The ten cloud genera by altitude
The World Meteorological Organization classifies clouds into ten genera grouped by the height of their base. High clouds (cirrus, cirrocumulus, cirrostratus) form above roughly 6 km and are made largely of ice crystals. Mid-level clouds (altocumulus, altostratus, and the precipitation-bearing nimbostratus) sit between about 2 and 6 km. Low clouds (stratus, stratocumulus, cumulus) form below 2 km.
Two genera develop vertically across the layers: cumulus and the towering cumulonimbus, the thunderstorm cloud that can span the full troposphere.
How the names are built
The naming uses Latin roots from Luke Howard's 1803 system. Cirrus means a curl of hair, cumulus means a heap, stratus means a layer, and nimbus indicates rain. Combining them, such as cumulonimbus, describes both the shape and the precipitation behavior.