Atmospheric Pressure and Wind Systems Atmospheric Pressure Global Distribution of Air Pressure: -Global Surface (Horizontal) Pressure Belts Nature of Winds

Atmospheric Pressure and Wind Atmospheric Pressure and Wind SystemsSystems

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 Atmospheric Pressure and Wind SystemsSystems

General Circulation of the Atmosphere- Global Surface Wind Systems- Regional Wind Systems- Local Wind Systems

Atmospheric PressureAtmospheric 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 PressureAtmospheric PressureStandard 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 TorricelliMercury Barometer Invented By Torricelli


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


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


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)


Strongly descending air often produces high pressure at the surface (a dynamic high)

Global Belts of Low & High PressuresGlobal 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 PressuresGlobal Belts of Low and High Pressures

Global Belts of Low & High PressuresGlobal Belts of Low & High PressuresEquatorial 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 Location of Major Deserts at Sub-Tropical Pressure BeltsPressure Belts


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 Global Belts of Low & High PressuresPressures

Polar High Pressure- centered at the poles- forma a circular pressure cap

over the polar region- thermally induced


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


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


Average Atmospheric Pressure (Winter)Average Atmospheric Pressure (Winter)

Canadian HighIcelandic Low Siberian High


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


Average Atmospheric Pressure (Summer)Average Atmospheric Pressure (Summer)

Bermuda HighAsian Low

Hawaiian High

The Nature of Atmospheric The Nature of Atmospheric PressurePressure

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 WindsNature of WindsSome Definitions:

Wind: Air in horizontal motionUpdraft: 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 WindThe 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 WindThe 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 Vertical Variations in Pressure and Windand 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 WindCauses of Wind

The principal causes of wind are:

- solar energy- pressure gradient force- coriolis force- frictional force


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 WindCauses of WindPressure 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 ForcePressure Gradient Force

Pressure Gradient Force OnlyPressure Gradient Force Only

Isobars: Pressure GradientIsobars: Pressure Gradient


- determines wind speed such that steeper pressure gradient force

produces stronger wind speed


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 EffectCoriolis Effect

Geostrophic WindGeostrophic Wind

Coriolis Effect and Geostrophic WindsCoriolis Effect and Geostrophic Winds


- 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


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 ResistanceFrictional Resistance


- 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


results in wind blowing at some intermediate angle between pressure gradient and coriolis forces

The Influence of Pressure Gradient Force (PGF), The Influence of Pressure Gradient Force (PGF), Coriolis Effect (CE), and Frictional Resistance (FR) Coriolis Effect (CE), and Frictional Resistance (FR)

on Wind Directionon Wind Direction

950mb

1000mb

1050mb

CE

900mb

Effects of Friction, Coriolis Effect, and Effects of Friction, Coriolis Effect, and Pressure Gradient Force on Wind DirectionPressure Gradient Force on Wind Direction

CyclonesCyclones

Cyclones describe wind flow pattern around a low pressure center

air converge at the low pressure center and rises to the upper level

clouds can easily form

in the N.H., airflow is a counterclockwise in-spiral into the low pressure center

CyclonesCyclones

N.H. Cyclone (surface)Counterclockwise Flow

S.H. Cyclone (surface)Clockwise Flow

CyclonesCyclones

Converging & Rising Air in a Cyclone

Descending & Diverging Air in a Anticyclone

CyclonesCyclones

in the Southern Hemisphere, airflow is a clockwise in-spiral into the low pressure center

commonly associated with bad weather

AnticyclonesAnticyclones

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


N.H. Anticyclone (surface)Clockwise Flow

S.H. Anticyclone (surface)Counterclockwise Flow

CyclonesCyclones

Converging & Rising Air in a Cyclone

Descending & Diverging Air in a Anticyclone


Southern Hemisphere: air flow is counterclockwise

Commonly associated with fine weather condition

Global Surface Wind SystemsGlobal 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 SystemsGlobal Surface Wind Systems


