GEOGRAPHY · GRADE 12

This is the heaviest single topic in Paper 1. Almost every question hangs on one skill: reading a synoptic weather chart or a satellite image and saying what the atmosphere is doing and why. Learn the systems as living things that move, and the map reads itself.

Learning outcomes · you must be able to

• Describe the four stages in the life of a mid-latitude cyclone and draw a labelled cross-section through its cold and warm fronts.
• State the conditions a tropical cyclone needs to form, and describe the weather across its eye, eye wall and rain bands.
• Name the three high-pressure cells that control South African weather and explain their seasonal shift.
• Explain ridging of the South Atlantic High, berg winds, the coastal low and line thunderstorms, and identify each on a synoptic chart.
• Explain slope aspect, anabatic and katabatic winds, inversions, the thermal belt, frost pockets and radiation fog in a valley.
• Explain the urban heat island and pollution dome, and read a station model off a synoptic map.

Core content · mid-latitude cyclones

• Where it forms: along the polar front, where a cold polar air mass and a warm subtropical air mass converge. The difference in temperature and density between the two masses supplies the energy.
• Four stages of development:
  • Family / frontal-wave stage: a wave (kink) develops on the polar front, with a cold front trailing a warm front.
  • Mature stage: the wave deepens into a clear low with a warm sector wedged between the two fronts; pressure is lowest and the system is most active.
  • Occlusion stage: the faster cold front catches up to the warm front and lifts the warm sector off the ground, forming an occluded front.
  • Dissipation stage: the warm air is fully lifted, the temperature contrast is gone, and the system weakens and dies.

Core content · tropical cyclones

• Eye: the calm centre of clear skies, light winds, highest temperature and lowest pressure, where air is gently descending.
• Eye wall: the ring of towering cumulonimbus around the eye, where the winds are strongest and the rain heaviest. This is the most destructive part.
• Spiral rain bands: curved arms of thunderstorms spiralling in toward the eye wall.
• Main impacts: storm surge (a wall of sea water pushed ashore), flooding, mudslides and violent wind damage.
• Management: early-warning systems, evacuation of low-lying coasts, storm-resistant building and disaster-response planning.

Core content · subtropical anticyclones & South African weather

• Seasonal shift: in summer the cells move about 6° south, weakening their grip on the country and allowing rain-bearing systems inland; in winter they move about 6° north, so their dry, stable air settles over more of the country.
• Ridging of the South Atlantic High: behind a passing cold front, the South Atlantic High extends a "finger" (a ridge) of high pressure south of the country and along the south coast. This pushes cool, moist, stable south-easterly air onshore, giving cloud, drizzle and cool conditions along the south and east coast.
• Coastal low: a shallow low-pressure cell that moves around the coast (west, then south, then east) ahead of an approaching cold front, and usually brings berg-wind conditions with it.
• Berg winds: hot, dry, offshore winds that blow from the interior toward the coast. Air descends from the high plateau, warming and drying as it sinks, so the coast becomes unusually hot and dry, with a high fire risk, ahead of the cold front. A cool southerly change follows once the front passes.
• Line thunderstorms / moisture front: in summer a north-south line of thunderstorms marks the moisture front, the boundary between dry continental air to the west and moist maritime air to the east, and it moves eastward across the interior.

Core content · valley climates

• Anabatic wind: a gentle upslope wind by day. The sun heats the valley sides, the warmed air becomes less dense and rises up the slope.
• Katabatic wind: a downslope wind by night. The slopes cool, the air above them becomes cold and dense, and it drains down into the valley floor.
• Temperature inversion: when the katabatic cold air pools on the floor, warmer air comes to lie above it, so temperature increases with height instead of decreasing. The warm air acts as a lid over the valley.
• Frost pocket: the coldest hollow on the valley floor where the drained cold air settles, so frost forms there first on still, clear nights.
• Radiation fog: fog that forms on the valley floor on still, clear nights when the cold, moist pooled air cools below its dew point.
• Thermal belt: a band of warmer air on the mid-slope, sitting above the cold frost pocket and below the top of the inversion. It is the best place to site orchards, crops and houses because it escapes both the night frost and the coldest floor air.

Core content · urban climates

• Why urban and rural climates differ: cities are built of tar and concrete that absorb and store heat, have little vegetation to cool the air, release waste heat from vehicles, factories and buildings, and trap pollution that holds warmth in.
• Urban heat island: a city centre is measurably warmer than the surrounding countryside, especially at night. Causes: heat-storing surfaces, lack of vegetation, waste heat and pollution. Reduce it with green roofs, parks, street trees and light, reflective surfaces.
• Pollution dome: a dome of polluted, dusty air that builds over a city when the air is calm and stable, often under an inversion. It is dispersed only when wind blows it away (then it stretches into a pollution plume). Reduce it with cleaner fuels, emission controls and better public transport.

Worked interpretation · reading the systems

Memory aids

• Cold vs warm front: "Cold is bold" (steep, tall storm cloud, brief and violent); "warm is calm" (gentle slope, flat layered cloud, long drizzle).
• Tropical cyclone formation: "WARM" - Warm ocean (26.5°C), Away from the equator, Rising moist air, Minimal wind shear.
• Anabatic vs katabatic: "kata" falls down (like a catastrophe) and happens at night; "ana" climbs up and happens by day.
• The three highs: "AIK" - South Atlantic High, South Indian High, Kalahari High.