ADVANCED LEVEL GEOGRAPHY
NATURAL LANDSCAPE: CLIMATIC SYSTEM
Topic 3: Atmospheric Circulation
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CONTENT (內容大綱)
1. Atmospheric circulation (大氣環流) / General Circulation:
- horizontal and vertical flow,
- driving forces of air movement,
- spatial variation of temperature, air pressure, pressure gradient, Coriolis force
- influences on surface wind system
2. Major wind systems:
- patterns and characteristics of the trades, westerlies, polar winds and monsoons
3. Air masses:
~ nature and types
~ influences on weather and climate
4. Atmospheric disturbances
~ typhoons and man responses
~ changes of wind speed and direction
EXPLANATORY NOTES
1. Atmospheric circulation is the mechanism through which the energy surpluses and deficits are
balanced, and the balance involves air movement of different scales.
2. Students are expected to :
□understand the flows and driving forces of atmospheric circulation
□relate atmospheric circulation to surface wind systems,
□understand the nature and characteristics of air masses and
□note how people response to atmospheric disturbances, e.g. typhoons
3.
Temporal and spatial scales for atmospheric motions
Name of Scale
1.
Time Scale
Length Scale
Example
weeks – years
1000-40000km
Waves in Westerlies
100-5000km
Cyclones, anticyclones, hurricanes
Macroscale
-
General
Circulation
-
Synoptic Scale
Days-weeks
2 Mesoscale
Minutes-days
3. Microscale
Seconds-minutes
1-100km
< 1 km
Land-sea breeze, thunderstorms, tornadoes
Turbulence
I. INTRODUCTION [引言]
! The atmosphere acts as gigantic heat engine in which the difference in the
temperature between the poles and the equator provides the energy supply to
drive the planetary atmospheric circulation.
! Large-scale air circulation transport heat, both sensible heat and latent heat
present in water vapor.
! Because of the global radiation imbalance -- a surplus in low latitudes and
a deficit in high latitudes -- atmospheric circulation must transport heat
across the latitudes from the regions of surplus to the region of deficit.
! The point concern here is that the variable heating of different parts of the
atmosphere sets up variations in pressure, which in turns sets the air in
motion.
! Wind is air in motion and it dominantly horizontal.
(II) ATMOSPHERIC CIRCULATION
(GENERAL CIRCULATION) (大氣環流)
A. HORIZONTAL AND VERTICAL AIR FLOW (橫向及直向流
動)
THE IMPORTANCE OF AIR MOVEMENT (空氣流動的重要
性)
The atmospheric system is a dynamic system characterized by ceaseless air
motion on a great variety of spatial and temporal scales.
Air movement is a vital determinant of weather and climate, and thus of
major human significance.
For convenience, air motion may be resolved into two components:
--- horizontal and vertical
1. HORIZONTAL MOVEMENT = WINDS(風)
Horizontal movement, or wind, is by far the faster and consists of air
movements parallel to the surface.
Horizontal movements or wind is an important climatic factor for a number
of reasons.
(1) Thermal
Re-distribution (熱力再分佈)
Wind action physically relocates warm and cold bodies of air, thereby
modifying the thermal characteristics of places related to their radiation
regime.
Such modification may have a considerable effect on the air temperature of a
place:
(a) On a local and daily/short-term scale, e.g. a change in wind direction ,
bringing a cool air from a lakeshore areas, may cause a sudden drop in
temperature of a place such as experienced in Chicago.
(b) Similar thermal effects can be observed over longer time-scales and over
larger areas, illustrating the importance of wind as an agent of horizontal heat
transfer, or advection. This was illustrated with reference to the Maritime
influences. A change in wind direction may cause changes in temperature,
either a rise or drop in temperature, suddenly or during change of season. For
example, HK in winter was affected by NW monsoon winds.
(2) Moisture Transfer (水份轉送)
Wind action transports water vapor, the source of life-giving precipitation. In
particular, moisture is brought from areas where it is abundant, such as over
the oceans, to areas where it is often deficient, such as over the interiors of
continents.
e.g. Onshore winds in E and SE China, including HK, in summer.
Example: Figure below illustrates the significance of a seasonal reversal of
wind direction in rainfall amount for Bombay. It shows the effects of
monsoon winds on rainfall.
(3) Environmental
hazards (環境災害)
Air in rapid motion is, a severe environmental hazard. On average, more lives
are lost each year as a result of tropical storms than from the combined effects
of fire, lightning, floods, tidal waves and earthquakes.
2. VERTICAL MOVEMENT
Vertical motions, on the other hand, involve sinking and rising masses of air
perpendicular to the surface and are usually 100-1000 times slower than their
horizontal counterparts.
Vertical movements of air, although normally less rapid than their horizontal
counterparts, are very important, since they strongly influence whether the
climate and weather will be cloudy and rainy or clear and dry.
Areas where air is sinking are relatively cloud-free and dry, e.g. TD; whereas
in areas characterized by rising air motion the opposite weather types tend to
prevail. e.g TRF.
3. RELATIONSHIP BETWEEN AIR MOTION (BOTH HORIZONTAL & VERTICAL)
AND THE GLOBAL ENERGY BUDGET.
Air in motion, however, has an even more fundamental function to fulfilled at
a global scale -- the transfer of heat.
It will be recalled from A.1a (energy budget) that THE UNEQUAL
HEATING OF THE EARTHSURFACE BY THE SUN PRODUCES A
LATITUDINAL CONTRAST IN ENERGY BUDGETS between about 40
N and 35 S, where the amount of incoming radiation exceeds that lost by the
cooling of the earth-atmosphere system, whereas towards the poles the
reverse applies.
Obviously, if such a situation persisted over very long time periods, it
would cause the low latitudes to be very much hotter than they are at present,
and the high latitudes to be very much more frigid.
The fact that so implies the existence of a mechanisms whereby heat is moved
from the surplus areas to the deficit areas to compensate for the shortfall in
the energy budget of the latter.
This vital role, without which only a small proportion of the global would be
habitable, is fulfilled by a global system of wind circulation, describe latter in
this set of notes.
B. DRIVING FORCES THAT CONTROL AIR MOVEMENT
1.
