section_path: Stratospheric classification and distribution
section_name: Stratospheric classification and distribution
paragraph_text: Stratospheric classification and distribution
  
  Polar stratospheric clouds (PSC's) are found in the lowest part of the stratosphere. Moisture is scarce above the troposphere, so nacreous and non-nacreous clouds at this altitude range are restricted to polar regions in the winter where and when the air is coldest.
  
  PSC's show some variation in structure according to their chemical makeup and atmospheric conditions, but are limited to a single very high range of altitude of about 15,000–25,000 m (49,200–82,000 ft) Accordingly, they are classified as a singular type with no differentiated altitude levels, genus types, species, or varieties. There is no Latin nomenclature in the manner of tropospheric clouds, but rather descriptive names of several general forms using common English.
  
  Supercooled nitric acid and water PSC's, sometimes known as type 1, typically have a stratiform appearance resembling cirrostratus or haze, but because they are not frozen into crystals, do not show the pastel colors of the nacreous types. This type of PSC has been identified as a cause of ozone depletion in the stratosphere. The frozen nacreous types are typically very thin with mother-of-pearl colorations and an undulating cirriform or lenticular (stratocumuliform) appearance. These are sometimes known as type 2.