Atmosphere
Part III
Circulation
Content
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Importance of Air Movement
Forces control atmospheric motion
Microscale Movement
Mesoscale Movement
Macroscale Movement
Global Pressure Patterns and Planetary Wind
System
 Synoptic Scale Movement
 Climatic Classification and Climatic Regions
Importance of Air Movement
 Air motion:
– Horizontal movement (wind): air movement
parallel to the surface.
– Vertical movement: sinking and rising masses
of air perpendicular to the surface and usually
100-1000 times slower than horizontal
movement
Horizontal Movement
 It is an very important climatic factor.
– Wind relocates warm and cold bodies of air.
Horizontal Movement
– Wind transport water vapour and giving
precipitation
– Air in rapid motion is a severe environmental
hazard. Eg. Typhoon.
Vertical Movement
 It strongly influence whether the climate
and weather will be cloudy and rainy or
clear and dry.
Air motion and
global energy budget
 Between about 40oN and 40oS, the amount of incoming
radiation exceeds that lost by the cooling of the earthatmosphere system.
 Heat energy must be moved from surplus areas to the
deficit areas .
 Global system of wind circulation plays this vital role.
Forces
control atmospheric motion
 Pressure-gradient force
 Coriolis force
 Centripetal force
 Friction
Pressure- Gradient Force
 Pressure can be considered as resulting from the
weight of air.
 Isobars are lines joining points that have same
air pressure.
 The change in barometer pressure across a
horizontal surface or vertical dimension
constitute a pressure gradient.
 Air move from high to low pressure location.
 The magnitude of the force is directly
proportional to the steepness of the gradient.
 Larger pressure gradient, the faster wind speed.
Pressure- Gradient Force
 Pressure gradient exists both vertically and
horizontally.
 Pressure decreases vertically.
 Pattern of air pressure close to the surface is
reversed in the upper atmosphere.
 Warm air expands and rises, so that vertical
pressure gradient is less steep. Therefore, above
areas of warm air, the pressure tends to be
relatively high.
Pressure- Gradient Force
Horizontal
Pressure- Gradient Force
Vertical
Coriolis Force
 It is a deflecting force which is produced by earth
rotation. The force tends to turn the flow of air.
 Direction
– Northern hemisphere: turn right
– Southern hemisphere: turn left
 The degree of the deflecting force depend on:
– Wind speed
– Latitude
• Near equator, it has also no effect.
• Higher latitudes, it has marked effect
• It is the greatest at the poles.
Coriolis Force
Coriolis Force
 Geostrophic wind:
– It is the balance effect with the pressure
gradient force and the coriolis force.
Coriolis Force
 Geostrophic Wind
Centripetal Force
 It applies when the isobaric pattern is markedly
curved.
 When air is following a curved path, a force is
acting centripetally, pulling the air inwards
 The balance with these three forces (pressure
gradient force, coriolis force and centripetal force)
is known as the gradient wind.
Low pressure centre
High pressure centre
Centripetal Force
 Gradient winds
Low pressure centre
High pressure centre
Frictional Force
 In the lower parts of atmosphere, the frictional drag
exerted by the ground on the air flow, which acts as a
force pulling against the direction of flow.
 Friction lessens the wind speed, so weakens the coriolis
force, and the pressure gradient force asserts its greater
strength and cause the air to flow more towards low
pressure area.
Frictional Force
Upper Level
Lower Level
Scales of air movement
Microscale Movement
 Above the friction layer:
– Air movement is controlled by pressure-gradient force
and Coriolis force. (Geostrophic Wind)
Microscale Movement
 Within the friction layer:
– Friction can reduce wind speed and so the effect of
Coriolis force will also be reduced.
– Pressure-gradient force becomes the dominant.
– The angle at which the wind will cross the isobars will
vary according to how much frictional reduction of
speed takes place.
