ximately 234 significant extratropical cyclones form each winter. In Europe, particularly the United Kingdom, recurring extratropical low-pressure weather systems are typically known as depressions. These tend to bring wet weather throughout the year. Thermal lows also occur over continental areas across the subtropics during the summer such as the Sonoran Desert, the Mexican plateau, Sahara, South America, and Southeast Asia. The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.
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.
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.
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.
The Coriolis force caused by the Earth's rotation is what gives winds within low-pressure systems their counter-clockwise (anticlockwise) circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically, toward their centers. A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with light winds and fair skies.
- Trough (meteorology)
- East Asian monsoon
- Intertropical convergence zone
- North American Monsoon
- Surface weather analysis
- Tropical wave
- Weather map
- High-pressure area
- Arctic Climatology and Meteorology (2009). Cyclogenesis. National Snow and Ice Data Center. Retrieved on 2009-02-21.
- Glossary of Meteorology (2009). "Cyclogenesis". American Meteorological Society. Retrieved 2009-02-21.
- Glossary of Meteorology (June 2000). "Cyclonic ci