GEOGRAPHY
REVISION
GUIDE
GRADE 12
, 2
Geography Revision Guide Gr 12/2018
Module 1
Climate & Weather
, 3
Geography Revision Guide Gr 12/2018
1. Mid-latitude Cyclones
Origin:
o Polar front (60 N/S)
o Cold polar easterlies meet warm westerlies and move parallel but do not mix.
Alternate names:
o Extra tropical cyclone
o frontal depression
o temperate cyclone
General characteristics:
o Forms between 30-60 N/S
o Moves eastward
o Has two fronts (warm and cold)
o Large diameter
o Lasts between 4-14 days
o Steered by westerlies
o Affects SA in winter
Conditions for formation:
o Frictional drag caused by difference in temperature and speed of the two air masses.
o Air masses must move opposite to each other and parallel.
o Warm subtropical air must meet cold polar air at the polar front.
Stages in formation:
o Initial:
• Cold polar air and warm tropical air move parallel to each other but in opposite directions
at the polar front.
o Wave formation:
• Frictional drag takes place.
• Warm air becomes uplifted.
• Fronts begin to form as air converges to the centre low pressure.
o Mature:
• Wave deepens.
• Cold and warm sectors and fronts fully developed.
• Warm air moves up steep pressure gradient of cold front to form towering cumulonimbus
clouds. (heavy rain over a small area)
• Warm air moves up gentle pressure gradient of warm front to form a broad band of
stratus clouds. (light rain over a large area)
o Occlusion:
• Cold front catches up to the warm front at the apex. (apex is the shortest distance between
the 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)
, 4
Geography Revision Guide Gr 12/2018
Cross section through a mid-latitude cyclone:
Weather associated with Mid-latitude cyclones:
o Cold front weather:
• Low air temperature
• Thunderstorms and hail
• Rise in air pressure
• Low humidity
o Warm front weather:
• Rising air temperature
• Soft rains
• Drop in air pressure
• High humidity
Cyclone families:
o Mid-latitude cyclones form in groups.
o Up to 5 can pass through a certain area in quick succession.
Impact of mid-latitude cyclones:
o Rain:
✓ Water for vineyards and deciduous fruits
Flooding
o Snow:
✓ Tourist attraction
Crop damage
o Storms:
A threat to fishermen at sea
Precautionary and management strategies:
o Build high
o Efficient drainage systems
o Enclose livestock
o Sufficient grain
o Update weather systems
o Secure boats
REVISION
GUIDE
GRADE 12
, 2
Geography Revision Guide Gr 12/2018
Module 1
Climate & Weather
, 3
Geography Revision Guide Gr 12/2018
1. Mid-latitude Cyclones
Origin:
o Polar front (60 N/S)
o Cold polar easterlies meet warm westerlies and move parallel but do not mix.
Alternate names:
o Extra tropical cyclone
o frontal depression
o temperate cyclone
General characteristics:
o Forms between 30-60 N/S
o Moves eastward
o Has two fronts (warm and cold)
o Large diameter
o Lasts between 4-14 days
o Steered by westerlies
o Affects SA in winter
Conditions for formation:
o Frictional drag caused by difference in temperature and speed of the two air masses.
o Air masses must move opposite to each other and parallel.
o Warm subtropical air must meet cold polar air at the polar front.
Stages in formation:
o Initial:
• Cold polar air and warm tropical air move parallel to each other but in opposite directions
at the polar front.
o Wave formation:
• Frictional drag takes place.
• Warm air becomes uplifted.
• Fronts begin to form as air converges to the centre low pressure.
o Mature:
• Wave deepens.
• Cold and warm sectors and fronts fully developed.
• Warm air moves up steep pressure gradient of cold front to form towering cumulonimbus
clouds. (heavy rain over a small area)
• Warm air moves up gentle pressure gradient of warm front to form a broad band of
stratus clouds. (light rain over a large area)
o Occlusion:
• Cold front catches up to the warm front at the apex. (apex is the shortest distance between
the 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)
, 4
Geography Revision Guide Gr 12/2018
Cross section through a mid-latitude cyclone:
Weather associated with Mid-latitude cyclones:
o Cold front weather:
• Low air temperature
• Thunderstorms and hail
• Rise in air pressure
• Low humidity
o Warm front weather:
• Rising air temperature
• Soft rains
• Drop in air pressure
• High humidity
Cyclone families:
o Mid-latitude cyclones form in groups.
o Up to 5 can pass through a certain area in quick succession.
Impact of mid-latitude cyclones:
o Rain:
✓ Water for vineyards and deciduous fruits
Flooding
o Snow:
✓ Tourist attraction
Crop damage
o Storms:
A threat to fishermen at sea
Precautionary and management strategies:
o Build high
o Efficient drainage systems
o Enclose livestock
o Sufficient grain
o Update weather systems
o Secure boats
