From AMS Glossary
The coldest temperature province in C. W. Thornthwaite's 1931 climatic classification. It is the climate of the ice cap regions of the earth, that is, those regions perennially covered with snow and ice.
It is equivalent to the more commonly used term perpetual frost climate, to the colder of Köppen's (1918) polar climates, and to Nordenskjöld's (1928) high arctic climate.
Thornthwaite, C. W. 1931. The climates of North America according to a new classification. Geogr. Rev.. 21. 633–655.
Köppen, W. P. 1918. Klassification der Klimate nach Temperatur, Niederschlag und Jahreslauf. Petermanns Geog. Mitt.. 64. 193–203; 243–248.
Nordenskjöld, O., and L. Meeking 1928. The Geography of the Polar Regions. American Geographical Society, Special Pub. No. 8, New York.