Geography Textbook Notes Matric
Climate and weather:
Chapter 1: Mid-latitude cyclones
Unit 1: characteristics, area of formation and conditions needed for formation
1. General characteristics of mid-latitude cyclones
A low pressure system
Develops in the westerlies
Travels west to east
Isobars are closed around a low-pressure center
Wind moves from high to low pressures and as the wind moves toward the low pressure, the
Coriolis force deflects it
o Left in the southern hemisphere
o Right in the northern hemisphere
Circulation
o Clockwise- Southern hemisphere
o Anticlockwise- Northern hemisphere
Systems where cold polar and warm subtropical air masses meet
o Boundary between the air masses is called a front
o Warm and cold air don’t mix
2 Fronts
o Cold font
o Warm front
Pressure at center- approx. 996 hPa
Diameter- 1500-3000 km
Occur in families- a few of them form one after the other and travel closely together
Travel at 50-60km/h
Last for 4-14 days
2. Areas where mid-latitude cyclones form
40°-60° North and South
Where moist air from tropics meets cold drier air from polar regions
2 different air masses meet along the polar front
In the path of the westerly winds in the global circulation of the atmosphere
In winter
o Mid-latitude cyclones move north as pressure belts and wind systems north following
the migration of the sun
o Cold fronts pas over the southern cape and may extend into the interior
In summer
o Pressure belts and wind systems move south and so do the mid-latitude cyclones
o The cold fronts pass to the south of South Africa
Cold fronts- associated with cold weather and rain
Seasonal movement of the mid-latitude cyclones explains why:
o The south-western part of South Africa has wet winters and dry summers
o There are sometimes very cold snaps in winter in the interior of the country
,3. Conditions necessary for the formation of mid-latitude cyclones:
Mid-latitude cyclones need the following conditions to form:
o A mass of warm moist air meeting a mass of colder drier air
o Upper air divergence- this is required to remove air that is ascending from below and to
make continued ascent possible
o Something to trigger the development of a low-pressure center
Unit 2: Stages of development and related weather conditions
Go through four stages from their initial formation to their degeneration
The first two stages make up the process of Cyclogenesis
o Cyclogenesis- the development and strengthening of a cyclone
Stages three and four- cyclone weakens and disappears
Stage 1: The initial stage
Warm topical air meets cold polar air along a polar front
Big difference in temperature across the front
The winds on either side of the front blow in opposite directions
The front itself does not move and is known as a stationary front
Stage 2: The developing stage
A kink/wave develops in the polar front
Wave develops because of disturbances in the jet streams (fast moving
upper winds) that affect the movement of air at the surface or because
a mountain range disturbs the motion of the air flowing over it
Small mass of warm air extends into the cold air and rises over it
Rising air causes the pressure to decrease, establishing a center of low
pressure
Related weather: clouds and some rain- because of the rising air cooling
and condensing
Stage 3: The mature stage
Approx. 24hrs after its initial formation
The wave increases a warm air moves further into the mass of cold air
– warm and cold sector are established
Two fronts develop, marking boundaries between cold and warm air
masses
Front takes the name of the air mass that moves behind it
o Warm front is ahead of the warm sector
o Cold from is leading the cold air behind it
Moves from west to east
Air rises along the warm front- warm air of the warm sector rises above colder air ahead of the
warm front
Air rises along the cold front – heavier colder air behind the cold front wedges in under the
warm air ahead of it
, Specific weather conditions
o Rising air along the fronts leads to condensation, cloud formation and rain
o Rising air causes the pressure to drop further in the centre of the mid-latitude cyclone-
pressure is lower than it was in the initial stage
o Clockwise airflow in the southern hemisphere
o Anticlockwise in the northern hemisphere
Stage 4: The occluded stage
Cold air behind the cold front travels faster than the air in the warm
sector- the cold front catches up with the warm front along most of its
length
Warm air is forced to rise leaving a small sector of the warm front left
Cold and warm fronts join
Widespread rain occurs as warm air rises along the occluded front, cools
and condenses
Stage 5: The dissipated stage
The entire warm sector is above the ground and there is not more warm air at the surface
No cold or warm fronts are present- there are no longer two distinctly different masses of
air- no boundaries marked by the fronts between the air masses
Isobars no longer have the pattern of a mid-latitude cyclone and resume a regular pattern-
