9.2 Structure and characteristics of anticyclones
4 min read•july 31, 2024
Anticyclones are high-pressure systems that bring fair weather and stability to regions they affect. They're characterized by descending air, clockwise rotation in the Northern Hemisphere, and can persist for days or weeks, influencing weather patterns across large areas.
Understanding anticyclones is crucial for grasping mid-latitude weather dynamics. They contrast with cyclones in structure and effects, often blocking storm systems and creating persistent conditions that can lead to heat waves or air quality issues in affected areas.
Anticyclones: Definition and Characteristics
Core Attributes of Anticyclones
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Top images from around the web for Core Attributes of Anticyclones
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Ventos e ciclones – Monolito Nimbus View original
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- Large-scale atmospheric circulation systems characterized by high pressure at the center
- Rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
- Central region experiences , light winds, and stable weather conditions due to descending air motions
- Range in size from a few hundred to several thousand kilometers in diameter
- Persist for days to weeks
- Feature weaker pressure gradients compared to cyclones, resulting in lighter winds flowing outward from the center
- Associated with leading to adiabatic warming and often temperature inversions near the surface (temperature increases with height)
- Play crucial role in global atmospheric circulation patterns
- Significantly influence regional weather and climate (Mediterranean summers)
Weather and Climate Impacts
- Bring periods of fair weather and clear skies to affected regions
- Can lead to heat waves during summer months (2003 European )
- Contribute to air quality issues by trapping pollutants near the surface (Los Angeles smog)
- Influence precipitation patterns by blocking or deflecting storm systems
- Affect wind patterns and intensity in surrounding areas
- Can create persistent weather conditions lasting for extended periods (blocking highs)
- Interact with other weather systems, influencing their development and movement (steering hurricanes)
Anticyclones vs Mid-latitude Cyclones
Structural Differences
- Anticyclones have high-pressure centers while mid-latitude cyclones have low-pressure centers
- Opposite wind circulation patterns anticyclones (clockwise in Northern Hemisphere) cyclones (counterclockwise in Northern Hemisphere)
- Vertical motion predominantly descending in anticyclones, rising in mid-latitude cyclones
- Anticyclones bring fair weather with clear skies, mid-latitude cyclones associated with cloudy conditions and precipitation
- Weaker pressure gradient in anticyclones results in lighter winds compared to mid-latitude cyclones
- Frontal structure key feature of mid-latitude cyclones, absent in anticyclones
- Temperature patterns differ anticyclones often cause warming at surface due to subsidence, mid-latitude cyclones lead to varied temperature changes associated with frontal passages
Stability and Evolution
- Anticyclones generally more stable and long-lasting compared to dynamic, rapidly evolving mid-latitude cyclones
- Anticyclones can persist for weeks, while mid-latitude cyclones typically last 3-7 days
- Mid-latitude cyclones undergo distinct life cycle stages (cyclogenesis, maturation, occlusion)
- Anticyclones often exhibit slower changes in intensity and structure
- Energy sources differ anticyclones maintained by large-scale subsidence, mid-latitude cyclones fueled by temperature gradients and latent heat release
- Anticyclones can block or steer the movement of mid-latitude cyclones (omega blocks)
- Interaction between systems can lead to complex weather patterns and forecasting challenges
Anticyclone Structure and Circulation
Vertical Structure
- Characterized by descending air motion throughout troposphere, strongest subsidence near center
- Upper-level divergence and lower-level convergence maintain high-pressure system
- often forms near surface due to radiative cooling and subsidence
- Traps pollutants and moisture (Beijing air pollution events)
- Vertical extent can reach tropopause, sometimes extending into lower stratosphere
- Horizontal wind speeds generally increase with height, forming expanding spiral pattern outward from center
- Coriolis effect influences circulation pattern, causing winds to deflect right in Northern Hemisphere, left in Southern Hemisphere
- Some anticyclones exhibit warm core structure, particularly in subtropical high-pressure systems (Bermuda High)
- Affects vertical stability and evolution
Circulation Patterns
- Surface winds flow outward from high-pressure center in spiral pattern