SPATIAL VARIATION OF TEMPERATURE (地面溫度差
異)
The energy required to drive the gigantic circulation of the earth surface is provided
by the temperature contrasts between cold polar region and warm tropical air region.
Why there is unequal heating on the earth surface ?
2. AIR PRESSURE (空氣壓力)
In moving about randomly, gas molecules collide with each other and with any
surface which confines them. The push exerted by this continuous bombardment is
known as pressure. The atmosphere is a mixture of gases, confined by the ground or
sea below and the force of gravity, which prevents escape above. Although not readily
noticeable, air exerts a pressure on every surface exposed to it. That pressure can
be considered as resulting from the weight of overlying air pressing down on a given
area.
AIR MOTION IS A RESPONSE TO A FORCE OR FORCES OF SOME KIND.
ATMOSPHERIC MOTION IS CONTROLLED BY THE INTERPLAY BETWEEN 5
FORCES:
1. THE PRESSURE-GRADIENT FORCE
2. GRAVITY
3. THE CORIOLIS FORCE
4. FRICTION
5. CENTRIPETAL FORCE
1. THE PRESSURE GRADIENT FORCE (大氣梯度)
-- INFLUENCE THE DIRECTION AND SPEED OF WIND
Air motion is initiated by a pressure gradient between places, with initial movement
occurring from high to low pressure locations. The air is pushed from areas of
high pressure to areas of low pressure. The air ought to move at right angles to the
isobars.
(Spatial variations of pressure are depicted on maps by means of isobars, which are
lines connecting places having the same barometric pressure).
A change in barometric pressure across a horizontal surface constitutes a pressure
gradient. In other words, the gradual change of pressure between different areas is
known as pressure gradient. Where a pressure gradient exists, air molecules tend to
drift in the same direction as that gradient. This tendency for mass movement of air is
referred to as the pressure gradient force.
The magnitude of the pressure gradient force is directly proportional to the steepness
of the gradient. A simple relationship between pressure-gradient and wind speed
exists: the steeper the pressure-gradient, the faster the wind speed.
Falling (low) pressure generally generate the onset of poor unstable weather, and a
rising barometer(high pressure) suggests a trend towards sunny stable weather
conditions.
~
A pressure gradient exists both vertically and horizontally.
(a) Vertical Pressure Gradient
Pressure decreases vertically. As we move upwards through the atmosphere, the
weight of overlying air diminishes.
Obviously, the layers closest to the surface will have the greatest weight overlying
them and thus the pressure will be greatest. Therefore, rapid decrease in air pressure
occurs with increasing height.
(b) Horizontal Pressure Gradient
Pressure varies laterally because of the temperature differences resulting from
differences in the intensity of solar heating of the atmosphere.
Where solar radiation is intense, the air warms up, expands and its density decreases.
As a result, air pressure falls.
Where cooling occurs, the air contracts, its density increase and air pressure becomes
greater.
(c) Pressure in the Upper Atmosphere
But the pattern of air pressure close to the surface is reverse in the upper atmosphere.
This is because as cold air contracts, the upward decline in pressure is rapid and at
any constant height above a zone of cool air the pressure is relatively low. (High
pressure at lower atmosphere, but low atmosphere at the upper)
Conversely, warm air expands and rises, so that the vertical pressure gradient is less
steep. Above areas of warm air, therefore, the pressure tend to be relatively high.
2. FORCE OF GRAVITY (地心吸力)
-- acts against vertical air motion
The greatest pressure gradients occur vertically in the atmosphere, not horizontally.
This would appear to suggest that the atmosphere should shoot away from the earth
into the vacuum of space. The fact that this does not happen is because the vertical
pressure-gradient force is almost always balanced by the force of gravity acting in
the opposite direction. These two great forces cancel each other to produce a state of
stability. Relatively small imbalances occur between them, producing rising and
sinking motions.
3. CORIOLIS FORCE [科奧利氏力]/Deflective force [地球自轉偏向
力]
The coriolis force is the effect produced by the rotation of the earth on a body moving on
its surface. It is this force that winds do not follow the direction of pressure gradient and
are deflected.
-- acts at right angles to wind direction
-- great at the poles
-- influences wind direction, but not wind speed.
Earth rotation produces the Coriolis effect, which tends to turn the flow (direction) of
air. The direction of action of this turning effect is stated in “Ferrel Law”:
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The Coriolis effect acts to turn winds or ocean currents to the right of its path in the
northern hemisphere and to the left of its path in the southern hemisphere.
- The Coriolis effect turns the winds so that it crosses the isobars at an angle.
- The degree of deflecting force varies with the speed of the moving air and with
latitude.
- Near the equator, where the earth surface is spinning around in a plane almost
parallel to the axis of rotation, the Coriolis force is very slight. However, it increase
in strength toward the poles.
- For surface winds, the angle of turning is limited by the force of friction of the air
with the ground.
- The magnitude of the deflection is directly proportional to
(a) the higher the speed, the greater the deflection.
(b) the force is maximum at the poles and becoming zero at the equator.
Geostrophic wind:
~ Wind moves down the pressure gradient from high to low pressure.
~ However, as the wind begins to move, it will start to be affected by the Coriolis
force. It will be deflected towards the right in the Northern Hemisphere.
~ When a wind ( in this case called geostrophic wind(地轉風) blows parallel with
isobars, the Coriolis force is just balanced by the pressure gradient force.
~ the geostrophic wind blows parallel with the isobars, with high pressure to the right
in Northern Hemisphere. Winds are essentially geostrophic in character above about
1000m.
4. CENTRIPETAL ACCELERATION (向心加速力)
~ For a body to follow a curved path there must be an inward acceleration towards the
center of rotation. This acceleration (C ) is expressed by:
C =
MV 2 / r
Where m = the moving mass
V = its velocity
R = the radius of curvature
-
this force applies when the isobaric pattern is curved as within cyclone
or around high-pressure centers (e.g. anticyclone). It pulls the air inwards.
Low Pressure System
High Pressure System
-
Low pressure centre : motion around a low-pressure centre, anticlockwise in the
northern hemisphere, is termed cyclonic, and in this case the result of the
centripetal effect is to make the Coriolis force weaker than the pressure
gradient force. The difference between the two gives the net centripetal
acceleration inward.
The wind is subgeostropic”.
