Bruce,

Absolutely, practically all characteristics of a thermal change rapidly
as it approaches the boundary layer, which is not always the same as
cloud base. (I'm not an expert on this subject, but I believe you could
call it a mere coincidence if the top of the lift is the same as
cloudbase.)

We have many days out west where the top of the lift will be several
thousand feet below cloudbase, but when there's enough moisture in the
air and the dewpoint is low enough, cloudbase will be below the top of
the lift, in which case you have to be most careful not go get sucked
into the clouds.

A well known German pilot whose name is escaping me at the moment (not
Reichmann) wrote several articles on thermal characteristics, one of
them is in the "Going farther and faster" books that you can purchase
from the Australian gliding federation. (Other RASers can hopefully
reply with those details.) The books are highly recommended for
newbies, and the stuff on thermals is quite informative.

~ted/2NO

ps the GFA web page where you can buy the books is he
http://www.gfa.org.au/sales/index.php#books. If Google mangles that
address, just Google for their web page, it's easy to find.