Chapter 5: Atmospheric Pressure and Wind. The Impact of Pressure and Wind on the Landscape Atmospheric pressure indirectly affects the landscape Changes

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


• 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


• 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


• 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


• 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)



• 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


• 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


– 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


– 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


– 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


– 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


• 7 components of the general circulation Figure 5-26


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


– 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

http://www.pmel.noaa.gov/tao/

http://www.pmel.noaa.gov/tao/

– Forecasting El Niño• Array of 70 buoys –

Galapagos Islands to New Guinea

• Forecasts?


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

Documents

Chapter 5: Atmospheric Pressure and Wind. The Impact of Pressure and Wind on the Landscape Atmospheric pressure indirectly affects the landscape Changes