text: why are cumulus clouds flat on the bottom?   it is called the cloud condensation level.  essentially it is when the dew point is reached at a certain altitude.  the dew point is the temperature at which saturation occurs & water will condensate on tiny particles in the atmosphere.  since temperature lowers at a steady rate when you go upwards, the cloud condensation level will essentially be parallel with the ground.  that is why clouds are flat on the bottom.