen atom through a strong bond-* * * valence bond. When solid ice crystals are formed, they are bonded together by hydrogen bonds. Relatively speaking, the most stable arrangement is that six water molecules stick together to form a hexagon, so most ice crystals are hexagonal.

How did the six-star snowflake we saw grow up? It turns out that the water vapor near the ice crystals will be consumed and the ice crystals will grow together in the collision. Therefore, the closer to the ice crystal, the less the water vapor content and the lower the supersaturation. Near the surface of the ice crystal, excess water vapor has condensed on the ice crystal, so it has just reached saturation. In this way, the water vapor content near the ice crystals is smaller than that far away from the ice crystals. Water vapor moves from a distance to the ice crystals. Water vapor molecules first meet the corners and protrusions of ice crystals and condense there. Therefore, the angles and protrusions of ice crystals will grow rapidly first and gradually branch. Then, for the same reason, water vapor from far away will grow new branchlets on newly formed branches and corners. In this way, flaky ice crystals evolved into the familiar star snowflakes.