Polar vortex
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The polar vortex is a persistent, large-scale cyclone located near the Earth's poles, in the middle and upper troposphere and the stratosphere. The vortex is most powerful in the hemisphere's winter, when the temperature gradient is steepest, and diminishes or can disappear in the summer. The Antarctic polar vortex is more pronounced and persistent than the Arctic one; this is because the distribution of land masses at high latitudes in the northern hemisphere gives rise to Rossby waves which contribute to the breakdown of the vortex, whereas in the southern hemisphere the vortex remains more undisturbed. The Arctic vortex is elongated in shape, with two centres, one roughly over Baffin Island in Canada and the other over northeast Siberia.
The chemistry of the Antarctic polar vortex has created severe ozone depletion. The nitric acid in polar stratospheric clouds reacts with CFCs to form chlorine, which catalyzes the photochemical destruction of ozone. Chlorine concentrations build up during the winter polar night, and the consequent ozone destruction is greatest when the sunlight returns in spring (September/October). These clouds can only form at temperatures below about -80°C, so the warmer Arctic region does not have an ozone hole.
Other astronomical bodies are also known to have polar vortices, including Venus, Mars, Jupiter, and Saturn's moon Titan.
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