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).
Cirrocumulus cloud
Cirrocumulus is a cloud of the "stratocumuliform" physical category that shows both stratiform and cumuliform characteristics and typically appears as white, patchy sheets with ripples or tufts without gray shading. Each cloudlet appears no larger than a finger held at arm's length. These often are organized in rows like other cumuliform and stratocumuliform clouds, but since they are so small, cirrocumulus patches take on a finer appearance, sometimes also referred to colloquially as "herringbone" or as a "mackerel sky". Cirrocumulus is coded C9 for the main genus-type and all subforms.