Here's why:

* Condensation: For clouds to form, water vapor in the air needs to cool down and condense into tiny water droplets or ice crystals.

* Condensation Level: This is the altitude in the atmosphere where the air has cooled enough for condensation to occur. The temperature at this level is called the dew point.

* Cloud Formation: When air rises and cools, it reaches the condensation level, and clouds begin to form. The bottoms of these clouds mark the altitude where the air has reached its dew point.

So, the bottoms of clouds are a visual indicator of where the air has cooled enough for water vapor to condense into visible clouds.