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Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Chapter 4A Atmospheric Circulation
Air Masses and Major Wind System
4.1 Introduction
Atmospheric Circulation
(a) Importance of Horizontal Air Movement
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(b) Importance of Vertical Air Movement
Vertical movements of air are no less important since they strongly influence whether
the climate and weather will be stable or unstable.
Associated Weather
Sinking air
Fine / unstable
Rising air
Fine / unstable
Consolidated Questions
What is atmospheric circulation???????
circulation come into existence?
Why
Chapter 4 Atmospheric Circulation –P.1
Written by Clement Cheng
does
atmospheric
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
4.2 Atmospheric Circulation
(A) Driving Forces of Horizontal Motion
1. Pressure Gradient Force (氣壓梯度力) / Isobaric Slope
What is pressure gradient?
It is the difference of atmospheric pressure between two adjacent areas.

Why is pressure different from one place to another?

Altitude
In general, the layers closest to the surface will have the greatest weight overlying
them and thus the pressure will be _______________ and vice versa for the layers at

the top of the atmosphere.

Temperature
Horizontal pressure differences mainly result from temperature differences that
produce the density contrast. The higher the temperature, the lighter and less dense
is the air.
The case when air is heated up
The case when the air is cooled
Air is heated
Air is cooled
Air is expanded and spread over
larger area
Air pressure decreases
What is the relationship between pressure gradients and wind speed?
The steeper the pressure gradient, the ______________ the wind speed.

How do we represent pressure gradient on an isobaric map?
In an isobaric map, line of equal pressure is joined as isobar. (等壓線)

The closer together the isobars are, the ______________ the pressure gradient. A
steep gradient, like a steep slope, causes more rapid acceleration of material down the
gradient than does a gentler gradient.
Chapter 4A Atmospheric Circulation - P.2
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
The Effect of Pressure Gradient Force on Winds
Pressure gradient force is the force that starts atmospheric motion.

Pressure gradient force causes the movement of air away from _____________
pressure area to ________________ pressure area.
How does the air move as a result of the pressure gradient force?
996 hPa
1000 hPa
1004 hPa
1008 hPa
Figure 4.1 Pressure Gradient Force
Two important points to note of the winds as a result of the pressure gradient force:
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2. Coriolis Force (科爾里奧力)
What are the attributes of the Coriolis Force?
 The Coriolis force influences only the ___________________ of motion, not the
speed.
 Most of the winds of the earth follow a generally curved path rather than a
straight line.

 The Coriolis force acts at right angles to the direction of a moving object towards
the ______________ in the Northern Hemisphere and towards the
_____________ in the Southern Hemisphere.
 When the pressure gradient has initiated the movement of the air, the resulting
wind is deflected more and more to the right in the Northern Hemisphere until it
is blowing at a right angle to the pressure gradient.
Chapter 4 Atmospheric Circulation –P.3
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
Geostrophic Winds (地轉風)– Winds above the Friction Layer
996 hPa
1000 hPa
1004 hPa
1008 hPa
Figure 4.2 Development of Geostrophic Wind
Necessary Conditions for the Development of Geostrophic Wind:
1. Geostrophic wind takes place above the influence of surface frictional drag, air
2.
movement is controlled by the horizontal ______________________ force and
the ________________ force.
For the purpose of explaining horizontal air movement, or wind, the forces of
gravity and the vertical pressure gradient may be assumed to cancel each other
out.
When an air parcel in the Northern Hemisphere is affected by the two controlling
forces above the friction layer, the following will happen:
1. From it starting point, the air would begin the move in response to the
2.
3.
4.
5.
6.
_____________________________ from the high to the low pressure.
Once it begins to move, however, it becomes subject to the influence of
__________________________, which displaces it to the right of its trajectory.
As the parcel speeds up as a result of the continued presence of the pressure
gradient force, the Coriolis force also intensifies.
As the parcel of air accelerates, the magnitude of the deflection grows.
Eventually, the wind will be turned so that it is blowing ___________________
to the isobars.
At this stage, the two forces are acting in opposite directions and the two forces
must be ________________________.
The result is that the air parcel continues to move parallel to the isobars. Since
no unbalanced force is now acting on the parcel, it continues to move at a
constant speed and in the direction it was travelling when balance was achieved.
The wind produced as a result of this balance between the pressure gradient force
and the Coriolis force is known as the Geostrophic Wind. Such winds, blowing
parallel to the isobars, for the Northern Hemisphere, the high pressure core on the
_________________ and the low pressure on the _____________ when viewed
downwind, are a feature of the upper air circulation.
Chapter 4A Atmospheric Circulation - P.4
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
3. Centrifugal Acceleration and Centripetal Acceleration
In which circumstance should we incorporate the centripetal acceleration?
_____________________________________________________________________
_____________________________________________________________________

