The most common type of natural ‘lift’ that the gider can use to prolong its flight time is the thermal.
As the sun’s rays heat the ground, the air near the surface warms up, and once the air is warm enough, it will start to form a bubble and rise. As the air rises, it starts to cool, until eventually it is at the same temperature as the surrounding air.
Thermals are often marked out by the formation of cumulus cloud at their top, where moisture condenses out of the now cooled air.
Depending on the weather, the bubble may rise as much as 7,000 ft or higher. The bubble will have areas of lift on the inside (at the core) and associated areas of sinking air on the outside.
By circling in the core, a pilot can use the rising air without flying through the sinking air.
Although thermals are weather dependant, they can be experienced for the majority of the year, with the main season being March-October, and pilots are taught from an early stage how to locate and use them effectively.
One of the easiest indicators is the frequent formation of cumulus cloud at the head of a thermal, and glider pilots will frequenty fly under these clouds in their search for thermal lift that can prolong their flight.
Other Forms of Lift
While thermals are the most usual form of lift encountered at Darlton, there are other natural phenomena that generate rising air currents that can be used by glider pilots.
- Ridge Lift: when a wind blows towards a hill ridge or cliff it gets deflected upwards as it strikes the slope. By flying along a ridge when the wind is in the right direction, birds (and gliders!) can keep themselves airborne without expending their own energy.
- Wave Lift: A strong wind blowing over a mountain range can be set into an oscillating flow, or wave. Such wave cycles can extend for miles behind the mountain range, and the wave peaks can reach up tens of thousands of feet into the atmosphere. By flying in the rising part of the wave, gliders can experience a smooth lift that permits record breaking flights. Distances of 1,000 km and more can be covered in a single flight using wave lift, while altitudes up to 50,000 feet can be achieved – all with no engine!