It is quite common for gliders to remain aloft for hours at a time. Within southern Ontario, gliders have known to be up in the air for over 6 hours at a time. Elsewhere in the world, it is possible to remain aloft for 12 hours or more. This is a possibility due to pilot skill in finding lift and using that lift to gain altitude.
There are three types of lift that glider pilots use to gain altitude. These are Thermal Lift, Ridge Lift, and Wave Lift.
Thermal Lift – You may notice that the sky is often filled with beautiful puffy cumulus clouds the afternoon after a cool clear night. Each of these clouds is a marker for the rising air currents that we call thermals. Thermals start at the ground where the sun warms the surface and the air close to the surface is warmed by the ground. Hot air rises and if the air contains enough moisture, the cooler air at altitude condenses the moisture and forms clouds. That is why the cumulus clouds are such good indicators of thermal lift. Skilled glider pilots can pinpoint these thermals and use them to sustain their flights for hours at a time.
Ridge Lift – In areas that have ridges, the wind is often deflected upwards with enough force that the sailplane can sustain itself with these rising air currents.
Wave Lift – In mountainous regions wind that is deflected upwards may provide wave-like rising and falling air currents. If glider pilots orient their sailplanes properly to these waves, flight can be sustained. At altitudes above 12,000 feet though supplemental oxygen is required. Also it’s very cold so pilots have to dress warmly.