Final answer:
Cloud classification is based on altitude and form or appearance, with the main altitude categories being high, middle, low, and vertically developed, while form categories are defined by their visual characteristics.
Step-by-step explanation:
The two criteria used as the basis for cloud classification are cloud altitude and cloud form or appearance. Meteorologists classify clouds into different types based on how high they are in the atmosphere and their visual characteristics. The main categories for altitude include high clouds (cirro-), middle clouds (alto-), and low clouds (strato-), and clouds with vertical development (cumulo-). Cloud forms, on the other hand, are categorized by appearance and texture, such as cirrus (wispy), cumulus (heaped or piled), stratus (layered), and nimbus (rain-bearing).