Written by Seven Kings
The Winter Olympics are the headlines of these two days. The Winter Olympics are good, and the snowflakes at the opening ceremony are also beautiful. But there is a problem that you may not know, the snowflakes at this year's Winter Olympics are not hexagonal at the opening ceremony, but round.
Opening ceremony of the 2022 Beijing Winter Olympics. Image source: wikipedia
The problem begins with global warming.
Due to the warming climate, natural snowflakes are now rare at the Winter Olympics and snow events. According to the European Environment Agency (EEA), the snow season in the Northern Hemisphere has been reduced by 5 days since the 1970s.
Because of the widespread lack of "snow," artificial snow was used in 88 percent of the National Ski Areas Association's ski resorts between 2009 and 2010. The Turin Winter Olympics, vancouver, Sochi and Pyeongchang all use artificial snowmaking. For example, at the 2018 Winter Olympics in Pyeongchang, artificial snow accounted for 90% of the total snow used.
Biathlon site of the 2022 Beijing Winter Olympics. Image source: wikimedia
This Beijing Winter Olympics is also the one with the highest artificial snow content, because this year's snowflakes are almost all artificial. In the Yanqing Division, Zhangjiakou Division and shougang Ski Jump, artificial snow is used.
So, what's the difference between natural snowflakes and artificial snow?
In principle, there is no difference between the two. Both natural and artificial snow are ice ice, crystals formed by water molecules.
Ice requires crystal nuclei to form. The crystal nucleus is equivalent to the singularity of snowflakes, and many materials such as ice itself and dust can become crystal nuclei.
If there are no impurities, pure water does not freeze until minus 39 degrees Celsius, and this process of freezing that relies on extremely low temperatures (or high pressures) is also called homogeneous nucleation, when the crystal nuclei are traitors from their own people — some water molecules that first become solid.
Pure water between minus 39 degrees Celsius and the freezing point remains supercool, i.e. water that does not freeze below freezing.
Supercooled water can quickly freeze when it encounters disturbances or nuclei.
For natural snow, the crystal nucleus is not a frozen water molecule, but some fine dust. Water vapor clings to the dust and condenses into a solid, thus turning into Xiaoice ice. In order to be symmetrical with the above, it has to be said that this icing process that relies on foreign matter as crystal nuclei is called heterogeneous nucleatio.
In the process of falling with gravity, this Xiaoice ice will encounter more water vapor, gradually growing open, becoming a large ice ice visible to the naked eye, that is, snowflakes.
Most people have the impression that the snowflakes are hexagonal like at the opening ceremony, but in fact, natural snowflakes have many forms, such as the needle-shaped "MSG snow" in Henan in January this year.
The relationship between snowflake morphology and temperature and air humidity is shown in the following figure.
Snowflake morphology at different temperatures (abscissant coordinates) and supersaturation (to the extent of air exceeding 100% relative humidity), in some cases natural snowflakes are MONOSG-like. Image credit: Kenneth Libbrecht
However, artificial snow is neither hexagonal nor cylindrical, but spherical.
Peter Wasilewsk, a scientist at NASA, has studied the difference between natural and artificial snow. Here's a picture he took —
Image source: USDA
You can see that on the left is natural snow with some artificial snow, and on the right is artificial snow, which is obviously spherical.
The discovery of artificial snow forms was also an accident. Wasilewsk said that in the 1990s, when the USDA was studying the differences between natural and artificial snow, it accidentally discovered the morphological differences with a microscope.
Snowflakes under an electron microscope. Image credit: pixabay
So why is artificial snow spherical? This is related to the formation process of artificial snow and the composition of the crystal nucleus.
Wasilewski, who has studied snowflakes for more than twenty years, explains that the nuclei of natural snowflakes are dust, and they do not accelerate condensation, but slowly condense in the process of natural falling, and according to the temperature and humidity differences in the path of each snowflake falling, they will grow "petals" of different shapes; but the nuclei of artificial snow are ice crystals formed by water droplets, and the crystallization process is very fast, which makes the form of artificial snow and natural snow significantly different.
