are referred to as borderline temperatures or limit temperatures. If the wet-bulb temperature drops, more snow can be produced faster and more efficiently. Snowmaking is an energy-intense process, and has environmental impacts, both of which inherently limit its use. History Art Hunt, Dave Richey, and Wayne Pierce invented the snow cannon in 1950, but secured a patent sometime later. In 1952, Grossinger's Catskill Resort Hotel became the first in the world to use artificial snow. Snowmaking began to be used extensively in the early 1970s. Many ski resorts depend heavily upon snowmaking. Snowmaking has achieved greater efficiency with increasing complexity. Traditionally, snowmaking quality depended upon the skill of the equipment operator. Today, computer control supplements that skill with greater precision, such that a snow gun operates only when snowmaking is optimal. Operation The key considerations in snow production are increasing water and energy efficiency and increasing the environmental window in which snow can be made. Snowmaking plants require water pumpsand sometimes air compressors when using lancesthat are both very large and expensive. The energy required to make artificial snow is about 0.6–0.7 kW h/m3 for lances and 1–2 kW h/m3 for fan guns. The density of artificial snow is between 400 and 500 kg/m3 and the water consumption for producing snow is roughly equal to that number. Snowmaking begins with a water supply such as a river or reservoir. Water is pumped up a pipeline on the mountain using very large electric pumps in a pump house. This water is distributed through an intricate series of valves and pipes to any trails that require snowmaking. Many resorts also add a nucleating agent to ensure that as much water as possible freezes and turns into snow. These products are organic or inorganic materials that facilitate the water molecules to form the proper shape to freeze into ice crystals. The products are non-toxic and biodegradable. The next step in the snowmaking process is to add air using an air plant. This plant is often a building which contains electric or diesel industrial air compressors the size of a van or truck. However, in some instances air compression is provided using diesel-powered, portable trailer-mounted compressors which can be added to the system. Many fan-type snow guns have on-board electric air compressors, which allows for cheaper and more compact operation. A ski area may have the required high-output water pumps, but not an air