<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> <P> Ridge lift rarely allows pilots to climb much higher than about 600 metres (2,000 ft) above the terrain; thermals, depending on the climate and terrain, can allow climbs in excess of 3,000 metres (9,800 ft) in flat country and much higher above mountains; wave lift has allowed a glider to reach an altitude of 15,447 metres (50,679 ft). In a few countries such as the UK, gliders may continue to climb into the clouds in uncontrolled airspace, but in many European countries the pilot must stop climbing before reaching the cloud base (see Visual Flight Rules). </P>

How long does a glider stay in the air