Mesoscale Movement
 Land and Sea Breeze
Day time
Mesoscale Movement
 Land and sea breeze
Night Time
Mesoscale Movement
Mesoscale Movement
 Katabatic Wind / Mountain Wind
Farmers should try
to avoid valleyfloor location for
frost-sensitive
crops and plant
them just above
the level of frost
pocket.
Mesoscale Movement
 Anabatic wind / Valley wind
Mesoscale Movement
 Mountain and valley winds
Macroscale Movement
 Pattern of atmospheric circulation
Macroscale Movement
 Pattern of atmospheric circulation
Macroscale Movement
 Upper Westerlies
– The existence and intensity of the upper westerly flow
is determined by the equator to pole temperature
gradient.
– This system of westerlies is blowing in a complete
circuit about the earth from about 25o almost to the
poles.
Macroscale Movement
 Upper air waves and the jet stream
– The uniform flow of the upper-air westerlies are
–
–
–
–
frequently disturbed by the formation of large
undulations, called upper air waves / Rossby waves.
The waves develop in a zone of contact between cold,
polar air and warm, tropical air.
Jet streams are also a product of the temperature
gradient between the equator and the poles.
It concentrated into a few narrow zones where cold
and warm air masses come into contact. (Polar front)
It will be best developed during winter for the greatest
equator-pole temperature gradient.
Jet Stream
Rossby Waves
Importance of Heat Transfer
– In the equatorial seas.
• Vast energy is used for evaporation so that the sensible heat
transfer to the atmosphere is small.,
– In equatorial trough
• Air rises and cools, the water vapour condenses and release
latent heat. The increased height of the air also represents an
increased potential energy.
• The equatorial air then diverges and flows polewards, so the
potential energy is exported to higher latitudes.
Importance of Heat Transfer
– About 30oN or 30oS (tropical deserts)
• The air subsides with adiabatic warming process, the
potential energy is converted to sensible heat, and produce
clear cloudless skies.
• Most incoming energy is used for heating ground surface
which then heats the atmosphere. (sensible heat)
• During night, energy loss rapidly by radiation, so net
surplus of radiation is also small.
– In 60oN and 60oS
• There is no general cellular circulation, but instead a
complicated pattern.
• Within these storms warm air masses rise, releasing latent
heat and gaining potential energy.
– All these transfer of energy through the atmosphere
are highly variable, and major differences in the
intensity and character of transfer occur over time.
Global Pressure Patterns
 Standard atmospheric pressure
– 1013 hPa
 Seven pressure belts
– Doldrums:
• In the equatorial zone is a belt of low pressure (equatorial
trough) – Inter tropical convergence zone (ITCZ)
– Subtropical High pressure belts
• Lying to the 30oN or 30oS (horse latitudes), air subsiding zone.
– Temperate low pressure belts / sub-polar low p.b.
• Locate on 60oN and 60oS, meeting place of polar air mass and
warm air mass from 30oN or 30oS.
• It is also called polar front
– Polar high pressure belts.
• Locate on North and South Poles.
 The pressure belts shift seasonally through several
degree of latitude.
Pressure Belts and Planetary
Winds systems
Polar High
Sub-polar Low
Sub-tropical High
Doldrum
Sub-tropical High
Sub-polar Low
Polar High
Planetary Wind Systems
Planetary Wind Systems
 Doldrums:
– Between 5oN and 5oS, variable winds and calms. There are no
prevailing surface winds here.
 Trade wind belts:
– Between Doldrums and sub-tropical high pressure belts in both
hemisphere.
– N. Hemisphere: North-east trade winds
– S. Hemisphere: South-east trade winds
– The system of doldrums and trades shifts several degrees
of latitudes seasonally north and south.
 Westerlies belts
– Between latitudes 35o and 60o in both hemisphere.
– In southern hemisphere, it is an unbroken of ocean.
– ‘the roaring forties’, ‘furious fifties’, ‘screaming sixties’
 Polar easterlies
– Locate on both polar zones.
Monsoon Winds in Asia
 Summer Monsoon
– In summer, southern Asia, warm, humid ocean air
moves northward and northwestward from Pacific
Ocean into Asia.
