Why can cloud's bottoms be flat?
That many types of clouds have flat bottoms is a function of decreasing air temperature with increasing height, and the water vapor pressure, commonly in this context called the dew point, of the air. When some force causes a non-cloudy parcel of air to rise, moisture will condense out as cloud particles just as soon as the air temperature reaches the dew point of the air mass.  The colder air (higher up) then contains a cloud, where the warmer air (even just slightly above the dew point) below the cloud remains clear.