High frequency waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances. The band is used by international shortwave broadcasting stations (2.31–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses. The dominant means of long distance communication in this band is skywave ("skip") propagation, in which radio waves directed at an angle into the sky refract back to Earth from layers of ionized atoms in the
Rarefaction local medium. This expansion behaviour is in contrast to the behaviour of pressure increases, which gets narrower with time, until they steepen into shock waves. A natural example of rarefaction occurs in the layers of Earth's atmosphere. Because the atmosphere has mass, most atmospheric matter is nearer to the Earth due to the Earth's gravitation. Therefore, air at higher layers of the atmosphere is less dense, or "rarefied", relative to air at lower layers. Thus rarefaction can refer either to a reduction in density over space at a single point of time, or a reduction of density over time for