Snowmaking is the production of snow by forcing water and pressurized air through a "snow gun", also known as a "snow cannon". Snowmaking is mainly used at ski resorts to supplement natural snow. This allows ski resorts to improve the reliability of their snow cover and to extend their ski seasons from late autumn to early spring. Indoor ski slopes use snowmaking. They can generally do so year-round as they have climate-controlled environments. The use of snowmaking machines has become more common as changing weather patterns and the popularity of indoor ski resorts create a demand for snow beyond that which is provided by nature. Snowmaking machines have addressed the shortage in the supply of snow; however, there are significant environmental costs associated with the artificial production of snow. According to the European Environment Agency, the length of snow seasons in the northern hemisphere has decreased by five days each decade since the 1970s, thus increasing the demand for the production of artificial snow. Some ski resorts use artificial snow to extend their ski seasons and augment natural snowfall; however, there are some resorts that rely almost entirely upon artificial snow production. Artificial snow was used extensively at the 2014 Winter Olympics in Sochi, the 2018 Winter Olympics in Pyeongchang and the 2022 Winter Olympics in Beijing to supplement natural snowfall and provide the best possible conditions for competition. The production of snow requires low temperatures. The threshold temperature for snowmaking increases as humidity decreases. Wet-bulb temperature is used as a metric since it takes air temperature and relative humidity into account. The bulb temperature is always below the outside temperature. The damper the air, the less moisture it can absorb. The higher the atmospheric humidity, the colder it must be to turn the small water droplets into snow crystals. Examples; dry temperature and a humidity of 90% are equal to a wet-bulb temperature of dry temperature and a humidity of 30% are equal to a wet-bulb temperature of dry temperature and a humidity of 90% are equal to a wet-bulb temperature of dry temperature and a humidity of 30% are equal to a wet-bulb temperature of To start a snowmaking system a wet-bulb temperature of is required. If the atmospheric humidity is very low, this level can be reached at temperatures slightly above , but if the air humidity is high, colder temperatures are required. Temperatures around freezing point