Atmospheric Pressure and Wind
Systems
 Atmospheric
Pressure
 Global
Distribution of Air Pressure:
- Global Surface (Horizontal)
Pressure Belts
 Nature
of Winds
- Causes of Wind
- Cyclones and Anticyclones
Atmospheric Pressure and Wind
Systems
 General
Circulation of the Atmosphere
- Global Surface Wind Systems
- Regional Wind Systems
- Local Wind Systems
Atmospheric Pressure
 It
is the force or weight of an air column
exerted on the surface
 It’s
measured using mercury Barometer
 Torricelli
(1643) measured air pressure
with a mercury barometer

Atmospheric Pressure
 Standard barometric pressure:
- at sea-level: 29.92 inches or 1013.2mb
or 14.7Ibs/in2 or 1kg/cm2
-
high pressure is any value higher than
1013.2mb (1013 - 1040mb)
-
low pressure is any value lower than
1013.2mb (980 - 1013mb)
 Note:
1cm = 13.3mb or 1in = 34mb
Mercury Barometer Invented By Torricelli
Atmospheric Pressure
 Air
pressure varies vertically and
horizontally
 Heating
or warming surface temperatures
cause air pressure to decrease due to:
- air expansion, and
- increased vibration/collision of air
molecules
Atmospheric Pressure
 Cooling
or cold surface temperatures
cause air pressure to increase due to:
- air contraction or crowding of air
molecules
- reduced vibration/collision of air
molecules
Atmospheric Pressure
 In
general, cold surfaces in winter
develop thermally induced areas of
high pressure
 while,
warm surfaces in summer
develop thermally induced low pressure
 Strongly
rising air often produces low
pressure at the surface (a dynamic low)
Atmospheric Pressure
 Strongly
descending air often produces
high pressure at the surface (a dynamic
high)
Global Belts of Low & High Pressures
 The
global belts of low and high pressures
include:
-
Equatorial trough of low pressure
Subtropical high pressure
Sub-polar low pressure
Polar high pressure
Global Belts of Low and High Pressures
Global Belts of Low & High Pressures
 Equatorial Trough of Low Pressure
- centered at the equator
- occurs between latitudes 5oN & S
- It’s thermally induced
 Sub-Tropical
-
Pressure Belt
centered at latitudes 30oN & S
occurs between lat. 25o and 35oN & S
dynamically induced
zone of air subsidence
zone of major deserts
Location of Major Deserts at Sub-Tropical
Pressure Belts
Global Belts of Low & High Pressures
 Sub-Polar
Low Pressure
- centered at lat. 60o N & S
- dynamically induced due to strong
lifting of warm air as it meets cold air
from the pole
Global Belts of Low & High
Pressures
 Polar
High Pressure
- centered at the poles
- forma a circular pressure cap
over the polar region
- thermally induced
Global Belts of Low & High Pressures
 The
global pressure belts represent
average pressure conditions
 Belts
shift several degrees of latitudes
annually following the overhead sun
 Belts
are better defined in the southern
hemisphere than in the north because of
large homogenous water body
Global Belts of Low & High Pressures
 Poorly
defined belts in the N.H. because
of remarkable land and water contrasts
 In
winter, the sub-polar low in N.H. is
not continuous, hence:
- over warmer oceans, the Aleutian low
and the Icelandic low persist
- but, over colder land surface, the
Siberian & Canadian high pressures
form instead
Global Belts of Low and High Pressures
Average Atmospheric Pressure (Winter)
Canadian High
Icelandic Low
Siberian High
Global Belts of Low & High Pressures
 In
summer, the subtropical high pressure
belt is not continuous in N.H., hence:
- over colder oceans, the Hawaiian and
Bermuda high pressure persist
- But, over warmer land surface, Asian
low pressure develops in this belt of
high pressure
Global Belts of Low and High Pressures
Average Atmospheric Pressure (Summer)
Hawaiian High
Bermuda High
Asian Low
The Nature of Atmospheric
Pressure

