Wind moving towards a slope. The flow produces an area of ascending air (in green) which can be used by gliders.
<a href="//commons.wikimedia.org/w/index.php?title=User:Dake&amp;action=edit&amp;redlink=1" class="new" title="User:Dake (page does not exist)">Dake</a>
CC BY 2.5
Self-made illustration