PSC (POLAR STRATOSPHERIC CLOUDS)

During the polar 'night' (i.e. the period in the middle of the winter when insolation does not penetrate to ultra-high latitudes), the stratosphere cools significantly leading to 'closed-loop' circulations (both vertical and horizontal) which virtually isolate these polar stratospheric regions - the "Polar night vortex" is found, within which temperatures can be found well below (minus)75degC. In these extremely cold conditions, clouds are observed to form in the stratosphere, which appear to be composed of a combination of nitric acid and water. Stratospheric clouds can also form from ordinary water ice (i.e. as in the troposphere) but these are much less common at these high altitudes as the stratosphere is normally very dry and water-ice clouds only form at the lowest temperatures. The presence of PSC's and the part they play in the chemical interactions at these levels have been a subject of much debate in recent years.