Global Surface Wind SystemsGlobal Surface Wind SystemsTrade 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 WindsHigh Pressure Belt as Source of Surface Winds

Hadley CellHadley Cell: Air Flow Between Equatorial Low : Air Flow Between Equatorial Low and Sub-Tropical High Pressure Beltsand Sub-Tropical High Pressure Belts

Trade Winds BeltTrade Winds Belt


- 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



- 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


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


The Westerlies BeltThe Westerlies Belt


- wind velocities increase poleward in the south and sailors describe these increases by terms like:

roaring fortiesfurious fiftiesscreaming sixties

- mid-latitude depressions are associated with this wind


Polar Easterlies- Global Surface Wind Systems- wind moves from east to west - it is cold and dry

Polar Easterlies BeltPolar Easterlies Belt


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


- ITCZ shifts north or south following the overhead sun - zone of instability and rising air (updraft) during thunderstorms

Local Wind SystemsLocal 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: Local Wind Systems: Land & Sea BreezeLand & 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


- 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 BreezeLand and 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: Local Wind Systems: Chinook/Foehn WindsChinook/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


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 WindChinook Wind


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


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 EffectChinook Wind and Rainshadow Effect

Local Wind Systems: Local Wind Systems: Drainage or Katabatic WindDrainage 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


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


It is also called Taku winds in southeastern Alaska

Local Wind Systems: Local Wind Systems: Mountain and Valley WindsMountain 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 BreezeMountain and Valley Breeze


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


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 breeze is more prominent in winter

Regional Wind Systems:Regional Wind Systems:Monsoonal Wind SystemsMonsoonal 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 SystemsMonsoonal 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 AfricaMonsoonal System in Africa

Minor Monsoonal System

Monsoonal System in IndiaMonsoonal System in India

Monsoonal System in ChinaMonsoonal System in China

El Nino and La NinaEl 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 NinaEl Nino and La NinaIn 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 PatternsEl Nino: Walker Circulation Patterns

El Nino and La NinaEl Nino and La NinaLocally, 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 NinaEl Nino and La NinaThe 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


- 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


- or warmer than normal winter in northern United States and Canada persists

El Nino and La NinaEl Nino and La NinaDuring 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 NinaEl Nino and La NinaWhat 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 NinaEl Nino and La NinaLa 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 5Review Questions for Topic 5

1)1) The force exerted by gas molecules on some area of The force exerted by gas molecules on some area of Earth’s surface or any other body is called what?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)1) The force exerted by gas molecules on some area of The force exerted by gas molecules on some area of Earth’s surface or any other body is called what?Earth’s surface or any other body is called what?

A. Density

B. Wind

C. Atmospheric pressure

D. Friction

E. Geotropism

Explanation: Gas molecules, when in contact with a surface, will exert a force on that surface. This force corresponds to atmospheric pressure.

Figure 5-1

2) 2) Lines drawn on maps joining areas of equal Lines drawn on maps joining areas of equal atmospheric pressure are called what?atmospheric pressure are called what?

A. Wavelengths

B. Isotherms

C. Contours lines

D. Isohyets

E. Isobars

Figure 5-4

2) 2) Lines drawn on maps joining areas of equal Lines drawn on maps joining areas of equal atmospheric pressure are called what?atmospheric pressure are called what?

A. Wavelengths

B. Isotherms

C. Contours lines

D. Isohyets

E. Isobars

Explanation: Lines of constant pressure, by definition, are called isobars.

Figure 5-4

3) 3) Due to Coriolis force, freely moving objects in the Due to Coriolis force, freely moving objects in the Northern Hemisphere appear to be deflected toNorthern Hemisphere appear to be deflected to

A. the left.

B. the right.

C. the ocean.

D. the east.

E. the west.

3) 3) Due to Coriolis force, freely moving objects in the Due to Coriolis force, freely moving objects in the Northern Hemisphere appear to be deflected toNorthern Hemisphere appear to be deflected to

A. the left.

B. the right.

C. the ocean.

D. the east.

E. the west.

Explanation: In the Northern Hemisphere, winds are deflected to the right by Coriolis. In the Southern Hemisphere, winds are deflected to the left

Figure 5-6

4) 4) The air that descends and spirals out of the subtropical The air that descends and spirals out of the subtropical high pressure belt is the source of 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) 4) The air that descends and spirals out of the subtropical The air that descends and spirals out of the subtropical high pressure belt is the source of 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.

