The cold temperatures in the polar regions cause air to descend, creating the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise instead and create the low pressure Intertropical Convergence Zone.
Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallel of latitude.
These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin in a stationary front.
Closely related to this concept is the polar vortex, a rotating low-pressure circle of cold air around the poles.
Air flows outwards from the poles to create the polar easterlies in the Arctic and Antarctic areas.