Mapping pressure with
isobars
– Pressure measured with
a barometer
– Typical units are
millibars or inches of
mercury
– Contour pressure values
reduced to sea level
– Shows highs and lows,
ridges and troughs
Nature of Winds
 Some
Definitions:
Wind:
Air in horizontal motion
Updraft:
Small-scale air in upward
vertical motion
Ascent:
Large-scale air in upward
vertical motion
Downdraft: Small-scale air in downward
vertical motion
Subsidence:Large-scale air in downward
vertical motion
The Nature of Wind

Origination of wind
– Uneven heating of
Earth’s surface creates
temperature and
pressure gradients
– Direction of wind
results from pressure
gradient
– Winds blow from high
pressure to low
pressure
25
The Nature of Wind

Wind speed
– Tight pressure gradients
(isobars close together)
indicate faster wind speeds
– Wind speeds are gentle on
average
26
Vertical Variations in Pressure
and Wind




Atmospheric pressure
decreases rapidly with
height
Atmospheric surface
pressure centers lean with
height
Winds aloft are much faster
than at the surface
Jet streams
27
Causes of Wind
 The
-
principal causes of wind are:
solar energy
pressure gradient force
coriolis force
frictional force
Causes of Wind
 Solar
Energy:
- differences in the distribution of solar
energy determines low and high
pressure belts
- air moves from high to low pressure
areas
Causes of Wind
 Pressure Gradient Force:
- spatial variation of pressure produce
pressure gradient force
-
the force causes air to move from high
to low pressure area across the isobars
-
determines wind direction and causes
wind to blow at right angle or
perpendicular to the isobars
Pressure Gradient Force
Pressure Gradient Force Only
Isobars: Pressure Gradient
Causes of Wind
- determines wind speed such that
steeper pressure gradient force
produces stronger wind speed
Causes of Wind

