The warm ground in turn heats the air above. The warm air at the ground is lighter and less dense than the surrounding air above, so it begins to rise. As the parcel of air rises, it experiences this drop in pressure. As the pressure on the air parcel drops, the parcel expands and increases in size.
Anytime air expands due to a drop in pressure, it also gets cooler.
So, as the parcel rises and expands, it also cools. This is called the dry adiabatic lapse rate . The cooling parcel is no longer able to hold all of the water vapor it was able to hold when it was warm. The excess water vapor begins to condense out of the air parcel in the form of liquid cloud droplets. The higher the parcel rises, the more it expands, the cooler it gets, and the greater the amount of moisture that condenses out of it.