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Chapter 5: Atmospheric Pressure and Wind
The Impact of Pressure and Wind on the Landscape
• Atmospheric pressure indirectly affects the landscape• Changes manifest primarily by changes in wind and
temperature• Wind has a visible
component to its activity
• Severe storm winds can drastically affect the landscape
Figure 5-2
The Nature of Atmospheric Pressure
• Gas molecules continuously in motion
• Force exerted by gas molecules is called atmospheric pressure
• Force exerted on every surface the gas touches
• Pressure is approximately 14 lbs per square inch
Figure 5-1
The Nature of Atmospheric Pressure
• Factors influencing atmospheric pressure– Density
• At higher density, particles are closer & collide more frequently, increasing pressure
– Temperature• Warmer particles
move faster & collide more frequently, increasing pressure
Figure 5-3
Figure 4-15
The Nature of Atmospheric Pressure
• Measurement of Pressure– Barometer – device to measure
atmospheric pressure– Average Sea Level Pressure
• 1013.25 MILLIBARS• 29.92 INCHES OF HG
(MERCURY)• 14.7 LBS / SQ. INCH• 760 MM OF HG
– No matter which unit of measurement is used, pressure ALWAYS decreases with altitude
• Mapping pressure with isobars– Isobar: line joining points of equal pressure– Pressure gradient: horizontal rate of pressure change
• Directly affects wind speed
Figure 5-4
The Nature of Wind
• Wind: horizontal air movement– In theory, why does air move?
• Energy imbalances on globe– Why does air actually move?
• Differences in horizontal atmospheric pressure
• Origination of wind– Uneven heating of Earth’s surface
creates temperature & pressure gradients
– Direction of wind results from pressure gradient
– Winds blow from high pressure to low pressure
• Winds are named for the direction they blow from!!!
Figure 5-5
The Nature of Wind
• Forces which govern the wind– Pressure gradient force
• Characterized by wind moving from high to low pressure, always• Winds blow at right angles to isobars
– Coriolis force• Turns wind to the right in
the N Hemisphere, left in S Hemisphere
• Only affects wind direction, not speed, but faster winds turn more
– Friction• Wind is slowed by Earth’s
surface due to friction, does not affect upper levels
Figure 5-7
The Nature of Wind
• Force balances– Geostrophic balance
• Balance between pressure gradient force and Coriolis
• Winds blow parallel to isobars
– Frictional balance• Winds blow slightly towards low
pressure and slightly away from high pressure
• Winds slowed by friction weaken Coriolis, so pressure gradient force is stronger and turns the winds
Figure 5-6
The Nature of Wind
• Anticyclones and cyclones
Figure 5-8
The Nature of Wind
• Cyclone: low-pressure cell– Air converges & ascends– Surface convergence & low pressure indicate rising motion– Rising motion results in clouds and storms
• Anticyclone: high-pressure cell– Air descends & diverges– Surface divergence &
high pressure indicate sinking motion
– Sinking motion results in sunny skies
Figure 5-9
The Nature of Wind
• Wind speed– Determined by pressure
gradients– Tight pressure gradients
(isobars close together) indicate faster wind speeds
– Wind speeds are gentle on average
Figure 5-11Figure 5-10
12
The Nature of Wind
Vertical Variations in Pressure &
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
The General Circulation of the Atmosphere
• Atmosphere is in constant motion
• Major semi-permanent 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
The General Circulation of the Atmosphere
• Simple example: Non-rotating Earth– Strong solar heating
at equator• Low pressure
forms over equator• Ascending air over equator
– Little heating at poles• High pressure forms
over poles• Descending air over poles
– Winds blow equatorward at surface, poleward aloft
Figure 5-12
16• Because Earth rotates
The General Circulation of the Atmosphere
• If Earth didn’t rotate
The General Circulation of
the Atmosphere• Observed general
circulation– Earth’s rotation increases
complexity of circulation– Hadley cells
• Complete vertical circulation cells where warm air rises, cools, moves poleward, & subsides
• Around 30° N/S
Figure 5-14
Figure 5-13
• 7 Surface Components1. Intertropical convergence
zone (ITCZ) (0°)
2. Trade winds (easterlies) (0° - 30° N/S)
3. Subtropical highs (30° N/S)
4. Westerlies (30° - 60° N/S)
5. Subpolar low (polar front) (60° N/S)
6. Polar easterlies (60° - 90° N/S)
7. Polar highs (90° N/S)
The General Circulation of the Atmosphere
The General Circulation of the Atmosphere
• Components of the general circulation– Subtropical highs (STH’s)
• Persistent zones of high pressure near 30° N/S
• Result from descending air in Hadley cells
• Regions of world’s major deserts
• No wind• AKA horse latitudes• Source of 2 of the 3 major wind systems: trade winds & westerlies
Figure 5-16
The General Circulation of the Atmosphere
• Winds are named for the direction they blow from!– Trade winds (easterlies)
• Diverge from equator-ward side of STH’s
• About 30°N to 30°S– SE in S Hemisphere,– NE in N Hemisphere
• Blow east to west• Most reliable/consistent
of wind systems• AKA Winds of commerce
Figure 5-17
The General Circulation of the Atmosphere
– 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
Figure 5-20
The General Circulation of
the Atmosphere– Intertropical Convergence
Zone (ITCZ)• Zone of rising air from surface
heating & convection• Zone of low pressure &
convergence toward which trade winds blow
• Near equator• Constant rising motion &
storminess in this region• Position seasonally shifts
