Windows to the Universe original image
Some clouds form when air at the surface of the Earth is forced to rise. There are three processes that force air to rise.
First, in a low pressure system, wind moves in towards the center from all directions because air moves from high to low pressure. When this air meets in the center, there is nowhere for the air to go but up. Air is also forced to rise when it is traveling over land that slopes upward. The air cools as it rises, and eventually clouds will form. Finally, weather fronts produce clouds by causing air to rise when the lighter warm air flows over the heavier cold air.
All of the cloud types are formed by these processes, especially altocumulus, altostratus, cirrocumulus, stratocumulus, or stratus clouds.
source: windows to the universe