What exactly is a cloud? Why do clouds form over some areas and not others? Take a look inside: learn about factors that cause clouds to develop in the first place and find out why there are so many different types.

NOVA is produced by WGBH in Boston. National corporate funding for NOVA is provided by The Boeing Company. Major funding for NOVA is provided by David H. Koch, the Corporation for Public Broadcasting, and PBS viewers.

Funding for the Cloud Lab is provided by NASA.