(more over land than water) • AKA Doldrums
Figure 5-21
The General Circulation of the Atmosphere
– Westerlies• Form on poleward sides of STH’s• Wind system of the midlatitudes
– Blow west to east
• 2 cores of high winds – jet streams– Subtropical Jet– Polar Jet
• Rossby waves
Figure 5-22
Figure 5-24
The General Circulation of the Atmosphere
– Polar highs• Develop over poles
due to extensive cold conditions
• Anticyclonic winds• Strong subsidence• Arctic desert
– Polar easterlies• Regions north of 60°N
& south of 60°S• Winds blow east to
west• Cold & dry
Figure 5-27
The General Circulation of the Atmosphere
– Polar front• Low pressure around 60° N/S• Air mass conflict between warm westerlies & cold polar easterlies• Rising motion =
clouds & precipitation
– Very strong winds over oceans
• Polar jet stream position typically coincident with the polar front
• AKA subpolar low
Figure 5-25
The General Circulation of the Atmosphere
• 7 components of the general circulation Figure 5-26
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
• Blow from SW in N Hemisphere, & from NW in S Hemisphere
Figure 5-28
Modifications of the General Circulation
• Seasonal modifications– 7 general circulation components shift seasonally– Components shift northward during Northern Hemisphere summer– Components shift southward during Southern Hemisphere summer
Figure 5-29
Modifications of the General Circulation
• Monsoons– Seasonal wind shift– Most significant disturbance of
general circulation• Offshore: wind movement from land to water• Onshore: wind movement from water to land
– Seasonal Winds• Wet summer• Dry winter
– Develop due to:• Shift in position
of ITCZ• Unequal heating
of land & water
Figure 5-30
Figure 5-31
Modifications of the General Circulation
• Major monsoon systems
Figure 5-32
Localized Wind Systems
• Convectional circulation from differential heating of land & water
• Sea breezes– Water heats more slowly than
land during the day– Lower pressure over land, higher
pressure over sea– Wind blows from sea to land
• Land breezes– At night, land cools faster– Higher pressure over land, lower
pressure over sea– Wind blows from land to sea
Figure 5-34
Localized Wind Systems
• Convectional circulation from differential heating of higher vs. lower elevations
• Valley breeze– Mountain top during the day heats
faster than valley, creating lower pressure at mountain top
– Upslope winds out of valley
• Mountain breeze– Mountain top cools faster at night,
creating higher spressure at mountain top
– Winds blow from mountain to valley, downslope
Figure 5-35
Localized Wind Systems
• Katabatic winds– Cold winds that originate from cold upland areas– Descend quickly down mountain, can be destructive
• Chinook winds– Warm downslope winds– H pressure on windward side, L pressure on leeward side– Warms adiabatically as it moves down leeward slope of Rocky
MountainsFigure 5-36
Localized Wind Systems
• Santa Ana winds– Hot, dry, high speed winds– Occur in S. California from Oct to
Mar– Spread wildfires in dry seasons
El Niño-Southern Oscillation
• Warming of waters in the eastern equatorial Pacific• Associated with numerous changes in weather patterns
worldwide• Typically occurs every 3 to 7 years for about 18 months• El Niño
– Reference to the Christ Child
• 1st signs usually appear around Christmas
Figure 5-37
El Niño-Southern Oscillation
• Circulation patterns– Normal– El Nino– Upwelling: winds
blowing offshore on W side of continents pushes water westward & brings up cold, nutrient-rich water from below
Figure 5-38
38
Upwelling• Phytoplankton
shows where upwelling occurs
El Niño-Southern Oscillation
• Patterns associated with El Niño– Teleconnections– Weather effects
• La Niña—opposite of El Niño– Weather effects– Circulation is similar to
“normal”
• Causes of El Niño– Which changes first,
the atmosphere or the ocean?
Figure 5-40
El Niño-Southern Oscillation
– Sea Levels & Sea Surface Temperatures during normal, El Niño & La Niña years (http://www.pmel.noaa.gov/tao/)
Figure 5-EFigure 5-D
Figure 5-CFigure 5-39
– Forecasting El Niño• Array of 70 buoys –
Galapagos Islands to New Guinea
• Forecasts?
El Niño-Southern Oscillation
Figure 5-B
Summary
• Atmospheric pressure and wind affect the geographic landscape in several ways• Atmospheric pressure is the force exerted by air molecules on all objects the air
is in contact with• Pressure is influenced by temperature, density, and dynamic• Isobars show areas of high pressure and low pressure• Vertical and horizontal atmospheric motions are called wind• Wind is affected by many forces• Geostrophic balance represents a balance between the Coriolis force and the
pressure gradient force• Friction slows the wind and turns it towards lower pressure• Wind patterns around high and low pressure systems are anticyclonic and
cyclonic, respectively• Areas of divergence at the surface are associated with sinking motion,
convergence at the surface with rising motion• Close isobar spacing indicates faster winds
Summary
• Winds increase rapidly with height, pressure decreases rapidly with height• The global atmospheric circulation is called the general circulation• There are seven components to the general circulation• Each component has associated weather conditions• Seasonal modifications to the general circulation exist, including monsoons• Localized wind systems affect wind direction locally on diurnal time scales• El Niño is a warming of eastern equatorial Pacific water and subsequent
switching of the high and low air pressure patterns • El Niño is associated with varied weather patterns in different locations
globally