Here are the relative heights at which various types of clouds generally exist.
CIRRUS = about 20,000 feet or higher
ALTOSTRATUS (mid-level clouds) = between 6500 and about 20,000 feet
STRATUS (low clouds) = below 6500 feet
CUMULUS (clouds of vertical development) = base could be below 6500 feet; tops can go as high as 50,000 feet or more
FOG = inches or feet (fog is a cloud that's sitting on or very near the ground)