Week 7 (March 10)Week 7 (March 10)• TonightTonight

– Air Pressure & Winds (Chp 6)– Atmospheric Circulation (Chp 7)– Classwork/Homework #7

• Next Week (Mar 17)Next Week (Mar 17)– Air Masses and Fronts (Chp 8)– El Niño/La Niña

• March 24March 24– No Class – Spring Break

• March 31March 31– No Class = Cesar Chavez Day

Wind

WeightPressure = Force / Area Force = Weight of overlying column of air = mass x gravity

Pressure• The steady exertions of atoms and molecules,

exchanging momentum with the walls of a container are “Pressure”.

Atmospheric PressureAtmospheric Pressure• More air near the surface

then number of molecules decreases with height

• Air pressure, Air Density and Air temperature are all interrelated.– If one changes then the other

2 will change

Pressure Changes• Horizontal: Changes ~ 1 mb over 6000 meters• Vertical: 1 mb over 10 meters (600 X greater)

• Vertical atmospheric motions are most important – Vertical pressure and temperature changes are much

more dramatic

Two columns of air– same temperature

same distribution of mass

1000 mb 1000 mb

500 mb level

Cool the left column; warm the right column

1000 mb

500 mb

500 mb

1000 mb

The heated columnexpands

The cooledcolumn contracts

original 500 mb level

The level of the 500 mb surface changes; the surface pressure remains unchanged

1000 mb

new 500 mblevel in warmair

new 500 mblevel in coldair

1000 mb

500 mb surface isdisplaced upward in the warmer column500 mb level is

displaced downward in the cooler column original 500 mb level

The surface pressure remains the same since

both columns still contain the same mass

of air.

1003 mb 997 mb

original 500 mb level HighLow

Air moves from high to low pressure in Air moves from high to low pressure in the middle of column, causing surface the middle of column, causing surface pressure to change.pressure to change.

Note the new surface

pressures

1003 mb 997 mb

original 500 mb level

Air now also moves from high to low Air now also moves from high to low pressure at the surface…pressure at the surface…

HighLow

High Low

Where would we have rising motion?

1003 mb 997 mb

original 500 mb level

Air now also moves from high to low Air now also moves from high to low pressure at the surface…pressure at the surface…

HighLow

High Low

What have we just observed?• Differential heating to uniform atmosphere• Different rates of expansion in the air• Results in hortizontal pressure differences• Pressure differences caused flow of air• Example of Atmosphere converting heating into

motion

Measuring Air PressureMercury

Barometer

Aneroid Barometer

Station Pressure v. Sea Level PressureStation Pressure v. Sea Level Pressure

Pressure MapsPressure Maps• a) Surface map has altitude-adjusted station pressures to construct sea level pressure

contours• b) Upper air map has constant pressure level delineated by height above sea level

Primary LevelsPrimary Levels

1000 mb = Surface

850 mb = 5,000’

700 mb = 10,000’

500 mb = 18,000’ (middle of the atmosphere)

300 mb = 30,000’

Troughs and RidgesTroughs and Ridges

• But contour lines are usually not straight.– Ridges (elongated

highs) occur where air is warm

– Troughs (elongated lows occur where air is cold

• Temperature gradients generally produce pressure gradients• Isobars usually decrease in value from south to north (cooler

temperatures)

Surface pressure and windsSurface pressure and windsNear the surface in the N

Hemisphere winds blow– counterclockwise

around and in toward the center of low pressure areas

– clockwise around and outward from the center of high pressure areas

Why doesn’t the wind blow directly

from high to low pressure?

Upper Level Pressure PatternsUpper Level Pressure Patterns• At upper levels, winds blow parallel to the

pressure/height contours

Forces and windsForces and winds• Differences in pressure produce fluid movementDifferences in pressure produce fluid movement

Forces Controlling the WindForces Controlling the Wind

• Pressure Gradient Force• Coriolis Force• Centrifugal Force• Friction Force

• Four forces act simultaneously to cause the wind

Pressure Gradient ForcePressure Gradient Force• Magnitude

– Inversely proportional to distance

– Closer together = stronger force

• Direction– Always directed toward lower pressure

and perpendicular to isobars

Coriolis ForceCoriolis ForceApparent force due to rotation

• Magnitude– Dependent on latitude and

speed of air parcel• Higher latitude = larger

Coriolis force– zero at the equator,

maximum at the poles• The faster the speed, the

larger the Coriolis force

• Direction– To the right he Northern

Hemisphere• To the left in S Hemi

• Does NOT influence speed

Coriolis ForceCoriolis Force• Acts to right in northern

hemisphere• Stronger (i.e. more deviation)

for faster wind

Geostrophic WindGeostrophic Wind• Geostrophic wind is flow in a straight line in

which the pressure gradient force balances the Coriolis force. PGF=CF

Lower PressureLower Pressure 994 mb994 mb

996 mb996 mb

998 mb998 mb

Higher PressureHigher Pressure

Geostrophic WindGeostrophic Wind• Wind speed constant if isobars are straightWind speed constant if isobars are straight• Speed is proportional to Pressure GradientSpeed is proportional to Pressure Gradient

