Knowledge about snow
Precipitation in the atmosphere that falls on the earth's surface in a solid form is called atmospheric solid precipitation. Snow is the most widespread, common and dominant form of solid precipitation in the atmosphere. Atmospheric solid precipitation is diverse. In addition to beautiful snowflakes, it also includes hail that can cause great damage, as well as snow graupel and ice particles that we do not often see.
The formation of snowflakes
How can water vapor moving in the sky form snowfall? Is it ok if the temperature is below zero? No, in order for water vapor to crystallize and form snowfall, two conditions must be met:
One condition is saturation of water vapor. The maximum amount of water vapor that air can contain at a certain temperature is called the saturated water vapor amount. The temperature at which air reaches saturation is called the dew point. When saturated air cools to a temperature below the dew point, excess water vapor in the air turns into water droplets or ice crystals. Because the saturated water vapor content of the ice surface is lower than that of the water surface, the water vapor saturation required for the growth of ice crystals is lower than that of water droplets. In other words, water droplets must grow when the relative humidity (relative humidity refers to the ratio of the actual water vapor pressure in the air to the saturated water vapor pressure of the air at the same temperature) is not less than 100%; as for ice crystals, the relative humidity is often less than 100% It can also grow over time. For example, when the air temperature is -20°C and the relative humidity is only 80%, ice crystals can grow. The colder the temperature, the less humidity required for ice crystals to grow. Therefore, in high-altitude and low-temperature environments, ice crystals are more likely to be generated than water droplets.
Another condition is that there must be condensation nuclei in the air. Someone has done experiments and found that if there are no condensation nuclei, the water vapor in the air can condense into water droplets only when it is supersaturated to a relative humidity of more than 500%. But such a large supersaturation phenomenon does not exist in the natural atmosphere. Therefore, without condensation nuclei, it would be difficult to see rain and snow on our earth. Condensation nuclei are tiny solid particles suspended in the air. The most ideal condensation nuclei are those material particles that absorb water the most. For example, particles of sea salt, sulfuric acid, nitrogen and other chemicals. That’s why we sometimes see clouds in the sky but no snowfall. In this case, people often use artificial snowfall.
Snowflakes that do not condense in the sky
Snow falls from the sky, so how can there be snowflakes that do not condense in the sky?
1773 In the winter of that year, a newspaper in Petersburg, Russia, reported a very interesting news. This news said that at a ball, due to the large number of people and the burning of hundreds of candles, the ballroom was so hot and stuffy that the ladies and ladies in poor health almost fainted in front of the God of Joy. . At this time, a young man jumped on the window sill and broke the glass with a punch. Then, an unexpected miracle appeared in the ballroom. Beautiful snowflakes danced in the hall with the cold air outside the window, falling on the hair and hands of people who were dizzy from the sweltering heat. Some people rushed out of the ballroom curiously to see if it was snowing outside. What is amazing is that the sky is bright with stars and the new moon is as silver as water.
So, where did the snowflakes in the hall come from? This is really a puzzling question. Could someone be playing some magic trick? But no matter how good the magician is, he can't make snowflakes appear in the hall.
Later, scientists solved this mystery. It turned out that the breath of many people in the ballroom was full of water vapor, and the burning of candles spread many condensation nuclei. When the cold air outside the window breaks in, it forces the saturated water vapor in the hall to immediately condense and crystallize, turning into snowflakes. Therefore, as long as there are conditions for snow, it will snow in the house.
The basic shapes of snowflakes
The scenery when it snows is beautiful, but what scientists and arts and crafts artists admire most are the small and exquisite snowflake patterns. More than a hundred years ago, glaciologists began to describe the shape of snowflakes in detail.
Tyndall, the originator of Western glaciology, described the snowflakes he saw on Roza Peak in his classic glaciology work: "These snowflakes... are all composed of small ice flakes, each one Each small ice flower has six petals. Some petals release beautiful little tongues like shansu flowers. Some are round, some are arrow-shaped, or zigzag-shaped, some are complete, and some are in a grid. shape, but they do not exceed the scope of the six-petal type."
In our country, as early as the era of Emperor Wen of the Western Han Dynasty more than 100 years ago, there was a poet named Han Ying who wrote a book. "Han Shi Wai Zhuan" clearly points out in the book, "Every plant and tree has five flowers, but snowflakes only have six."
