Functions and forms of snowflakes

Snowflake is a kind of beautiful crystal, which sticks together to form snowflake during falling. The size of a single snowflake is usually between 0.05 and 4.6 mm, and the snowflake is very light, with a single weight of only 0.2-0.5 g. No matter how small a snowflake is, it is wonderful. Its crystals are all regular hexagons. Therefore, the ancients said that "there are five flowers in vegetation and six snowflakes". How beautiful and light the snowflake is! I like snowflakes more and more. If I can witness the snow-covered earth and the silver-covered green trees again, it will really be a pleasure.

"Xue Rui heralds a bumper harvest year" is an agricultural proverb widely circulated in China. In the north, a thick layer of loose snow is like covering wheat with a quilt to keep out the cold. The cold nitrogen in snow is easily absorbed and utilized by crops. The low temperature of snow water can freeze the overwintering pests on the surface and bring benefits to agricultural production. So there is another agricultural proverb, "wheat covers three layers in winter and sleeps with steamed bread in the next year."

Snow has a wide range of functions, but it is of great benefit to mankind. First, it is beneficial to the growth and development of crops. Because of the poor thermal conductivity of snow, covering the soil surface with a layer of snow cover can reduce the diffusion of soil heat and block the invasion of cold air on the snow surface, so the crops protected by snow can survive the winter safely. Snow can also store water for crops. In addition, snow can enhance soil fertility. According to the determination, every 1 liter of snow water contains about 7.5 grams of nitride. Snow water permeates the soil, which is equivalent to applying nitrogen fertilizer once. Feeding livestock and poultry with snow water and irrigating crops can obtain obvious benefits.

Snow is both beneficial and harmful to people. In mid-spring in March and April, if it suddenly snows because of the cold wave. It will cause severe cold. Therefore, the agricultural proverb says, "La Xue is a treasure, but Chun Xue is not good."

Snowflakes do not come out of thin air. They must rely on dust particles invisible to the naked eye in the air below the stratosphere as crystal nuclei. Water molecules of water vapor condense around it layer by layer under the action of cold air, and crystal nuclei grow outward from the center. It takes about 5 minutes to form snow crystals. During this time, the airflow in the snow-making environment is always fluctuating, but the water vapor must act on the periphery of the crystal nucleus in equal amount. The thickness, humidity and temperature of clouds in the air have great influence on the shape of snowflakes. The formation of star-shaped snowflakes requires high humidity, while clouds with low humidity are easy to form flake and powdery snowflakes. In fact, a single snowflake is extremely tiny, with a diameter of 0.5-3mm, and 5000 snowflakes are only 1 g on the precision balance, which is very beautiful under the microscope. The quality of ordinary water depends on the content of heavy water, which is high and poor. Instead, the water quality is better. Under normal circumstances, 7 kg of water contains 1 g of heavy water, while 7 kg of snow water only contains 0.25 g of heavy water, which shows that the biochemical performance of snow water is much better. If there is enough snow and water, the wheat field will grow well in spring. Soaking seeds in spring ploughing has high proportion of heavy water and low germination rate. If seeds are soaked in snow water, they will be like a duck to water.

Clouds are places where snowflakes are bred. Snowflakes are produced by these small crystal nuclei in the clouds. There are three growth trends of crystal nucleus: slender hexagonal column, crystal needle with needle tip, and slender hexagonal sheet. If the concentration of water vapor around them is low, the growth of ice crystals is slow and uniform on all sides; If the surrounding water vapor concentration is high, not only the volume will increase, but also the shape will change during the growth process. The most common is hexagonal snowflakes falling from the sky. Why do people tend to be hexagonal? It turns out that when the ice crystal grows, it consumes the nearby water vapor, so the closer it is to the ice crystal, the thinner the water vapor is, and the farther water vapor will naturally be replenished. They first encounter sharp corners that extend forward, so each sharp corner is rapidly elongated and gradually becomes dendritic. In the same way, new twigs have grown on these "branches", and the hexagonal snowflakes we see have been formed again and again. Before the formation of snowflakes, the crystal growth rate of regular hexagon on the bottom and cuboid on the side is different, and the shape changes accordingly. For example, temperature will bring subtle changes to the crystalline surface. When the temperature is close to 0℃, the bottom surface expands horizontally into a hexagon, when it reaches -5℃, it forms a needle, and when it drops to -5 ~- 10℃, it begins to form regular hexagonal prisms and hexagonal prisms with hollow sides. Dendrites form at-15℃, and when the temperature drops to-10 ~-2 1℃, the regular hexagon begins to expand again, and then hexagonal prisms are formed.

When the surrounding water vapor content is less, the formation process is also slow, and complex shapes are not easy to appear. On the contrary, the greater the water vapor content, the faster the formation speed and the more complicated the shape. Dendritic snow crystals, also known as "snowflakes", are often generated in an environment containing a lot of water vapor at about-15℃. Although the speed of crystal formation depends on temperature and water vapor concentration, other gases in the air will also affect its formation. Experiments show that almost all the ice crystals formed in the vacuum space with only water vapor are single triangular prism, while the crystals formed in the sky are needle-shaped and hexagonal prism. After computer calculation, the shape of ice crystals extending in six directions can be reproduced, but the reason why they diverge in the middle can never be explained. As shown in the figure, the mechanism of the wonderful point symmetric branching mode is still a mystery.