Geography Revision Guide Grade 12
Geography Revision Guide - Grade 12
Module 1: Climate & Weather
1. Mid-latitude Cyclones
Origin:
Polar front (60° N/S)
Cold polar easterlies meet warm westerlies; move parallel but do not mix.
Alternate Names:
Extra tropical cyclone
Frontal depression
Temperate cyclone
General Characteristics:
Forms between 30°-60° N/S
Moves eastward
Contains two fronts (warm and cold)
Large diameter
Duration: 4-14 days
Steered by westerlies
Affects South Africa in winter
Conditions for Formation:
Frictional drag caused by differences in temperature and speed of two air masses.
Air masses must move opposite and parallel to each other.
Warm subtropical air must meet cold polar air at the polar front.
Stages in Formation:
Initial:
Cold polar air and warm tropical air move parallel to each other at the polar front.
Wave Formation:
Frictional drag occurs.
Warm air is uplifted.
Fronts begin to form as air converges to the center of low pressure.
Mature Stage:
Wave deepens.
Cold and warm sectors/ fronts fully developed.
Warm air moves up steep pressure gradient of cold front, forming towering cumulonimbus clouds (heavy rain over a small area).
Warm air rising slowly along the gentle pressure gradient of the warm front forms a broad band of stratus clouds (light rain over a larger area).
Occlusion:
Cold front catches up to warm front at the apex (shortest distance between fronts).
Cold Front Occlusion: Warm air moves up the cold front (cold front on the ground).
Warm Front Occlusion: Cold air moves up the warm front (warm front on the ground).
Cross Section of a Mid-latitude Cyclone
Weather associated with Mid-latitude Cyclones:
Cold Front Weather:
Low air temperature
Thunderstorms and hail
Rise in air pressure
Low humidity
Warm Front Weather:
Rising air temperature
Soft rains
Drop in air pressure
High humidity
Cyclone Families:
Mid-latitude cyclones form in groups; up to 5 may pass through an area in quick succession.
Impact of Mid-latitude Cyclones:
Rain:
Water for vineyards and deciduous fruits
Can cause flooding
Snow:
Tourist attraction
Can cause crop damage
Storms:
A threat to fishermen at sea
Precautionary and Management Strategies:
Build infrastructure high
Implement efficient drainage systems
Enclose livestock
Ensure sufficient grain storage
Update weather systems in real-time
Secure boats prior to storms.
2. Tropical Cyclones
Origin:
Warm oceans
Occur between 5°-25° N/S where the Coriolis force is strong enough to form a vortex.
Alternate Names:
Hurricanes
Willy Willies
Typhoons
Cyclones
General Characteristics:
Characterized by circular isobars enclosing intense low pressure
Moves westwards
Steered by tropical easterlies
Diameter: 300-500 km
Follows erratic paths; unpredictable.
Conditions for Formation:
Sea temperature above 27°C (for high evaporation)
Warm air rising; unstable atmospheric conditions (for convection)
Latitude between 5°-25° N/S (Coriolis force is strong)
Calm conditions (lack of wind allows vortex to form)
Upper air divergence (to maintain low pressure center)
Stages in Formation:
Initial:
Center pressure above 1000 mb; isobars far apart; gale-force winds; presence of cirrus and cumulus clouds.
Immature Stage:
Pressure drops below 1000 mb; eye forms; wind reaches hurricane strength; diameter approximately 100 km; cumulonimbus clouds around the eye.
Mature Stage:
Center pressure well below 1000 mb; isobars close together; diameter between 300-500 km; dangerous semicircle effects of intense winds combine with cyclone movement.
Dissipating Stage:
Center pressure rises above 1000 mb; occurs when cyclone encounters land or moves over cold oceans.
Cross Section of a Tropical Cyclone
Weather Associated with Tropical Cyclones:
As Storm Approaches:
Cumulus clouds
Rainy conditions
Windy weather
Presence of cumulonimbus clouds, particularly in the dangerous semicircle with torrential rain and hurricane winds.
Eye:
Calm
Cool
Clear conditions.
Impact of Tropical Cyclones:
Flooding
Storm surges
Crop losses
Disruption of transport
Silt accumulation in dams
Disruption of ecosystems.
Precautionary and Management Strategies:
Stock supplies of water and canned food
Maintain a first aid kit
Keep livestock on higher ground
Use sandbags along river banks
Have evacuation plans prepared
Establish early warning systems.
3. Factors Influencing the Weather of South Africa
Influence of the Plateau:
During Summer:
Kalahari High is weakly developed
Low subsidence
Inversion layer develops above the escarpment
Moist air ridges from the Indian Ocean over the plateau.