- High pressure centre: motion around a high pressure centre, clockwise in the
northern hemisphere, is termed anticyclone, and in this case the result of the
centripetal effect is to make the Coriolis force stronger than the pressure gradient
force. The inward acceleration is provided by the Coriolis force exceeding the
pressure force. The wind is supergeostrophic”.
5. FRICTIONAL FORCE (磨擦力)
- For wind movement close to the ground surface, frictional force reduce wind
velocity and hence the deflective force decreases as well. Therefore at lower levels,
the wind blows more obliquely across the isobar..
- surface wind does not follow parallel with the isobars, but instead crosses them at an
oblique angle.
- the greater the friction, the wider is the angle the wind direction makes with the
isobars; i.e. the closer the wind direction is to the direction of the pressure gradient.
- Therefore, winds over irregular land surface usually form angles from 20 to 45 with
the isobars. But over oceans, where the frictional drag is less, the angle may be as
small as 10.
- observations in the free atmosphere (above the level affected by surface friction at
about 500 to 1000 m) show that the wind blows more or less at right angles to the
pressure gradient (i.e. parallel to the isobars). It is called ________________. For the
northern hemisphere the high pressure on the right and the low-pressure on the left
when viewed downwind. This implies that , in the absence of frictional force, the
pressure gradient force is exactly balanced by the Coriolis force in the opposite
direction.
C. PRIMARY CIRCULATION / PLANETARY
CIRCULATION
1. Global Distribution of Surface Pressure Systems
-- in order to understand the planetary surface wind system, we must study the
global pressure system distribution.
The above diagram also show the global pattern of surface
winds.
2. The Global Pattern of Planetary Surface Winds
~ assume that the Earth’s surface is a uniform body of water/land
~ pressure gradient force and Coriolis force cause the planetary wind system.
Planetary Wind System
Trades
Origin
From subtropical high pressure
Types
Characteristics
NE Trades in northern hemisphere ~ they blow from 30N and 30S
belt to Equatorial low pressure
SE Trades in southern hemisphere ~ they are constant in strength and
direction
~ trades from the two hemisphere
converges to form the
Intertropical Convergence Zone
(ITCZ)
Westerlies
From subtropical high pressure
South–westerlies in the northern
~ they blow between 30N and
belt
hemisphere
60N or 30S and 60S.
To temperate low pressure belt
North—westerlies in the southern ~ westerlies meet polar easterlies
hemisphere
along a polar front
~ owing to the great sea surface in
the southern hemisphere, the
westerlies are more constant on
strength and direction than in the
northern hemisphere.
Polar Easterlies
From polar high pressure belt to
North—easterlies in the northern ~ they blow between 60N and
temperate low pressure belt
hemisphere
90N or 60S and 90S
South—easterlies in the southern ~ easterlies meet westerlies
hemisphere
along a polar front
~ owing to the great sea surface in
the southern hemisphere, the
easterlies are more constant on
strength and direction than in the
northern hemisphere.
(a) The trade winds:
~ The trades are important because of the great extent of their activity.
~ They blow over nearly half of the globe.
~ They originate from the low latitudes on the margins of subtropical high pressure
cells, and their constancy of direction and speed is remarkable. The two trade-wind
systems tend to converge in the equatorial trough.
~ Over the oceans, particularly the central Pacific Ocean, the convergence of these air
streams is pronounced and in this sector the term Inter-Tropical Convergence Zone
(ITCZ) is applicable.
~ The two trades wind belts are the North-east trades and South-east trades.
(b) The Mid-latitudes Westerlies:
~ In comparison to the winds of the tropics, the westerlies of the mid-latitudes seem
unreliable and fickle, They are westerlies only on the average,
~ Westerlies are the poleward movement of air from the sub-tropical anticyclone.
These systems cover vast areas and can clearly be seen from space, identified by their
characteristic spiral of clouds. They tend to move north-northeastwards.
~The westerlies of the southern hemisphere are stronger and more constant in
direction than those of the northern hemisphere because the broad expanses of ocean
rule out the frictional effect.
(c ) The Polar easterlies:
~ This term is applied to winds which are supposed to occur between a polar high
pressure and the belt of low pressure of the higher mid-latitudes.
~ Around the poles, there is some evidences of prevailing easterlies. The winds are
variable and linked to the shallow polar anticyclones.
Planetary pressure belts shift northward in June but southward in December following
the movement of the overhead sun.
The planetary wind system follows the shift of the planetary pressure belts.
In June, with the northward shift of the equatorial low pressure belt, SE trades cross
the Equator and are deflected as SW trades.
In December, with the southward shift of the equatorial low pressure belt, NE trades
cross the Equator and are deflected as NW trades.
3. TRI-CELLULAR MODEL
The general circulation in the atmosphere is conceptualized as TRI-CELLULAR
MODEL. It shows the relationship between the general atmospheric circulation and
the major surface wind systems
a. LOW-LATITUDE CIRCULATION - THE HADLEY CELL(哈得來
環流圈)
~ In tropical latitudes, atmospheric general circulation is dominated by the meridional
Hadley cell, named after the Englishman George, who in 1735 first proposed the idea of a
tropical cellular circulation.
~ The Hadley cell establishes a pattern of pressure and a system of winds that
attempts to explain the weather of low latitudes, e.g. tropical rain forest and tropical
desert.
~ The rising limbs of the Hadley cells generally lie on either side of a belt of
equatorial low pressure belt known as the Doldrums. It is the Inter-Tropical
Convergence Zone (ITCZ) where the Trade Winds from the two hemispheres are
drawn together in a zone of rising air. This convergence zone moves north and south
with the following the movement of overhead sun.
~ It travels further over the land than over the sea because, for land warms faster than
water.
~ In this cell, air near the earth moves equatorward as the surface winds called
Trade winds. Trade winds pick up moisture and latent heat over the oceans and this
heat is liberated when the water vapor condenses in towering clouds in the rising limb
of the cell. (The ascending limb is a broad area of rising air)
~ The heat liberated in the ascending air on the equatorward side of each Hadley
Cell then drifts poleward at the top of the cells as upper winds.
Discuss the climate of tropical rain forest .
~ In the poleward limb of the Hadley cell, sinking air produces the high pressure cells
(anti-cyclones) at subtropical high pressure belt ( horse latitudes).