• Bernoulli EffectBernoulli Effect– Same as nozzle on water hoseSame as nozzle on water hose

Geostrophic flowGeostrophic flow• With the inclusion of the Coriolis Force, air flows With the inclusion of the Coriolis Force, air flows

parallel to isobars of constant pressure. parallel to isobars of constant pressure.

Centripetal ForceCentripetal Force• Object on a curved path has an apparent Object on a curved path has an apparent

inward force: centripetal forceinward force: centripetal force• MagnitudeMagnitude

– depends upon the radius of curvature of the depends upon the radius of curvature of the curved path taken by the air parcelcurved path taken by the air parcel

– depends upon the speed of the air parceldepends upon the speed of the air parcel• DirectionDirection

– at right angles to the direction of movementat right angles to the direction of movement

Friction near Earth’s surfaceFriction near Earth’s surface• Friction of the ground slows wind downFriction of the ground slows wind down

– Magnitude depends onMagnitude depends on• Speed of the air parcel Speed of the air parcel • Roughness of the terrainRoughness of the terrain• How uniform the wind field isHow uniform the wind field is

– DirectionDirection• Always oppositeAlways opposite to air movement to air movement

– Importance of friction layer Importance of friction layer (aka PBL = Planetary Boundary Layer)(aka PBL = Planetary Boundary Layer)• Approx. lowest 3,000 ft of the atmosphereApprox. lowest 3,000 ft of the atmosphere

Frictional EffectsFrictional Effects• AGAIN Friction only slows wind speed, does not

change wind direction• Therefore, in the Northern Hemisphere

– Wind speed decreased by frictionWind speed decreased by friction– Coriolis force thus decreased and thus will not quite Coriolis force thus decreased and thus will not quite

balance the pressure gradient forcebalance the pressure gradient force– Force imbalance (PGF > CF) pushes wind in toward Force imbalance (PGF > CF) pushes wind in toward

low pressurelow pressure– Angle at which wind crosses isobars depends on Angle at which wind crosses isobars depends on

surface roughnesssurface roughness» Average ~ 30 degreesAverage ~ 30 degrees

Frictional EffectsFrictional Effects• Retards wind Retards wind

speed near the speed near the surfacesurface

• Lowers the Lowers the Coriolis ForceCoriolis Force

• Therefore, wind Therefore, wind direction is direction is altered from altered from parallel to parallel to crossing isobarscrossing isobars.

Cyclonic & Anticyclonic WindsCyclonic & Anticyclonic Winds

Isobar Surface MapIsobar Surface Map

Winds and vertical air motionWinds and vertical air motion• Surface winds blow

– Toward low pressure (convergence)– Outward from high pressure (divergence)

• Vertical movement to compensate– Surface convergence leads to divergence aloft– Surface divergence leads to convergence aloft

VERY IMPORTANT

CONCEPT

Naming WindsNaming Winds• Named for direction of originNamed for direction of origin

– North wind comes from the northNorth wind comes from the north– Seabreeze comes from the seaSeabreeze comes from the sea– Exceptions: offshore/onshoreExceptions: offshore/onshore upslope/downslopeupslope/downslope

Measuring WindsMeasuring Winds• InstrumentsInstruments

– Wind vanesWind vanes– AnemometersAnemometers– ComboCombo

• AerovaneAerovane• Wind sockWind sock

– ProfilersProfilers– RadarRadar

Wind MeasurementsWind Measurements• SpeedsSpeeds

– Sustained: 2 minute average in past 10 minutesSustained: 2 minute average in past 10 minutes– Gusts: greatest 5-second speed in past 10 minutesGusts: greatest 5-second speed in past 10 minutes– Peak: greatest 5-second speed since last observationPeak: greatest 5-second speed since last observation

• DirectionDirection– 2 minute average direction2 minute average direction– +/- 10 degrees+/- 10 degrees

Wind DirectionWind Direction• Directional

names • (16-point

compass)

Beaufort ScaleBeaufort ScaleForceForce DescriptionDescription MphMph SeaSea LandLand

00 CalmCalm < 1< 1 Sea like a mirrorSea like a mirror Smoke rises vertically.Smoke rises vertically.

11 Very LightVery Light 1-31-3 Ripples like scales, Ripples like scales, Direction of wind shown by smoke drift Direction of wind shown by smoke drift

