Courtesy of CMMAP
Some clouds form when air encounters a mountain range or other types of terrain. When this happens, the air will rise and cool, and this cooler air is no longer able to hold all of the water vapor it was able to hold when it was warm. The extra water vapor begins to condense out of the air parcel in the form of liquid water droplets and a cloud is formed.
The types of clouds that form from encounters with mountains are stratus clouds and lenticular clouds.
The image on this page shows how winds can blow into a mountain range and then rise higher in the atmosphere. The side of the mountains where the wind starts is called the windward side. The side of the mountains where the wind leaves the area is called the leeward side.
Another way that mountains cause cloud formation is when air rises because the mountain is warmer than the surrounding air and causes the air to rise. Once the air rises, it follows the same process to form clouds as described above. The types of clouds that form in this case are cumulonimbus (and associated mammatus clouds), and cumulus.
source: windows to the universe