Snow removal or snow clearing is the job of removing snow after a snowfall to make travel easier and safer. This is done by both individual households and by governments and institutions. De-icing and anti-icing De-icing is defined as removal of existing snow, ice or frost from a roadway, airport runway, roof, or other surface. It includes both mechanical means, such as plowing, vacuuming or scraping, and chemical means, such as application of salt or other ice-melting chemicals. Anti-icing is treatment with ice-melting chemicals before or during the onset of a storm in order to prevent or delay the formation and adhesion of ice and snow to the surface. Brine, or wetted salt, is usually applied shortly before the beginning of a snowstorm. When properly performed, anti-icing can significantly reduce the amount of salt required and allow easier removal by mechanical methods, including plowing. The de-icing of roads has historically been accomplished by snowplows or specially designed dump trucks that spread salt, often mixed with sand and gravel, onto slick roads. Rock salt is normally used because it is inexpensive and readily available in large quantities. However, brine freezes at −18 °C (0 °F), and so it is ineffective at these low temperatures. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar in concrete bridges. More recent snowmelters use other salts, such as calcium chloride and magnesium chloride, which not only decrease the freezing point of water to a much lower temperature but also produce an exothermic reaction, whose dissipated heat further aids in melting. In addition, they are somewhat safer for concrete sidewalks, but excess should still be removed. Recently, organic compounds have been developed that reduce the environmental impact associated with salts and that have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are generated as byproducts of agricultural operations, such as sugar beet refining or ethanol distillation. A mixture of some selection of these organic compounds with a combination of salts results in a substance that is both more easily spread and more effective at lower temperatures (−34 °C or −30 °F). Since the 1990s, use of liquid chemical melters has been increasing, sprayed on roads by nozzles instead of a spinning spreader used with salts. Liquid melters are more effective at preventing the ice from bonding to