Coriolis Force:
- produced by earth rotation
-
it turns wind to the right of its direction in
the N.H. and to the left in the S.H.
-
Coriolis effect increases in strength
poleward
-
It only affects wind direction, not wind speed,
though faster winds turn more
Coriolis Effect
Geostrophic Wind
Coriolis Effect and Geostrophic Winds
Causes of Wind
- prevents the wind from flowing down
the pressure gradient
- when pressure gradient force is equal
to coriolis force, GEOSTROPHIC
WIND develop and moves parallel to
the isobars
Causes of Wind
 Frictional
Resistance:
-
caused by surface roughness or
molecular friction within the air
stream
-
reduces wind speed by 60% on land and
produces wind turbulence (eddying and
swirling motions)
-
Does not affect upper levels
Frictional Resistance
Causes of Wind
- wind speed is faster over water
bodies because of smoother surface
(only 40% reduction in speed)
- interferes with coriolis force by
causing a less than 90o deflection
- produces 10 - 35o change in wind
direction
Causes of Wind
 results
in wind blowing at some
intermediate angle between pressure
gradient and coriolis forces
The Influence of Pressure Gradient Force (PGF),
Coriolis Effect (CE), and Frictional Resistance (FR)
on Wind Direction
900mb
950mb
1000mb
1050mb
CE
Effects of Friction, Coriolis Effect, and
Pressure Gradient Force on Wind Direction
Cyclones
 Cyclones
describe wind flow pattern
around a low pressure center
 air
converge at the low pressure center
and rises to the upper level
 clouds
 in
can easily form
the N.H., airflow is a counterclockwise
in-spiral into the low pressure center
Cyclones
N.H. Cyclone (surface)
Counterclockwise Flow
S.H. Cyclone (surface)
Clockwise Flow
Cyclones
Converging & Rising Air in
a Cyclone
Descending & Diverging
Air in a Anticyclone
Cyclones
 in
the Southern Hemisphere, airflow is a
clockwise in-spiral into the low pressure
center
 commonly
associated with bad weather
Anticyclones
 air
flow pattern around a high pressure
center
 air
divergence at the high pressure
center leading to air subsidence from the
upper level
 Northern
Hemisphere: air flow is a
clockwise out-spiral from the high
pressure center
Anticyclones
N.H. Anticyclone (surface)
Clockwise Flow
S.H. Anticyclone (surface)
Counterclockwise Flow
Cyclones
Converging & Rising Air in
a Cyclone
Descending & Diverging
Air in a Anticyclone
Anticyclones
 Southern
Hemisphere: air flow is
counterclockwise
 Commonly
condition
associated with fine weather
Global Surface Wind Systems
 Main
-
prevailing surface winds are:
Trade winds
Westerly
Polar Easterlies
 there
are four zones of variable winds
and calms over the four existing
pressure belts in each hemisphere
 Monsoon
winds
Global Surface Wind Systems
Global Surface Wind Systems
Global Surface Wind Systems
 Trade Winds
- originates from the equator ward
side of the subtropical high pressure
belt
- the wind is deflected to the west in its
movement to the equator to blow as
an easterly wind
- in N.H., it is the North-East (NE)
Trade wind
High Pressure Belt as Source of Surface Winds
Hadley Cell: Air Flow Between Equatorial Low
and Sub-Tropical High Pressure Belts
Trade Winds Belt
Global Surface Wind Systems
- in S.H., it’s South-East Trade wind
- trade winds are persistent with a
steady direction
- originates as warm dry winds and
prevails between the equator & lat. 30º
- could cause heavy precipitation over
tropical oceans
High Pressure Belt as Source of Surface Winds
Global Surface Wind Systems
- The trade winds become monsoon
wind in SE Asia and Africa once they
cross the equator
- Could cause dry and dusty winds
when it blows over continents,
especially over desert areas
- Causes Harmattan wind in West
Africa in December through March
Global Surface Wind Systems
 Westerlies
- originates on the poleward side of the
Subtropical High Pressure Belt
- wind prevails between lat. 30º and 60º
- disrupted by landmasses in N.H.
- they are strong and persistent winds
in Southern Hemisphere
High Pressure Belt as Source of Surface Winds
The Westerlies Belt
Global Surface Wind Systems
- wind velocities increase poleward in
the south and sailors describe these
increases by terms like:
roaring forties
 furious fifties
 screaming sixties

