South African Anti-Cyclones

• Anti-cyclones are high-pressure, anti-clockwise rotating cells
• They have a significant influence on South African weather
• South Africa experiences these due to its location beneath the sub-tropical high pressure belt
• These cells contribute to the semi-arid conditions in South Africa

Travelling Disturbances

• The study of anti-cyclones includes associated weather events

General Anti-Cyclone Characteristics

• Large systems, spanning 3000km to 4000km in diameter
• Present throughout the year, though positions may shift slightly
• Air circulates anti-clockwise and outward, descending from these cells
• Descending air promotes stable atmospheric conditions, clear skies, and sunshine

Three Anti-Cyclones Affecting South Africa

South Atlantic Anti-Cyclone (SAAC)

• Located off Namibian coast
• Semi-permanent and semi-stationary
• Changes are seasonal, linked to the seasonal shift of ITCZ
• Brings clear, stable weather
• Air moves anti-clockwise
• Cool, dry air affects western and southern coasts
• Can deflect mid-latitude cyclones in summer

Kalahari Anti-Cyclone

• Located above the central plateau

Summer

• Warmer temperatures above the plateau
• Inversion forms above escarpment
• Moist air flows from the Indian Ocean inland, resulting in rainfall

Winter

• Temperatures drop
• Cool, dense air sinks inland
• Sinking air compresses and heats, causing warmer lower layers
• Inversion forms below the escarpment
• Pressure system prevents warm air from the Indian Ocean from entering

South Indian Anti-Cyclone (SIAC)

• Located off east coast of South Africa
• Semi-permanent and semi-stationary
• Air moves anti-clockwise
• Close to SA, brings moist winds to the coast
• Air blows over warm ocean
• Warmer, moist onshore winds occur and cooler land causes precipitation

Inversion Layers

• Created by descending air that warms as it descends
• Present year-round, higher in summer (approx. 2000m above sea-level) than winter (approx. 1000m above sea-level)

Summer

• Temperatures increase, causing air to rise and reduce pressure
• Kalahari anti-cyclone displaced, lifting the inversion layer approx. 2000m
• This allows warm moist air from the Indian Ocean to feed the thermal low, causing clouds and rain

Winter

• Absence of thermal low increases the power of subsiding air from the Kalahari anti-cyclone
• The Kalahari anti-cyclone becomes dominant, pushing inversion layer down to approx. 1000m
• Effectively puts a lid on moisture trying to reach South Africa's interior
• This generally creates dry atmospheric conditions in winter

Winter Conditions

• Anticyclones positioned northerly, allowing cold fronts to affect South Africa
• Mid-latitude cyclones closer to the southern coast
• Frontal rainfall over south-western Cape
• Interior is cool, dry, and clear due to Kalahari anti-cyclone dominance
• Inversion layer forms below escarpment
• Berg wind conditions develop

Summer Conditions

• Anticyclones position southerly, preventing cold fronts from affecting the country
• Kalahari anticyclone is in upper air, not on the surface
• Warm air rises, forming thermal low, causing thunderstorms over the interior
• Warm, moist coast air moves to the escarpment, bringing clouds and rain inland
• Temperatures and humidity are higher
• Moisture front and squall line thunderstorms develop

Travelling Disturbances

• Pertains to adjacent weather phenomenon

Moisture Front & Line Thunderstorms

• Moisture front cross-section is similar to a mid-latitude cyclone, but moisture fronts differ by bashing head-on
• Develop with thermal/heat low pressure and a coastal low over South Africa
• Low-pressure trough feeds moist air into the interior in summer when low pressure dominates
• Moisture front develops when cool, dry air meets warm, moist air
• Cool air lifts warm air, cooling adiabatically, condensing, and forming cumulonimbus clouds
• Rain falls northeast of the moisture front, with little rain falling to the southwest

Negative impacts

• Summer thunderstorms cause damage
• Torrential downpours can cause flooding
• Hail, thunder, and lightning can damage property and crops

Positive Impacts

• May replenish water in dams, relieving drought
• Heat and pressure from ligtning turns nitrogen into natural fertilizer

Line Thunderstorms

• Develop along moisture front, often when a thermal low-pressure cell and coastal low-pressure cell are located over South Africa
• Pressure trough feeds warm, moist air from the northeast interior, converging with cold, dry air from the southwest
• Cold air lifts warm air, cooling adiabatically, condensing, and forming tall cumulonimbus clouds
• Can occur at night, not caused by heating of the atmosphere directly, but convergence of two air masses
• Replenish dams and relieve drought
• Heat and pressure convert nitrogen in the air into natural fertilizer

Coastal Low-Pressure Systems

• Small low-pressure system occurring on the west coast, moving east
• Not associated with fronts, occurring entirely within the warm sector of a mid-latitude cyclone
• Clockwise circulation around a coastal low creates onshore winds on its western side (bringing rain and fog) and offshore winds on its eastern side (clear conditions)
• Different weather will occur on either side of the low

West Coast

• Onshore winds cold and dry with limited moisture
• Offshore winds warm and dry with no moisture

East Coast

• Onshore winds warm with moisture, causing rainfall
• Offshore winds warm and dry with limited moisture
• Systems move from west to east along the coastline

Berg Winds

• Develop during winter, bringing gusty, hot, and dry winds
• Berg wind conditions occur ahead of the mid-latitude cyclones,.
• Air flows from the Kalahari High-Pressure cell to the coastal low-pressure
• Air subsides from the plateau and down the escarpment (off-shore winds), heating and drying adiabatically
• Can cause uncomfortable, dry, and hot conditions
• Cold conditions can replace the hot quickly
• Encourages veld fires

Reading and Interpreting Satellite Images and Weather Maps

• Large high-pressure systems associated with fair weather appear as large cloudless areas

• Berg wind marked by plume of dust going out to sea

• Line thunderstorms and associated low-pressure trough appear as a band of cloud covering across the country from south-east to north-west

• Coastal low appears as a patch of cloud on the coast

• The anticyclones appear as large systems of isobars, with an H in the centre

• A berg wind presents with offshore winds and higher temperatures ahead of a cold front

• Ridging isobars indicate high temperatures and extending pressures

• Coastal low displays relatively low pressures on the coast

• South easterly winds are parallel in isobar direction