Soaring

Soaring is usually achieved by flying through a mass of air that is ascending as fast or faster than the sailplane is descending, and thus gaining potential energy. The most commonly used rising masses of air are thermals (updrafts of warm air), ridge lift (found where the wind blows against the face of a hill and is forced to rise), and wave lift (standing waves in the atmosphere, analogous to the ripples on the surface of a stream).

Ridge lift rarely allows pilots to climb much higher than about 2,000 ft (600 m) above the terrain; thermals, depending on the climate and terrain, can exceed 10,000 ft (3,000 m) in flat country and much higher in the mountains; wave lift has allowed gliders to achieve altitudes approaching 50,000 ft (15,000 m).

On rare occasions, glider pilots have been able to use a technique called "dynamic soaring", where a sailplane can be made to gain kinetic energy by repeatedly crossing the boundary between air masses of different horizontal velocity. However, such zones of high "wind gradient" are usually much too low to be used safely by aircraft, so dynamic soaring is a technique only really useful to radio control model aircraft.

In thermal flight, the glider pilot attempts to find streams of air that are moving upwards as a result of being heated by contact with sun-lit earth. If the air contains enough moisture, the water will condense from the rising air and form cumulus clouds. Well-formed cumulus clouds (the fluffy, cotton-wool type of cloud) with sharply defined flat bases, often form at the tops of strong thermals. Once a thermal is encountered, the pilot banks sharply to keep the plane turning in a small circle within the thermal and so can ride upward. Rates of climb depend on conditions, but several metres per second is common.

As it requires rising heated air, thermalling is typically only effective in mid-latitudes from spring through into late summer. Other latitudes often have a layer of warm air, an inversion, which stops the air in the thermals from rising higher. During winter the solar heat can only create weak thermals.

In a few countries gliders can continue to climb into the clouds in uncontrolled airspace but in many countries the pilot must stop climbing at cloud-base.

Sometimes thermals do not create cumulus clouds. This can happen when the air has little moisture or when an inversion stops the thermal from rising high enough for the moisture to condense. Without clouds to mark the thermals, the pilot must use his skill and luck to find them. Typical locations to find thermals are over towns, freshly ploughed fields and asphalt roads, however most of the time thermals are hard to associate with any feature on the ground.

A pilot who is ridge soaring looks for air that is being lifted as it flows up the sides of hills. Ridge lift is present whenever the wind blows in any weather but sometimes it is augmented by thermals when the slopes also face the sun.

Mountain waves give long stretches of rising air and allow gliders to climb high, long before the sun has started heating the ground. Most sailplane altitude records have therefore been set by using in mountain waves from long mountain ranges all over the world.

The current World Distance Record of 3008 km by Klaus Ohlmann (on 21 Jan 2003) was also flown in the mountain wave in South America. Long, stationary lenticular (lens-shaped) clouds, perpendicular to the wind direction, frequently mark the crests of atmospheric waves.