In the early to mid 20th century, the sport of snow skiing skyrocketed in popularity. As more and more people discovered the sport and made it a yearly tradition, ski slopes became extremely profitable businesses. But resort owners had a major problem: In their line of work, success was completely dependent on the weather. If it didn't snow, or didn't snow enough, they had to close everything down until the weather decided to cooperate. In many parts of the world, the skiing season was limited to a month or two.

The solution, of course, was to figure out a way to make snow when you needed it, in case nature failed to provide. The result was an invention called the snow gun. This device proved useful even when natural snow was plentiful because it gave resort owners greater control over the consistency of the top layer of snow, allowing them to create better skiing conditions. These days, snow-making machines are standard equipment in the vast majority of ski resorts around the world. They have made it possible for many resorts to stay open four months or more a year, and there are even a few indoor slopes that stay open year-round!

Photo courtesy SMI Snow Makers
Super Wizzard snow guns at work in Deer Valley, Utah

Now, we'll look at these snow guns to see how they work, what they can do and how operators use them to create ideal skiing conditions. So the next time you go skiing, you'll know exactly where all that perfect snow is coming from!

How Nature Makes Snow
One common notion is that machine-made snow is artificial. This is not really the case -- it's actually the same stuff that falls out of the sky, it's just created by a machine rather than by weather conditions. The machine works very differently than a weather system, but it accomplishes exactly the same thing. To understand how machines make snow, it's a good idea to first look at how snow occurs naturally.

Snow comes from water vapor in the atmosphere. Clouds form when the water vapor (water in gas form) in the atmosphere cools to the point that it condenses -- that is, changes from a gas into a liquid or solid. The droplets in a cloud are so light that the air in the atmosphere keeps them aloft. If the droplets get too heavy, they fall in the form of precipitation. If it is cold enough, this water vapor doesn't condense as liquid water droplets, but instead as tiny ice crystals. In most parts of the world, rain generally starts out as snow but melts as it falls through the atmosphere (it is very cold at cloud level, even in the summertime).

Oddly enough, water doesn't automatically freeze at "freezing temperature" -- 32 degrees Fahrenheit / 0 degrees Celsius. You have to cool pure water to a much lower temperature (as low as -40 F / -40 C) for it to lose enough heat energy to change form. Usually, however, water in a cloud does freeze around 32 F / 0 C because of the work of nucleators, tiny bits of naturally-occurring material that help water molecules coallesce. The nucleators attract water molecules, which reduces their energy to the point that they form ice crystals. The nucleators in snow crystals are just dirt bits, bacteria and other material floating around in the atmosphere. Water condenses onto the nucleator, which becomes the nucleus -- the center -- of the snow crystal.

As the snow crystal moves around the cloud, more water particles condense onto it and freeze into crystals. The collection of individual crystals forms a snow flake. As the snow flake grows heavier, it falls toward the earth. If it is cold enough the whole way down, the flake will still be frozen when it reaches the surface.

Manmade Snow
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.

Photo courtesy SMI Snow Makers
A Kid Wizzard airless snow gun

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.

Photo courtesy SMI Snow Makers
Super Wizzard snow guns mounted on towers

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.

Snow-Making Conditions
As we've seen, the main job of snow-making machines is to do the work of snow-making clouds that occur naturally in the atmosphere. These machines do not make snow under just any conditions, however -- you need to have the right kind of ground weather, just as you need the right kind of ground weather for natural snow to make it to the earth and then stick. To figure out when to make snow, and to make sure they get the right kind of snow, ski resorts depend on the expertise of experienced snow machine operators, commonly called snow-makers.

So how do snow-makers determine if the conditions are right? It turns out they need a lot more information than they can get from an ordinary thermometer. Standard thermometers measure the dry bulb temperature of the atmosphere; but the most important factor for snow conditions is the wet bulb temperature.

The wet bulb temperature is a function of the dry bulb temperature and the relative humidity, the amount of water vapor in the air. Liquid or solid water cools itself by evaporating some water as water vapor. This releases heat, and so lowers the energy level in the water. When there is more water vapor in the atmosphere, water or snow can't evaporate as much because the air is already saturated with water to a high degree. Consequently, water cools more slowly when the humidity is high, and more quickly when the humidity is low.

Photo courtesy Wintergreen Resort, Wintergreen Virginia
A snow-maker checks out an air snow gun at Wintergreen Resort in Virginia.

For this reason, humidity is a very important factor in determining snow conditions. If the humidity level is low enough, you can actually get snow even when the dry bulb temperature is several degrees above freezing. If the relative humidity is 100 percent, than the wet bulb temperature and the dry bulb temperature will be exactly the same. But even if both are at the freezing temperature, you might get rain instead of snow because the air saturation slows the cooling process down so much.

If the temperature is around 30 F (-1 C), you need a fairly low relative humidity (less than 30 percent) for good snow-making conditions. If the temperature is less than 20 F (-6.7 C), you can make snow fairly easily even if the relative humidity is 100 percent. A temperature in the teens is ideal for snow-making.