Easterly Trade winds blow towards the equatorward side and westerlies towards the
poleward sides of these subtropical high pressure belts.
~ Anticyclonic subsidence leads to the sinking, compression and warming of the air,
but this can occur only down to the upper limit of strong convection rising from the
warm earth (1800 to 6000 m) in the anticyclone. It frequently damps down or halts
currents of rising air from the ground. In consequence, clouds are often stunted and
only occasionally give. This is the fundamental explanation of the tropical deserts.
Discuss the climate of tropical desert ?
b. MID-LATITUDE CIRCULATION -- THE FERREL
WESTERLIES
~ In the mid-latitudes, weather is dominated by the consequences of air movement
through the migratory waves of the Ferrel westerlies in the middle and upper
troposphere.
~ In the middle latitudes, air in the Ferrel cell, westerlies moves from west to east
through a series of waves.
~ The upper westerlies are disturbed by a number of waves called Rossby waves (羅
斯貝波), which stretch from the sub-polar to tropical latitudes. There are generally
about four in each hemisphere. These exist because of:
~ the existence of large mountain barriers,
~ the uneven distribution of land and sea,
These waves may grow, change in form, remain stationary for many days or drift
slowly, and then disappear. They develop where cold polar air meets warm tropical air,
as the wave forms, the sinuosity may increase, developing troughs and ridges.
Troughs may become detached and form a low pressure centre with cold air
circulating anticlockwise. A ridge may also detached forming a high pressure centre
of warm air in the mid-latitudes, bringing great quantities of heat to polar regions.
c. POLAR CIRCULATION -- THE POLAR CELL
~
In polar regions, there is a weak cellular overturning known as the polar cell
~
but this frequently disappears to give high pressure over the poles.
~
This is because in high altitude the troposphere is only half as deep as in the
tropics, hence the circulation systems are shallow and often short-lived.
SUMMARY :
i.
The general circulation of the atmosphere is powered by solar radiation.
ii.
The circulation operates in response to the disparities in energy inputs between
different parts of the world. It acts to reduce these disparities;
iii.
Through these various links in the general circulation, set up by radiation on a
rotating earth producing thermal and pressure differences, energy is
transferred in a variety of forms from areas of surplus to areas of deficit.
In so doing the fundamental character of the world patterns of winds and
climates are established.
iv.
The circulation represents one of the main processes of energy transfer
through the global system. By this process, the earth preserves as steady state
within the system, preventing the accumulation of heat in the tropics or the
perpetual cooling of the poles.
v.
Energy is transferred in a variety of forms, and during these transfers energy
undergoes numerous transformations.
Four main forms of energy exists in the atmospheric circulation:
- sensible heat
vi.
- latent heat
- potential energy
-kinetic energy
Winds are thus a consequence of inequalities in net radiation receipts (input)
and at the same time a means of balancing them out by transfers of energy in
various forms. Energy is transferred both vertically and horizontally
(latitudinally) in the Hadley cell.
3. Winds in the Upper troposphere
a. In general, airflow at high altitude parallel the isobars. It is because friction
force is largely absent. The ideal case is the pressure gradient force and
Coriolis exactly balanced each other and geostrophic wind result.
b. Jet Stream: It is a narrow zone of very high wind speed, usually formed along
the line of contact of cold and warm air. It forms at the high level of the
troposphere. The very steep pressure gradient at the frontal zone causes the
high-speed flow of air (up to 350 to 450 km per hour).
Two major jet stream:
(i) Polar Front Jet Stream : Between polar tropopause and mid-latitude
tropopause.
(ii) subtropical Jet Stream: Between mid-latitude tropopause and tropical
tropopause.
D. SECONDARY CIRCULATION (次環流)
These include the monsoon system, trough, ridge, anticyclone, and cyclone.
1.
Trough (低壓槽)
!
a trough is a extension of low pressure centre or a linking up of different low
pressure centres.
!
Weather under trough :
!
-
temperature slightly lower
-
low pressure
-
thunderstorm, convectional rain, cloudy and unstable weather.
-
Calm conditions and variable winds
-
High relative humidity
-
Poor visibility
Effect of trough :
-
Landslides / rockfall
-
Flooding on farmland
-
Damages / loss of crops an livestocks, e.g. pigs and fish (in pond)
-
Damage / loss of man’s life and property, e.g. houses.
-
Temporary closure of schools, shops and offices.
2. Ridge (高壓脊)
!
Ridge is an extension of a high pressure center / anticyclone.
!
Weather under a ridge :
~ same as anticyclone
!
Weather under ridge:
- temperature slightly higher
-
high pressure
3. Anticyclone (反氣旋) and Cyclone (氣旋)
Anticyclone
------
Types
Shape of Isobars
Pressue center
Movement of air
Wind
Circular
Cyclone
Tropical, temperate
Circular
High (pressure decrease
Low (pressure decrease
outward)
inwards)
Sinking
Rising
In the Northern
hemisphere
Out-blowing in a clockwise
In-blowing in an
direction
anticlockwise direction
In the Southern
hemisphere
Out-blowing in a
In-blowing in an clockwise
anticlockwise direction
direction
In summer, warm, dry and
Unstable weather
Associated weather
sunny weather
Rain and thunderstorm
In winter, cold, dry and stable
weather
Condensation & rain:
- Clear sky with few cirrus
- Dew, frost fond in the
morning
Other characteristics
- slow-moving, long-lasting
Tropical cyclone is fast
and large in extent
moving, short-lived and
small in extent
-
-
in the center, calm
condition
In the center, calm
close isobars at the
condition
edges and the
-
resulting wind is
Close isobars near the
strong
center and the resulting
wind of tropical cyclone is
strong
4. Tropical Cyclone (熱帶氣旋)
Place of Origin:
~ found between 10 and 20 N and between 10 and 20S
~ in East Asia, often formed in the east/ around Luzon Island of the
Philippines which is a tropical area.
~ formed over humid ocean surfaces where there are strong convection
and water vapor for the cyclone
~ formed along the ITCZ where the two trades converge.
Path of Movement:
~ the tropical cyclone rotates by Coriolis force
~ often moves westwards following the westwards movement of trades
~ due to Coriolis force, its path gradually curves to the right in the
northern hemisphere and to the left in the south hemisphere.
