Shortwave bands are frequency allocations for use within the shortwave radio spectrum (the upper medium frequency [MF] band and all of the high frequency [HF] band). Radio waves in these frequency ranges can be used for very long distance (transcontinental) communication because they can reflect off layers of charged particles in the ionosphere and return to Earth beyond the horizon, a mechanism called skywave or “skip” propagation. They are allocated by the ITU for radio services such as maritime communications, international shortwave broadcasting and worldwide amateur radio. The bands are conventionally named by their wavelength in metres, for example the ‘20 meter band’. Radio propagation and possible communication distances vary depending on the time of day, the season and the level of solar activity. International broadcast bands These bands are used by powerful long range AM radio stations, many operated by governments, which broadcast to multiple countries. Most international broadcasters use amplitude modulation with 5 kHz steps between channels; a few use single sideband or reduced carrier single sideband modulation. The World Radiocommunication Conference (WRC), organized under the auspices of the International Telecommunication Union, allocates bands for various services in periodic conferences. The most recent WRC took place in 2012. At WRC-97 in 1997, the following bands were allocated for international broadcasting: Particularly in the United States and at frequencies under 10 MHz, shortwave broadcasters may operate in between those bands. WTWW (whose main programming is on 5.085), WBCQ (5.130) and WRMI (9.335, 7.73, 7.78) all operate at least some of their frequencies outside the designated shortwave bands. Amateur HF bands Amateur radio operators in many countries are allocated several shortwave bands for private, non-commercial use. Amateur radio is a communications service, educational tool and hobby. It is particularly useful in providing emergency communication where standard telecommunications infrastructure is compromised or nonexistent, such as a disaster area or remote region of the globe. Marine, air, land mobile and fixed allocations Designated bands in the shortwave spectrum are used for ships, aircraft, and land vehicles. Shortwave (HF radio) is used by transoceanic aircraft for communications with air-traffic control centers out of VHF radio range. Most countries with HF citizens'-band allocations use 40 or 80 channels between approximately 26.5 MHz and 27.9 MHz, in 10 kHz steps. Due to antenna-length requirements and the band's long-distance propagation characteristics (undesirable in these cases), much land-mobile radio activity has moved to VHF or UHF and