Because that's (roughly) the Lifted Condensation Level: https://en.wikipedia.org/wiki/Lifted_condensation_level https://en.wikipedia.org/wiki/Lifted_condensatio

Because that's (roughly) the Lifted Condensation Level: https://en.wikipedia.org/wiki/Lifted_condensation_level https://en.wikipedia.org/wiki/Lifted_condensation_level Water vapor around the LCL starts condensing and turning from a gas into liquid cloud droplets. This process happens considerably faster once it begins for a variety of reasons, so once you can have cloud droplets, you get a ton of cloud droplets - not a gradual transition from water vapor to cloud. It's almost like a light switch. Most air masses are relatively homogenous anyways, so unless there are underlying processes causing things like undulatus asperatus, it will certainly appear very, very flat over a large area.