Structure of tropical cyclone:
Weather conditions with the passage of a tropical cyclone
The day before the arrival of a tropical cyclone at X:
(1) Temperature and humidity are high
(2) There is dead wind and still air
(3) Pressure starts to drop
(4) It is sunny
(5) The sky is clear with cirrus
X at the front vortex:
(1)
There is a distant fall of temperature.
(2)
Wind force increase from gentle breeze to
hurricane
(3)
Wind direction is either NE or NW
(4)
Pressure is low
(5)
High and thick cumulonimbus develops together
with thunderstorm
(6)
Visibility is low
X in the “eye”:
(1)
Temperature rises slightly
(2)
There is gentle to moderate breeze
(3)
Pressure is the lowest (e.g. 980 hPa)
(4)
Cool air descends with no condensation, therefore
rain stops
(5)
The sky is clear, and sunny
(6)
Visibility is good
X at the rear vortex:
(1)
Temperature drops
(2)
Wind speed increases to hurricane force
(3)
Wind comes mainly from SE or SW.
(4)
Pressure is still low but starts to rise
(5)
Dense cumulonimbus brings torrential rain
(6)
Visibility is low
The day after the passage of a tropical cyclone:
(1)
Temperature rises to normal conditions
(2)
Wind speed decreases to light breeze
(3)
Pressure is relatively high
(4)
Cirrus clouds give no rainfall
(5)
The weather is fine
(6) Visibility is good.
Effects of tropical cyclone
~ heavy rainfall provides much water for reservoirs
~ landslides / rockfall
~ flooding by rainwater / river water, e.g. on farmland
~ storm surge: rise in sea level causes flooding by sea water in lower-lying areas
~ damage / loss of crops and livestock, e.g. pigs and fish (in ponds),
~ damage / loss of life and property, e.g. houses and boat
~ temporary closure of schools, shops and offices,
~ destruction of trees / plants
~ disturbance of land, sea and air transport
Tropical cyclones may cause considerable loss of life and damage to property and
crops. This may be the direct result of the following features associated with
hurricanes.
~ High winds: which often exceed 160 km/hk, in extreme cases, over 300 km/hr.
Whole villages may be destroyed in less developed countries while even reinforced
buildings in the southern USA may be damaged. Countries whose economies relie
largely on a single crop may suffer serious economic problems, while the electricity
and communications links can be severed causing further difficulties.
~ Ocean storm surges: may inundate coastal areas, many which are densely populated.
~ Flooding : may be caused either by storm surge or the intensive rainfall.
5. Temperate Cyclone (溫帶氣旋)
Places of Origin:
~ These temperate cyclone originate in the higher mid-latitudes probably 40-60 N & S
especially in the belt of westerlies.
~ These are the latitudes where the polar air and the tropical air meet.
(Note: (a) The polar air, originating over cold land or water in high latitudes, moves
southerward as their masses; (b)The tropical air. Originating over hot land or water in
low latitudes, moves northward as air masses)
~ often travels westward following the path of westerlies
~ They may extend their influence to high and low latitudes
Formation Processes :
Stage 1:
In the temperate latitudes, two contrasting air masses – one polar and one tropical – move
towards each other.
When the two air masses meet, the zone of contact is called a front.
This zone of contact between the polar and the tropical air mass is called a polar front.
Along the polar front, the cold polar air and the tropical air masses are of contrasting
temperature, humidity and density.
Polar air is colder, drier, denser and heavier than the tropical air.
Stage 2:
When the two air masses meet together, there is friction created.
As a result, a wave is developed along the frontal surface and become larger..
Stage 3:
As the bulge (waves) continues to develop, it tend to form two distinctive fronts.
In the eastern portion of the front, warm air is moving towards cold air and is known as
warm front.
In the western portion of the front. Cold air is moving towards warm air and is known as
cold front.
The point where the two fronts meet forms the center of the system.
The warm air which lies between the two fronts is called warm sector.
By this stage, the temperate cyclone is its mature stage.
On the Weather Chart:
Structure of temperate cyclone:
Stage 4:
The cold front moves faster than the warm front.
The warm sector decreases in size as more and more warm air is uplifted.
Finally, the cold front overtakes the warm front.
The heavier cold air undercuts the warm air and lifts it aloft.
The uplift of warm air leads to condensation and formation of light rain.
Later, it mixes with the cold air above and disappears.
Neither front exists on the ground level.
This stage is called an occlusion and the front is an occluded front.
The temperate cyclone is weakened and soon dies away.
The life cycle of a temperate cyclone may last for 24 hours to as many
as five days.
Weather Associated with the passage of temperate cyclone:
X is under the warm front:
(1) T℃is low since the place is under the
effect of cold air
(2) Pressure is relatively high but starts to
drop
(3) Winds mainly come from the southeast
(4) Stratus or nimbostratus develops
Therefore the sky is overcast and light rain
falls for a long duration.
X is under the warm sector:
(1) T℃ rises
since the place is under the
effect of warm air
(2) Pressure is low due to rising air
(3) Winds mainly come from the SW
(4) Thin cloud cover
(5) The sky is sunny
Air humidity is high
develops
Under cold front in N.Hemisphere:
- temperature decrease suddenly due to the
cold northerly wind
- pressure rise gradually due to cold air
- wind become stronger and come from NW
- As the warm air is forced to rise over the
cold air, thick cumulonimbus develops.
- cloudy and unstable weather condition
- Heavy rain or thunderstorm is experienced
but lasts only a short duration.
- cyclonic rain
- poor visibility
After the passage of cold front:
-
cold conditions exist fore a few days
-
pressure is high
-
NW or N wind direction
-
fine, sunny and stable weather condition
-
rain stops, air humidity is low
-
good visibility
6. The Monsoons (季侯風)
The word “Monsoon” is derived from the Arabic for a season but the term is more
commonly used in meteorology to denote a seasonal reversal of wind direction. The
major monsoon occurs in SE Asia and results from some factors:
Definition : A type of wind system in which there is a complete or almost complete
reversal of prevailing direction from season to season. It is formed by the difference
in heat dynamics between land and sea.
= The northward movement and southward movement of the ITCZ during the
northern hemisphere summer.
