It is commonly called alumina, and may also be called aloxide, aloxite, or alundum depending on particular forms or applications. It commonly occurs in its crystalline polymorphic phase Î±-Al 2 O 3, in which it composes the mineral corundum, varieties of which form the precious gemstones ruby and sapphire.
rain cloud. (meteorology). Any cloud from which rain falls; a popular term having no technical denotation. 1  Facebook. 2  Twitter.
Clouds are classified into four basic categories, depending largely on the height of their bases above the ground. High-level clouds, called cirrus clouds, can reach heights of 20,000 feet (6,000 meters) and are typically thin. They do not produce rain and often indicate fair weather.