- mid-latitude depressions are
associated with this wind
Global Surface Wind Systems
 Polar
Easterlies
- Global Surface Wind Systems
- wind moves from east to west
- it is cold and dry
Polar Easterlies Belt
Global Surface Wind Systems
 Intertropical
Convergence Zone (ITCZ)
- it is where the 2trade winds meet at or
close to the equator
- zone of calm and variable winds
(Doldrums)
- zone of weak horizontal airflow
Global Surface Wind Systems
- ITCZ shifts north or south following
the overhead sun
- zone of instability and rising air
(updraft) during thunderstorms
Local Wind Systems
 The
main types of local winds described
are:
-
Land and Sea Breeze
Chinook/Foehn Winds
Drainage Winds or Katabatic Winds
Mountain and Valley Winds
Local Wind Systems:
Land & Sea Breeze
 Land
and Sea Breeze
- commonly experienced along tropical
coastlines any time of the year and in
summers in mid-latitudes
- involves sea breeze on land during the
day and land breeze over the ocean
surface at night
Local Wind Systems:
Land & Sea Breeze
- caused by differential heating of land
and water to form a small-scale
convectional circulation
- low pressure develops over warm
land in the day causing low pressure
to develop over land
- high pressure develops over relatively
colder ocean surface during the day
Land and Sea Breeze
Local Wind Systems:
Land & Sea Breeze
- hence, sea breeze develops and blows
- at night, low pressure develops over
relatively warmer ocean surfaces
and high pressure over relatively
colder land surface
- hence land breeze develops & blows
offshore over the oceans at night
Local Wind Systems:
Chinook/Foehn Winds
 It
is a local downslope wind
 It’s
called Chinook (snow-eater) in the
Rockies and foehn wind in the Alps
 It
begins as moisture laden wind that is
forced to rise the windward side of the
slope
Local Wind Systems:
Chinook/Foehn Winds
 It
causes heavy precipitation on the wind
ward side & a relatively dry wind is pulled
over the leeward side of the mountain
 the
descending air is compressed and
warmed up adiabatically
 the
wind arrives the base of the mountain
on the leeward side as a warming, drying
wind
Chinook Wind
Local Wind Systems:
Chinook/Foehn Winds
 It’s
capable of melting snow very rapidly
and makes it possible to keep the animals
longer in the field in winter
A
similar wind is called Santa Anas in
California
 Santa
Anas is noted for its high speed,
high temperature and extreme dryness
Local Wind Systems:
Chinook/Foehn Winds
 Santa
Anas provides ideal condition for
wildfires in summers and fall in
California to spread rapidly
 Chinook
wind causes extreme dryness or
the rain shadow effect on the leeward
side
Chinook Wind and Rainshadow Effect
Local Wind Systems:
Drainage or Katabatic Wind
 Common
in cold uplands, high plateaus
or high interior valleys of high latitudes,
examples: Greenland and Antarctica
 it
involves the spill over of cold & dense
air across low mountain divides (or
through passes) downslope to lowland
regions under the force of gravity
Local Wind Systems:
Drainage or Katabatic Wind
 Hence,
it is called Gravity-Flow Winds
 It
is called the Mistral wind along the
Rhone Valley in southern France
 Mistral
wind originates in the Alps and
channeled through the Rhone valley at
high velocity to the Mediterranean Sea

Local Wind Systems:
Drainage or Katabatic Wind
 It
is also called Taku winds in
southeastern Alaska
Local Wind Systems:
Mountain and Valley Winds
 It’s
a daily cycle of airflow between the
valley side slopes and the valley bottoms
 Valley
side slopes are heated more rapidly
during the day than the valley bottom
 Hence,
low pressure develops on the valley
side slopes and high pressure at the valley
bottom receiving less heat during the day
Mountain and Valley Breeze
Local Wind Systems:
Mountain and Valley Winds
 Hence,
valley breeze invades the slope at
daylight when pressure is low
 Valley
breeze are prominent during
summer
 An
opposite process, the mountain breeze,
operates at night

Local Wind Systems:
Mountain and Valley Winds
 Valley
side slopes lose heat very rapidly
and become chilled at night
 Hence,
high pressure develops on the
slopes & causing chilled and dense
mountain side air to slip downslope as
mountain breeze at night
 Mountain
winter
breeze is more prominent in
Regional Wind Systems:
Monsoonal Wind Systems
 Monsoon winds are seasonal wind shift of up
to 180o

Monsoonal winds blow onshore in summer

Monsoonal winds blow offshore in winter

Well developed in the trade wind belts due to
shifts in positions of ITCZ an unequal heating
of land and water
Monsoonal Wind Systems

Best developed along the West African coast,
India, and China. Minor systems are
recognized in Northern Australia

It brings heavy monsoonal rain to these
regions in summer and dry dusty winds in
winter
Monsoonal System in Africa
Minor Monsoonal System
Monsoonal System in India
Monsoonal System in China
El Nino and La Nina

Warming of waters in
the eastern equatorial
Pacific

Associated with changes
in weather patterns
worldwide

Typically occurs on time
scales of 3 to 7 years for
about 18 months
El Nino and La Nina

In normal years, the coasts of Ecuador and
Peru are washed by cold Peruvian current

But in some years when the Equatorial
currents are weak, warm ocean currents flow
southward to replace the cold Peruvian current

This happens close to the end of the year and
the natives named it El Nino (the child) after
the child Jesus because of the Christmas season
El Nino: Walker Circulation Patterns
El Nino and La Nina


