css clouds animation issue : im having trouble integrating this css clouds animation into my 
website. the overflow : hidden and scroll are causing my problems.
and I don't want the clouds scrolling outside of the blue box background area, but don't know how . please see http://www.filehostfree.com/cloudcsstest/cssanimation.html

ive left a comment in the source code. please see ,highlights my issue.
any help is appreciated.thanks