= Due to different rates of heating and cooling of the land and sea , it cause the
extreme heating and cooling of large landmass in relation to smaller heat changes
experienced over adjacent sea areas. This in turn affect pressure and winds.
= this then cause extensive areas of low pressure in the summer and high pressure in
the winter of the Asian mainland.
In July, (Summer Conditions in Northern Hemisphere)
~ The overhead sun moves to the Tropic of Cancer,
~ It is summer in northern hemisphere,
~ Due to different rate of heating of the land and sea, the air over the Asia inland heats
more quickly than over the ocean,
~ heat air over the land rises, and an extensive low pressure system develops over central
Asia.
~ cool air over the ocean sinks, so the pressure is higher.
~ It is winter in southern hemisphere,
~ air over central Australia cools more rapidly than over the ocean due to differential
rates of cooling of the land and sea.
~ cold air over the land sinks, and an intense high pressure system develops over
central Australia.
~ Winds blow anticlockwise from the Australian high pressure system across the
equatorial low pressure belt to the ocean and to the low pressure system of central
Asia.
~ these winds blow SE from Australia as SE Monsoon
~ after crossing the Equator, they are deflected to the right in the northern hemisphere
by Coriolis force
~ they blow SW over SEAsia as SW Monsoon
~ as they continue northwards they enter the anticlockwise circulation of the Asian
low pressure system.
~ they blow SE over South China (including Hong Kong) and Japan as SE Monsoon.
Effects on Hong Kong:
~ warm onshore SE/SW/S monsoon winds
~ they are wet and bring much rainfall
~ alternately cloudy sky
In January, (Winter Conditions in Northern Hemisphere)
~
The overhead sun moves to the Tropic of Capricorn,
~ It is winter in northern hemisphere,
~
Due to different rate of cooling of the land and sea, the air over the Asia inland cools
more quickly than over the ocean,
~
cold air over the land sinks, and an extensive high pressure system
develops over central Asia.
~
warm air over the ocean rises, so the pressure is lower.
~ It is summer in southern hemisphere,
~
air over central Australia heats more rapidly than over the ocean due to
differential rates of heating of the land and sea.
~
hot air over the land rises, and an intense low pressure system develops over
central Australia.
~ Winds blow clockwise from the Asian high pressure system to the ocean, across the
equatorial low pressure and to the low pressure system of central Australia.
~ these winds blow NW from northern China and Japan as NW Monsoon
~ they are deflected to the right and blow NE over South China (including Hong
Kong) and SE Asia as NE Monsoon
~ After crossing the Equator, they are deflected to the left in the southern hemisphere
by Coriolis force.
~ they blow NW to Northern Australia as NW Monsoon.
Effects on Hong Kong:
~ cold offshore NE/NW/N monsoon winds
~ they are dry and bring little rainfall
~ fine weather and cool winter
7. Air masses (氣團)
An important agent for transporting heat, moisture, and energy from one part of the
earth to another is the air mass. Air masses are large “Parcels” of air, which have
uniform characteristics of temperature, moisture and stability in a horizontal direction
within them.
The interaction of these air mass with the earth’s surface , gives a characteristic type
of weather which usually persists over a place for a period of time, until replaced by
conditions associated with another air mass of different characteristics. Over a longer
period, the passage of a sequence of air mass produces the distinctive climate of a
region.
Definition :
It is a body of air in which the upward gradient of
temperature, moisture content and lapse rate are fairly more or less uniform
horizontally over a large area.
In other words, a given air mass is characterized by a distinctive combination of
temperature, specific humidity. It usually extends horizontally for hundreds of
kilometers and vertically through the troposphere.
Main factors tend to determine the nature and degree of uniformity of air mass
characteristics. They are:
(a) the nature of the source area and the direction of movement
(b) changes that occur in the structure of air mass as it moves over long distances.
(a) Source regions and classification. (來源地及分類)
Two basic conditions are needed for the formation of an air mass.
== A region with more or less uniform geographical environment in which the air
mass obtain its original qualities; e.g. oceans, desert, snowed covered areas.
==
A region in which the air subsides and diverges. A region in which an air mass
grows is called the Source of air mass. Such land or ocean surface in different
latitudes can strongly impress the temperature and moisture characteristics of the
overlying air masses.
Air mass acquire their characteristics by remaining over a particular land or ocean
surface for a considerable period, so that the temperature and moisture conditions of
that surface are transferred to the overlying air mass. This surface is the air mass
source region. (Such regions is large and homogeneous in terms of temperature and
moisture.)
These source regions are found where the air is still, or stagnating, as for
example at the centres of cells of high pressure (called anticyclones) where air is
descending and diverging.
Air masses classified according to two categories of source regions:
==
the latitudinal position on the globe
(determines thermal properties)
== the underlying surface, whether continents or oceans (determines moisture
content)
Five types of air masses are as follows:
Air Mass
Symbol
Source Region/ Chief Characteristic
Arctic
A
Arctic ocean and fringing lands (Cold)
Antarctic
AA
Antarctica
Polar
P
Continents and Ocean
(Cold)
(Lat. 50-60 N &S)
Tropical
T
Continents and Ocean
(Lat. 20-35 N &S)
Equatorial
E
(Cold)
(Warm)
Oceans close to Equator
(Hot)
With respect to type of underlying surface (ocean or continents), two further
subdivisions are imposed on the preceding types:
Air Mass
Symbol
Source Region / Chief Characteristics
Maritime
m
Oceans
(moist)
Continental
C
Continents (dry)
By combining types based on latitudinal position with those based on underlying
surface a list of six important air masses results.
Properties of Six typical Air Masses
Air Mass
Symbol
Properties Temperature Temperature Specific
℃
Continental
(cA)
Very cold,
very dry
Humidity
(g/kg)
F
- 46
-50
0.1
-11
12
1.4
4
39
4.4
24
75
11
75
17
Arctic
(winter)
(Continental (cAA)
- Antarctic)
Continental
(cP) Cold, dry
(winter)
Polar
Maritime
Polar
Continental
(mP) Cool, moist
(winter)
(cT) Warm, dry
Tropical
Maritime
(mT) Warm,
moist
24
(mE) Warm, very
27
Tropical
Maritime
equatorial
Moist
80
19
Cold air mass:
The principle sources of cold air mass in the northern hemisphere are:
~ the continental anticyclones of Siberia and northern Canada, which originate
continental polar (cP) air masses, and
~ the Artic Basin, when it is dominated by high pressure, this air mass is also
designated as continental arctic (cA).
