This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
In addition to the horizontal wind we're accustomed to on the
ground, air moves vertically as well. If a glider encounters an rising
chunk of air, it will go up along with it. The whole trick of soaring
a hang glider (or any other glider for that matter) is to figure out
where the air is going up and then to get there. While there are many
sources of rising air or "lift", the most commonly used by hang
gliders are ridge lift and thermal lift. Ridge lift occurs when
horizontal wind hits an obstruction (like a ridge, for instance) and
is deflected upward. Thermal lift occurs when terrain is heated by the
sun and transfers this heat to the surrounding air - which then rises.
Typically ridge lift exists in a "lift band" on the windward side of a
ridge and pilots get up by flying back and forth through this
band. Thermal lift on the other hand usually starts at some local
"trigger point" on the ground and then rises as a column or bubble of
air. To get up in a thermal, pilots typically circle in this region of