- Wind speed generally increases with distance from center due to increasing pressure gradient
- Upper-level winds converge towards center, maintaining mass balance
- Subsidence warming creates temperature gradient between center and periphery
- Friction near surface causes wind to cross at slight angle, affecting boundary layer flow
- Diurnal variations in circulation strength due to heating and cooling cycles
- Interaction with topography can create local wind patterns (Santa Ana winds in California)
Formation and Maintenance of Anticyclones
Large-scale Atmospheric Dynamics
- Descent of air in downward branch of Hadley and Ferrel cells plays crucial role in anticyclone formation
- Surface heating and cooling patterns, especially over large land masses, initiate and strengthen anticyclonic circulation (Siberian High)
- Topographic features induce lee-side anticyclogenesis through formation of stationary (Colorado High)
- Upper-level divergence associated with jet streaks contributes to development and intensification of surface anticyclones
- Conservation of potential vorticity in atmosphere influences formation and evolution of anticyclonic systems
- Seasonal variations in solar radiation and land-sea temperature contrasts affect positioning and strength of semi-permanent anticyclones (subtropical highs)
Maintenance Mechanisms
- Diabatic processes including radiative cooling at surface and in cloud-free regions aloft help maintain temperature structure
- Subsidence inversion acts as "cap" on vertical motion, reinforcing stability
- Positive feedback between clear skies and radiative cooling strengthens anticyclonic circulation
- Large-scale convergence in upper levels compensates for surface divergence, maintaining mass balance
- Interaction with underlying surface (land or ocean) affects heat and moisture fluxes, influencing anticyclone persistence
- Vorticity advection and temperature advection contribute to anticyclone intensification or weakening
- Blocking patterns in upper-level flow can anchor anticyclones in specific locations for extended periods (European heat waves)
Key Terms to Review (18)
Barometric Pressure: Barometric pressure, also known as atmospheric pressure, is the weight of the air above a specific point on Earth. It plays a critical role in meteorological phenomena, influencing weather patterns, climate conditions, and the behavior of various atmospheric layers.
Clear Skies: Clear skies refer to atmospheric conditions characterized by minimal cloud cover, allowing for unobstructed visibility and abundant sunlight. This phenomenon is often associated with high-pressure systems that promote stable air, inhibiting cloud formation and precipitation. The presence of clear skies can also indicate atmospheric stability, where the air mass remains uniform and undisturbed, contributing to specific weather patterns.
Drought: Drought is a prolonged period of abnormally low precipitation, leading to a water shortage that can have significant impacts on agriculture, ecosystems, and water supply. Understanding drought involves examining how different precipitation types and formation processes contribute to dry conditions, as well as the atmospheric patterns like anticyclones that can exacerbate these situations. Drought can also be influenced by larger climate phenomena such as El Niño and La Niña, which affect weather patterns across the globe. Its implications for agricultural practices are critical, as insufficient moisture affects crop growth and food production.
Dynamic anticyclone: A dynamic anticyclone is a large-scale atmospheric circulation system characterized by high pressure at its center, where air descends and spreads outward, leading to stable weather conditions. This phenomenon plays a crucial role in the Earth's climate system, influencing weather patterns, temperature distribution, and precipitation rates across various regions.
Heat wave: A heat wave is an extended period of excessively hot weather, which may be accompanied by high humidity. This phenomenon can lead to significant increases in temperature that exceed average seasonal norms, creating a stressful environment for both humans and ecosystems. Heat waves are often associated with high-pressure systems, particularly anticyclones, which can trap heat and inhibit cloud formation, resulting in prolonged sunny and dry conditions.
High Pressure System: A high pressure system is a weather phenomenon characterized by an area where the atmospheric pressure is higher than that of the surrounding areas. These systems are often associated with clear skies and stable weather conditions as they promote descending air, which inhibits cloud formation and precipitation. The presence of high pressure systems can significantly influence local weather patterns, making them an essential feature in meteorology.