Classification of snowflake forms by the International Association of Cryospheric Sciences in 1951, by Vincent J. Schaefer
The difference in the morphology between artificial snow and natural snow has important physical significance, that is, natural snow is softer, but also more likely to collapse, while artificial snow is more compact. The artificial snow density of the snowmaker is 450 kg per cubic meter, and the natural snow is 250 kg per cubic meter. In fact, high-density artificial snow is more suitable for slalom skiing.
Steeplex skiing requires solid snow, so artificial snow is better.
At this point, you must be wondering how artificial snow is made.
No matter what kind of snow machine, the manufacturing process of artificial snow can be compressed into one sentence: first use water droplets to make tiny ice crystals, which are crystal nuclei, and then shoot out the crystal nuclei, so that the water mist and the crystal nuclei are in contact to form snowflakes.
The snowmaker (Figure 1) uses compressed air and water to blow out an icy crystal nucleus (Figure 3), which encounters another wave of water mist to form snowflakes (Figure 4). Image credit: TechnoAlpin
The principle of snowmaking is simple to say, but there are only a few companies in the world that can make snow machines. According to a 2017 report in The Economist, the global snowmaker market is currently dominated by several European and American companies, namely TechnoAlpin, Demaclenko and SUFAG.
You may be familiar with one of the names, the artificial snow for this Beijing Winter Olympics is made using Tianbingjia's machines. Michael Mayr, Sky Ice's Asia Manager, said: "We are the sole supplier of snowmaking systems for the Beijing 2022 Winter Olympics. ”
Snow machines make snow. Image credit: TechnoAlpin
The reason why these companies can monopolize the snow machine market is because they master the core technology of the snow machine - nuclear devices.
The nuclear apparatus spews out a mixture of crystal nuclei and compressed air, which is also the key to snowmakers. Image source: See watermark
In the snow machine, the part that uses water to make the crystal nucleus is called the nuclear device, and the nuclear device is one of the key technical bottlenecks of the snow machine.
This is because, to become a crystal nucleus, the water droplets must be as small as 30-70 microns, almost as fine as a human hair, and this precision is the technical difficulty of the nuclear device.
This process relies on the high-pressure gas inside the reactor. High-pressure gases can make small water droplets. At the same time, relying on the ideal gas equation of state, high-pressure gases can also cool the water droplets when they expand, turning them into ice crystals. The pressure of the high-pressure gas that can complete these operations is about 7 times that of the atmosphere, and the hydraulic pressure is more than 100 times the atmospheric pressure.
According to the ideal gas equation of state, the temperature decreases as the gas volume increases, and the temperature increases in turn. The volume of the high-pressure gas is rapidly increased when it is ejected, which cools the water droplets. Image source: chemistrygod.com
This is also why at this Winter Olympics, the carbine snow kirin M2 snow machine independently developed by the mainland was placed in the center of the Science and Technology Winter Olympic Hall as a major technological breakthrough.
So in a sense, artificial snow is more precious than natural snow.
Every cubic meter of snow produced by a snow machine requires 0.6-2 kWh of energy, which is almost equivalent to the energy consumption of a household air conditioner for one hour. In U.S. ski resorts, snowmaking consumes half of the total cost.
The Stubai Alps are used to store water for artificial snow. Image source: wikipedia
Water, the raw material for artificial snow, is also a big problem. According to the Global Times reported on February 8, the total planned water consumption for the Winter Olympics is 390,000 tons, but fortunately, artificial snow comes from surface runoff and rainwater, as well as the reuse of melted snow.
In short, the Winter Olympics not only show the ceiling of human quality physique, but also the stage of cutting-edge technology, not only athletes, but also the snowflakes under their feet.
Fake ice ice is an ice heart, and real ice ice heart is like dead ash.