There are two polar vortices in the atmosphere of planet Earth, one around the North Pole, and one around the South Pole. Each polar vortex is a persistent, large-scale cyclone, circling the planet's geographical pole. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. They surround the polar highs and lie in the wake of the polar front. These cold-core low-pressure areas strengthen in the winter and weaken in the summer due to their dependence upon the temperature differential between the equator and the poles. They usually span less than 1,000 kilometers in diameter within which the air circulates in a counter-clockwise fashion in the Northern Hemisphere, and in a clockwise fashion in the Southern Hemisphere. As with other cyclones, their rotation is caused by the Coriolis effect. The Northern Hemisphere vortex often contains two low pressure centers, one near Baffin Island, Canada and the other over northeast Siberia. Within the Antarctic vortex in the Southern Hemisphere a single low pressure zone tends to be located near the edge of the Ross ice shelf near 160 west longitude.