<P> Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic . The Arctic oscillation provides an index used to gauge the magnitude of this effect in the Northern Hemisphere . Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans . A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels there are an average of 37 cyclones in existence during any 6 - hour period . A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter . In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low - pressure weather systems are typically known as depressions . These tend to bring wet weather throughout the year . Thermal lows also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert, the Mexican plateau, the Sahara, South America, and Southeast Asia . The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains . </P> <P> Elongated areas of low pressure form at the monsoon trough or intertropical convergence zone as part of the Hadley cell circulation . Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest . It can reach as far as the 40th parallel in East Asia during August and 20th parallel in Australia during February . Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure over the warmest part of the various continents . The large - scale thermal lows over continents help create pressure gradients which drive monsoon circulations . In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, oriented along a west - northwest / east - southeast axis . Many of the world's rainforests are associated with these climatological low - pressure systems . </P> <P> Tropical cyclones generally need to form more than 555 km (345 mi) or poleward of the 5th parallel north and 5th parallel south, allowing the Coriolis effect to deflect winds blowing towards the low - pressure center and creating a circulation . Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest . However, each particular basin has its own seasonal patterns . On a worldwide scale, May is the least active month while September is the most active month . November is the only month that activity in all the tropical cyclone basins is possible . Nearly one - third of the world's tropical cyclones form within the western Pacific Ocean, making it the most active tropical cyclone basin on Earth . </P> <P> Wind is initially accelerated from areas of high pressure to areas of low pressure . This is due to density (or temperature and moisture) differences between two air masses . Since stronger high - pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low - pressure areas in advance of their associated cold fronts . The stronger the pressure difference, or pressure gradient, between a high - pressure system and a low - pressure system, the stronger the wind . Thus, stronger areas of low pressure are associated with stronger winds . </P>

The wind system associated with a low pressure area in the northern hemisphere is