Explanation: As seen in Figure 5-14, the subtropical high is the source location for both the mid-latitude westerlies and the trade winds.

Figure 5-14

5) 5) A sea breeze is experienced 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) 5) A sea breeze is experienced 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.

Explanation: During the day, the land heads faster than the water, creating athermal low on land and a thermal high over water. The winds will blow fromhigh to low pressure, creating a sea breeze.

Figure 5-34a

6) 6) The semi-permanent area of high pressure over the The semi-permanent area of high pressure over the poles of Earth is an example of 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) 6) The semi-permanent area of high pressure over the The semi-permanent area of high pressure over the poles of Earth is an example of 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.

Explanation: The area of high pressure over the poles is an example of a thermal high.When the air gets extremely cold and dense over these regions, it becomes very heavy, thus exerting higher pressure on the surface as a result of its temperature.

Figure 5-14

7) 7) An El Niño is observed asAn 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) 7) An El Niño is observed asAn 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.

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.

Figure 5-37

8) 8) Waves in the jet stream pattern are calledWaves 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) 8) Waves in the jet stream pattern are calledWaves in the jet stream pattern are called

A. Kelvin waves.

B. electromagnetic waves.

C. shallow water waves.

D. Coriolis waves.

E. Rossby waves.

Explanation: The wave patterns which give the midlatitudes a majority of its weather that are embedded in the jet stream pattern are called Rossby waves.

Figure 5-24

9) 9) Which type of wind is an example of a katabatic wind?Which type of wind is an example of a katabatic wind?

A. Foehn

B. Gale

C. Bora

D. Chinook

E. Santa Ana

9) 9) Which type of wind is an example of a katabatic wind?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 wind.

10) 10) Air which has decreased in density and temperature willAir 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) 10) Air which has decreased in density and temperature willAir which has decreased in density and temperature will

Explanation: Air temperature and density are directly related to pressure. If air temperature and density decrease, the pressure must also decrease as a result of the temperature and density decrease.

A. be warmer.

B. have a higher air pressure.

C. have a lower air pressure.

D. sink.

E. compress.

Figure 5-3

The General Circulation of the The General Circulation of the AtmosphereAtmosphere

131

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


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


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

133

Figure 5-14


Seasonal differences in the general circulation

134

Figure 5-15


Components of the general circulation– Subtropical highs

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

135

Figure 5-16


– 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”

136Figure 5-17


– Trade winds (cont.) Heavily laden with

moisture Do not produce rain

unless forced to rise If they rise, they produce

tremendous precipitation and storm conditions

137

Figure 5-20


– 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

138

Figure 5-21


– Westerlies Form on poleward sides of

subtropical highs Wind system of the

midlatitudes Two cores of high winds –

jet streams Rossby waves

139

Figure 5-22

Figure 5-24


– 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


– Polar front 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

141

Figure 5-25


The seven components of the general circulation

142Figure 5-26


Vertical wind patterns of the general circulation– Most dramatic

differences in surface and aloft winds is in tropics

– Antitrade winds

143

Figure 5-28

Modifications of the General Modifications of the General CirculationCirculation

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


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

145

Figure 5-30


Major monsoon systems

146

Figure 5-32


Minor monsoon systems

147

Figure 5-33

Localized Wind SystemsLocalized 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


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

149

Figure 5-35


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

Documents

Atmospheric Pressure and Wind Systems Atmospheric Pressure Global Distribution of Air Pressure: -Global Surface (Horizontal) Pressure Belts Nature of Winds