Download Chapter 1 Climatic System and

Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Chapter 4B Atmospheric Circulation
Air Masses and Major Wind System
4.5 Monsoon Circulation (Continental Scale / Secondary Wind
System)
(a) What you have learnt last year (Monsoon in South China Region)
In January
Overhead sun shifts to the _______________
_______________________________ .
It is ______________ in the northern
hemisphere.
Air
over
Asian
inland
cools
more
______________ than over the ocean.
Cold air over land sinks and an extensive
___________ pressure centre develops over
central Asia.
Warm air over the ocean rises, so the
pressure is ______________ .
Australia is in summer, then an intensive
______________ pressure centre develops
over central Australia because land is heated
up _________________ than sea in summer.
Winds blow from the Asian high pressure centre to the ocean and across the
equatorial low pressure belt to the low pressure centre of central Australia.
Because of deflection, wind direction over North China is ______________________
but becomes _________________ over South China(including Hong Kong) and SE
Asia.
1
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
After crossing the Equator, winds are deflected to the ___________ in the southern
hemispere by Coriolis force.
Wind direction over northern Australia is ____________________ .
Effect on Weather
South China(including Hong Kong)
Northern Australia
Cold offshore wind in NE direction
wet
Relief rain
Unstable weather
In July
Overhead sun shifts to the _______________
_______________________________ .
It
is
______________
in
the
northern
hemisphere.
Air
over
Asian
inland
heats
more
______________ than over the ocean.
Hot air over land rises and an extensive
___________ pressure centre develops over
central Asia.
Cool air over the ocean sinks, so the pressure is
______________ .
Australia is in winter, then an intensive
______________ pressure centre develops over
2
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
central Australia because land is cooled down _________________ than sea in
winter.
Winds blow from the Australia high pressure centre to the ocean and across the
equatorial low pressure belt to the low pressure centre of central Asia.
Because
of
deflection,
wind
direction
over
northern
Australia
is
____________________ .
After crossing the Equator, winds are deflected to the ___________ in the northern
hemispere by Coriolis force. Therefore wind direction over SE Asia is
_____________________ .
Wind direction over South China(including Hong Kong) is ____________________ .
Effect on Weather
South China(including Hong Kong)
Northern Australia
Cold offshore wind in NE direction
Dry
Very little rain
Fine weather
(b) What is Monsoon?
The monsoon is a flow pattern of the general atmospheric circulation over a large
geographical area, in which there is a clearly dominant wind in one direction in every
part of the region concerned, but in which this prevailing direction of wind is reversed
from season to season.
(c) General Cause of Monsoon
Due to the co-existence of both land masses and oceans on the surface of the earth,
planetary wind systems will inevitably be modified and form the monsoon circulation.
(d) Spatial Distribution of Monsoon
It usually occurs along the coast which is backed with large land mass, e.g.
southern Asia and northern Indian Ocean.
3
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
-
It also occurs with the existing of great mountain barriers.
-
Monsoon circulation is expected to be much stronger whenever and wherever
prevailing zonal winds are weak.
Monsoon weather in many parts of the world is also limited to lands between the
equator and the ITCZ.
-
4.6 Local Winds
These winds do not prevail for long periods nor do they affect extensive areas.
1. Land and Sea Breeze
(i)
Sea Breeze

General Principle:
Different heat capacity between land and sea leading to differential heating speed of
them

1.
2.
3.
4.
Processes:
Warming of the air column over the land causes its expansion and increases the
vertical distance between isobaric surfaces.
The result of this is that, although the surface pressure is not changed, at similar
levels aloft, air over the sea is slightly lower than over the land.
Therefore air begins to drift seawards aloft in response to pressure gradient.
At the surface, therefore, a pressure gradient now exists from the sea to the land,
and it is this which results in the onset of the onshore sea breeze so characteristic
of coastal areas on warm summer days.
4
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Figure 4.17

1.
2.
Formation of Sea Breeze
Associated Effects on Weather:
Moderating influence on the temperatures in coastal areas and the cooling effect
of these breezes generally reaches a maximum of 100km inland in the tropics and
often half that distance in the middle latitudes.
Also affect the amount of cloud cover and rainfall
Duration
These cool sea breezes generally begin shortly before noon and reach their greatest
intensity, about 10 to 20 km/hour, in the mid-afternoon.

5
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
(ii) Land Breeze
Reversal of the temperature gradient, as occurs during the night when the sea is
warmer than the land, may produce an offshore flow at the surface, known as land
breeze. Land breezes are usually about 5km/h because the pressure gradient is
greater in day time.
2. Mountain Wind and Valley Wind
(a) Mountain Wind / Katabatic Winds

Formation
During the night / winter, rapid radiation heat loss along the mountain slopes results
in colder and denser air at drains gently down slope under gravity at speeds at about 1
m/s. Such down slope movements of air is known as katabatic winds.
It is marked over surfaces such as glaciers, where intense chilling of the air occurs.

