Soaring

Thermals

Thermals are streams of rising air that are formed on the ground through the warming of the surface by sunlight. If the air contains enough moisture, the water will condense from the rising air and form cumulus clouds. When the air has little moisture or when an inversion stops the warm air from rising high enough for the moisture to condense, thermals do not create cumulus clouds. Without clouds or dust devils to mark the thermals, thermals are not always associated with any feature on the ground. The pilot must then use both skill and luck to find them using a sensitive vertical speed indicator called a variometer that quickly indicates climbs and descents. Occasionally reliable thermals can be found in the exhaust gases from power stations or from fires.

Once a thermal is encountered, the pilot can fly in tight circles to keep the glider within the thermal, so gaining altitude before flying towards the destination or to the next thermal. This is known as "thermalling". Alternatively, glider pilots on cross-country flights may choose to 'dolphin'. This is when the pilot merely slows down in rising air, and then speeds up again in the non-rising air, thus following an undulating flight path. Dolphining allows the pilot to minimize the loss of height over great distances without spending time turning. Climb rates depend on conditions, but rates of several meters per second are common and can be maximized by gliders equipped with flaps. Thermals can also be formed in a line usually because of the wind or the terrain, creating cloud streets. These can allow the pilot to fly straight while climbing in continuous lift.

As it requires rising heated air, thermalling is most effective in mid-latitudes from spring through late summer. During winter the sun's heat can only create weak thermals, but ridge and wave lift can still be used during this period.

 

Ridge (Hill) lift

A ridge soaring pilot uses upward air movements caused when the wind blows on to the sides of hills. It can also be augmented by thermals when the slopes also face the sun. In places where a steady wind blows, a ridge may allow virtually unlimited time aloft, although records for duration are no longer recognized because of the danger of exhaustion. 

Wave lift

The powerfully rising and sinking air in mountain waves was discovered by glider pilot, Wolf Hirth, in 1933. Gliders can sometimes climb in these waves to great altitudes, although pilots must use supplementary oxygen to avoid hypoxia.

This lift is often marked by long, stationary lenticular (lens-shaped) clouds lying perpendicular to the wind. Mountain wave was used to set the current altitude record of 15,453 metres (50,699 ft) on 29 August 2006 over El Calafate, Argentina. The pilots, Steve Fossett and Einar Enevoldson, wore pressure suits. The current world distance record of 3,008 kilometres (1,869 mi) by Klaus Ohlmann (set on 21 January 2003) was also flown using mountain waves in South America.

A rare wave phenomenon is known as Morning Glory, a roll cloud producing strong lift. Pilots near Australia's Gulf of Carpentaria make use of it in springtime

Sea Breeze Lift

The boundaries where two air masses meet are known as convergence zones. These can occur in sea breezes or in desert regions. In a sea-breeze front, cold air from the sea meets the warmer air from the land and creates a boundary between two masses of air like a shallow cold front. Glider pilots can gain altitude by flying along the intersection as if it were a ridge of land. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.

Glider pilots have occasionally been able to use a technique called "dynamic soaring"allowing a glider 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 too close to the ground to be used safely by gliders.

Other sources of lift

The boundaries where two air masses meet are known as convergence zones. These can occur in sea breezes or in desert regions. In a sea-breeze front, cold air from the sea meets the warmer air from the land and creates a boundary between two masses of air like a shallow cold front. Glider pilots can gain altitude by flying along the intersection as if it were a ridge of land. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.

Glider pilots have occasionally been able to use a technique called "dynamic soaring"allowing a glider 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 too close to the ground to be used safely by gliders.

Sometimes forms of ridge lift can be got of cliffs like amazing flight shown in this video on the South coast of England.