In recent days, Beijing has been frozen and snowy (people are going to freeze stupid), and ski resorts have started to open one after another...
Dream back to the Beijing 2022 Winter Olympics.
Some time ago, I secretly went to the Forbidden City to see a heavy snowfall all over the city, which was indescribably beautiful. I really envy the cats in the Forbidden City, eat and sleep and visit the Forbidden City, meow
Swipe right to see the picture taken to evaluate the scientific level of the evaluation.
This beautiful snow scene is not enough for one person, and wittily sent a friend in Shenzhen a cup of snow from the Forbidden City, which can make the child happy....
pyq classic copywriting: Nanfang baby also has a snowman pile.
In the past, there was fresh air from the Hulunbuir prairie, and now there is fresh snow from the side of the road in Haidian, Beijing, and suddenly a new business opportunity has been discovered!
Wait, there are even bigger business opportunities, if you can make snow directly in the south, and then lie to them and say that this is the snow ...... in the north?
So here comes the secret, how is the snow formed?Is it okay as long as the temperature is below freezing?
A joy that can be felt through the screen.
part.1 Snow conditions.
Actually, no, there are two conditions that must be met for the formation of snowfall
The first condition is saturation with water vapor. The maximum amount of water vapor that can be contained in the air at a certain temperature is called saturated water vapor. When the water vapor and air pressure are constant, the temperature is lowered so that the air can reach the temperature when the water vapor is saturated, which is called **. When the saturated air cools to a temperature below **, the 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. For example, when the air temperature is -20 and the relative humidity is only 80%, the ice crystals can grow.
Relative humidity is the percentage of water vapor pressure in the air compared to the saturated water vapor pressure at the same temperature:
U is the relative humidity, E is the water pressure (HPA), and Ew is the saturated water pressure (HPA) corresponding to the dry bulb temperature t( ).
Water droplets must have a relative humidity of at least 100% for them to grow. The lower the temperature, the less humidity is required for the ice crystals to grow. Therefore, ice crystals are more likely to be produced than water droplets in high-altitude and low-temperature environments.
Another condition is that there must be contrail nuclei in the air. Condensation nuclei are tiny particles suspended in the air, and ideally condensation nuclei are particles of matter that absorb water strongly. For example, sea salt, sulfuric acid, nitrogen, and so on. Some people have done experiments that if there is no condensation nucleus, the water vapor in the air can be condensed into water droplets when it is supersaturated to the level of relative humidity of more than 500%, but this supersaturation condition is difficult to achieve in nature.
When there are condensation nuclei in the air, or when water droplets in the atmosphere pass through freezing to form crystal nuclei, ice crystals can grow on the basis of crystal nuclei, and when their gravity is greater than buoyancy, they fall to the ground. Atmospheric temperature and water vapor supersaturation directly affect the shape of snow crystals, resulting in the structural diversity of snowflakes.
Symmetrical snowflakes.
Only when the above two conditions are met can ice crystals be formed. Climatically speaking, the climate in the south is warm and humid, with frequent rainfall, but the temperature is difficult to reach below **, so it is difficult for water vapor to condense into ice crystals. Even if the temperature over the atmosphere is low, if the ambient temperature is high, the snowflakes will melt into water as they fall, so it is rare to see snowfall in the south.
Aside from the "geographical advantage", we can also wait for the "time of day", and wait for a wave of cold wave that is rare in a century to come, if the cold air from Siberia goes south, causing the temperature in some places in the south to plummet, there will also be snowfall.
part.2 Artificial snowmaking.
Of course, the fastest way is to "humanize" and rub the snowflakes directly!
In fact, the snow in the ski resort is basically artificial snow, but the temperature is low in winter and it is not easy to melt, so it is generally open in the cold winter. Whether it is the northeast region with abundant snowfall, or the Altay region of Xinjiang, which is known as the "snow capital of China", ski resorts still need artificial snow.