22 Light breezeLight breeze 4-74-7 Wavelets, pronounced. Wavelets, pronounced. Wind felt on face , leaves rustle, Wind felt on face , leaves rustle,

33 Gentle breezeGentle breeze 8-128-12 Large wavelets, crests break. Large wavelets, crests break. Leaves and twigs in constant motion, Leaves and twigs in constant motion,

44 Mod. breezeMod. breeze 13 - 1813 - 18 Small waves becoming longer, Small waves becoming longer, Wind raises dust and loose paper, Wind raises dust and loose paper,

55 Fresh breezeFresh breeze 19 - 2419 - 24 Moderate waves of long form. Moderate waves of long form. Small trees in leaf start to sway, Small trees in leaf start to sway,

66 Strong breezeStrong breeze 25 - 3125 - 31 Some large waves, extensive Some large waves, extensive white foam crests, some spray.white foam crests, some spray.

Large branches in motion, whistling in telegraph Large branches in motion, whistling in telegraph wires, wires,

77 Near galeNear gale 32 - 3832 - 38 Sea heaped up, white foam from Sea heaped up, white foam from breaking wavesbreaking waves Whole trees in motion, Whole trees in motion,

88 GaleGale 39 - 4639 - 46 Moderately high and long waves. Moderately high and long waves. Twigs break from trees, difficult to walk.Twigs break from trees, difficult to walk.

99 Strong galeStrong gale 47 - 5447 - 54 High waves, dense foam streaks High waves, dense foam streaks in wind, in wind, Slight structural damage occursSlight structural damage occurs

1010 StormStorm 55 - 6355 - 63 Very high waves with long Very high waves with long overhanging crests. overhanging crests.

Trees uprooted, considerable structural damage Trees uprooted, considerable structural damage occurs.occurs.

1111 Violent stormViolent storm 64 - 7364 - 73Exceptionally high waves, Exceptionally high waves, sometimes concealing small and sometimes concealing small and medium sized ships. medium sized ships.

Widespread damage.Widespread damage.

1212 HurricaneHurricane >74>74 Air filled with foam and spray, Air filled with foam and spray, sea white with driving spray, sea white with driving spray, Widespread damage.Widespread damage.

Wind RoseWind RoseNORTH

SOUTH

WEST EAST

4%

8%

12%

16%

20%

WIND SPEED (Knots)

>= 22

17 - 21

13 - 17

9 - 13

4 - 9

1 - 4

Calms: 0.15%

Wind Rose ApplicationWind Rose Application

Atmospheric Atmospheric CirculationsCirculations

Scales of MotionScales of Motion• MicroscaleMicroscale: meters: meters

– Turbulent eddiesTurbulent eddies• Mechanical disturbance or convectionMechanical disturbance or convection• MinutesMinutes

• MesoscaleMesoscale: km’s to 100’s of km’s: km’s to 100’s of km’s– Local winds and circulationsLocal winds and circulations

• Land/sea breezes, mountain/valleyLand/sea breezes, mountain/valleywinds, thunderstorms, tornadoeswinds, thunderstorms, tornadoes

• Minutes to hoursMinutes to hours• Synoptic scaleSynoptic scale: 100’s to 1000’s of km’s: 100’s to 1000’s of km’s

– High and low pressure circulationsHigh and low pressure circulations• Days to weeksDays to weeks

• Global scaleGlobal scale: systems ranging over entire : systems ranging over entire globeglobe

Surface Friction and WindsSurface Friction and Winds• Planetary Boundary Layer (PBL)• Wind speeds typically increase with height but rate

depends on PBLB) smooth terrain =

stable

A) rough terrain = unstable

EddiesEddies

EddiesEddies

• Produced by flow past a mountain range in Produced by flow past a mountain range in a stable atmospherea stable atmosphere– Can form lenticular and rotor cloudsCan form lenticular and rotor clouds

• Large gradients in wind speed over short Large gradients in wind speed over short distances cause strong wind sheardistances cause strong wind shear– Clear air turbulence (CAT) can result, producing Clear air turbulence (CAT) can result, producing

dangerous conditions for aircraftdangerous conditions for aircraft

EddiesEddies

EddiesEddies

Von Karmann Eddies

Sea BreezesSea Breezes• Sea breezeSea breeze

– Differential heating/Differential heating/coolingcooling of adjacent land of adjacent land and water surfacesand water surfaces

• Land BreezeLand Breeze– Weaker gradients, Weaker gradients,

weaker breezeweaker breeze

Florida Sea BreezesFlorida Sea Breezes

The MonsoonThe Monsoon• Seasonal wind (Arabic word "mausim” = season)

– Eastern and southern Asia– Arizona monsoon– Synoptic scale land/sea breeze systems

• Differential heating and pressure patterns

What is NOT mentioned here?What is NOT mentioned here?

