Anticyclone

An anticyclone is a large-scale weather system characterised by high atmospheric pressure at its centre, from which air moves outward in a spiral pattern. It is the opposite of a cyclone, which features low pressure and inward-spiralling winds. Anticyclones are associated with stable weather conditions, clear skies, and calm winds, and they play a vital role in the global circulation of the atmosphere.

Formation and Structure

Anticyclones form when air in the upper atmosphere descends towards the surface, a process known as subsidence. As the air descends, it compresses and warms, reducing its relative humidity and inhibiting cloud formation. This results in clear, dry, and settled weather over the affected region.
The structure of an anticyclone typically includes:

• High-pressure centre: The core region where atmospheric pressure is highest compared to surrounding areas.
• Descending air currents: Air sinks at the centre and spreads outward near the surface.
• Outward and clockwise flow (Northern Hemisphere): Due to the Coriolis effect, winds circulate clockwise around the high-pressure centre in the Northern Hemisphere.
• Outward and anticlockwise flow (Southern Hemisphere): In the Southern Hemisphere, winds circulate anticlockwise.

Characteristics of an Anticyclone

Anticyclones exhibit several distinct meteorological features:
1. High Atmospheric Pressure:

• The central pressure is higher than that of the surrounding air, usually exceeding 1013 millibars.
• The pressure gradient is gentle, leading to light or moderate winds.

2. Clear Skies and Dry Conditions:

• Descending air suppresses cloud formation, resulting in sunny and dry weather.
• In summer, this can lead to prolonged heat waves, while in winter, it may cause cold, frosty nights due to radiational cooling.

3. Temperature Effects:

• Summer: Anticyclones produce warm and dry weather as clear skies allow maximum solar heating during the day.
• Winter: Clear skies lead to rapid cooling at night, often resulting in fog, frost, or temperature inversions.

4. Wind Patterns:

• Winds in an anticyclone are generally light because the pressure gradient is weak.
• They blow outward from the centre, diverging and turning under the influence of the Coriolis force.

5. Stability of Air:

• The descending air produces a stable atmosphere, suppressing vertical air movement.
• This stability prevents the development of convective clouds or thunderstorms.

6. Size and Duration:

• Anticyclones can span several thousand kilometres across.
• They are typically long-lived systems, sometimes persisting for several days or even weeks.

Types of Anticyclones

Anticyclones can occur in different forms depending on their location and origin:
1. Cold Anticyclones (Thermal Anticyclones):

• Found mainly in high-latitude regions such as Siberia or Canada during winter.
• They form due to intense surface cooling, which increases air density and surface pressure.
• These anticyclones are shallow, cold, and often associated with extremely low temperatures.

2. Warm Anticyclones (Dynamic Anticyclones):

• Common in subtropical and mid-latitude regions.
• They develop due to large-scale subsidence of air within high-pressure belts, such as the subtropical high-pressure zones at around 30° latitude.
• Examples include the Azores High, the Bermuda High, and the South Pacific High.
• These systems are deeper and more persistent than cold anticyclones.

Associated Weather Conditions

The weather under an anticyclone depends on the season and the nature of the surface below:

• Summer: Clear skies, light winds, strong sunshine, and high daytime temperatures. However, prolonged periods may cause droughts and heatwaves.
• Winter: Calm, cold conditions with fog, frost, or smog in urban areas due to stagnant air and lack of vertical mixing.
• Coastal Areas: Temperature inversions can lead to sea fogs, while inland areas experience clear and dry weather.

Effects on Environment and Climate

Anticyclones have both beneficial and adverse impacts on the environment and human activities:
Positive Effects:

• Promote clear skies favourable for outdoor activities and agriculture.
• Contribute to pleasant, settled weather for travel and tourism.

Negative Effects:

• Prolonged anticyclones can cause droughts, water shortages, and crop failures.
• In winter, they can lead to poor air quality and health hazards due to trapped pollutants (temperature inversion).
• They may intensify wildfires due to dry and hot conditions in summer.

Anticyclones and Global Circulation

Anticyclones form an integral part of the Earth’s general circulation pattern. They are commonly found in the subtropical high-pressure belts, such as the Horse Latitudes, where descending air from the Hadley Cell creates persistent high-pressure systems. These regions influence global wind patterns and ocean currents, contributing to the development of desert climates in areas such as the Sahara, Arabian, and Australian deserts.