If the expanding air contains so much water vapor that it can cool this vapor to the dew point, clouds will form. The air can rise in several ways. When the sun heats the ground, the air next to the ground is warmed. Because warmer air is lighter than cooler air, the warm air rises. The upward flow of the warm air is called a convection current, and a cloud formed by this process is called a convective cloud.
Clouds also form by lifting.
When warm, moist air moves up the side of a hill or over a mountain range, it is lifted, and it cools by expansion. This cooling causes the water vapor to condense, forming clouds that hang over the mountains. Frontal lifting, the lifting of air at weather fronts, also produces clouds. At a front, a region where cold air and warm air meet, the warm air rises above the cold air. As the rising air cools, its water vapor condenses and clouds are formed.