Artificial snow

Ski-resort operators may resort to making snow artificially when the natural snowfall is insufficient. Theoretically, snow (machine-made or natural) will accumulate if both the wet-bulb temperature of the air and the ground temperature are less than 0° C. In this case, the snow will not melt while it falls nor after it has settled. Practically, operators simply require that the air temperature is below freezing. Snowmaking machines are usually operated only at night, when the ground is colder than the air.

A large supply of compressed air is needed, commonly from a (noisy) petrol-fuelled compressor. Compression causes the air’s temperature to rise. It is then cooled by conduction, for instance by funneling the compressed air through tubes in a nearby pond. Sudden expansion of the air to ambient air pressure then cools the air well below 0° C, possibly as low as -40° C. (Any remaining supercooled droplets freeze spontaneously at this temperature.) Making snow involves releasing the compressed air into the atmosphere in combination with a spray of cold water. The expanding air freezes the water, producing ‘snow’ at the nozzles. The finer the spray droplets, the more crystalline the resulting snow. The higher the compression ratio, the colder the expanding air, and the less rimed the snow crystals will be.

A single snow gun typically has a large number of spray nozzles. Sometimes ice nuclei are released into the compressed air, to make sure that all droplets freeze even when the expanding air does not cool below -40° C. When ice nuclei are plentiful, freezing of the supercooled droplets occurs at much higher temperatures, and this allows lower compression and flow rates of air. Typically the snow is made by machines on top of towers, perhaps 10 m high, to create snow drifts which are then smoothed out by bulldozer.