During the summer months glider pilots can take advantage of rising air currents known as thermals that usually manifest themselves as white puffy clouds.
Clouds
These can be used to stay aloft for prolonged periods of time and to gain thousands of feet in altitude. By flying from thermal to thermal the glider pilot uses these stepping stones in the sky travel across the countryside to cover long distances.
High altitude flying above 15000ft
Gliders can also make use of wave lift which occurs when air is forced up, over and down a mountain range creating a wave like pattern or ripple like a stone in a stream, on the lee side. By using the up-going part of this wave gliders can climb to tens of thousands of feet