Cumulus clouds develop in a vertical direction from the base up. They have flat bases and dome- or cauliflower-shaped upper surfaces. They develop due to vertical winds, like thermical lift or mountain breeze: air masses ascend, expand and cool down. In a special height, when the dewpoint is reached, the humidity of the air precipitates. That's the reason why the bases of cumulus clouds are flat and the bases of these clouds in the surrounding are in the same height.