Why do large cumulus clouds have flat bottoms?

Dear Tom,
Why do large cumulus clouds have flat bottoms?
— Barbara Fanta, Oak Park

Dear Barbara,
Cumulus clouds, those puffy clouds common in the sky during warm days, especially in the summer, are indeed flat-bottomed. It’s because of the way they form.
They exist at the top of columns of warm air rising into the sky from the ground. Sunshine heats the ground, and the ground heats the air above it. If the heating is sufficient, bubbles or columns of warm air begin rising.
The air cools as it rises, though it’s still warmer than the surrounding air. The rising air eventually cools to its saturation temperature, the temperature at which its relative humidity hits 100 percent, and condensation begins. The height of the cumulus cloud bases varies with the weather situation, typically 1,500 to 4,500 feet in our area.