A description of cloud formation and of the processes that take place within
clouds can be found in any good meteorology text. I recommend F. K. Lutgens and
E. J. Tarbuck, THE ATMOSPHERE (Prentice-Hall).
As an instrument-rated pilot who has seen the inside of quite a few clouds
(though not as many as I would like!), I can attest that many clouds are quite
smooth inside, with nary an updraft or downdraft to be found. But cumulus clouds
typically have both updrafts and downdrafts within them. A long out-of-print
book by W. H. Rankin (THE MAN WHO RODE THE THUNDER, Prentice-Hall, 1960)
describes the author's harrowing experience when he had to bail out of his
Marine jet fighter at 47,000 feet --- into an active thunderstorm. He spent over
half an hour going up and down in the vertical circulation, hanging on to his
parachute for dear life! See if your library has this book --- it's quite a
read.