Anthropogenic cloud
The intermediate layers of the troposphere are the regions with less influence of human activity. This region is far enough from the surface for not being affected by the surface emissions. Additionally, commercial and military flights only cross this region during ascending or descending maneuvers. Moreover, in this region there exist two types of clouds with a large horizontal extension: "Nimbostratus" and "Altostratus", which cannot originate from human activity. Consequently, it is assumed that there are no anthropic clouds of these two "genera". However, what can occur is enhancing existing "Nimbostratus" or "Altostratus" due to the additional water vapor or condensation nuclei emitted by a thermal power plant, for instance.
Cloud
The genitus and mutatus categories have been expanded to include certain types that do not originate from pre-existing clouds. The term "flammagenitus" (Latin for 'fire-made') applies to cumulus congestus or cumulonimbus that are formed by large scale fires or volcanic eruptions. Smaller low-level "pyrocumulus" or "fumulus" clouds formed by contained industrial activity are now classified as cumulus "homogenitus" (Latin for 'man-made'). Contrails formed from the exhaust of aircraft flying in the upper level of the troposphere can persist and spread into formations resembling any of the high cloud genus-types and are now officially designated as cirrus, cirrostratus, or cirrocumulus homogenitus. If a homogenitus cloud of one genus changes to another genus type, it is then termed a "homomutatus" cloud. Stratus cataractagenitus (Latin for 'cataract-made') are generated by the spray from waterfalls. "Silvagenitus" (Latin for 'forest-made') is a stratus cloud that forms as water vapor is added to the air above a forest canopy.