<P> Gliding did not return to the Olympics after the war for two reasons: a shortage of gliders, and the failure to agree on a single model of competition glider . (Some in the community feared doing so would hinder development of new designs .) The re-introduction of air sports such as gliding to the Olympics has occasionally been proposed by the world governing body, the Fédération Aéronautique Internationale (FAI), but has been rejected on the grounds of lack of public interest . </P> <P> In many countries during the 1950s a large number of trained pilots wanted to continue flying . Many were also aeronautical engineers who could design, build and maintain gliders . They started both clubs and manufacturers, many of which still exist . This stimulated the development of both gliding and gliders, for example the membership of the Soaring Society of America increased from 1,000 to 16,000 by 1980 . The increased numbers of pilots, greater knowledge and improving technology helped set new records, for example the pre-war altitude record was doubled by 1950, and the first 1,000 - kilometre (620 mi) flight was achieved in 1964 . New materials such as glass fiber and carbon fiber, advances in wing shapes and airfoils, electronic instruments, the Global Positioning System and improved weather forecasting have since allowed many pilots to make flights that were once extraordinary . Today over 550 pilots have made flights over 1,000 kilometres (620 mi). Although there is no Olympic competition, there are the World Gliding Championships . The first event was held at the Samedan in 1948 . Since World War II it has been held every two years . There are now six classes open to both sexes, plus three classes for women and two junior classes . The latest worldwide statistics for 2011 indicate that Germany, the sport's birthplace, is still a center of the gliding world: it accounted for 27 percent of the world's glider pilots, and the three major glider manufacturers are still based there . However the meteorological conditions that allow soaring are common and the sport has been taken up in many countries . At the last count there were over 111,000 active civilian glider pilots and 32,920 gliders, plus an unknown number of military cadets and aircraft . Clubs actively seek new members by giving trial flights, which are also a useful source of revenue for them . </P> <P> Glider pilots can stay airborne for hours by flying through air that is ascending as fast or faster than the glider itself is descending, thus gaining potential energy . The most commonly used sources of rising air are </P> <Ul> <Li> thermals (updrafts of warm air); </Li> <Li> ridge lift (found where the wind blows against the face of a hill and is forced to rise); and </Li> <Li> wave lift (standing waves in the atmosphere, analogous to the ripples on the surface of a stream). </Li> </Ul>

How long do gliders stay in the air