Low-pressure area

In meteorology, a low-pressure area is a region where the atmospheric pressure is lower than in surrounding places. These areas are important because they are often linked to cloudy, windy weather and sometimes rain or storms. In contrast, high-pressure areas usually bring lighter winds and clear skies.

Winds around low-pressure areas move in different directions depending on where you are. In the northern hemisphere, winds circle counter-clockwise, while in the southern hemisphere, they circle clockwise. This happens because of the Earth's rotation, known as Coriolis forces.

Low-pressure areas form when winds spread out higher up in the sky, a process called cyclogenesis. This spreading out of winds can happen on the east side of large weather patterns called Rossby waves or ahead of smaller weather disturbances. As air moves upward away from the ground, it creates a region of lower pressure at the surface.

These areas can also form over hot land or water. When the sun heats up places like deserts, the warm air rises and creates lower pressure near the ground. This helps drive big weather patterns, like monsoons. Sometimes, strong thunderstorms over warm water can also create low-pressure areas, and if they get strong enough in the tropics, they can turn into a tropical cyclone.

Low-pressure areas usually bring clouds because the rising air cools and forms them. Clouds can keep daytime temperatures cooler by blocking sunlight and nighttime temperatures warmer by trapping heat. The stronger the low-pressure area, the stronger the winds around it. These systems are often found over places like the Tibetan Plateau or downwind of big mountain ranges like the Rocky Mountains. In Europe, especially around the British Isles and Netherlands, these weather systems are commonly called "low levels".

Formation

Main article: Cyclogenesis

Cyclogenesis is the process where low-pressure areas form in the atmosphere. These areas have lower pressure than the air around them, which often brings cloudy, windy weather and sometimes rain or storms.

Low-pressure systems can form in many ways. Big systems, like those that affect large areas, are called extratropical cyclones. Smaller systems, such as tropical cyclones (like hurricanes), form over warm ocean waters when conditions are right. Other types include polar lows over cold ocean areas and mesocyclones over land. These systems all share one thing: air rising upward, which lowers the pressure at the surface.

In deserts, intense heating of the ground can create low-pressure areas called thermal lows. Monsoons are large seasonal low-pressure systems that form over land in summer, bringing moist air and rain from the ocean.

Climatology

See also: Arctic oscillation, Extratropical cyclone, and Thermal low

Large polar cyclones help guide weather systems moving through mid-latitudes, south of the Arctic and north of the Antarctic. Extratropical cyclones often form east of troughs near continents' east coasts or west of oceans. In the Southern Hemisphere, there are usually about 37 of these cyclones at any time in a six-hour period between the 30th and 70th parallels. In Europe, especially in the United Kingdom and the Netherlands, these weather systems are called depressions and often bring wet weather.

Elongated low-pressure areas form at the monsoon trough or Intertropical Convergence Zone as part of the Hadley cell circulation. In the western Pacific, this reaches its farthest north in late summer. It can extend to the 40th parallel in East Asia in August and the 20th parallel in Australia in February. These low-pressure systems help drive monsoon rains and are linked to many of the world's rainforests.

See also: Monsoon trough

See also: Tropical cyclone

Tropical cyclones need to form more than 555 km or poleward of the 5th parallel north and 5th parallel south to allow the Coriolis effect to create their spinning winds. Worldwide tropical cyclone activity is strongest in late summer. May is the quietest month, while September is the busiest. Nearly one-third of all tropical cyclones form in the western Pacific Ocean, making it the most active area on Earth for these storms.

Associated weather

See also: Geostrophic wind and Precipitation (meteorology)

Wind moves from places with high pressure to places with low pressure. This happens because of differences in how warm or moist the air is. When there is a big difference between high pressure and low pressure, the wind blows harder.

The way the Earth spins changes the direction of the wind around low-pressure areas. In the northern part of the world, winds move counter-clockwise around these areas, while in the southern part, they move clockwise. Big storms like hurricanes, cyclones, and typhoons are examples of this.