The basic shape of snowflakes is hexagonal, but they are almost impossible to find in nature. Two identical snowflakes are just like there are no two identical people on earth. Many scholars have observed thousands of snowflakes with microscopes. These studies have finally shown that snowflakes with exactly the same shape and size and complete symmetry of each part cannot be formed in nature.
Among the snowflakes that have been observed by people, no matter how regular and symmetrical the snowflakes are, there are also deformities. Why are snowflakes deformed? Because the water vapor content in the atmosphere around the snowflake cannot be the same in all directions. As long as there is a slight difference, the side with more water vapor content will always grow faster.
There are many collectors of snowflake patterns in the world. They collect all kinds of snowflake photos like stamp collectors.
There was an American named Bentley who spent his life taking nearly 6,000 photos. Soviet photography enthusiast Sigshan was also a photographer of snowflake photos. His ecstatic works are often used by industrial artists as models for structural patterns. The Japanese Ukichiro Nakatani and his colleagues worked hard for twenty years in a cold room in the laboratory of Hokkaido University in Japan and in a tent on the snowfields of northern Japan, photographing and studying thousands of snowflakes.
However, although the shapes of snowflakes come in all shapes and sizes, they remain the same, which is why scientists can summarize them into the seven shapes mentioned above. Among these seven shapes, hexagonal snow flakes and hexagonal prismatic snow crystals are the most basic forms of snowflakes, and the other five are just the development, metamorphosis or combination of these two basic forms.
The effect of snow on human health
"Medicine and Health Care News" In winter, there is heavy snowfall and endless sky. When people appreciate Yushu and Qionghua, they often ignore the role of snow. Snow has many benefits for human health. "Compendium of Materia Medica" has long recorded that snow water can detoxify and cure plague. Folks use snow water to treat fire burns and frostbite.
Frequent bathing with snow water can not only enhance the resistance of the skin and body, reduce diseases, but also promote blood circulation and enhance physical fitness. If you drink clean snow water for a long time, you can live longer. This is one of the "secrets" for the longevity of those long-lived people living in the deep mountains and forests.
Why does snow have such strange functions? Because the amount of heavy water contained in snow water is 1/4 less than that in ordinary water. Heavy water can seriously inhibit biological life processes. Someone has done experiments and found that 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 ideal condensation nuclei. If the water vapor, temperature and other meteorological factors in the air reach certain conditions, the water vapor will condense into snowflakes around these condensation nuclei. Therefore, snowflakes can clean a large amount of pollutants in the air. Therefore, the air seems particularly fresh after a heavy snowfall.
According to measurements, the density of new snow is generally 0.05-0.10 grams per cubic centimeter. Therefore, the reflectivity of snow on the ground to sound waves is extremely low, and it can absorb a large amount of sound waves and contribute to reducing noise.
The thermal insulation effect of snow
Snow is like a wonderful carpet covering the ground, preventing the ground temperature from dropping too low due to the severe cold in winter. The thermal insulation effect of snow is inseparable from its own characteristics.
We all know that wearing a cotton-padded jacket is very warm 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. The thermal conductivity of air is very poor. This layer of air prevents the heat of the human body from diffusing outward. The snow covering the chest of the earth is very much like cotton. The porosity between the snowflakes is very high. It is this layer of air that gets into the pores of the snow that protects the ground temperature from falling too low. Of course, the thermal insulation function of snow changes with its density at any time. This is very similar to the situation where wearing a new cotton-padded jacket is very warm, but an old cotton-padded jacket is not very warm. The density of new snow is low, so there is more air stored in it, and the insulation effect is particularly strong. The old snow is like an old cotton-padded jacket, with a high density and less air stored inside, so its thermal insulation effect is weak.
Why does the more air stored in an object have a stronger thermal insulation effect?
This is because air is a poor conductor. We know that any object can pass heat. This ability to pass heat is called the thermal conductivity of the object. Among several common substances in nature, air has the worst thermal conductivity. So the more air an object holds, the less conductive it is. Since the amount of air that can be accommodated in the snow changes greatly, the thermal conductivity of the snow also changes greatly. Generally, newly fallen snow has large pores and has the best thermal insulation effect. In the later stages of snowmelt in spring, the snow is soaked in water. At this time, its thermal conductivity is closer to that of water, and the thermal insulation effect of the snow tends to disappear.