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