Eocene
Another method considered for producing the warm polar temperatures were polar stratospheric clouds. Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and traps outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II).
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.