Knowledge about snow

The precipitation that falls on the earth's surface in the form of solid in the atmosphere is called atmospheric solid precipitation. Snow is one of the most extensive, universal and main forms of atmospheric solid precipitation. There are many kinds of atmospheric solid precipitation, including beautiful snowflakes, hail that can cause great harm, and snow graupel and ice particles that we don't often see.

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.

Non-condensing snowflakes in the sky

Snow falls from the sky. How can there be non-condensing snowflakes in the sky?

1in the winter of 773, a newspaper in Petersburg, Russia reported a very interesting news. According to news reports, at a dance, due to the large number of people and the burning of hundreds of candles, the ballroom was hot and stuffy, and the ladies and gentlemen in poor health almost fainted in front of the God of Joy. At this time, a young man jumped on the windowsill and smashed the glass with one punch. As a result, an unexpected miracle appeared in the ballroom. Beautiful snowflakes danced in the hall with the cold airflow outside the window and fell on the hair and hands of people who were dizzy with heat. Someone rushed out of the ballroom curiously to see if it was snowing outside. Surprisingly, the sky is dotted with stars and the crescent moon is as silver as water.

So, where did the snowflakes in the hall come from? This is really a puzzling question. Is anyone playing any magic tricks? But no matter how clever a magician is, he can't play with snowflakes in the hall.

Later, scientists solved the mystery. It turns out that the breath of many people in the ballroom is full of water vapor, and the burning of candles has scattered many condensation nuclei. When the cold air outside the window breaks through the window, it forces the saturated water vapor in the hall to condense and crystallize immediately and turn into snowflakes. So as long as it snows, it will snow in the house.

The basic shape of snowflakes

The scenery is beautiful when it snows, but scientists and artists appreciate the exquisite snowflake patterns. Glaciologists began to describe the shape of snowflakes in detail more than a hundred years ago.

Ding Duoer, the originator of western glaciology, described the snowflakes he saw on Luoza Peak in his classic glaciology works: "These snowflakes ... are all composed of small ice flowers, each of which has six petals, some of which release beautiful little lateral tongues like Su Hua, some are round, some are arrow-shaped, or zigzag, some are complete, and some are lattice-shaped, but they are not beyond.

In China, as early as 100 BC in the era of Emperor Wendi of the Western Han Dynasty, there was a poet named Han Ying who wrote a biography of Han Poetry, which clearly pointed out that "there are six snowflakes everywhere."

The basic shape of snowflakes is hexagonal, but there are almost no two identical snowflakes in nature, just as there are no two identical people on earth. Many scholars have observed thousands of snowflakes with microscopes. These studies finally show that it is impossible to form snowflakes with the same shape and size and partial symmetry in nature.

Among these observed snowflakes, even the regularly symmetrical snowflakes are deformed. Why do snowflakes deform? Because the water vapor content in the atmosphere around snowflakes can't be the same in all directions, as long as there is a slight difference, the side with more water vapor content always grows fast.

There are many collectors of snowflake patterns in the world. They collect all kinds of snowflake photos like stamp collectors. An American named Bentley took nearly 6,000 photos in his life. Soviet photographer Siger Mountain is also a photographer of snowflake photos. His charming works are often used as models of structural patterns by artists. Japanese Nakatani Yoshijiro and his colleagues worked hard for 20 years, shooting and studying thousands of snowflakes in the cold room of the laboratory of Hokkaido University in Japan and in the tent on the snowfield in northern Japan.

However, although snowflakes have various shapes, they remain unchanged, so scientists may classify them into the above seven shapes. Among these seven shapes, hexagonal snowflake and hexagonal prism snowflake are the most basic forms of snowflake, and the other five are only the development, transformation or combination of these two basic forms.

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.