Locally, El Nino causes:
-
abnormal weather patterns with
abnormally high amount of rain inland
-
hence, abnormal high crop yields and
devastating floods in Ecuador and Peru
observed
-
but the fishing industry is usually
devastated because the warm waters blocks
the upwelling of nutrient rich cold waters
El Nino and La Nina

The effects of El Nino are felt across the globe:
-
Causes severe drought in Australia,
Indonesia, the Philippines and Africa Sahel
-
1997-98 El Nino brought severe storms
accompanied by unusual beach erosion,
landslide and floods to California
-
Heavy rains and flood in Texas and the
Gulf states and less hurricane events
El Nino and La Nina

The effects of El Nino are felt across the globe:
-
Suppression of Atlantic hurricanes
-
allows a pool of warm water over the Pacific
to develop which in turn displaces the paths
of both the polar and subtropical jet
streams
-
hence, subtropical jet brought heavy winter
precipitation to the Gulf coast and the polar
jet brought milder winter far north
El Nino and La Nina

The effects of El Nino are felt across the globe:
-
or warmer than normal winter in northern
United States and Canada persists
El Nino and La Nina

During an El Nino year, high pressure develops
in the western pacific near Australia and low
pressure in east pacific

When El Nino comes to an end, the pressure
situation reverses such that east pacific has
high pressure and the west low pressure

This phenomenon is referred to as El Nino
Southern Oscillation (ENSO)
El Nino and La Nina

What was once regarded as the normal
condition with high pressure and cold current
in eastern pacific is now referred to as La Nina

Researchers restrict La Nina to periods when
surface temperatures are colder than average
El Nino and La Nina