What are the factors that affect the centripetal acceleration?
1. __________________________________________
2. __________________________________________
3. __________________________________________
The magnitude of the centripetal acceleration is generally small, and it only becomes
really important where _______________ winds are moving in very _____________
paths. Here the curved path of the air is maintained by an inward-acting
acceleration.
Gradient Winds - Cyclonic Rotation above the Friction Layer

What is gradient winds (梯度風)?
It is the curved flow of air around a low pressure system or a high pressure above the
friction layer.
Movement of Wind around the High and Low Pressure Systems
The development of the gradient wind also explains why the winds always blows
counterclockwise around a low pressure centre in the Northern Hemisphere and
clockwise around a low pressure centre in the Southern Hemisphere.
In the following diagrams, add arrows to show the balance of gradient to show the
situation in the Northern Hemisphere.
HIGH
Figure 4.3
LOW
Gradient Winds in the Case of Circular Pressure Pattern
Chapter 4 Atmospheric Circulation –P.5
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems

Significance of the Gradient Winds - Persistent Establishment of Low
Pressure System: Is it right?
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4. Force of Friction
What is Friction?
All types of obstacles protruding into the air contribute to the frictional dray, which is
obviously greatest close to the surface (boundary / frictional layer).
Where is the friction-free layer (also called free atmosphere)?
Above a height of about 1000 m
Any exceptions:
1. _________________________________________________________________
_________________________________________________________________
2. _________________________________________________________________
_________________________________________________________________
Gradient Winds within the Friction Layer
Friction exerts an influence only after the air is in motion. Frictional drag acts in a
direction opposite to the path of motion and can cause only deceleration.
996 hPa
1000 hPa
1004 hPa
1008 hPa
Figure 4.4
Forces controlling wind direction at the surface
Chapter 4A Atmospheric Circulation - P.6
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
HIGH
LOW
Figure 4.5 Surface winds in a cyclone and anticyclone in the Northern
Hemisphere
Two major points to note about surface wind:
1. _________________________________________________________________
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2.
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(B) Vertical Motion
1. The Effect of High and Low Pressure System
Under the influence of the frictional force, surface winds show an anti-clockwise in
spiral in a low pressure system (cyclone) in the Northern Hemisphere. The crossing
of the isobars by winds coming from a number of different directions produces a net
inflow of air at the centre. This may only escape upwards and diverge aloft.
(a) Case of Low Pressure System:
Thus an important relationship is apparent between low pressure flow, convergence of
the surface, upward motion and divergence aloft at the centre of the depression:
 Surface convergence could be maintained if divergence aloft occurred at a rate
equal to the inflow below.


Divergence aloft may occasionally even exceed surface convergence, thereby
resulting in intensified surface inflow and increased vertical motion.
Usually, it is divergence aloft that first creates the surface low. Spreading out
Chapter 4 Atmospheric Circulation –P.7
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
aloft initiates upflow in the layer directly below, and it eventually works its way
to the surface where inflow is encouraged.
(b) Case of High Pressure System

A clockwise outspiral of surface winds is found in a high pressure system
(anticyclone) in the Northern Hemisphere, under the influence of frictional force.

A net outflow of air at the centre of an anticyclone necessitates air being down
from aloft as the centre.

Therefore, high pressure flow is associated with divergence of air at the surface,
convergence aloft and subsidence of air from higher up at the centre of the
anticyclone.
2. The Effect of Friction
Friction can also causes mass convergence when the flow is straight. When air
moves from the relatively smooth ocean surface onto land, for instance, the increased
friction causes an abrupt drop in wind speed. This reduction of wind speed
downstream results in a pile up of air upstream. Thus converging winds and
ascending air accompany flow off the ocean. This effect contributes to the cloudy
conditions over the land often associated with a sea breeze in a humid region.
As expected, general divergence and subsidence accompany the flow of air seaward
because of increasing wind speeds over the winter.
(C) Circulation Models (大氣環流模型)
1. Hadley's Model: Motion on a Stationary Earth
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Proponent: Edmund Hadley
Year: 1686
Essence: He was aware of the fact that __________________ drives the winds.
The large temperature contrast between the poles and the equator would create a
thermal circulation.
Model:
He proposes the existence of two great convective cells, one in either hemisphere.
They are driven by rising masses of air overlying the area of most intense solar
heating close to the equator.
As these masses of warm air rose, they would cool by radiation to space and lose
their buoyancy.
Encouraged by the continuing up-draught from below, they would begin to flow
towards the poles, undergoing further cooling as they went.
Chapter 4A Atmospheric Circulation - P.8
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System



