Water vapor condensing and creating a cloud. That happens at a certain point, where temperatures and dewpoints are roughly equal… so you have saturation. You are referring to cumulus clouds, the most common daytime clouds in the summer sky. Morning sunshine warms the ground, and most of that heat transfers into the air immediately above, forming a layer of warm air several hundred feet in depth. If the temperature of the heated layer surpasses a critical value, as it invariably does, currents of warm air begin to ascend, carrying water vapor along. Because air pressure decreases at the rate of 0.91 inches per 1,000 feet of ascent, rising air expands and cools. The flat bottom of cumulus clouds defines the exact height at which a critical combination of temperature and air pressure causes water vapor within the rising current to condense into a visible cloud.