La Nina has its distinct weather patterns:
-
colder than normal winter of the Pacific
Northwest and Northern Great Plains
-
Warming experienced in the rest of the
United States
-
Great hurricane activity producing more
than 20 times more damage than El Nino
years
Review Questions for Topic 5
1) The force exerted by gas molecules on some area of
Earth’s surface or any other body is called what?
A. Density
B. Wind
C. Atmospheric pressure
D. Friction
E. Geotropism
Figure 5-1
1) The force exerted by gas molecules on some area of
Earth’s surface or any other body is called what?
A. Density
B. Wind
C. Atmospheric pressure
D. Friction
E. Geotropism
Figure 5-1
Explanation: Gas molecules, when in contact with a surface, will
exert a force on that surface. This force corresponds to
2) Lines drawn on maps joining areas of equal
atmospheric pressure are called what?
A. Wavelengths
B. Isotherms
C. Contours lines
D. Isohyets
E. Isobars
Figure 5-4
2) Lines drawn on maps joining areas of equal
atmospheric pressure are called what?
A. Wavelengths
B. Isotherms
C. Contours lines
D. Isohyets
E. Isobars
Figure 5-4
Explanation: Lines of constant pressure, by definition, are called isoba
3) Due to Coriolis force, freely moving objects in the
Northern Hemisphere appear to be deflected to
A. the left.
B. the right.
C. the ocean.
D. the east.
E. the west.
3) Due to Coriolis force, freely moving objects in the
Northern Hemisphere appear to be deflected to
A. the left.
B. the right.
C. the ocean.
D. the east.
E. the west.
Figure 5-6
Explanation: In the Northern Hemisphere, winds are deflected to the
right by Coriolis. In the Southern Hemisphere, winds are deflected to
4) The air that descends and spirals out of the subtropical
high pressure belt is the source of
A. polar easterlies.
B. trade winds and
the westerlies.
C. Chinooks.
D. land and sea breeze.
E. the monsoons.
4) The air that descends and spirals out of the subtropical
high pressure belt is the source of
A. polar easterlies.
B. trade winds and
the westerlies.
C. Chinooks.
D. land and sea breeze.
E. the monsoons.
Figure 5-14
Explanation: As seen in Figure 5-14, the subtropical high is the
source location for both the mid-latitude westerlies and the trade
5) A sea breeze is experienced
A. in the night.
B. in winter.
C. during the day.
D. at dawn only.
E. when air over land is too
heavy to be lifted by
convective currents.
5) A sea breeze is experienced
A. in the night.
B. in winter.
C. during the day.
D. at dawn only.
E. when air over land is too
heavy to be lifted by
convective currents.
Figure 5-34a
Explanation: During the day, the land heads faster than the water, crea
thermal low on land and a thermal high over water. The winds will blow
high to low pressure, creating a sea breeze.
6) The semi-permanent area of high pressure over the
poles of Earth is an example of
A. a subtropical high.
B. a sea breeze.
C. a dynamic high.
D. a thermal high.
E. a midlatitude
anticyclone.
Figure 5-14
6) The semi-permanent area of high pressure over the
poles of Earth is an example of
A. a subtropical high.
B. a sea breeze.
C. a dynamic high.
D. a thermal high.
E. a midlatitude
anticyclone.
Figure 5-14
Explanation: The area of high pressure over the poles is an example of a t
When the air gets extremely cold and dense over these regions, it becomes
thus exerting higher pressure on the surface as a result of its temperature.
7) An El Niño is observed as
A. a cooling of equatorial
Pacific waters.
B. high pressure over
western South America.
C. a warming of eastern equatorial
Pacific waters.
D. low pressure over Australia.
E. rainy conditions for Australia.
Figure 5-37
7) An El Niño is observed as
A. a cooling of equatorial
Pacific waters.
B. high pressure over
western South America.
C. a warming of eastern
equatorial Pacific waters.
Figure 5-37
D. low pressure over Australia.
E. rainy conditions for Australia.
Explanation: During an El Niño event, water over the eastern
equatorial Pacific (off the west coast of South America) becomes
abnormally warm, resulting in a switch of the Walker Circulation.
8) Waves in the jet stream pattern are called
A. Kelvin waves.
B. electromagnetic waves.
C. shallow water waves.
D. Coriolis waves.
E. Rossby waves.
Figure 5-24
8) Waves in the jet stream pattern are called
A. Kelvin waves.
B. electromagnetic waves.
C. shallow water waves.
D. Coriolis waves.
E. Rossby waves.
Figure 5-24
Explanation: The wave patterns which give the midlatitudes a
majority of its weather that are embedded in the jet stream pattern
9) Which type of wind is an example of a katabatic wind?
A. Foehn
B. Gale
C. Bora
D. Chinook
E. Santa Ana
9) Which type of wind is an example of a katabatic wind?
A. Foehn
B. Gale
C. Bora
D. Chinook
E. Santa Ana
Explanation: A bora wind is a strong cold wind that affects the lee
slopes of a mountain range. It is katabatic because it is a cold
10) Air which has decreased in density and temperature will
A. be warmer.
B. have a higher air pressure.
C. have a lower air pressure.
D. sink.
E. compress.
Figure 5-3
10) Air which has decreased in density and temperature will
A. be warmer.
B. have a higher air pressure.
C. have a lower air pressure.
D. sink.
E. compress.
Figure 5-3
Explanation: Air temperature and density are directly related to
pressure. If air
The General Circulation of the
Atmosphere

Atmosphere is in constant motion
 Major semipermanent conditions of wind and
pressure—general circulation
 Principal mechanism for longitudinal and latitudinal
heat transfer
 Second only to insolation as a determination for
global climate
131
The General Circulation of the
Atmosphere

Simple example: A non-rotating
Earth
– Strong solar heating at equator
– Little heating at poles
– Thermal low pressure forms over
–
–
–
–
equator
Thermal high forms over poles
Ascending air over equator
Descending air over poles
Winds blow equatorward at
surface, poleward aloft
132
Figure 5-12
The General Circulation of the
Atmosphere

Observed general circulation
– Addition of Earth’s rotation
increases complexity of
circulation
– One semipermanent convective
cell near the equator
– Three latitudinal wind belts per
hemisphere
– Hadley cells
Figure 5-14
133
The General Circulation of the
Atmosphere