Ultimately, their increased density would induce them to sink back to the surface,
creating a high pressure area.
From the high pressure area, a return flow at the surface back to the equatorial
low pressure zone would complete the convective cell.
Thus the proposed Hadley circulation for non-rotating earth has upper-level air
flowing poleward and surface air flowing equatorward.
Equator
Figure 4.6
Hadley Cell

Validity of the model only if
1.
2.
3.
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2. Hadley's Model: Motion on a Rotating Earth
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-
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-
Modified by: George Hadley
Year: 1735
Essence:
He sought to include the effects that the earth’s rotation would have on wind
direction. He reasoned that the poleward-flowing currents of air aloft would be
deflected to the right in the Northern Hemisphere and to the left in the Southern
Hemisphere to become south-westerlies and north-westerlies respectively.
Model:
Still one cell.
The surface return flows would become north-easterlies in the Northern
Hemisphere and south-easterlies in the Southern Hemisphere.
Chapter 4 Atmospheric Circulation –P.9
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
3. The Models of Ferrel and Rossby – Tricellular Model
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
Proponents and Year of Establishment: by Ferrel in 1856 and elaborated-on by
Rossby in 1941
Essence: Three cells
(a) Low-latitude direct cell / Hadley cell
From equator to about 30 latitude to the equatorial region
The rising and consequent cooling of these warm, normally humid air masses
induces them to shed their excess water vapour as cloud. This releases large
amounts of heat energy, the latent heat of evaporation, which increases the
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instability of the air masses, producing further uplift and cooling.
As the upper flow in this cell moves away from the equatorial region, radiation
cooling would result in increased density of the air aloft and contribute to the
general subsidence found between 20-30 latitude.
(b) Mid-latitude Indirect Cell / Ferrel Cell
Between the horse latitudes and the rising limb of the high-latitude cell, it is
driven by friction with the two direct cells adjacent to it.
On the equatorward side, this friction would be provided by the descending
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motion of the air within the subtropical highs. Some of this air would also
“spills” polewards to produce the westerlies of the middle latitudes.
On the poleward side, the upward limb of the polar cell could be envisaged as
providing further frictional energy, enabling a return flow aloft equatorwards to
occur within this indirect cell.
The poleward boundary of this indirect cell marked an important junction
between tropically-derived air and the cold polar circulation. It was later
included in the model as the Polar Front.
(c) High-latitude Direct Cell / Polar Cell
A second direct cell was postulated as existing in high latitudes, driven by the
chilling-induced subsidence of air immediately over the polar ice-caps.
This cell develops between 90 and 60 degrees latitude. The air near the poles
sinks and near the surface, flows southward in the Northern Hemisphere
(northward in the Southern Hemisphere) until it encounters air from the opposite
direction at about 60 degree north latitude.
Chapter 4A Atmospheric Circulation - P.10
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
60N
30N
Equator
30S
60S
Figure 4.7 The Planetary Wind and Pressure System
4.3 Major Wind System
1.
The Trade Winds (信風/貿易風)
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Originate at the Horse Latitudes and convergence in the equatorial trough.
NE direction in the Northern Hemisphere
-
SE direction in the Southern Hemisphere
Associated with the formation of Intertropical Convergence Zone (ITCZ)
 Why does it form?
- Convergence of NE trades of the Northern Hemisphere and SE trades from the
Southern Hemisphere at the equatorial trough

-
Characteristics:
ITCZ = thermal equator
-
Average position is _________.
Light and variable winds
Deep convectional rain
2.
Westerlies (西風帶)
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Originate at Horse Latitudes and move to the sub-polar low pressure belts.
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They are in the direction of SW in the N. Hemisphere and NW in the S. Hemisphere.

