The bottom of clouds is flat but the top is fluffy. Why?

Question

Looking through a window from the plane I can see that the base of the clouds is flat and the top is fluffy.

Answer 1

The clouds you are talking about are formed by rising bubbles of air.  The flat bottom is determined by the level at which water vapor condenses out of the bubbles.  Since each bubble of air probably starts out more or less the same at the surface, each one's water vapor will start condensing at the same level--that makes the flat bottom.  The fluffy top is evidence of each individual bubble.

Answer 2

In a given volume of air,the temperature is usually uniform horizontally,but it varies vertically because temperature decreases with altitude.This means that the condensation point(where condensation of water vapour occurs) is also horizontally uniform at a given altitude.When air rises and cools at the above mentioned condensation point,clouds start to form.So,this happens at a fairly constant level.Above this level,water vapour condenses to form clouds and below this level water remains in gaseous state.So, the bottoms of most clouds are flat.

As the cloud stretches high up into the sky,it will draw in drier surrounding air.Then the cloud will start getting evaporated and eroded on its sides,giving the fluffy look.