– It is accompanied by heavy rainfall in southeast Asia.
 Winter monsoon
– In winter, Asia is dominated by a strong centre of high
pressure, air blows southward and southeastward
toward the equatorial oceans.
– It brings dry, clear and cool weather for a period of
several months.
Monsoon in Asia
Synoptic Scale Movement
 Air Masses and Fronts
 Disturbances in the Mid-Latitude
Circulation (Temperate Cyclones)
 Disturbances in the Low-Latitude
Circulation (Typhoons)
Air masses and fronts
 Air Masses
– A body of air in which the physical properties
(temperature, specific humidity) are fairly
uniform over a large area is known as an air
mass.
– The properties of an air mass are derived in
part from the regions located.
– Two categories of generalized source regions:
• Latitudes: thermal properties
• Underlying surface: moisture content
Air mass
Latitudinal Position
Underlying surface
Air mass
 Combination of latitudinal position and
underlying surface.
 6 important air masses results
Air mass
Air mass
 When an air mass moves out from its source
region, it brings its distinctive properties with it to
influence weather at distant locations.
 But it also tends to undergo progressive
modification.
 Planetary wind circulation system makes
interaction between air masses more likely to
occur in some areas. (eg. 60oN)
 When tropical and polar air masses collide at the
high-middle latitudes in each hemisphere, which is
called polar front.
 Fundamentally different air masses are coming
into conflict along the boundary zone.
Fronts
 It is a rather sharply defined boundary between 2 air masses.
 Fronts may be nearly vertical (also no motion) and may be
inclined at an angle (it is sliding over another)
 When the cold air mass moves forward, the boundary is
cold front (about 2o of gradient)
 When the cold air mass retreats, the boundary is warm front
(below 1o of gradient)
Disturbances in the MidLatitude Circulation
 Depression / Temperate cyclone
 It is formed by two convergence of
contrasting air masses along the polar front.
Cyclones paths
 Tropical Cyclones - Typhoons
Temperate cyclone
Temperate cyclone - development
Temperate cyclone - development
Weather changes associated with
the passage of a warm and cold
front
Anticyclones and cyclones
Disturbances in the Low-latitude
Circulation - Typhoon
 They are maritime phenomena, originating over
tropical oceans.
 Prerequisite of typhoon formation:
– Sea-surface temperature are in excess of 27oC.
Constant supply of warm, humid air thus appears to
be primary nutrient for typhoon.
– They usually form at 5-8oN and 5-8oS, over tropical
oceans.
– Small islands, which is a intensive low pressure
centre, on the ocean will be excellent for typhoon
development.
Typhoon
 Typhoon information provided by HKO.
Paths of Tropical Cyclones
Nature and
Structure of
Typhoon
English Version
Chinese Version
Typhoon Structure
Weather Changes associated
with Typhoon
Before
Front Vortex
Eye
Rear Vortex
Landed
Pressure
Begin to drop
Drops further
Drops to the
lowest
Suddenly
increases
Continues to
rise to normal
Wind speed
Very slow or
clam
Increasing.
Gusty to
violent
Reduces to
gentle or Clam
Increases
again / violent
to gusty
Decreases to
normal
Wind
direction
Still air
NE or NW
Still air
SE or SW
SE or SW
Rain
No rain to
occasional
showers
Heavy rain
Rain stop for
clear sky
Thick cloud
and heavy rain
again.
Heavy rain
may continue
to last 1 or 2
days
Remarks
Temperature
and humidity
are high.
Thick cloud /
cumulonimbus
Clam period
may last for
one or few
hours
Typhoon is
leaving
Normal
weather
resumes after
the typhoon
has completely
disappear.
Wind speed changes associated
with the Typhoon
Climatic Classification –
Koppen Model
Climatic Classification –
Koppen Model
Koppen’s Climate Classification
http://www.uwmc.uwc.edu/geography/100/koppen_web/koppen_map.htm