First of all, the continuous growth of snowflakes needs the ambient temperature to reach below **, according to statistics, the annual average relative humidity of southern cities is mostly above 65%, Guangzhou, Haikou, Guiyang, Chengdu annual average humidity is more than 82%, the higher the humidity, the higher the environmental value, the easier it is for water vapor to condense or condense, so you can choose a cold night to snow, the snowmaking time of the general ski resort is also to choose the night.
With this, the freedom of snowmaking can be realized.
At present, the mainstream artificial snowmaking technology also uses the principle of natural snowmaking, mixing water and air in the nozzle of the snow gun and spraying out to form tiny atomized water droplets, the droplets in the cold air are frozen in the low temperature air due to the temperature difference and water vapor concentration difference between the external environment and the external environment, becoming the key element of snowflake growth - "crystal nucleus", the following figure is the four processes of the freezing process
Temperature curve of the freezing process.
The first part is the droplet cooling to supercooling stage, and the droplet temperature is continuously reduced until the supercooling state is reached
The second part is the nucleation and re-ignition stage, in which the droplet is in a metastable state, and once this state is destroyed, the droplet will partially crystallize, changing from a pure liquid droplet to an ice-water mixture. During the re-ignition process, the liquid water continues to undergo a phase transition, and the latent heat of this part of the phase change increases the temperature of the droplet itself, and finally reaches the equilibrium temperature
The third stage is the freezing stage, in which the droplets are still in the state of ice-water mixture, and while continuing the phase change process, the heat transfer and mass transfer process with the external environment are carried out, so that the temperature is always maintained at an equilibrium temperature until the liquid water is completely frozen
The fourth stage is the re-cooling stage, in which the droplets have been completely frozen into ice crystals, and the heat and mass transfer process with the external environment continues, and the temperature of the ice crystals continues to decrease until it reaches a steady state.
After the nucleus is formed, the nucleus then gathers the surrounding atomized water droplets, causing them to solidify on the surface of the nucleus and grow before hitting the ground. In general, artificially manufactured crystal nuclei are smaller in size, harder and have a higher moisture content of artificial snow, about 40%-50%, which is about 40% higher than that of natural snow.
So, what if someone sells you fake snow?Actually, there is a difference between artificial snow and natural snow:
Natural snow is flake-like, relatively small in density, fluffy and soft, and is crystallized by water vapor and grows fully along the crystal plane, so it has many regular shapes.
Artificial snow is ice granular, heavier, solidified and crystallized by water droplets, and contains a lot of water, which looks like a mixture of ice and snow. It doesn't matter what kind of snow it is, the snow that can build a snowman is good snow!
part.3 Artificial snowfall.
In addition to small-scale snowmaking, large-scale artificial snowfall can be achieved by using aircraft to spread silver iodide on the clouds. The crystals of silver iodide are very similar in size to the hexagonal monomers of snow crystals, the atomic arrangement in their monomers is also very similar, and the lattice spacing between the two is also very similar (silver iodide is 458 angstroms, snow crystal is 452 angstroms).
Image via Bing AI
Therefore, by scattering silver iodide particles in clouds with poor precipitation ability, so that they can "impersonate" the ice crystals, the water vapor and small water droplets in the clouds can be condensed and crystallized on the "impostor" crystals, turning into snowflakes.
Image via Bing AI
So, pick a cold night and create a romantic snowflake that is just right for you
References. 1] Jia Huan, Zhou Xiaoqing, Cai Aifeng, etc. Experimental study and application of artificial snow crystal nucleus diameter for airworthiness verification[J ol].Vacuum and Low Temperature, 1-8 [2023-12-22].
2] Wu Yanli. Study on the cooling process of heat and mass transfer of small droplets in artificial snowmaking[D].Beijing University of Civil Engineering and Architecture, 2023
3] Dong Peiwen, Liu Guoqiang, Zhang Bowen, et al. "Ice and Snow Magic" Turning Water into Snow: Artificial Snowmaking Technology[J].Chinese Science Bulletin,2022,67(21):2447-2449
4] Yuan Weihua. Snowmaking experts explain the snow on the track in detailHebei**,2022-02-17(T03).
Edit: tt