Valley WindsValley Winds• Sunlight heats mountain

slopes during the day• Air in contact with surface

is heated• A difference in air density

is produced between air next to the mountainside and air at the same altitude away from the mountain

• Density difference produces upslope (day) or downslope (night) flow

Valley Wind

Mountain/Valley WindsMountain/Valley Winds• Sunlight heats mountain

slopes during the day and they cool by radiation at night

• Air in contact with surface is heated/ cooled in response

• A difference in air density is produced between air next to the mountainside and air at the same altitude away from the mountain

• Density difference produces upslope (day) or downslope (night) flow

Mountain Wind

Katabatic WindsKatabatic Winds• Colder denser air descending

downslope– Channeled by terrain– Mistral from the Alps thru the

Rhone Valley to the Mediterranean

– Bora from Russia through Yugoslavia to the Adriatic

– Coho from Columbia Basin to the Pacific

Foehn WindsFoehn Winds• High pressure over the mountains• Low pressure over the plains• Strong winds aloft - above 15000 ft• Chinook – “snow eater”

– Blackfoot Indian name– 1/22/43 Spearfish, SD

• 0730 = -4 deg• 0732 = 47 degrees!!

Santa Ana WindsSanta Ana Winds• High pressure over the

Great Basin• Low pressure off Calif.

Coast• Compressional warming• Peak Season = Fall• High Fire danger

Santa Ana Winds Santa Ana Winds (10/23/2003)(10/23/2003)

Diablo WindsDiablo Winds• Winds from direction of Mt. Diablo

– also Spanish term for “devil”• Higher pressure over Idaho and N. Nevada• An “Offshore” wind• Oakland Hills Fire

– Sunday, October 20, 1991– Temperature low 90s– Dry fuel from Dec 1990– Northeast winds 25 mph

for 48 hours– 25 fatalities, 150 injured– Destroyed 4,000 homes & 2,000 vehicles– Total damage $1.6 Billion

Nor’eastersNor’easters• Strong low

pressure systems moving up Atlantic seaboard– Strong winds

and heavy precipitation

Dust DevilsDust Devils• Dust devils are NOT Tornadoes

– Surface heating produces convection and eddies

– Wind blowing past object twists rising air– Air rushes into rising column lifting dirt and

debris

Global WindsGlobal Winds

Global CirculationGlobal Circulation• Atmospheric and oceanic circulations are ultimately

driven by …. differential solar heating • Solar Radiation

– Incoming radiation from the sun (short wavelength or solar radiation)

– Outgoing radiation from the earth (long wavelength or terrestrial radiation)

• Equator more, Poles receive much less solar radiation– Difference in the sun’s angle of incidence – Tilt of the earth’s axis results in no solar radiation pole-ward

of the arctic circle for six months each year– Arctic and Antarctic ice reflect considerable solar radiation

back to space

Single Cell ModelSingle Cell Model• Assume Non-Rotating Earth• Equatorial Convection leads to formation of convection cell in each

hemisphere• Energy transported from equator toward poles with return flow• Hadley CellBUT THE EARTH DOES ROTATE!!

Three Cell ModelThree Cell Model• Hadley cell

– air rises near equator and descends near 30 deg

– explains deserts; trade winds; ITCZ

• Ferrel Cell (indirect thermal cell)

- air rises near 60 deg and descends near 30 deg

- explains surface westerlies• Polar Cell

– Boundary between cold polar air and mid-latitude warmer air is the polar front

The Real WorldThe Real World• Disruptions to 3-Cell model by

– continents, mountains, and ice fields• Semi-permanent Highs and Lows persist throughout large

periods of the year– Winter: highs form over land; lows over oceans. – Summer: lows over land and highs iover oceans. – Bermuda High and Pacific High near 30° shrink in winter– Features change from winter to summer.

• The Inter-Tropical Convergence Zone (ITCZ) shifts toward south in January and toward north in July.

Winter PatternWinter Pattern

Summer PatternSummer Pattern

Jet StreamsJet Streams• Fast rivers of air• 1000’s of mi’s long, a few

hundred mi wide, a few mi thick

• Typically two jet streams• Polar

– Stronger• Subtropical

Jet StreamsJet Streams

JetJetStreamStream

MapMap

Rossby Waves – Global ScaleRossby Waves – Global Scale

The dishpan experimentThe dishpan experiment• A dishpan with a hot equator and a cold pole is

rotated– Troughs, ridges and eddies are produced, similar to

patterns observed in earth’s general circulation

Wind Patterns and OceansWind Patterns and Oceans

Winds and UpwellingWinds and Upwelling

Ekman Spiral

Winds and UpwellingWinds and Upwelling