More variable than the trades in terms of DIRECTION and INTENSITY because:
They are frequently affected by cells of low and high pressure (cyclones and
anticyclones)
Chapter 4 Atmospheric Circulation –P.11
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems

The preponderance of land areas with their irregular relief and changing seasonal
pressure patterns in the Northern Hemisphere tends to obscure the westerly
airflow.
- Usually, the westerlies in the Southern Hemisphere are stronger and more
constant in direction than those of Northern Hemisphere. Why?
_____________________________________________________________________
3.
Polar Easterlies (極地東風帶)
Polar easterlies originate from the Poles. They are in the direction of NE in the
Northern Hemisphere and SE in the Southern Hemisphere.
Polar Front
The polar easterlies collide with the westerlies at polar fronts which are located
around 60 degrees north or south. The collision of the air streams results in the
frequent development of frontal depressions (temperate cyclones) (for detail, please
refer Section 4.4 Upper Winds).
** Shifting of Wind Belts and Pressure Belts



Due to the seasonal variation in the distribution of solar radiation, the pressure
belts migrate northward or southward accordingly which in turn results in the
shifting of wind belts.
In July, the pressure is low over the northern hemisphere but high over the
southern hemisphere. The southeast trades flow across the equator. Under the
influence of the Coriolis force, they are deflected to the right and become
southwest winds.
The condition is reversed in January. The northeast trades are deflected to the
left on crossing the equator and become northwest winds.
(Recall from your certificate materials)
4.4 Upper Winds
1.

Upper Westerlies
Upper Condition of Mid-latitudes according to the Prediction of Tricellular
Model:
The tricellular model shows that the flow aloft in the mid-latitudes is equatorward.
In conjunction with the Coriolis force, an east wind is produced.
However, numerous observations during WWII indicated that a general westerly flow
exists aloft in the mid-latitudes as well as at the surface.
Chapter 4A Atmospheric Circulation - P.12
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
 Actual Upper Condition of Mid-latitudes
Upper westerlies blow in a complete circuit around the earth from about 25ºN and S
almost to the poles.
 Why Upper Westerlies?
The existence and the intensity of the upper westerly flow are determined by the
equator-to-pole temperature gradient. Because warm air is less dense than cold air,
air pressure decreases more slowly with height in a column of warm tropical air than
in a column of cold polar air. Consequently, at levels above the earth’s surface,
higher pressure exists over the tropics and lower pressure is the norm above poles.
This produces a _____________ gradient from equator to pole aloft.
This gradient increases with increasing height; air movement along is modified by the
Coriolis force to produce a westerly motion aloft.
This wind is called
___________________ winds.
Figure 4.8
Upper Westerlies
Coriolis force
Pressure gradient
wind
Pressure
decreases
force
North Pole

1.
2.
3.
4.
5.
Cold
Warm
Equator
Characteristics
Travel from west to east slowly
The
wind
speeds
tends
to
increases
with
altitude
as
___________________________.
They reach their maximum speed at approximately 12 km between 30º and 40º
latitude.
The mean speed is as much as 35m/s (125km/hour) and maximum speeds of
several hundred kilometres per hour are common.
Wavy pattern: between two and five long waves
Chapter 4 Atmospheric Circulation –P.13
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
2.
Jet Streams (噴射氣流)
-
Jet streams are the integral parts of the westerlies and are the fast core of the
overall westerly flow.
(a) Polar Front Jet
A particularly well-defined frontal zone exists at the poleward limit of the surface
westerly circulation, where the tropical and polar air masses interact.
(b) Subtropical Jet
It forms as a result of frontal zone aloft at about 25 degree latitude, where the Hadley
cell and mid latitude circulation interacts. This meanders much less than the polar
jet and seldom exhibits the very high speeds of the latter.
(c) Rossby Waves
The upper westerlies are disturbed by a number of waves called Rossby waves.
These exist because of:
1) the existence of large mountain barriers
2) the uneven distribution of land and sea
3) the steep temperature gradient in the mid-latitudes of the upper atmosphere
Figure 4.9
Rossby wave and jet stream
Chapter 4A Atmospheric Circulation - P.14
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Further Development of Rossby Wave (Figure 4.10)
Chapter 4 Atmospheric Circulation –P.15
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
(e) The Interplay of Upper Winds Systems (Figure 4.11)
Polar Front
Cell
(b)
Cell Z
Cell Y
(a)
(a)
Cell X
Chapter 4A Atmospheric Circulation - P.16
Written by Clement Cheng