Average temperature of the warmest month < 10°C. Zones indicated in violet (e.g. Siberia) are tundra with an average temperature in the warmest month greater than 0°C, while those in blue-green (the coast of Greenland) have no month with temperature above 10°C.
A subtype of the latter subclass is the Greenland interior (light blue), where the average temperature of the coldest month is less than -38°C.