Grade 11
Global Air Circulation
Geography Notes
,Global Wind and Pressure Belts as a Response to the Unequal Heating of the Atmosphere:
Relationships between air temperature, air pressure and wind:
Warm air is "lighter" than cold air
Air that rises on its own (or wants to rise) is known
as unstable air
Unstable air is associated with the possibility of
cloud formation
Cold air is normally regarded as stable and is an
indicator of cloud free conditions
The EXCEPTION is where stable air is forced to rise
in the case of a mountain or surface convergence
amongst others
Direction of Movement and the Speed of Winds are Related to Pressure Gradient, Friction,
Pressure Gradient Force, Coriolis Force ad Geostrophic Flow:
Pressure Gradient and Friction
Pressure gradient affects the wind SPEED
Pressure gradient is a HORIZONTAL
CHANGE IN PRESSURE between areas of
high and low pressures
Horizontal changes in pressure arise
mainly as a result of uneven heating of
the earth's surface
An isobar is a line joining places of equal
atmospheric pressure
Closely spaced isobars indicate a strong
wind
Isobars spaced further apart indicate a
weak wind
, Friction affects the wind SPEED If we compare pressure gradients over the land, ocean, and the upper
atmosphere as being equal, the speeds of wind would be different.
The land surface is rough and uneven. Wind speeds are the slowest here.
The ocean surface has considerably less friction and wind speeds over the
ocean are much higher with the same pressure gradient.
As one measures wind speeds in the upper atmosphere, this is regarded
as an almost friction free zone. With the same pressure gradient (as for
land and ocean) the winds speeds are the greatest and very strong
Pressure Gradient Force (PGF) and Coriolis Force influence the DIRECTION of wind
Wind direction is influenced by TWO forces:
1. Pressure Gradient force
2. Coriolis Force
Pressure Gradient Force (PGF)
Pressure gradient force (PGF) acts directly
from a high pressure to a low pressure at
90º to the isobars.
It is this force that initiates (starts) the
horizontal movement of air (wind)
Coriolis Force (CF):
Coriolis force only affects the horizontal
movement of air (wind) once it has started to
move. (NB. PGF is the primary cause of air
starting to move)
Coriolis force is set up by the rotation of the
earth and will only react once on-air
movements start,
Coriolis force DEFLECTS winds away from the
direction from PGF to the left in the Southern
Hemisphere and to the right in the Northern
Hemisphere
Global Air Circulation
Geography Notes
,Global Wind and Pressure Belts as a Response to the Unequal Heating of the Atmosphere:
Relationships between air temperature, air pressure and wind:
Warm air is "lighter" than cold air
Air that rises on its own (or wants to rise) is known
as unstable air
Unstable air is associated with the possibility of
cloud formation
Cold air is normally regarded as stable and is an
indicator of cloud free conditions
The EXCEPTION is where stable air is forced to rise
in the case of a mountain or surface convergence
amongst others
Direction of Movement and the Speed of Winds are Related to Pressure Gradient, Friction,
Pressure Gradient Force, Coriolis Force ad Geostrophic Flow:
Pressure Gradient and Friction
Pressure gradient affects the wind SPEED
Pressure gradient is a HORIZONTAL
CHANGE IN PRESSURE between areas of
high and low pressures
Horizontal changes in pressure arise
mainly as a result of uneven heating of
the earth's surface
An isobar is a line joining places of equal
atmospheric pressure
Closely spaced isobars indicate a strong
wind
Isobars spaced further apart indicate a
weak wind
, Friction affects the wind SPEED If we compare pressure gradients over the land, ocean, and the upper
atmosphere as being equal, the speeds of wind would be different.
The land surface is rough and uneven. Wind speeds are the slowest here.
The ocean surface has considerably less friction and wind speeds over the
ocean are much higher with the same pressure gradient.
As one measures wind speeds in the upper atmosphere, this is regarded
as an almost friction free zone. With the same pressure gradient (as for
land and ocean) the winds speeds are the greatest and very strong
Pressure Gradient Force (PGF) and Coriolis Force influence the DIRECTION of wind
Wind direction is influenced by TWO forces:
1. Pressure Gradient force
2. Coriolis Force
Pressure Gradient Force (PGF)
Pressure gradient force (PGF) acts directly
from a high pressure to a low pressure at
90º to the isobars.
It is this force that initiates (starts) the
horizontal movement of air (wind)
Coriolis Force (CF):
Coriolis force only affects the horizontal
movement of air (wind) once it has started to
move. (NB. PGF is the primary cause of air
starting to move)
Coriolis force is set up by the rotation of the
earth and will only react once on-air
movements start,
Coriolis force DEFLECTS winds away from the
direction from PGF to the left in the Southern
Hemisphere and to the right in the Northern
Hemisphere
