system
Given the following post, respond as if you're commenting on Reddit:
human
Why do clouds have flat bottoms? Why do they seem to just stop?

When I look at clouds, cumulus to be exact, I notice that at the bottom, they appear to be flat. I just want to know why that is. I assumed that it had to do with the atmosphere but Im not sure.
gpt
As air rises, warmed up by the ground, the temperature falls by adiabatic effect (fall in pressure) to an average rate of 0.65 C per 100 m of altitude. 

At 15 C (the average temperature worldwide) a cubic meter of air can contain 14 grams of water. No more. 

As the rising air cools down, it has less and less place for water vapour and soon the temperature reaches what is called the dew point, i.e. when the air is 100% saturated and cannot cools any further without condensing in droplets; the cloud. 

The base of the cumulus is flat because it is exactly the place where the air reaches the dew point. 

But once 'in the cloud' the air cools down at a lower rate because the condensation of water creates energy released as heat. Because of that, the air inside the cloud continues to rise until, eventually the temperature becomes even with the surrounding temperature and the convective rise stops. 

The reason the top of the cumulus is cauliflower shaped is that, each 'bump' is in fact the top of one cell of warm air that has been rising. 

Such cells, often called thermic cells are what glider pilots are looking for because they use it to circle in and climb. You will also see birds like seagulls and eagles taking those 'elevators' to gain altitude without effort.