Mid-Latitude Cyclones
Low-pressure cell which develops in the mid-latitudes and
travels from west to east
Characteristics :
- Cells of low pressure
- Bring cold, windy wet weather (caused by the pressure and wind patterns of
mid-latitude cyclones)
- Winds converging in MLC in southern hemisphere (coming together)
- Pressure is low at the centre and increases towards the outside.
- Wind blows into a low pressure cell in a clockwise direction.
- The winds do not blow straight along the pressure gradient - deflected by
coriolis forces.
- Between 40 and 60 degrees north and south of equator
- Where cold polar air meets warm subtropical air mass - in polar front
- cover a large area of between 1500km to 3000km
Effect of air masses on MLC
1. The cold polar air mass meets the warm subtropical air mass in a zone called
The Polar front
2. the warm subtropical air rises and the cold polar air wedges in underneath
3. the warm air cools and condensation of water vapour acres
4. this produces the clouds and rain
Where mid-latitude cyclones form
- Along the region known as the polar front
- 35 degrees - 70 degrees north and south of the Equator
- The MLC move from West to East in both the North and South hemispheres
- occur mainly over the oceans and coastal areas
- do not spread across large areas of land
Mid-Latitude cyclones in South Africa
- Only cross the Southern cape
- The warm fronts are bent southwards away from South Africa
- Cold fronts only reach South Africa in winter
- the pressure belts and wind systems move North in winter
- this is why the south western Cape receives winter rainfall
- the rain is brought by cold fronts passing from West to East
, Conditions necessary to form MLC
- A pressure difference exists between the warm air and cold air (A pressure
gradient )
- Westerly winds and polar easterlies blow along these pressure gradient
towards the polar front
- a band forms in the polar front
- this is due to a disturbance in the winds high in the atmosphere in the jet
stream
- this can also be caused by a mountain range or the shape of the coastline
- this change in condition of the polar front causes the isobars to form more of
a circular pattern
- a cell of low pressure begins to form
- the winds then deflect and blow into the low pressure cell
Stages of development + weather conditions
INITIAL STAGE MATURE STAGE OCCLUDED STAGE DEGENERATI
ON STAGE
Bend forms in polar Bend in polar front The warm sector air rises and Ends up as a
front deepens the cold air wedges little gust of
underneath it cold wind at
Cell of low pressure Pressure gradient ground level
begins to form becomes steeper Causes warm sector to narrow
and cold front overtakes the All the warm air
Wind starts to deflect Air blows in clockwise warm has lifted above
and blow into low direction ground
pressure cell Almost all warm air has risen
Polar front now forms above ground level Low pressure
Stronger wind and more cold front and warm cell disappears
clouds front Weather conditions - nimbus and the isobars
clouds and rain even out
Warm moist air is Fronts divide area into
starting to rise and cool cold sector and warm Weather
down sector conditions -
cold with clouds
and rain
clearing.
, INITIAL STAGE :
MATURE STAGE :
OCCLUDED STAGE :
Low-pressure cell which develops in the mid-latitudes and
travels from west to east
Characteristics :
- Cells of low pressure
- Bring cold, windy wet weather (caused by the pressure and wind patterns of
mid-latitude cyclones)
- Winds converging in MLC in southern hemisphere (coming together)
- Pressure is low at the centre and increases towards the outside.
- Wind blows into a low pressure cell in a clockwise direction.
- The winds do not blow straight along the pressure gradient - deflected by
coriolis forces.
- Between 40 and 60 degrees north and south of equator
- Where cold polar air meets warm subtropical air mass - in polar front
- cover a large area of between 1500km to 3000km
Effect of air masses on MLC
1. The cold polar air mass meets the warm subtropical air mass in a zone called
The Polar front
2. the warm subtropical air rises and the cold polar air wedges in underneath
3. the warm air cools and condensation of water vapour acres
4. this produces the clouds and rain
Where mid-latitude cyclones form
- Along the region known as the polar front
- 35 degrees - 70 degrees north and south of the Equator
- The MLC move from West to East in both the North and South hemispheres
- occur mainly over the oceans and coastal areas
- do not spread across large areas of land
Mid-Latitude cyclones in South Africa
- Only cross the Southern cape
- The warm fronts are bent southwards away from South Africa
- Cold fronts only reach South Africa in winter
- the pressure belts and wind systems move North in winter
- this is why the south western Cape receives winter rainfall
- the rain is brought by cold fronts passing from West to East
, Conditions necessary to form MLC
- A pressure difference exists between the warm air and cold air (A pressure
gradient )
- Westerly winds and polar easterlies blow along these pressure gradient
towards the polar front
- a band forms in the polar front
- this is due to a disturbance in the winds high in the atmosphere in the jet
stream
- this can also be caused by a mountain range or the shape of the coastline
- this change in condition of the polar front causes the isobars to form more of
a circular pattern
- a cell of low pressure begins to form
- the winds then deflect and blow into the low pressure cell
Stages of development + weather conditions
INITIAL STAGE MATURE STAGE OCCLUDED STAGE DEGENERATI
ON STAGE
Bend forms in polar Bend in polar front The warm sector air rises and Ends up as a
front deepens the cold air wedges little gust of
underneath it cold wind at
Cell of low pressure Pressure gradient ground level
begins to form becomes steeper Causes warm sector to narrow
and cold front overtakes the All the warm air
Wind starts to deflect Air blows in clockwise warm has lifted above
and blow into low direction ground
pressure cell Almost all warm air has risen
Polar front now forms above ground level Low pressure
Stronger wind and more cold front and warm cell disappears
clouds front Weather conditions - nimbus and the isobars
clouds and rain even out
Warm moist air is Fronts divide area into
starting to rise and cool cold sector and warm Weather
down sector conditions -
cold with clouds
and rain
clearing.
, INITIAL STAGE :
MATURE STAGE :
OCCLUDED STAGE :
