Also known as weiyang flower, a beautiful crystal, it sticks together in the process of falling to form snowflakes. 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".
Why are snowflakes hexagonal?
Why are snowflakes mostly hexagonal and have so many patterns?
Snowflakes are formed by magnifying small ice crystals, and the molecules of ice are mostly hexagonal, so snowflakes are mostly hexagonal. The diversity of snowflake shapes is closely related to the water vapor conditions when it is formed.
Hexagonal flake ice crystals have different saturated vapor pressures because of the different bending degrees of their surfaces, edges and corners, of which the saturated vapor pressure on the edges is the largest, followed by the edges and the plane is the smallest. When the actual vapor pressure is the same, the growth of sublimation is different because of the different saturated vapor pressures on the surface, edge and corner of ice crystals. If the water vapor in the cloud is not very rich, the actual water vapor pressure is only greater than the saturated water vapor pressure on the plane, and the water vapor only condenses on the surface, then a columnar snowflake is formed; If there is a little more water vapor, the actual water vapor pressure is greater than the saturated water vapor pressure at the edge, and the water vapor will condense at the edge and surface. Because the speed of condensation is also related to the degree of bending, the place with a large degree of bending will condense faster, so the condensation at the edge of ice crystals is faster than that on the surface, and more flake snowflakes will be formed at this time; If the water vapor in the cloud is rich, and the actual water vapor pressure is greater than the saturated water vapor pressure on the corner, then there is water vapor condensation on the surface, edges and corners, but the sharp corners are prominent, and the water vapor supply is the most sufficient and the condensation grows fastest, so many branches or star-shaped snowflakes are formed.
Coupled with the constant movement of ice crystals, their temperature and humidity conditions are constantly changing, which makes the growth rates of various parts of ice crystals inconsistent and forms various snowflakes.
Formation of snowflakes
How does the water vapor in the air movement form snowfall in the sky? Is the temperature below zero enough? No, if water vapor wants to crystallize, two conditions must be met to form snowfall:
One condition is that water vapor is saturated. The maximum amount of water vapor that air can hold at a certain temperature is called saturated water vapor. The temperature at which air reaches saturation is called dew point. When saturated air is cooled to a temperature below the dew point, the excess water vapor in the air becomes water droplets or ice crystals. Because the saturated water vapor content of ice surface is lower than that of water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. That is to say, water droplets can grow only when the relative humidity (relative humidity refers to the ratio of the actual vapor pressure in air to the saturated vapor pressure of air at the same temperature) is not less than 100%; As for ice crystals, they tend to grow when the relative humidity is less than 100%. For example, when the temperature is -20℃ and the relative humidity is only 80%, ice crystals can grow. The lower the temperature, the less humidity is needed for the growth of ice crystals. Therefore, in the high-altitude and low-temperature environment, ice crystals are more likely to be produced than water droplets.
Another condition is that there must be condensation nuclei in the air. Some people have done experiments, if there is no condensation nucleus, the water vapor in the air will only condense into water droplets if it is supersaturated to a relative humidity of more than 500%. But such a large supersaturation phenomenon will not exist in the natural atmosphere. So without condensation nuclei, it is difficult for us to see rain and snow on the earth. Condensation nuclei are tiny solid particles suspended in the air. The ideal condensation nuclei are those particles that absorb the most water. Such as sea salt, sulfuric acid, nitrogen and other chemicals. So we sometimes see clouds in the sky, but there is no snow. In this case, people often use artificial snowfall.
Why is snow a good year?
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."
Effects of Snow on Human Health
In winter, it snows endlessly. People often ignore the role of snow when watching Yushu Qionghua.
Snow has many benefits to human health. Compendium of Materia Medica has long recorded that snow water can detoxify and cure the plague. There is a folk prescription for treating fire burns and frostbite with snow water.
Taking a bath with snow water often can not only enhance the resistance between skin and body, reduce diseases, but also promote blood circulation and enhance physical fitness. Drinking clean snow water for a long time can prolong life. This is one of the "secrets" of those who have lived in the deep mountains and forests for a long time.