mid-latitude cyclone has dissipated
Light gusts of cold air on the ground
Cross section of a mature mid latitude cyclone
, Unit 3: Weather patterns associated with cold, warm and occluded fronts
Weather patterns associated with warm fronts
Relatively gentle angle- therefore covers a wide area
Warm moist air from the warm sector rises over the cold air ahead of the warm front
causing clouds to form- high level cirrus, altostratus and nimbostratus
Soft rain falls because of relatively gentle uplift along the front
o Rain covers a large area- ahead of the front
Changing cloud types will indicate to an observer that the warm front is approaching
o Will first see cirrus, then altostratus then nimbostratus
As warm front passes over an observer
o Temperature increases- warm sector arrives behind the front replacing the cold air
ahead of the front
o Pressure decreases- warmer, rising air
o Humidity increases- warm air behind the front holds more water vapour than the
cold air ahead of it
o Wind direction backs as the front arrives- changes direction
Backs- a change in wind direction in an anticlockwise direction
Weather patterns associated with cold fronts:
Cold front is steeper so uplift of air is rapid
Tall cumulonimbus clouds form- associated with heavy rain and thunderstorms ahead and
behind the front
Area over which rain falls is usually smaller than the rain associated with the warm front
Most rain falls in the area behind the front
As the cold front passes over an observer
o Temperature decreases as cold air behind the front arrives
o Pressure increases- cold air
o Humidity decreases- cold air holds less water vapour
o The wind backs
2. Weather associated with occluded fronts
Two different types of occlusion
o Cold front occlusion- cold air behind the front is colder than the cold air ahead of the
warm front
o Warm front occlusion- cold air ahead of the warm front is colder than the cold air
behind the cold front
Weather associated with cold front occlusion
o Cold front has caught up with warm front- air in warm sector to rise
o Cold front touches the ground while warm front is lifted off the surface
o Colder air behind cold front undercuts the cold air ahead of the warm front forcing it
to rise
o Cold wet conditions similar to cold front
Weather associated with a warm front occlusion
o Cold front has caught up with the warm front causing air in the warm sector to rise
above ground
o Warm front touches the ground while cold front is lifted off the surface
Climate and weather:
Chapter 1: Mid-latitude cyclones
Unit 1: characteristics, area of formation and conditions needed for formation
1. General characteristics of mid-latitude cyclones
A low pressure system
Develops in the westerlies
Travels west to east
Isobars are closed around a low-pressure center
Wind moves from high to low pressures and as the wind moves toward the low pressure, the
Coriolis force deflects it
o Left in the southern hemisphere
o Right in the northern hemisphere
Circulation
o Clockwise- Southern hemisphere
o Anticlockwise- Northern hemisphere
Systems where cold polar and warm subtropical air masses meet
o Boundary between the air masses is called a front
o Warm and cold air don’t mix
2 Fronts
o Cold font
o Warm front
Pressure at center- approx. 996 hPa
Diameter- 1500-3000 km
Occur in families- a few of them form one after the other and travel closely together
Travel at 50-60km/h
Last for 4-14 days
2. Areas where mid-latitude cyclones form
40°-60° North and South
Where moist air from tropics meets cold drier air from polar regions
2 different air masses meet along the polar front
In the path of the westerly winds in the global circulation of the atmosphere
In winter
o Mid-latitude cyclones move north as pressure belts and wind systems north following
the migration of the sun
o Cold fronts pas over the southern cape and may extend into the interior
In summer
o Pressure belts and wind systems move south and so do the mid-latitude cyclones
o The cold fronts pass to the south of South Africa
Cold fronts- associated with cold weather and rain
Seasonal movement of the mid-latitude cyclones explains why:
o The south-western part of South Africa has wet winters and dry summers
o There are sometimes very cold snaps in winter in the interior of the country
,3. Conditions necessary for the formation of mid-latitude cyclones:
Mid-latitude cyclones need the following conditions to form:
o A mass of warm moist air meeting a mass of colder drier air
o Upper air divergence- this is required to remove air that is ascending from below and to
make continued ascent possible
o Something to trigger the development of a low-pressure center
Unit 2: Stages of development and related weather conditions
Go through four stages from their initial formation to their degeneration
The first two stages make up the process of Cyclogenesis
o Cyclogenesis- the development and strengthening of a cyclone
Stages three and four- cyclone weakens and disappears
