In the last section we saw that snow forms when water vapor condenses in cold enough temperatures, often around a nucleator, and becomes an ice crystal. So, the main things you need to manufacture snow are water and cool temperatures.
It helps the process along if you mix a nucleator of some sort into the water supply. The water will already contain lots of stuff that can act as nucleators, but increasing the count is a good idea because it ensures that more water droplets will freeze before they reach the ground. One of the most widely used nucleators is a natural protein called Snowmax that is especially good at attracting water molecules.
The traditional type of snow gun produces water droplets by combining cooled water and compressed air. On a ski slope, you'll notice that these guns are attached to two different hoses that run to air and water hydrant stations, respectively. The hydrants are hooked up to two different lines which run under the snow or even underground. One pumps in water from a lake, pond or reservoir and the other pumps in high-pressure air from an air compressor.
The compressed air serves three major functions:
- It atomizes the water -- that is, disrupts the stream so that the water splits into many tiny droplets.
- It blows the water droplets into the air.
- It helps cool the water droplets as they fly into the air.
This last step is an added bonus of using compressed air. When air is compressed, the different air particles are pushed tightly together, which means they don't move around as much. When the air is released, the particles spread out and move more freely. This means the particles are using more energy, absorbing heat from the area around them and thus cooling the air around the water droplets.
Another common type of snow machine is called an airless snow gun. Airless snow guns use simple nozzles (similar to the ones you find on household spray bottles) to atomize the water into a fine mist. The water droplets are then blown up into the air by a powerful fan. The main advantage of this design is that you don't have to hook the snow gun up to a compressed-air supply -- you only have to provide water and a power source. Some other snow gun designs actually atomize the water with high-speed fans.
It takes a lot of energy to change water from a liquid to a solid. You have to remove the water's heat of fusion, the large amount of heat energy required to change ice into liquid water at 32 F (0 C). If it's cold enough, the natural conditions outside will be sufficient for freezing the water; but if it is only a little below freezing, you may need additional components to help the process along. Some snow machines have special cooling units to speed the freezing process when the natural conditions aren't cold enough to do the job.
To give the water enough time to freeze before it falls to the ground, many resorts use snow gun towers. These are simply sturdy poles that elevate the snow gun above the slope. Another advantage of this set-up is that the snow guns can be less disruptive to skiers. And the snow falls from above, as it would naturally.
In most resorts, workers will accumulate a big pile of manmade snow and then disperse it along the trail with snow-grooming equipment. Snow groomers are just tractors with very wide tracks that spread the snow around and compact it to make it dry and powdery. For ski resorts, regular grooming is an essential part of the snow-making process.