Why do clouds clump together like they do?

Actually, clouds aren’t evaporated water; they’re water that has condensed from the atmosphere’s water vapor and is held in suspension as an aerosol. They require certain conditions to form. Since the amount of water that air can hold in gaseous form varies with its temperature, there will be a certain temperature (the “dew point”) at which the water can no longer stay vaporized, and tiny drops of liquid water will condense out. You now have a cloud.

The temperatures in the atmosphere are not uniform. The colder zones will be where clouds form. Once the condensation process begins in a given cool spot, the fog that forms further shields the interior of the nascent cloud from radiant heat, so that the inside of the cloud becomes cooler still.

In other words, the clouds are born out of subtle temperature differences, but their formation continues to exaggerate those differences. In this way, the clouds become increasingly more defined as the process continues.

To piggy back off of thorn, as a general rule, and depending on the saturation of the air, the temperature will cool approximately 2.5–4.5 degrees per 1000 feet.

If you look at some clouds. You will see the distinct level where all of the requirements have been met for clouds to form. This is when you see layers of clouds with a very flat base.