Leads to cloud formation and widespread rain.
During Winter:
Kalahari High is well developed
High subsidence
Inversion layer forms below the escarpment.
Moist air ridges from the Indian Ocean prevented from reaching the plateau.
Results in clear weather.
Influence of the Oceans:
Warm Mozambique Current:
East coast
Raises temperatures
Contributes to high rainfall.
Cold Benguela Current:
Decreases temperatures
Associated with dry weather.
4. High Pressure Systems Over South Africa
South Atlantic High:
Located on the west coast
Brings dry, clear, stable weather conditions
Winds diverging are dry over the cold ocean; fog and mist may develop.
Ridging of the South Atlantic High:
Occurs in summer
Diverts moist air from the Indian Ocean onto the land
Results in rain on the southeast coast and eastern plateau.
South Indian High:
Located on the east coast
Brings rainfall in summer.
When it lies in the path of a mid-latitude cyclone, it is referred to as a blocking high.
Kalahari High:
Located on the interior
Dominates land in winter
Interacts with the South Indian High to influence South Africa's climate differently in summer and winter.
Resultant Weather of High Pressure Cells
Moisture Front: Zone between two air masses with differing moisture content.
Line Thunderstorms:
Form over the interior when a cold air mass from the Atlantic Ocean meets a warm air mass from the Indian Ocean without mixing.
Cold dry air sinks below warm moist air, resulting in cumulonimbus cloud formation and flash floods on the right of the front.
Impact of Line Thunderstorms:
Torrential rain causes damage
Leads to soil erosion.
Gale-force winds can destroy infrastructure
Fills up dams.
5. Low Pressure Systems Over South Africa
Thermal Low:
Occurs in summer
Causes convective thunderstorms over the interior;
Typically more than one thermal low over the interior.
Cut-off Low:
Develops when ridging of the South Atlantic High and South Indian High prevents eastward movement of a mid-latitude cyclone.
Cold front is separated from the cyclone, extending over land, which draws in moist air resulting in prolonged rainfall.
Resultant Weather of Low Pressure Cells
Berg Winds:
Hot gusty winds blowing from the interior to the coast.
Air moves from Kalahari High to coastal low.
Diverging air warms at DALR (dry adiabatic lapse rate).
Effects of Berg Winds:
Can lead to forest fires.
Result in livestock death.
Cause lethargy in workers.
May lead to respiratory problems.
6. Synoptic Weather Maps
Isobaric Patterns:
Ridge: High-pressure area.
Trough: Low-pressure area.
Saddle: Zone between two high pressures or low pressures.
Weather Stations record:
Wind speed and direction
Air temperature and dew point
Precipitation and cloud cover.
7. Valley Climate
Slope Aspect:
The direction a slope faces; slopes facing the equator receive direct sunlight and are warmer.
Shadow zones receive no sunlight due to relief blocking.
Influence of Aspect:
Economic:
North-facing slopes favorable for fruit farming.
Pole-facing slopes ideal for forestry.
Settlement:
North-facing slopes have higher land value.
Middle slopes are often used for development.
Vegetation:
South-facing slopes are cool and moist, supporting dense vegetation.
North-facing slopes are warm and dry, leading to sparse vegetation.
Katabatic Winds
Winds descending down a valley during the night,
Cold air sinks to the valley floor while warm air rises,
Results in temperature inversions within valleys, and can lead to the formation of radiation fog when valley air cools below the dew point temperature.
Influence on Farming:
Citrus farms on valley floors mature fruit better due to warm air.
Deciduous fruit is ideal on middle slopes.
Frost-resistant crops thrive on the valley floor.
Effect on Settlement:
Higher land value on middle slopes within thermal belts.
Low value on valley floors due to trapped pollutants.
Impact on Transport:
Fog can reduce visibility leading to vehicular accidents.
Anabatic Winds
Upslope winds occurring during the day when slopes are heated.
Significance includes dispersing smoke and improving air quality.
8. Urban Climate
Differences Between Urban and Rural Climates:
Cloud Cover:
More prevalent in urban areas where pollutants act as condensation nuclei.
Precipitation:
Generally higher in urban areas due to increased cloud formation.
Humidity:
Higher in rural areas due to more vegetation, which encourages evaporation.
Wind Speed:
Tends to be higher in rural areas; urban areas have airflow obstructed by tall buildings.
Temperature:
Urban areas generally heat up more due to population density (e.g., more geysers, stoves, heaters).
Construction materials like concrete retain heat; glass reflects it.
Geometric shapes of buildings affect heat absorption and reflection.