Clouds are condensed water vapor.  The bottom of cumulus clouds are
fairly flat.  If the sea level temperature is 31 degrees Celsius and
the dew point is 11 degrees Celsius, show why the bottom of a cloud
would form at 2500 meters above sea level and explain why the bottom
of the cloud would appear flat.

Hi -

Water will condense out of the air to form a cloud at the dew-point of
the water - so at 11 degrees in this case, 20 degrees colder than the
surface.  Dry air cools at the dry adiabatic rate of 10 degrees/1000
meters - however, the dew point also decreases at a rate of around 2
degrees/ 1000 meters - thus the rate of convergence of dew point and
air parcel temperature can be approximated at 8 degrees/1000 meters -
or 2500 meters in this case.  As the air cools at a relatively
constant rate, the dew point temperature will be reached for all of
the air forming the cload at approximately the same height, leading to
a flat bottom for the cloud.

A nice powerpoint presentation explaining cloud formation (takes a
while to load though)
file:///E:/Documents%20and%20Settings/dan/Local%20Settings/Temporary%20Internet%20Files/Content.IE5/43X3MQNT/297,26,Lenticular
clouds

A review page from a meterology course explaining some of these
concepts:
http://www.atmo.arizona.edu/students/courselinks/spring00/atmo171-flittner/q6_review.html