Boreal

A period of more continental climate 5000 to 7000 years ago identified as part of the Blytt– Sernander sequence of inferred climates in northern Europe.

Sernander, R. 1908. On the evidence of postglacial changes of climate furnished by the peat-mosses of northern Europe. Geol. Fören. Förh.. 30. 456–478.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.

With respect to aviation weather observations, same as collective.