1.
2.
Effects
These flows of cold air are very shallow and easily obstructed by obstacles such
as forests, walls, buildings, behind which pond of cold air may develop before
overflowing occurs.
In relatively enclosed low-lying areas, katabatically induced chilling of the air
overlying the surface renders such areas more susceptible to fog and frost.
(b) Valley Wind / Anabatic Winds
During the day, a reversal of this circulation occurs. The air along the slopes of the
mountains is heated more intensely than the air at the same elevation over the valley
floor. This warm air moves upslope in response to the greater heating of the air at
lower elevations. This air flow is known as an anabatic winds (valley winds). The
occurrence of these daytime upslope breezes can often be identified by the cumulus
clouds that develop over adjacent mountain peaks.
(c) Occurrence Preference of Mountain and Valley Winds
Mountain and valley winds have seasonal preferences. Although valley winds are
most common during the warm season when solar heating is most intense, mountain
winds tend to be more dominant in the cold season.
6
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
3. Fohn / Foehn / Chinook Winds
(a) Where?
Fohn is a wind blowing down slope on the leeward side of a mountain ridge.
(b) Characteristics
Higher temperature
Lower relative humidity
than the windward slope
(c) Special Terminology
Fohn is very common in Alps in spring and autumn.
In the Rockies, it is known as Chinook.
Draw a stability diagram to show that fohn is usually warmer and drier than winds in
the windward slope.
Stability Diagram
4.7 Air Masses
(a) Formation
The atmosphere is warmed primarily by the interception of outgoing long-wave
radiation from the surface and gains its moisture by evaporation from below. It is
therefore reasonable to expect the temperature and moisture-content characteristics of
air over a region of the earth’s surface to reflect conditions below.
(b) Definition
Air mass is a mobile homogenous (uniform in a horizontal direction at different levels)
immense body of all bounded by a front.
7
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
(c) Factors Affecting the Nature and Degree of Uniformity of Air
Mass Characteristics



The nature of the source region and the direction of movement
Changes that occur in the constitution of an air mass as it moves over distances
Age of the air mass
(d) Source Regions
(i) Definition
Source regions are usually places where only minimal changes in surface geography
occur over large expanse (extensive and homogenous surface), for example, ocean
surface, ice-covered areas, desert, large plains.
Figure 4.19
Air Mass Regions
(ii) Where?
Major source regions are confined to subtropical and polar locations.
These will be particularly effective areas for forming air masses if the air above them
is slow moving or gently subsidence. This gives time for the transmission of
surface-related characteristics to proceed.
8
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Follow-up Question
Do you think we can find air mass there frequently?
How about in mid latitude?
Why?
_____________________________________________________________________
_____________________________________________________________________
(iii) Types of Source Regions
Primary Source Region
It is an extensive and homogenous surface where air masses develop.
E.g. the interior Siberia, the Sahara Desert, the Arctic Ocean, the Canadian Plain, the
tropical Pacific and Atlantic
Secondary Source Region
It refers to the surfaces (probably smaller in size) where air masses undergo rapid
changes in nature (the homogeneity of the air mass remains).
E.g The Northern Pacific and Atlantic Oceans, the Siberian Plain and the southeastern
part of the United States are typical examples of secondary source region.
(e) Classification of Air Mass
A.
(i)
-
Two criteria of classification:
With respect to Moisture Content
“m” (maritime)
“c” (continental)
(ii) With respect to Thermal Characteristics
An indicator of the thermal characteristics of the air masses is derived on a latitudinal
basis:
Arctic or Antarctic
“A” or “AA”
Polar
“P”
Tropical
Equatorial
-
“T”
“E”
B.
(i)
Types and Properties of Air Mass
Warm Air Mass

Where?
These have their origins in the sub-tropical high pressure cells and during the summer
9
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
season, in the great accumulations of warm surface air which characterize the heart of
large land areas.
The tropical sources are either
1. maritime (mT)
2. continental (cT)
Tropical maritime (mT)
Tropical contential (cT)
Temperature
Humidity
Stability
Stable stratification since the air is warm Stable air mass (in summer, warming of
and moist near the surface, stratiform the lower layers by the heated land
cloud commonly develops as the air generates a steep lapse rate, but despite
moves polewards from its source
its instability the low relative humidity
prevent the development of cloud and
precipitation)
(ii) Cold Air Mass

Where?
The principal sources of cold air in the northern hemisphere are
The Arctic Basin, which originates continental Arctic (cA)
The continental anticyclones of Siberia and northern Canada which originate
continental polar (cP) air masses

