Without knowing the EXACT answer, just to point out they are not perfectly flat at the bottom, they just seem that way from as distance. Flying nearer to them will show that. But like all clouds they suck big time! the pressure below the cloud is very strong and will 'suck' you up into the cloud unless you control your pitch / speed.
Edit for bad spelling..as usual
[ 06-24-2004, 09:21 AM: Message edited by: wellard ]