Origin: In northern hemisphere mP results primarily from modification of cP by flow over oceans: Siberia flowing over north Pacific, northern Canada & Greenland flowing over north Atlantic.
In summer the Arctic icecap – significant areas of melt water, and open leads in ice, provide effective water surface.
In southern hemisphere: oceans surrounding Antarctica.
In winter modified cA provides colder mP than modified cP.
Cool and moist, extensive cloud cover.