why are clouds flat on the bottom

Clouds are flat on the bottom because this flat base marks the altitude where rising warm air cools to its dew point, causing water vapor to condense into visible water droplets. This altitude is called the "lifting condensation level" (LCL). The temperature and humidity of the air near the ground are often fairly uniform over an area, so condensation (cloud formation) starts at a consistent height, making the cloud bottoms appear flat. Above this level, air rises and moves irregularly, creating the fluffy, uneven tops of clouds. The flat bottom is essentially the boundary where the air first reaches saturation and condensation begins, forming cloud droplets.