week 7 (march 10) tonighttonight – air pressure & winds (chp 6) –atmospheric circulation...
TRANSCRIPT
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