Orographic clouds are the result of the Earth's surface topography, mountains for example. Air passing over the mountains is forced upwards. As it ascends, the air cools and, if it cools to it's saturation point, condensation occurs and the water contained within the air becomes visible as a cloud. When the air mass clears the mountain top, it descends, warming as is goes. With its downward momentum, this warm air is now lighter than the surrounding air and ascends, cooling as it goes. With stable conditions, this cycle may be repeated several times, resulting in bands of cloud downwind of the mountain. These bands are often called "rotor clouds" or "rotor orographic" due to their rolling motion.

Orographic Clouds

Rotor clouds