Stage 1: The initial stage
Warm topical air meets cold polar air along a polar front
Big difference in temperature across the front
The winds on either side of the front blow in opposite directions
The front itself does not move and is known as a stationary front
Stage 2: The developing stage
A kink/wave develops in the polar front
Wave develops because of disturbances in the jet streams (fast moving
upper winds) that affect the movement of air at the surface or because
a mountain range disturbs the motion of the air flowing over it
Small mass of warm air extends into the cold air and rises over it
Rising air causes the pressure to decrease, establishing a center of low
pressure
Related weather: clouds and some rain- because of the rising air cooling
and condensing
Stage 3: The mature stage
Approx. 24hrs after its initial formation
The wave increases a warm air moves further into the mass of cold air
– warm and cold sector are established
Two fronts develop, marking boundaries between cold and warm air
masses
Front takes the name of the air mass that moves behind it
o Warm front is ahead of the warm sector
o Cold from is leading the cold air behind it
Moves from west to east
Air rises along the warm front- warm air of the warm sector rises above colder air ahead of the
warm front
Air rises along the cold front – heavier colder air behind the cold front wedges in under the
warm air ahead of it
, Specific weather conditions
o Rising air along the fronts leads to condensation, cloud formation and rain
o Rising air causes the pressure to drop further in the centre of the mid-latitude cyclone-
pressure is lower than it was in the initial stage
o Clockwise airflow in the southern hemisphere
o Anticlockwise in the northern hemisphere
Stage 4: The occluded stage
Cold air behind the cold front travels faster than the air in the warm
sector- the cold front catches up with the warm front along most of its
length
Warm air is forced to rise leaving a small sector of the warm front left
Cold and warm fronts join
Widespread rain occurs as warm air rises along the occluded front, cools
and condenses
Stage 5: The dissipated stage
The entire warm sector is above the ground and there is not more warm air at the surface
No cold or warm fronts are present- there are no longer two distinctly different masses of
air- no boundaries marked by the fronts between the air masses
Isobars no longer have the pattern of a mid-latitude cyclone and resume a regular pattern-
mid-latitude cyclone has dissipated
Light gusts of cold air on the ground
Cross section of a mature mid latitude cyclone
, Unit 3: Weather patterns associated with cold, warm and occluded fronts
Weather patterns associated with warm fronts
Relatively gentle angle- therefore covers a wide area
Warm moist air from the warm sector rises over the cold air ahead of the warm front
causing clouds to form- high level cirrus, altostratus and nimbostratus
Soft rain falls because of relatively gentle uplift along the front
o Rain covers a large area- ahead of the front
Changing cloud types will indicate to an observer that the warm front is approaching
o Will first see cirrus, then altostratus then nimbostratus
As warm front passes over an observer
o Temperature increases- warm sector arrives behind the front replacing the cold air
ahead of the front
o Pressure decreases- warmer, rising air
o Humidity increases- warm air behind the front holds more water vapour than the
cold air ahead of it
o Wind direction backs as the front arrives- changes direction
Backs- a change in wind direction in an anticlockwise direction
Weather patterns associated with cold fronts:
Cold front is steeper so uplift of air is rapid
Tall cumulonimbus clouds form- associated with heavy rain and thunderstorms ahead and
behind the front
Area over which rain falls is usually smaller than the rain associated with the warm front
Most rain falls in the area behind the front
As the cold front passes over an observer
o Temperature decreases as cold air behind the front arrives
o Pressure increases- cold air
o Humidity decreases- cold air holds less water vapour
o The wind backs
2. Weather associated with occluded fronts
Two different types of occlusion
o Cold front occlusion- cold air behind the front is colder than the cold air ahead of the
warm front
o Warm front occlusion- cold air ahead of the warm front is colder than the cold air
behind the cold front
Weather associated with cold front occlusion
o Cold front has caught up with warm front- air in warm sector to rise
o Cold front touches the ground while warm front is lifted off the surface
o Colder air behind cold front undercuts the cold air ahead of the warm front forcing it
to rise
o Cold wet conditions similar to cold front
Weather associated with a warm front occlusion
o Cold front has caught up with the warm front causing air in the warm sector to rise
above ground
o Warm front touches the ground while cold front is lifted off the surface
