From AMS Glossary
As defined by A. Supan (1879), a region of the earth within which the mean temperature of the warmest month is less than 10°C. This limiting condition closely approximates the temperature at the arctic tree line, and was later adopted by W. Köppen (1918) as his boundary between the polar climates and tree climates.
Supan, A. 1879. Die Temperaturzonen der Erde. Petermanns Geog. Mitt.. 25. 349–358.
Köppen, W. P. 1918. Klassification der Klimate nach Temperatur, Niederschlag und Jahreslauf. Petermanns Geog. Mitt.. 64. 193–203; 243–248.