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Concept
Lift (soaring)
Summary
Lift is a meteorological phenomenon used as an energy source by soaring aircraft and soaring birds. The most common human application of lift is in sport and recreation. The three air sports that use soaring flight are: gliding, hang gliding and paragliding.
Energy can be gained by using rising air from four sources:
Thermals (where air rises due to heat),
Ridge lift, where air is forced upwards by a slope,
Wave lift, where a mountain produces a standing wave,
Convergence, where two air masses meet
In dynamic soaring it is also possible to gain energy, though this uses differences in wind speeds rather than rising air.
Thermal
Thermals are columns 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.
Thermal lift is often used by birds, such as raptors, vultures and storks. Although thermal lift was known to the Wright Brothers in 1901, it was not exploited by humans until 1921 by Wilhelm Leusch at the Wasserkuppe in Germany. It was not until about 1930 that the use of thermals for soaring in gliders became commonplace.
Once a thermal is encountered, the pilot flies in circles to keep within the thermal, so gaining altitude before flying off to the next thermal and towards the destination. This is known as "thermalling". Climb rates depend on conditions, but rates of several meters per second are common. Thermals can also be formed in a line usually because of the wind or the terrain, creating cloud streets. These can allow flying straight while climbing in continuous lift.
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. Typical locations to find thermals are over towns, freshly ploughed fields and asphalt roads, but thermals are often hard to associate with any feature on the ground. Occasionally thermals are caused by the exhaust gases from power stations or by fires.
Official source
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