From Glossary of Meteorology
Generally, the climate that produces tundra vegetation; it is too cold for the growth of trees but does not have a permanent snow–ice cover.
In W. Köppen's 1936 climatic classification it is one of the polar climates, defined as having a mean temperature for the warmest month of less than 10°C (50°F) (in contrast to snow forest and temperate rainy climates) but higher than 0°C (32°F) (which is the limit of perpetual frost climate). Tundra is designated ET. Tundra climate appears as a temperature province in C. W. Thornthwaite's 1931 classification.
Köppen, W. P., and R. Geiger 1930–1939. Handbuch der Klimatologie. Berlin: Gebruder Borntraeger, 6 vols.
Thornthwaite, C. W. 1931. The climates of North America according to a new classification. Geogr. Rev.. 21. 633–655.