Seasonal differences in the
general circulation
Figure 5-15
134
The General Circulation of the
Atmosphere

Components of the general
circulation
– Subtropical highs





135
Persistent zones of high
pressure near 30° latitude in
both hemispheres
Result from descending air in
Hadley cells
Subsidence is common over
these regions
Regions of world’s major
deserts
No wind, horse latitudes
Figure 5-16
The General Circulation of the
Atmosphere
– Trade winds





Diverge from subtropical
highs
Exist between 25°N and 25°S
latitude
Easterly winds: southeasterly
in Southern Hemisphere,
northeasterly in Northern
Hemisphere
Most reliable of winds
“Winds of commerce”
Figure 5-17
136
The General Circulation of the
Atmosphere
– Trade winds (cont.)



137
Heavily laden with
moisture
Do not produce rain
unless forced to rise
If they rise, they produce
tremendous precipitation
and storm conditions
Figure 5-20
The General Circulation of the
Atmosphere
– Intertropical Convergence
Zone (ITCZ)
 Region of convergence of
the trade winds
 Constant rising motion
and storminess in this
region
 Position seasonally shifts
(more over land than
water)
 Doldrums
Figure 5-21
138
The General Circulation of the
Atmosphere
– Westerlies




139
Form on poleward sides of
subtropical highs
Wind system of the
midlatitudes
Two cores of high winds –
jet streams
Rossby waves
Figure 5-22
Figure 5-24
The General Circulation of the
Atmosphere
– Polar highs
Thermal highs that develop over poles due to extensive
cold conditions
 Winds are anticyclonic; strong subsidence
 Arctic desert
– Polar easterlies
 Regions north of 60°N and south of 60°S
 Winds blow easterly
 Cold and dry

140
The General Circulation of the
Atmosphere
– Polar front




141
Low pressure area between
polar high and westerlies
Air mass conflict between
warm westerlies and cold
polar easterlies
Rising motion and
precipitation
Polar jet stream position
typically coincident with the
polar front
Figure 5-25
The General Circulation of the
Atmosphere

The seven components of the general circulation
Figure 5-26
142
The General Circulation of the
Atmosphere

Vertical wind patterns of
the general circulation
– Most dramatic
differences in surface and
aloft winds is in tropics
– Antitrade winds
Figure 5-28
143
Modifications of the General
Circulation

Seasonal modifications
– Seven general circulation
components shift
seasonally
– Components shift
northward during
Northern Hemisphere
summer
– Components shift
southward during
Southern Hemisphere
summer
144
Figure 5-29
Modifications of the General
Circulation

Monsoons
– Seasonal wind shift of up to
180°
– Winds onshore during summer
– Winds offshore during winter
– Develop due to shifts in
positions of ITCZ and unequal
heating of land and water
Figure 5-30
145
Modifications of the General
Circulation

Major monsoon systems
Figure 5-32
146
Modifications of the General
Circulation

Minor monsoon systems
Figure 5-33
147
Localized Wind Systems

Sea breezes
– Water heats more slowly than
land during the day
– Thermal low over land, thermal
high over sea
– Wind blows from sea to land

Land breezes
– At night, land cools faster
– Thermal high over land, thermal
low over sea
– Wind blows from land to sea
148
Figure 5-34
Localized Wind Systems

Valley breeze
– Mountain top during the day heats
faster than valley, creating a thermal
low at mountain top
– Upslope winds out of valley

Mountain breeze
– Mountain top cools faster at night,
creating thermal high at mountain
top
– Winds blow from mountain to
valley, downslope
Figure 5-35
149
Localized Wind Systems

Katabatic winds
– Cold winds that originate from
cold upland areas, bora winds
– Winds descend quickly down
mountain, can be destructive

Foehn/Chinook winds
– High pressure on windward side
of mountain, low pressure on
leeward side
– Warm downslope winds
– Santa Ana winds
150
Figure 5-36