In the traditional system, water is forced through a nozzle under high pressure by compressed air. You've probably observed two pipelines alongside a ski trail serviced by snowmaking; one is to carry the water, and the other to transport compressed air. This compressed air serves two purposes: to atomize the water into tiny droplets, and to thrust the moisture skyward to allow it to cool and freeze before falling to the surface as snow. Simple, right? Nope. The exact mix of water and air determines how wet or how dry the resultant snow will be, or even if it will have time to freeze at all. In the old "jacket sleeve" test, snowmakers would stand in the gun's fallout area and raise their sleeve to have a look. If the snow bounced off their sleeve, it was dry. If it stuck, it was wet. The snowmaker would adjust the mix at the hydrant which serviced each gun. The proper mix of air and water is dependent upon two factors: temperature and humidity. These factors can vary dramatically at different points on the mountain, or even from one snowgun to another. Due to the complicated relationship between temperature and humidity, it's actually possible to make snow at temperatures above freezing. Likewise, on a humid day, it may not be possible to make snow even though the thermometer on your ski jacket indicates that it's well below freezing. Let me explain. When you look at a traditional thermometer, you're looking at what meterologists refer to as the "dry bulb" temperature. Remember how hot you felt on an 80-degree summer day that was also very humid? It certainly felt hotter. Hotter even than a dry, 90-degree day the week before. You body utilizes "evaporational cooling," whereby you are cooled by moisture in the form of perspiration evaporating from your body. If the ambient air is too humid, the moisture does not evaporate from your body quickly in its effort to reach equilibrium, as the air is already saturated. Your body doesn't cool very effectively. The same thing happens when you make snow. Water vapor gives off heat as it evaporates, accelerating the freezing process. Snowmakers are more interested in the "wet-bulb" temperature, which is a mathematical function of the dry bulb temperature and the level of humidity. Warmer air holds more moisture than cooler air, all other things equal, and the temperature at which 100% humidity is reached is called the "dewpoint". At the dewpoint, the wet bulb temperature and the dry bulb temperature are identical. On a very dry day, however, the wet bulb temperature may be below freezing when the dry bulb temperature is above freezing, and you can actually make snow! Likewise, if the temperature is near the dewpoint, although both are a bit below freezing, the water droplets will not freeze quickly enough to create snow. Additional cooling is provided by the energy released as the compressed air returns to ambient pressure. Think back to high school physics, and you'll recall that temperature is directly related to pressure; all other things being equal, a substance will increase in temperature as pressure increases. The release of the compressed air through the snowgun nozzle transforms some of that heat energy into kinetic energy, which is used to propel the water vapor skyward. The rapid release of heat energy quickly cools the water vapor as well. An alternative to the traditional water/compressed air mix is offered by such manufacturers as Hedco and Lenko. Using a snow cannon, a large amount of water is atomized through many tiny nozzles surrounding a giant fan blade. This fan propels the moisture into the air, where it condenses and freezes to form snow crystals. There are both distinct advantages, and disadvantages to this technology. Snow cannons generate a tremendously greater amount of snow per hour than a traditional air/water gun. For example the Hedco Super "D", can pump out a foot of snow over 2.1 acres in 12 hours, at 10º F and 60% relative humidity using 400 gallons of water per minute. They are also much quieter to operate. There's also no need for costly air compressors and a dual pipeline to service the hydrants - instead, the resort runs a relatively inexpensive electrical power line alongside the water pipes to provide the energy necessary to run the fan. The snow created by snow cannons, however, is much wetter and denser than that created by traditional snow guns. Info found at this site. Please see for further information http://www.firsttracksonline.com/welcome2.htm