Warm air mass:
~ These have their origins in the subtropical high pressure cells and, during the
summer season, in the accumulations of warm surface air characterize the heart of
large land areas.
~ The tropical (T) sources are either maritime (mT), originating in the oceanic
subtropical high pressure cells or continental (cT), originating from the major
continental in summer.
~ The maritime type is characterized by high temperature, high humidity .
~ The continental type is characterized by warm, dry.
In winter, air mass of North Hemisphere
In summer, air mass of North Hemisphere
In winter, air mass of South Hemisphere
In summer, air mass of South Hemisphere
b. Modification of air masses (氣團特性的改變)
! Air masses do not always remain in their source regions. They move out from
these areas along a path called trajectory, taking with them the characteristics they
acquired in their source region.
!
Air masses move from one region to another, following the pattern of
barometric pressure (pressure gradient).
!
Frequently, the boundaries of the air body may not be well defined, and in such
cases it is identified as an air stream.
!
In moving from their source regions air masses and air stream are modified by
the surface over which they pass.
-- During these migration, lower levels of the air mass undergo gradual
modification, taking up or losing heat and water vapor to the surface beneath.
-- The change of the physical properties in an air mass begins from the lower layer
and spread to the higher layer.
!
A cold air mass passing over a warmer surface, for example, tends to be heated
from below and so becomes unstable due to the convective uplift. Such air
masses have their lapse rates increased and are given the notation “K” to
indicate their instability.
An example of this occurs in North America where stable polar continental air
masses, originating over the cold land surface of northern Canada, migrate
south and east across the Mississippi lowlands. These polar outbreaks may
brings cold weaves to as far south as Florida where they are known as
“northers”.
Commonly, these polar continental air masses cross relatively warm water
surfaces such as the Great lakes causing them to be warmed from below, to pick
up moisture, and to become unstable (i.e. PcK). Thus they may bring snow to
lakeside, even though they were originally cold, dry and stable in their source
region. In other word, the weather they have produced has been partly the result
of their modifications along their trajectory.
!
When warm air mass passes over a cooler surface, thereby being cooled from
below and in consequence, made more stable since convective uplift. The
notation “W” is used to indicate instability.
For example, in North America, moist tropical air masses originating over the
eastern pacific and moving northwards towards the Californian coast are cooled
by the cool California Current (i.e. TmW). This increases their stability, and
may even produce a surface inversion. Because these masses are of maritime
origin, and thus are moist, this surface cooling produces low stratus clouds and
frequently fogs. Over such cities as Los Angeles, the fog may combine with
particles in the polluted air to form SMOG. Precipitation will only occur if this
stable air is force to rise over some large barrier such as the pacific Coast
Ranges, which is sufficient to overcome the natural stability of the air mass.
CONDITION
1. When air mass advances to a warmer region, its air is colder
SYMBOL
K
than that beneath, therefore called cold air mass.
2. Air mass moves towards a colder region, therefore called
W
warm air mass
3. When air mass will stay close to its source region, the air
S
remains stable
4. When air mass enters a region where the ground is heated up
and the air rises rapidly, the air is unstable.
U
e.g..
8. Frontogenesis (氣旋學說)
- A front was regarded as a relatively narrow zone separating two air masses.
- In the course of their migration air masses and air stream frequently come into
contact with other air masses of dissimilar character. Because the air masses are of
different temperature and moisture characteristics, and thus different densities, they
do not mix.
- Like warring arms, they confront each other along a surface of contact called front.
(A given air mass usually has a sharply defined boundary between itself and a
neighboring air mass. This discontinuity is termed a front.)
- Being areas of conflict, fronts are characteristically associated with unsettled and
changeable weather and frequently frontal precipitation.
- Fronts may be nearly vertical, as in the case of air masses having little motion
relative to one another. Fronts may be inclined at an angle where one air mass is
sliding over another.
Frontogenesis : - the process of growth of front.
-
When cold air is invading the warm air mass, it is called a cold front.
-
When warm air is moving into a region of colder air, it is called a warm front.
In both cases, the cold air mass remains in contact with the ground and the warm
air mass is forced to rise over it.
Along the cold front, since the cold air has a greater density than the warm air, the
gradient of the frontal surface for a cold front is usually greater. The invading cold air
forms a wedge which pushes the warm air vigorously and steeply upwards, usually
resulting in high cumulus clouds, heavy rain, and thunderstorms along the frontal
surface.
Along the warm front, warm air invades a cooler air mass. Being denser, the colder
air will remain closer to the ground, but the warmer air will force it back by sliding up
and over it on a gently sloping frontal surface. Warm fronts, therefore, are more likely
to give rise to stratus clouds, often apparent high in the sky. Rain from a warm front
tends to be more widespread and steady than from a cold front, but is less intense and
heavy.
Occluded front:
When cold and warm fronts shift towards the same direction, and the cold front
advances in a higher speed, the cold front will catch up with the warm front to form
the occluded front. In an occlusion the cold air will occupy the land surface and forces
the warm air to rise. Because of the extra uplift of warmer air at an occlusion very
heavy rain may fall in its vicinity.
There are two main regions in the world in which frontogenesis takes place most
frequently:a. Polar front: In the higher mid-latitudes, between the westerlies wind belt and
the polar easterlies wind belt. The polar front separate poleward flowing
tropical air and equatorward flowing polar air. Many temperate cyclones are
generated throughout the year.
b. Intertropical Front: In the low equatorial latitudes, it is a zone of convergence
of tropical air masses from the low latitudes of both hemisphere. It is ITCZ
(Intertropical Convergence Zone) between the two trade wind belts in both
hemisphere.
Since the converging tropical air masses are similar in nature, they do not
produce variable weather, but their convergence does produce uplift and heavy
precipitation throughout this broad zone. Tropical cyclones are formed.
9. Tornadoes:
Characteristics:
~ A small but intense cyclone in which air is spiraling at tremendous speed. They
are caused by the convection along the fast moving fronts in middle latitudes.
