Why do cumulus clouds have flat bottoms?

Question

Why do cumulus clouds have flat bottoms?

Answer 1

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.

Answer 2

I think the winds aloft have a larger effect on the situation which dicitates that seperation.

Answer 3

In a given volume of air in the atmosphere,the temperature is usually uniform horizontally,but it varies vertically because temperature normally decreases with height.This means that the condensation point(ie the height at which the condensation takes place and cloud forms) is horizontally uniform at a given altitude.Above this altitude, water vapour will condense out to form clouds,below which,water vapour will remain in its gaseous state.Therefore,the bottoms of most clouds(particularly cumulus clouds due to their vertical growth which makes the base appear clearly) appear flat.