Created by: Cassandra Hanscombe Matric 2022
Climate and Weather
Factors influencing horizontal differences in pressure:
1) Temperature
Over warm surfaces pressure will be less Over cold surfaces pressure will be more
Low atmospheric
pressure High atmospheric
pressure
Hot surface
Cold surface
2) Humidity
Dry air is denser than moist air and therefore moist air has a lower air pressure
Pressure gradient: the amount of change in pressure between two areas. This is shown by isobars on a
synoptic chart.
Isobars: Lines on a map which join all places of equal pressure. Pressure is measured in hectopascals
(hPa).
Pressure gradient force: (indicates strength of wind) the amount of force causing air to move from a high-
pressure area towards a low-pressure area. The closer the isobars, the steeper the gradient and
therefore the stronger the wind.
Wind: Air moving from a high pressure to a low pressure
Coriolis force:
Coriolis force: A deflecting force that deflects wind to the left in
the southern hemisphere and to the right in the northern
hemisphere. This is caused by the rotation of the Earth. This is
strongest at the poles and absent at the equator
Rotation around the HP/LP according to the
hemisphere.
Southern: Northern:
Out anticlockwise and in clockwise Out clockwise and in anticlockwise
Page | 1
,Created by: Cassandra Hanscombe Matric 2022
Global Air circulation:
Southern hemisphere:
Low pressure cells High pressure cells
Ascending unstable air converges Descending stable air diverges
• Circular shape • Oval shape
• Ari ascends and converges at the centre • Air descends and diverges at the centre
• Circulates anticlockwise • Circulates anti clockwise
• Known as cyclone • Known as anticyclone
• Isobars decrease towards centre • Isobars increase towards the centre
Associated weather: Associated weather:
• Unstable air • Stable air
• Cloud cover • Clear/cloudless
• Rainfall • Dry conditions
Planetary winds
Tropical easterlies • Known as Trade winds
• 0 – 30 degrees north and south
• Blow east to west
• Warm winds that blow towards the equator form 30 degrees n and s
• they converge at the equator to form the ITCZ
Westerlies • Aka Warm westerlies
• 30 – 60 degrees n and s
• Blows from west to east
• Collide with polar easterlies
Polar easterlies • 60 – 90 n and s
• Blow form east to west
• Cold , dry winds move from the high pressure at each pole towards the 60
degrees
Page | 2
, Created by: Cassandra Hanscombe Matric 2022
Tri-cellular model:
Hadley cell 1) Hot air rises for the surface, it cools producing tall cumulonimbus clouds
2) The risen air diverges, moves poleward, and cools further
3) At about 30 degrees n and s, the cooled air subsides
4) The subsiding air diverges towards the equator or 60 degrees n and s
Ferrell cell 5) Air subsides at 30 degrees, it warms and diverges at the surface
6) Some of the warm air move poleward
7) At about 60 degrees , the poleward moving warmer air meets cold air that is
moving equator ward from the pole. Convergence causes air to rise
8) In the upper air the rising air diverges. Some move equator wards and then
subsides at 30 degrees.
Polar cell 9) Cold air subsides over the pole
10) At the surface it moves equatorward and meets warm air form the Ferrell cell
at about 60 degrees
11) Converging air at the 60 degrees rises
12) The air then moves poleward completing the polar cell
World pressure belts:
ITCZ: The area where the tow Hadley cells
90° N converge. Warm air rises resulting in
cloudiness and high rainfall. The ITCZ shifts to
60° N the north of the equator in June and to the
south of the equator in December
30° N
Boundary between air
0° masses that have
different characteristics is
called a front.
30° S
60° S
90° S
Page | 3
Climate and Weather
Factors influencing horizontal differences in pressure:
1) Temperature
Over warm surfaces pressure will be less Over cold surfaces pressure will be more
Low atmospheric
pressure High atmospheric
pressure
Hot surface
Cold surface
2) Humidity
Dry air is denser than moist air and therefore moist air has a lower air pressure
Pressure gradient: the amount of change in pressure between two areas. This is shown by isobars on a
synoptic chart.
Isobars: Lines on a map which join all places of equal pressure. Pressure is measured in hectopascals
(hPa).
Pressure gradient force: (indicates strength of wind) the amount of force causing air to move from a high-
pressure area towards a low-pressure area. The closer the isobars, the steeper the gradient and
therefore the stronger the wind.
Wind: Air moving from a high pressure to a low pressure
Coriolis force:
Coriolis force: A deflecting force that deflects wind to the left in
the southern hemisphere and to the right in the northern
hemisphere. This is caused by the rotation of the Earth. This is
strongest at the poles and absent at the equator
Rotation around the HP/LP according to the
hemisphere.
Southern: Northern:
Out anticlockwise and in clockwise Out clockwise and in anticlockwise
Page | 1
,Created by: Cassandra Hanscombe Matric 2022
Global Air circulation:
Southern hemisphere:
Low pressure cells High pressure cells
Ascending unstable air converges Descending stable air diverges
• Circular shape • Oval shape
• Ari ascends and converges at the centre • Air descends and diverges at the centre
• Circulates anticlockwise • Circulates anti clockwise
• Known as cyclone • Known as anticyclone
• Isobars decrease towards centre • Isobars increase towards the centre
Associated weather: Associated weather:
• Unstable air • Stable air
• Cloud cover • Clear/cloudless
• Rainfall • Dry conditions
Planetary winds
Tropical easterlies • Known as Trade winds
• 0 – 30 degrees north and south
• Blow east to west
• Warm winds that blow towards the equator form 30 degrees n and s
• they converge at the equator to form the ITCZ
Westerlies • Aka Warm westerlies
• 30 – 60 degrees n and s
• Blows from west to east
• Collide with polar easterlies
Polar easterlies • 60 – 90 n and s
• Blow form east to west
• Cold , dry winds move from the high pressure at each pole towards the 60
degrees
Page | 2
, Created by: Cassandra Hanscombe Matric 2022
Tri-cellular model:
Hadley cell 1) Hot air rises for the surface, it cools producing tall cumulonimbus clouds
2) The risen air diverges, moves poleward, and cools further
3) At about 30 degrees n and s, the cooled air subsides
4) The subsiding air diverges towards the equator or 60 degrees n and s
Ferrell cell 5) Air subsides at 30 degrees, it warms and diverges at the surface
6) Some of the warm air move poleward
7) At about 60 degrees , the poleward moving warmer air meets cold air that is
moving equator ward from the pole. Convergence causes air to rise
8) In the upper air the rising air diverges. Some move equator wards and then
subsides at 30 degrees.
Polar cell 9) Cold air subsides over the pole
10) At the surface it moves equatorward and meets warm air form the Ferrell cell
at about 60 degrees
11) Converging air at the 60 degrees rises
12) The air then moves poleward completing the polar cell
World pressure belts:
ITCZ: The area where the tow Hadley cells
90° N converge. Warm air rises resulting in
cloudiness and high rainfall. The ITCZ shifts to
60° N the north of the equator in June and to the
south of the equator in December
30° N
Boundary between air
0° masses that have
different characteristics is
called a front.
30° S
60° S
90° S
Page | 3