~ it is appears as a dark funnel hanging from the base of a dense cumulonimbus
cloud. At its lower end, the funnel may be 100 to 450 m in diameter.
~ The funnel appears dark because of the density of condensing moisture, dust and
debris swept up by the wind, which can reach a speed of 400 kilometers per hour.
While moving, the funnel writhes and twists, sweeping the ground.
Formation:
~ They occurs as parts of cumulonimbus clouds travelling in advance of a cold front.
~ They are most common in spring and summer, often found over the continental
plains of the USA.
~ There exists layers of air with great contrasts in temperature, moisture and wind. In
summer, when maritime polar air mass lifts warm, moist tropical air mass at a cold
front, a tornadoes occurs.
Effects:
Tornadoes cause great wind stress and a sudden reduction of air pressure within the
cyclonic spiral. Houses will explode as the funnel passes overhead. Its passage can
lead to the complete destruction of anything in its path.
E. Tertiary Circulation / Local Circulation
Local winds are generated by immediate influence of the surrounding environment
(e.g. terrain and micro-climate) rather than by the large-scale pressure systems that
produce global winds and large traveling storms.
Tertiary circulation includes local winds:
Fohn/Chinook,
Land and sea breezes,
Mountain and valley winds.
1. Fohn/ Chinook: Refer to the “Formation of Relief Rain”
Place of Occurrence and Characteristics:
-
This is another local wind found on the leeward slopes of high mountains.
-
When strong regional winds passing over a mountain range are forced to
descend on the leeward side,
-
with the result that air is heated adiabatically and dry. It is called Fohn wind.
-
It is relatively warm, dry wind which descends a mountain slope.
-
Various names are applied to such winds found in different places:
-
Chinook (N. America)
-
Fohn (European Alps)
-
Sirocco (North Africa)
-
Bora (Yugoslavia)
Formation:
- When an air flow is strong enough to cross over a mountain, gravity makes the air
sinks.
- It becomes warmer as it drains lower the mountain slope.
- Warming of the air is unfavorable for condensation as relative humidity reduces.
- So this side is much drier and is know as rainshadow side.
- In addition, such descending warm wind sometimes has the effect of melting winter
snow at the foothill and the adjacent lowland.
2. Land and sea breeze:
Land and sea breeze are caused by differential rates of heating and cooling of the
land and sea.
Sea Breeze:
- During the daytime, land heated more rapidly than sea. So the air in contact
with the land is heated, rises and forms an area of lower pressure. The sea heats
more slowly. So the air in contact with the sea is cooler, sinks and forms an area of
higher pressure.
- Air moving landward in response to this gradient from higher to lower pressure
constitutes sea breeze.
Land Breeze:
- During the night time, land cools more rapidly than sea. So the air in contact
with the land is cold, sinks and forms an area of higher pressure. The sea cools more
slowly. So the air in contact with the sea is warmer, rises and forms an area of lower
pressure.
- Air moving seaward in response to this gradient from higher to lower pressure
constitutes land breeze.
3. Mountain and Valley wind
Mountain Wind
Valley Wind
Places of Occurrence and Characteristics:
~They are also winds if daily periodicity.
~A local wind following a daily alternation of direction between mountain and valley.
~ They are the reversal of wind direction between the mountain and the valleys within
a day.
Mountain Wind:
On a clear nights, the mountain sides radiate heat and are cooled.
The air in contact with the mountain sides is cooled quickly as well and produce high
pressure.
The valley floor cools slowly and produce low pressure.
The cool, denser air then flows down the mountain slopes and drains into the valleys.
This is called the mountain wind.
Valley Wind:
On a warm sunny day, the exposed hillslope are intense heated.
The air in contact with the hillslope is heated quickly as well and produce low
pressure.
The valley floor heats up slowly and produce high pressure.
This results in a strong rising of air along mountain slope.
Upslope flow of air from the valley bottom begins.
This is called valley wind.
F. Ocean Current (洋流)
- An ocean current is any persistent, dominantly horizontal flow of ocean water.
- Important regulators of thermal environment at the earth’s surface.
- On a global scale, the vast current systems aid in exchange of the heat between
low and high latitudes and are essential in maintaining the global energy balance.
- On a global scale, warm water currents bring a moderating influence to coasts in
arctic latitudes, cool currents greatly alleviate the heat of the tropical deserts
along narrow coastal belts.
- Practically, all of the important surface currents of the ocean are set in motion by
prevailing surface winds.
- Energy is transferred from wind to water by the frictional drag of the air blowing
over the water surface.
- because of the Coriolis effect, the water drift is impelled toward the right in the
North Hemisphere.
(III) CONCLUSION (總結)
The Global Circulation and Man Environment
Global Circulation in form of winds as well ocean currents, are the major mechanisms
circuits within which sensible heat is transported from low latitudes to high latitudes.
In low latitudes, the Hadley cell operates like a simple heat engine to transport heat
from the equatorial zone to the subtropical zone. Upper-air waves take up the
transport and move warm air poleward in exchange for cold air.
The global atmospheric circulation also transports heat in the latent form held by
water vapor. The heat is released by condensation.
Wind systems of the lower troposphere have direct environmental significance.
Arriving at a mountainous coastal zone after a long travel path over a great ocean,
these winds carry a large amount of water vapor, which is deposited as precipitation
on the coast. In this way, the distribution of our water resources is partly determined
by the atmospheric circulation or pollution.
However, some convectional activity such as cyclonic storms and tornadoes may
produce environmental hazards to life. As yet, man has not been able to exert
far-reaching control over the enormous releases of energy in dangerous storms. In
future more accurate prediction and modification of atmospheric circulation can be
anticipated. In doing so, one has to be careful not to create any undesirable side-effect
of human activity.
[END]
References:
Barry, RAG. & Chorley, R.J.
Bradsaw, M.J.
Atmosphere, weather and climate (Ch.3 &4)
Earth, the living Planet (Ch.5)
Bunnett, R.B. and Whiteley, J. Physical Geography
Strahler, A.N. Physical Geography (Ch.9 & 11)
Strahler, A.N. & Strahler, A. H. Modern Physical Geography
Yung, Y.K. Complete Notes on Physical Geography (Ch.11)