Mid latitude cyclones
Low pressure weather systems which develop in the middle latitudes of the earth, 40 - 60
degrees north & south of the equator.
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consists of a warm & cold front / warm sector & cold sector
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Low pressure system = air converges into the centre of this system.
Southern H = circulation is clockwise
Northern hemisphere = circulation is anti clockwise
This is due to the Coriolis force
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travel from west to east
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Family of cyclones = all connected, no breakages
Conditions for formation:
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form at the polar front ( warm subtropical westerly winds meet cold polar easterly winds )
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Disturbance in jet stream = cod air pushes into warm air
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Both air masses move in opposite directions = frictional drag ( causing a disturbance )
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Warm air gets pushed up polar front / cold air wedges underneath
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Upper air divergence = removes air that is rising from below
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Pressure gradient forms due to difference in pressure = both winds blow along this
pressure gradient towards polar front
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If a strong enough bend forms in the polar front = low pressure system develops
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Polar front
Stages of development:
Cyclogenesis: the development and strengthening of a cyclone
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Stage one: initial stage
Blows opp. direction to cold polar
easterly winds
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do not mix
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= friction
Warm westerly winds
Polar front
Polar front forms
a wave
Polar front = stationery
Cold polar easterly winds
Stage two: developing / wave stage
Warm rising air over cold = low pressure @ center
Wave develops @ polar front
Stage three: mature stage
Warm sector
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Cold + warm sector develop
Cold sector = larger
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cold air moves and lifts warm air
Stage four: occluded stage
co
ld
Oc
cl
ud
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se
ct
or
fro
n
Cold front
t
Warm front
m
ar
W
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Cold front overtakes warm front
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cold air wedges under warm air
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warm air = isolated from ground
Warm sector narrows
= occlusion
Stages ve: dissipated stage
Warm air has lifted above ground
= cold air left at ground level
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cold weather conditions + clearing clouds & rain
Backing: when wind direction changes ( anti clockwise ) in southern hemisphere
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Veering: wind direction changes ( clockwise ) in northern hemisphere
Associated weather patterns:
Warm front passes over:
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temperature increases
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Pressure decreases ( warm air rises )
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Humidity increases ( warm hair holds more water vapour )
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gentle front = gentle uplifting of warm air = nimbostratus & altostratus clouds form
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Soft, soaking rain
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North easterly wind ahead of front // backing occurs // north westerly wind behind front
Cold front passes over:
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temperature decreases
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Pressure decreases at cold front & increases with arrival of cold sector
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Humidity increases at cold front, decreases at cold sector
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Steep front = string uplifting of warm air = cumulonimbus clouds
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Torrential rainfall / hail
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North west wind // backing (SH) // west south wind // south winds speeds increase to gale
force
Reasons for weather changes at cold front:
1. Temp decreases as cold air behind front has arrived
2. Clockwise rotation of air around low pressure
3. Cloud cov er increases as warm air rises over approaching cold air, cools & condenses
4. Cumulonimbus clouds : cold air forces warm air to rise sharply = condensation occurs
5. Pressure drops then rises: pressure at its lowest just before cold front then rises as cold
air arrives
6. Strong convection and cumulonimbus clouds: heavy rainfall over small area
Warm front does NOT pass over South Africa:
Warm fronts
positioned too far
south to effect SA
Warm front is too short
( compared to cold front )
Occluded front conditions: cold front catches up with warm front
and war sector is closed off.
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warm air is forced up by the cold air of the cold front = condensation and rainfall
1. Cold front occlusion: slightly warmer air rises up the occluded front. It cools, condenses
and forms nimbostratus clouds and rainfall ( cold front conditions)
Cold front occlusion
Warm front occlusion
2. Warm front occlusion: air behind cold front and warm air in warm sector rise up occluded
front as air ahead of the cold front is colder than air behind cold front. = nimbostratus clouds
and steady rainfall ( warm front conditions )
Mid latitude cyclones are more effective in during South African winters:
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in winter the ITCZ is higher north, creating low pressure points further north, therefore mid
latitude cyclones are more likely to pass over South Africa.
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allows for growth of winter crops
softens soil for spring planting
fills dams
Rain four human use
Production of winter
crops = food security
Positive impacts on humans,
environment & economy
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Barriers to
communication ( no
transport )
String winds /
thunderstorms
destroys
infrastructure
Gale force winds =
storms conditions
production of winter crops
benefits the GDP
Negative impacts on humans,
environment & economy
cold snaps & snow kill young animals
Storms can destroy crops
Gale force winds can destroy crops
Stock losses on farms
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floods can damage crops &
impact economy negatively
Impact on tourism:
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heavy rainfall = tourist destinations inaccessible
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Outodoor activities will be affected
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Travel arrangements ( ights etc ) will be affected by negative weather conditions
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Reduced income of tourism due to poor weather conditions
Management strategies:
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avoid building on low lying areas
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Stock up on essentials
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Earl;t warning systems and communication for people to prepare
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Monitoring developments of mid latitude cyclones
Impact of South Indian and south Atlantic high on movement of
the cyclone:
High pressure systems such as (SI & SAH ) can form a ridge behind a cold front, blocking off
its movement. More cold air will converge into low pressure = cold weather, heavy rain and
fl
snow even.