The climate zone whose warmest-month average remains below 10°C is the polar climate. In the Köppen system, this temperature limit defines Group E climates, where even the warmest part of the year stays very cold.
Group E is divided into tundra climate, ET, and ice-cap climate, EF. Tundra climates have a warmest-month average between 0°C and 10°C, allowing a short season in which mosses, lichens and low shrubs can grow. Ice-cap climates keep every month below 0°C and support virtually no natural vegetation.
This criterion is based on monthly average temperature, not the highest temperature recorded during a day. A polar location may occasionally experience a brief warm spell, yet still qualify as polar if its warmest month averages below the threshold.
Polar climates occur around the Arctic and Antarctic, as well as on high mountain summits and ice sheets. “Alpine” is often used descriptively for high-elevation environments, but it is not the primary Köppen group name. That makes Polar the standard answer for this question’s classification wording.