I once had a teacher write me about a student of hers who realized fog was a cloud of sorts and during a class walk around the schoolyard, made that lovely comment.
Basically, you give each group of students a blue piece of paper. Then they get all or part of a white piece of paper. It's a good idea to have very small groups so you can double up on the cloud cover assignments.
For example: The group doing Overcast (100%) gets the total piece of white paper; the group doing Partly Cloudy gets half (50%) of the white paper. And so on...
They are instructed to rip the paper up into small pieces, all greater than a postage stamp, of course, and then glue the pieces onto the blue paper.
The blue papers with the white clouds are put up for display. The class then gets to guess which paper represents which type of cloud cover. After they are all recorded, the teacher puts them in the correct order. (Code the backs of the blue papers, maybe?)
The kids then sketch the appropriate types of cloud cover on their paper and add the Cloud Cover Symbols.
The next time you have your kids look out the window to assess the cloud type and cloud cover for the Weather Observations Activity, they should have a much better idea as to what 50% cloud cover really looks like!
CONSIDER THIS: Add a Cloud Cover Simulation Activity to your Weather unit!
FUN LINK: From the University of Illinois.