The combination of a drastic temperature and corresponding pressure drop along with the rotation of the Earth on its axis produces a spinning/rotating volume of air. The rotational speed of the winds commonly reaches as high as 180 mph. The motion of these winds form an impenetrable barrier such that the trapped air inside is unmixed, as it is separated from the air outside, and remains quite cold (temperatures drop below -80° Celsius) until October. Inside the whirling volume of freezing air, the cold temperatures facilitate the condensation of gases into particles that eventually form polar stratospheric clouds.
[Colin Baird. Environmental Chemistry; 1999; Freeman Press.]
Source: Atmospheric Chemistry Glossary