Long faint clouds which form in the stratosphere only when the temperature falls below about -80°C. They are common above the poles in winter. These clouds appear to play a role in the depletion of stratospheric ozone. The ice particles in the cloud provide surfaces on which a reaction takes place to release free chlorine. The chlorine then reacts with ozone to form chlorine monoxide and oxygen.

Clouds that form during the winter months in Antarctica and sometimes in the Arctic. Extremely cold temperatures are necessary for PSCs to exist, and it is on the surface of these clouds that reactions take place that contribute to the seasonal ozone loss.

PSCs. Ice clouds which form during the long, dark and very cold Antarctic winter. These cloud crystals provide a surface for chemical reactions that result in the release of reactive chlorine and bromine. When the sun rises in late August, UV radiation triggers reactions with chlorine and bromine that destroy ozone. The result is an ozone hole. A similar, but not as extensive destruction of ozone takes place in the Arctic under the same conditions, but the temperatures do not get as cold, therefore fewer PSCs form.