Getting the Perfect Mix
So we've seen that the basic idea of snow guns is pretty simple. Actually making snow can be a bit more complex, however. The wet bulb temperature has to be low enough, and snow-makers must carefully balance the levels of water and air to get the desired results.

There are all different kinds of snow. The main difference between snow types is how much water a certain volume of snow holds. Snow-makers often talk about dry snow and wet snow. Dry snow has a relatively low amount of water, so it is very light and powdery. This type of snow is excellent for skiing because skis glide over it easily without getting stuck in wet slush.

Photo courtesy SMI Snow Makers
A Super Wizzard snow gun in New Zealand

One of the advantages of using a snow-maker is that you can make sure the slope's surface level always has this ideal, powdery snow. Wet snow also plays an important role on ski slopes, however. Resort owners use this denser snow to build up the snow level on well-travelled trails. Many resorts build up the snow depth this way once or twice a year, and then regularly coat the trails with a layer of dry snow throughout the winter.

The density, or wetness, of snow is dependent on the temperature and humidity outside, as well as the size of the water droplets launched by the gun. Snow-makers have to adjust the proportions of water and air in their snow guns to get the perfect snow consistency for the outdoor weather conditions. Since temperature and humidity levels vary considerably from point to point on a ski slope, snow-makers have to adjust each machine accordingly.

This process has gotten a little bit easier recently with the introduction of new technology. Many ski slopes now control their snow guns with a central computer system that is hooked up to weather-reading stations all over the slope. The computers make a determination of the best snow-and-air mix based on the temperature and relative humidity at a given point. These systems do not always get the mix right, of course, but they are a great snow-making aid.

Other Uses of Manmade Snow
The most widespread market for snow-making machines is the ski resort industry. But manmade snow has a number of other uses as well. For example, snow-making machines have played a big part in many movie productions. Movie producers often take several months to shoot scenes that take place in the span of a few days. If the movie takes place in a snowy setting, the set decorators have to get the right amount of snow for each day of shooting. Depending on the weather, this could mean melting natural snow or adding manmade snow.

Snowmaking plays a part in agriculture as well. A good coating of snow keeps a lot of the earth's heat from escaping into the atmosphere, so farmers often use it as an insulator for winter crops. The insulation effect keeps the crops from freezing, even when the temperature drops well below 32 degrees Fahrenheit (0 C).

Another use of manmade snow is for testing aircraft equipment. Because they fly so high in the atmosphere, airplanes must be able to hold up in very cold, very snowy conditions. Snow-making machines let aircraft designers test how airplane equipment will react to these conditions.

There is also a consumer market for small snow-making machines and manmade snow services. Some homeowners will shell out hundreds of dollars to give their yard and house an attractive coating of snow for the Christmas season, or just to give the kids a special treat. People who really love a snowy yard can invest in a personal snow-making machine. These models, which typically cost thousands of dollars, hook up to an ordinary garden hose and a standard electrical outlet. Just like the ski slope machines, these models use a lot of water and a lot of power; but they're fairly easy to operate.

Costs of Snow-Making
To cover several ski trails with manmade snow, you need a lot of water. According to
SMI Snow Makers, it takes about 75,000 gallons (285,000 liters) of water to create a 6-inch blanket of snow covering a 200x200-foot area (61x61 meters). The system in a good-sized ski slope can convert 5,000 to 10,000 gallons (18,927 to 37,854 liters) of water to snow every minute!

Water is not a huge expense for ski resorts, however; and pumping this much water isn't incredibly bad for the environment. Most resorts pump water from one or more reservoirs located in low-lying areas. The run-off water from the slopes feeds back into these reservoirs, so the resort can actually use the same water over and over again. Moving this water around can have some negative effects on plant and animal life, though, so ski slopes must work hard to keep water levels fairly balanced.

A significant environmental concern, and one of a resort's biggest expenses, is power consumption. If a slope uses compressed air in its snow guns, it has to provide a lot of energy to run the large air-compressing pumps. It also needs a pump system to provide the water to the snow-makers. These pumps are often run by diesel engines, which expel a high level of air pollution.

Ski resorts that use airless snow guns also need a good amount of power to run the machines' fans. These types of snow guns consume a lot less energy for every foot of snow they produce, but they are still major power draws. For most ski resorts, power consumption is the second biggest operating expense, just behind labor costs (snow-making alone requires a lot of manpower). No matter what sort of technology a resort uses, snow-making accounts for a high percentage of this power use.

Photo courtesy Wintergreen Resort, Wintergreen Virginia
Air snow guns coat a slope at Wintergreen Resort in Virginia.

Because of the expense of making snow, ski resorts have to develop a good strategy for when and where they are going to use their machines. A lot of the work involved in snow-making is the task of balancing the cost of running the machines with the benefits of extending the ski season. Efficient snow-makers make sure they don't waste power making snow where it won't do any good, and they are very careful to make snow only when it will stick around.

As we've seen, snow-makers have to take many variables into account to cover a slope with ideal skiing snow. The idea behind manmade snow is extremely simple; but actually getting it to work effectively is quite a feat. Many snow-makers describe the job as a challenging marriage of science and art -- the basic elements are precise weather measurements and expensive machinery, but you need instinct, improvisation and creativity to get it exactly right.

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