Why does snow have such strange functions? Because snow water contains less heavy water than ordinary water 1/4. Heavy water will seriously inhibit the life process of living things. After testing, fish will die quickly in water containing 30-50% heavy water. The most basic condition for the formation of rain and snow is the existence of "condensation nuclei" in the atmosphere, and solid impurities such as dust, coal particles and minerals in the atmosphere are the most ideal condensation nuclei. If meteorological factors such as water vapor and temperature in the air reach certain conditions, water vapor will condense into snowflakes around these condensation nuclei. Therefore, snowflakes can clean a lot of pollutants in the air. So every time after the heavy snow, the air is particularly fresh.
According to the measurement, the density of new snow is generally 0.05-0. 10g per cubic centimeter. Therefore, the reflectivity of ground snow to sound waves is extremely low, which can absorb a lot of sound waves and help reduce noise.
Thermal insulation function of snow
Snow is like a wonderful carpet, covering the earth, so that the temperature of the ground will not drop too low because of the cold in winter. This heat preservation effect of snow is inseparable from its own characteristics.
As we all know, it is warm to wear a cotton-padded jacket in winter. Why is it warm to wear a cotton-padded jacket? This is because the porosity of cotton is very high, and the pores of cotton are filled with a lot of air, which has poor thermal conductivity. This layer of air prevents the heat of human body from spreading outward. The snow covering the breast of the earth is very similar to cotton, and the porosity between snowflakes is very high. It is this layer of air that drills into the pores of the snow to protect the ground temperature from falling very low. Of course, the thermal insulation function of snow changes with the density at any time. It's like wearing a new cotton-padded jacket, which is especially warm, and the old cotton-padded jacket is not very warm. The density of fresh snow is low, and there is a lot of air stored in it, so the heat preservation effect is particularly strong. Chen Xue is like an old cotton-padded jacket, with high density and weak heat preservation effect, because there is less air stored in it.
Why does the more air an object stores, the stronger the heat preservation effect?
This is because air is a bad conductor. As we know, any object itself can transfer heat, and this property of transferring heat is called the thermal conductivity of the object. Among several common substances in nature, air has the worst thermal conductivity. Therefore, the more air an object contains, the worse its thermal conductivity. Because the amount of air contained in snow changes greatly, the thermal conductivity of snow also changes greatly.
Generally, the newly fallen snow has large pores and the best heat preservation effect. In the late stage of snow melting in spring, snow is soaked by water, and its thermal conductivity is closer to that of water, so the thermal insulation effect of snow tends to disappear.
nivation
Erosion caused by frequent melting and frost heaving of snowfields in periglacial climate. Snow erosion occurs in polar and sub-polar regions without ice sheets, as well as in alpine regions below the snow line and above the tree line. The annual average temperature there is about 0℃, which belongs to permafrost region. On the one hand, the alternate freezing and thawing of the edge of the snowfield destroys the surface material through ice cracking; On the one hand, snow melting will take away the crushed fine substances, so snow erosion includes two functions: denudation and transportation. With the deepening of the bottom of the snowfield and the expansion of its periphery, a wide and shallow basin-shaped concave land with small peripheral slope is gradually formed on the hillside, that is, the snow melting concave land. Its shape, origin and spatial distribution are different from those of ice buckets, but they are related. When the climate becomes cold and the snow line drops, the snow-eroded depression can develop into an ice bucket; On the contrary, when the climate warms and glaciers recede, ice buckets can degenerate into snow-melting depressions. Under different natural and geographical conditions, the way and speed of snow erosion are different. Where the latitude is low, the precipitation is high, and the freezing and thawing days are many, the snow erosion rate is faster, and the snow erosion is deep and large. For example, snow erosion is very common in Xiaoxing 'anling area of Northeast China. On the contrary, snow erosion is weak in places with high latitude, little precipitation and low temperature in summer. The influence of ground slope is: the slope is steep > 40, and the snow field is not easy to exist; Snow erosion on the flat land is slow; Snow erosion is most active on the slope of about 30.