The polar highs are areas of high atmospheric pressure around the north and south poles; the south polar high being the stronger one because land gains and loses heat more effectively than sea. The cold temperatures in the polar regions cause air to descend to create the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise to 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 parallels 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. This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere. Closely related to this concept is the polar vortex.