You can ski even in the hot summer. How do you achieve “snow freedom”?

Skiing as a sport originated in Northern Europe. China's skiing industry began in 1996. In recent years, with the growth of the national economy, people's living standards have been getting higher and higher. At the same time, influenced by the Beijing Winter Olympics, the public's enthusiasm for skiing has increased sharply, and the south's desire for ice and snow has also increased. The Winter Sports Management Center of the State General Administration of Sports has proposed the strategic development goal of "moving snow from the north to the south", and my country's skiing industry is getting bigger and bigger.

In many people’s minds, skiing is just a standard winter sport. However, in recent years, in addition to outdoor ski resorts in the north, nearly 40 all-weather indoor ski resorts have been built across the country [1]. Regardless of the season, we can enjoy the passion and joy that ice and snow bring us even in the hot summer.

Due to the continuous accumulation of the greenhouse effect and global warming, the winter time has a direct impact, resulting in a reduction in natural snowfall time.

As far as the global ski industry is concerned, the dependence on artificial snow is increasing . The snow surface of the above-mentioned indoor ski resorts is all made by artificial snow. So, do you know how snow is formed? How can we "make snow"?

The rendering of a domestic indoor ski resort project

How is natural snow formed?

Snow is a form of water in solid form, and snowfall can also be considered a form of precipitation. When the temperature in the cloud is below 0°C, the water vapor in the atmosphere forms ice crystals, and many small ice crystals collide and condense into large ice crystals. When the temperature remains unchanged or drops, the ice crystals continue to grow. When the ice crystals can overcome the air resistance by their own weight, and when the ambient temperature under the cloud is below the freezing point, the white ice crystals will fall to the ground and form snowfall.

Natural snow formation process

What are the necessary conditions for natural snowfall?

▶ 1. Water vapor saturation

The maximum amount of water vapor that air can contain at a certain temperature is called the saturated water vapor. The temperature at which air reaches saturation is called the dew point. When saturated air is cooled to a temperature below the dew point, the excess water vapor in the air turns into water droplets or ice crystals.

▶ 2. Condensation nuclei

Condensation nuclei are very tiny solid particles suspended in the air. The most ideal condensation nuclei are those particles of substances that absorb water the most, such as particles of sea salt, sulfuric acid, nitrogen and some other chemicals.

▶ 3. Low temperature environment

Natural snow snowfall snow scene

In 1996, my country successfully held the 3rd Asian Winter Games. It is understood that at that time, the snow in the snow track was transported from the snowfall area to the competition venue, and then covered on the snow track for athletes to compete. However, with the development of science and technology, snow sports no longer have to rely on the weather, and can all be completed through artificial snowmaking technology.

The 2022 Beijing Winter Olympics was successfully held, presenting the world with a wonderful, extraordinary and outstanding ice and snow event. The Beijing Winter Olympics has a total of 15 events, and about 7 sports need to be completed on snow, including alpine skiing, cross-country skiing, freestyle skiing, Nordic combined, ski jumping, snowboarding, etc. The snow used in the Winter Olympics is all made by artificial snow.

Among them, the National Alpine Skiing Center project is known as the "crown jewel of the Winter Olympics". The venue has 3 competition ski slopes, 4 training ski slopes and other competition auxiliary ski slopes. The total length of the ski slopes is about 23.1km, and the maximum vertical drop is about 925m. A total of about 170 artificial snow-making equipment are used to meet the snow-making needs of this project.

The starting point of the competition track is 2,179 meters above sea level, with a slope length of approximately 3,045 meters, an average slope of 30%, and a maximum slope of 68%. It extends all the way down along the ridge, with a vertical drop of approximately 900 meters from the starting point to the end point, setting a record for the highest drop on domestic ski trails.

National Alpine Ski Center

How is artificial snow made?

Artificial snowmaking technology was invented in the 1950s. Its principle and technical route is to artificially imitate and create the process of natural snow formation, and to create the conditions for the formation and growth of snow crystals through technical means. The snowmaking system is a compressor that generates compressed air to send water to the nozzle of the nuclear generator, and uses the volume contraction behind the nozzle to take away the heat and condense the mist droplets into ice crystals, which are suspended in the air for cooling and solidification. The ice crystals form snowflakes when they encounter a low temperature environment in the air.

Snowmaking equipment snowmaking status

Artificial snow making process

The most important component in the artificial snowmaking system is the nucleator nozzle. The nucleator nozzle is the key to artificial snowmaking. Without it, only water will be produced. The nucleator nozzle mixes the compressed air through the mixing chamber inside it. The liquid and gas form tiny bubbles during the mixing process. In the mixture, the tiny bubbles expand, the liquid ruptures, and the gas expands, causing the outlet temperature to drop, forming flakes, strips and small droplets at the outlet, thereby realizing the atomization phenomenon and forming ice crystal snow nuclei in the cold environment.

Schematic diagram of snowmaking system principle [2]

Schematic diagram of the structure of the nuclear device nozzle equipment [2]

Can it snow at 4℃?

Environmental conditions directly affect whether droplets will freeze and form snow. At the same temperature and humidity, the smaller the droplet size, the easier it is to freeze. If you want to achieve snowfall at a high ambient temperature, you must reduce the droplet size as much as possible. As the ambient temperature and relative humidity decrease, the maximum droplet diameter that can be used for artificial snowmaking increases. When the ambient temperature is 4°C and the relative humidity is 20%, a complete freezing phase transition occurs when the droplet diameter is 100μm, and droplets of other diameters are all in a mixed state of ice and water. As the ambient temperature drops to -4°C, the droplet diameter that undergoes a complete freezing phase transition increases to 200μm [2]. Therefore, snow can be achieved at an ambient temperature of 4°C.

Critical snowfall particle size map [2]

As people's living standards are getting higher and higher, based on the continuous development of science and technology, we can enjoy the passion that ice and snow bring us in the hot summer.

However, artificial snow is different from natural snow:

First of all, their shapes are different. Natural snow is mostly hexagonal, star-shaped, flake-shaped, etc., which are intertwined with each other after falling to the ground. Its porosity is high and it is soft. Artificial snow is similar to spherical or conical crystals, and the accumulated snow has low porosity, good density and hardness.

Secondly, the density of natural snow is relatively low, generally around 250kg/m3; while the density of artificial snow is relatively high, generally above 300kg/m3. In order to ensure the snow surface requirements for competitions, outdoor ski resorts such as the National Alpine Ski Center require a snowmaking density of nearly 600kg/m3.

Summer vacation is coming. If you have plans to travel, you might as well search for indoor ice and snow venues during your trip. You can feel the feeling of pushing open the door of the ice and snow venue from the hot outdoors, instantly entering the sub-zero ice and snow environment and enjoying skiing. I believe it will bring everyone a different sense of freshness.

Of course, it is also possible that an ice and snow venue is right next to you.

References:

1. China Ski Industry Development Report; Social Sciences Academic Press

2. Liu Bin; Hu Hengxiang; Bi Lisen; et.al “Effect of the arrangement of two nozzles on morphology,velocity, and particle size distribution of artificial snow making spray field”, Physics of Fluids 35, 053326 (1~14);

END

Author: Li Kun, Senior Engineer, Deputy Director of the Mechanical and Electrical Design Institute of Huashang International Engineering Co., Ltd., Deputy Director of the Ice and Snow and Special Refrigeration Division (Selected for the Beijing Association for Science and Technology 2020-2022 Youth Talent Support Project)

Source: Beijing Refrigeration Society