1.
Characteristics
Stable condition: The stability produced by the effect of surface cooling
prevents vertical mixing so that further cooling occurs more slowly by radiation
losses only. The effect of this radiative cooling and the tendency for air-mass
subsidence in high pressure region combine to produce a prominent temperature
inversion from the surface up to about 850 hPa in typical cA and cP air.
2.
Small cloud amounts
3.
Occasional light snowfalls
10
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
Summary on the Nature and Properties of Major Air Masses
Characteristics at Surface
Air Mass
Temperature
(C)
Specific Humidity Typical Properties
(g/kg)
Maritime Tropical (mT)
Summer
22-30
15-20
Mild, moist,
Unstable
Martime Polar (mP)
Winter
Summer
0-10
2-14
3-8
5-10
Cool, moist
Cool, moist
Continental Tropical (cT)
30-42
5-10
Warm, dry, cloudless
Continental Polar (cP)
Winter
Summer
-35 - -20
5 - 15
0.2-0.6
4-9
Cold, dry
Cold, dry, stable,
cloudless
Continental Arctic (cA)
Winter
Maritime Equatorial
(mE)
-55 - -35
Approx. 27
0.05-0.2
Very cold, very dry,
very stable
Approx. 19
Warm, very moist,
unstable
(f) Influence and Interaction
A. Air Mass Weather
When an air mass moves out from its source region, it brings its distinctive properties
with it to influence weather at distant locations.
Because it may take several days for an air mass to traverse an area, the region under
its influence will experience generally constant weather conditions, a situation called
air mass weather.
B. Interaction of Air Mass
(i) Location of air mass interaction
The planetary wind circulation system makes interaction between air masses more
likely to occur in some areas:
1)
The convergence of air in the vicinity of the equator (ITCZ / thermal
equator)
Since the source regions from which these air streams originate are at similar latitudes
in either hemisphere, both air masses will have similar properties will mix freely.
11
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
2) Polar front
It refers to the zone where tropical and polar air masses collide at the high-middle
latitudes in each hemisphere. In this case, fundamentally different air masses are
coming into conflict along a boundary zone which has become known as the polar
front.
(ii) Frontal Depression / Temperate Cyclone / Mid-latitude Depression /
Extratropical Cyclone

How to form? (Brief)
The life of temperate cyclone
begins with the meeting of
cold, dry polar air mass and
warm, moist tropical air mass.

1.
2.
3.
4.
5.
Characteristics
Low pressure
Circular isobars
1500-2000km in diameter
Life spans of 4-7 days
Moves from west to east under the influence of the upper westerlies within which
its circulation is embedded
(iii) Fronts
Fronts are interfaces or transitional zones between two air masses with distinctive
differences in temperature and humidity.
Recall: Front Classification
1. Cold front: a front moving from cold area to warm area
2. Warm front: a front moving from warm area to cold area
3. Stationery front: a front is not moving or is moving slowly or irregularly
4. Occluded front: when a cold front catches up to and overtakes a warm front
12
Written by Clement Cheng
Shun Lee Catholic Secondary School
Advanced Level – Geography – Climatic System
(g) Air Mass Modification
The movement of air mass away from the source region would result in the exchanges
of heat and moisture with the ground surface take place.
A.
1)
Mechanisms of Modification
Thermodynamic Modification

What is it?
It results from the exchange of heat (sensible and latent) between the bottom of the air
mass and the overlying surface which the air mass travels.
An air mass may be heated from below either by
1. passing from a cold to warm surface; or
2. solar heating of the ground over which the air is located.
Similarly, it can be cooled from below.
What are the directions of sensible heat exchange?
They can be either heating from surface or cooling from surface.

What are the effects of heating from surface?
Heating from below acts to increase air mass instability so that the effect may be

spread rapidly through a considerable thickness of air.
What are the effects of cooling from surface?
When cooling from below, surface cooling produces a temperature inversion which
greatly limits the vertical extent of the cooling. For this reason, cooling mainly
occurs through radiative heat loss by the air, a process which takes place only very
gradually.

2)
Dynamic (Mechanical) Modification

What is it?
Dynamic modification involves the mixing and pressure change of the air mass.


-
How does it occur?
Collision of air masses with different natures
Convergence of air streams
Examples
The distribution of physical properties of air masses has been shown to be
considerably modified by a prolonged period of turbulent mixing. This process
13
Written by Clement Cheng
Chapter 4 Atmospheric Circulation: Air Masses and Major Wind Systems
is particularly important at low levels where surface friction intensifies the natural
-
turbulence of airflow, providing a ready mechanism for the upward transfer of the
effects of thermodynamic processes.
When cold and dense air mass collides with warm and light air mass, the latter
will be uplifted and adiabatically cooled.
When it becomes saturated,
condensation and precipitation take place. The nature of the warm air mass will
be partially or completely changed.
(h) Air Mass Dissipation
Air mass dissipation is associated with
(1) the loss of its original identity until finally its features merge with those of
surrounding airstreams (e.g. temperature, climatic characteristics); and
(2) the air may again become subject to the influence of a new source region.
14
Written by Clement Cheng