page_title: Polar stratospheric cloud
section_title: Types
hierarchical_section_title: Polar stratospheric cloud / Types
caption_attribution_description: English: Polar Stratospheric Cloud type I above Cirrus at sunrise Français&#160;: Nuages Nacrés de type I au dessus de Cirrus à l'aube
context_page_description: Polar stratospheric clouds are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 m. They are best observed during civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes. The other main type consists only of ice crystals which are not harmful. This type of PSC is also referred to as nacreous.
context_section_description: PSCs are classified into two main types each of which consists of several sub-types
  Type I clouds have a generally stratiform appearance resembling cirrostratus or haze. They are sometimes sub-classified according to their chemical composition which can be measured using LIDAR. The technique also determines the height and ambient temperature of the cloud. They contain water, nitric acid and/or sulfuric acid and are a source of polar ozone depletion. The effects on ozone depletion arise because they support chemical reactions that produce active chlorine which catalyzes ozone destruction, and also because they remove gaseous nitric acid, perturbing nitrogen and chlorine cycles in a way which increases ozone depletion.
  Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT).
  Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulfuric acid, nitric acid and water.
  Type Ic clouds consist of metastable water-rich nitric acid in a solid phase.
  Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types  and consist of water ice only.
  Only Type II clouds are necessarily nacreous whereas Type I clouds can be iridescent under certain conditions, just as any other cloud.  The World Meteorological Organization no longer uses the alpha-numeric nomenclature seen in this article, and distinguishes only between super-cooled stratiform acid-water PSC's and cirriform-lenticular nacreous PSC's.
caption: A stratiform type I PSC (white cloud above the orange tropospheric clouds), showing fine horizontal structures in the veil
exif: {"Image PhotometricInterpretation": "32803", "Image Make": "NIKON CORPORATION", "Image Model": "NIKON D5100", "Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Aperture 3.3.2", "Image DateTime": "2012:08:09 11:22:15", "Image ExifOffset": "216", "EXIF ExposureTime": "1/125", "EXIF FNumber": "28/5", "EXIF ExposureProgram": "Unidentified", "EXIF ISOSpeedRatings": "400", "EXIF DateTimeOriginal": "2012:08:09 11:22:15", "EXIF DateTimeDigitized": "2012:08:09 11:22:15", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "1", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash did not fire, auto mode", "EXIF FocalLength": "50", "EXIF SubSecTime": "70", "EXIF SubSecTimeOriginal": "70", "EXIF SubSecTimeDigitized": "70", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2460", "EXIF ExifImageLength": "860", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "1", "EXIF FocalLengthIn35mmFilm": "75", "EXIF SceneCaptureType": "Standard", "EXIF GainControl": "Low gain up", "EXIF Contrast": "Normal", "EXIF Saturation": "Normal", "EXIF Sharpness": "Normal", "EXIF SubjectDistanceRange": "0"}