Inversion Layer: An inversion layer is a meteorological phenomenon where the normal temperature gradient in the atmosphere is reversed, causing cooler air to be trapped near the ground under warmer air above. This situation can lead to a variety of effects on weather and air quality, influencing factors such as pollution levels and cloud formation. It plays a significant role in temperature distribution and impacts atmospheric stability, particularly in the context of high-pressure systems.
Isobars: Isobars are lines on a weather map that connect points of equal atmospheric pressure. They help meteorologists visualize and analyze pressure systems, revealing important features such as the structure of anticyclones. Understanding isobars is crucial for interpreting weather patterns and forecasting, as they indicate how pressure changes across an area, affecting wind flow and weather conditions.
Jet streams: Jet streams are narrow bands of strong winds in the upper levels of the atmosphere, typically found at altitudes of 30,000 to 40,000 feet. They play a crucial role in influencing weather patterns and temperature distribution across the globe, as they can steer weather systems and impact surface conditions.
John Dalton: John Dalton was an English chemist and physicist best known for his pioneering work in the field of atomic theory in the early 19th century. His contributions laid foundational principles that not only advanced chemistry but also had significant implications for understanding atmospheric composition, weather phenomena, and various processes in meteorology.
Radar: Radar, which stands for Radio Detection and Ranging, is a technology that uses radio waves to detect and locate objects, measure distances, and determine the speed of moving targets. In meteorology, radar is crucial for observing precipitation patterns, monitoring severe weather events, and providing real-time data that enhances our understanding of atmospheric phenomena.
Rossby waves: Rossby waves are large-scale, meandering patterns in the jet stream that influence weather and climate systems across the globe. They are essential for understanding the dynamics of anticyclones and cyclones, playing a crucial role in the distribution of high and low-pressure systems in the atmosphere. These waves are formed due to the Earth's rotation and variations in temperature, resulting in the characteristic undulating motion that impacts weather patterns.
Stable air: Stable air refers to a condition in the atmosphere where air parcels resist vertical movement and tend to remain in their original position. This stability typically occurs when a layer of warmer air overlays cooler air, creating a temperature inversion that prevents upward motion. This phenomenon impacts weather patterns, cloud formation, and atmospheric circulation, as stable air generally leads to clear skies and minimal precipitation.
Subsidence: Subsidence refers to the downward movement of air in the atmosphere, particularly in high-pressure systems. This phenomenon is closely associated with anticyclones, where the sinking air leads to stable atmospheric conditions, clear skies, and often dry weather. As air descends, it warms adiabatically, further inhibiting cloud formation and promoting fair weather, which are hallmark characteristics of anticyclonic conditions.
Temperature Inversion: Temperature inversion is a meteorological phenomenon where a layer of warmer air traps cooler air near the ground, preventing it from rising. This inversion can significantly affect weather patterns, air quality, and the stability of the atmosphere, as it influences how heat is distributed within different layers of the atmosphere.
Thermal anticyclone: A thermal anticyclone is a high-pressure system formed primarily due to the cooling of air near the surface, often associated with clear skies and calm weather conditions. These systems typically arise in regions where radiative cooling occurs, leading to denser, colder air that sinks and creates a stable atmospheric environment. This process not only influences weather patterns but also plays a significant role in local climate characteristics.
Weather balloon: A weather balloon is a large, inflatable balloon filled with a lighter-than-air gas, typically helium or hydrogen, used to carry instruments aloft to collect atmospheric data. These balloons ascend through the atmosphere, transmitting information on temperature, humidity, pressure, and wind speed, which is vital for understanding various weather patterns and forecasting. Weather balloons play an essential role in observing frontal systems and anticyclonic conditions by providing real-time data on the vertical structure of the atmosphere.
William Ferrel: William Ferrel was an American meteorologist known for his work in atmospheric dynamics, particularly for formulating the Ferrel cell concept, which describes the mid-latitude circulation patterns in the atmosphere. His contributions help explain the interactions between the Earth's rotation and wind patterns, influencing both local weather and global circulation systems.