<Li> South - West Indian Ocean cyclone </Li> <P> A polar, sub-polar, or Arctic cyclone (also known as a polar vortex) is a vast area of low pressure that strengthens in the winter and weakens in the summer . A polar cyclone is a low - pressure weather system, usually spanning 1,000 kilometres (620 mi) to 2,000 kilometres (1,200 mi), in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern hemisphere . The Coriolis acceleration acting on the air masses moving poleward at high altitude, causes a counterclockwise circulation at high altitude . The poleward movement of air originates from the air circulation of the Polar cell . The polar low is not driven by convection as are tropical cyclones, nor the cold and warm air mass interactions as are extratropical cyclones, but is an artifact of the global air movement of the Polar cell . The base of the polar low is in the mid to upper troposphere . In the Northern Hemisphere, the polar cyclone has two centers on average . One center lies near Baffin Island and the other over northeast Siberia . In the southern hemisphere, it tends to be located near the edge of the Ross ice shelf near 160 west longitude . When the polar vortex is strong, its effect can be felt at the surface as a westerly wind (toward the east). When the polar cyclone is weak, significant cold outbreaks occur . </P> <P> Under specific circumstances, upper level cold lows can break off from the base of the Tropical Upper Tropospheric Trough (TUTT), which is located mid-ocean in the Northern Hemisphere during the summer months . These upper tropospheric cyclonic vortices, also known as TUTT cells or TUTT lows, usually move slowly from east - northeast to west - southwest, and their bases generally do not extend below 20,000 feet (6,100 m) in altitude . A weak inverted surface trough within the trade wind is generally found underneath them, and they may also be associated with broad areas of high - level clouds . Downward development results in an increase of cumulus clouds and the appearance of a surface vortex . In rare cases, they become warm - core tropical cyclones . Upper cyclones and the upper troughs that trail tropical cyclones can cause additional outflow channels and aid in their intensification . Developing tropical disturbances can help create or deepen upper troughs or upper lows in their wake due to the outflow jet emanating from the developing tropical disturbance / cyclone . </P> <P> The following types of cyclones are not identifiable in synoptic charts . </P>

When does a low pressure system become a cyclone