Air Pressure and Winds, Atmospheric Circulations

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Transcript Air Pressure and Winds, Atmospheric Circulations

Week 7 (March 10)
• Tonight
– Air Pressure & Winds (Chp 6)
– Atmospheric Circulation (Chp 7)
– Classwork/Homework #7
• Next Week (Mar 17)
– Air Masses and Fronts (Chp 8)
– El Niño/La Niña
• March 24
– No Class – Spring Break
• March 31
– No Class = Cesar Chavez Day
Wind
Weight
Pressure = 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 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
500 mb level
1000 mb
1000 mb
Cool the left column;
warm the right column
The heated column
expands
The cooled
column contracts
500 mb
original 500 mb level
500 mb
1000 mb
1000 mb
The level of the 500 mb surface changes; the
surface pressure remains unchanged
500 mb surface is
displaced upward in the
warmer column
500 mb level is
displaced downward
in the cooler column
new 500 mb
level in warm
air
original 500 mb level
new 500 mb
level in cold
air
The surface pressure
remains the same since
both columns still contain
the same mass of air.
1000 mb
1000 mb
Air moves from high to low pressure in
the middle of column, causing surface
pressure to change.
original 500 mb level
Low
1003 mb
High
997 mb
Note the new
surface pressures
Air now also moves from high to low
pressure at the surface…
original 500 mb level
Low
High
High
Low
1003 mb
997 mb
Where would we
have rising
motion?
Air now also moves from high to low
pressure at the surface…
original 500 mb level
Low
High
High
Low
1003 mb
997 mb
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 Pressure
Mercury Barometer
Aneroid Barometer
Station Pressure v. Sea Level Pressure
Pressure 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 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 Ridges
• Temperature gradients generally produce pressure gradients
• Isobars usually decrease in value from south to north (cooler
temperatures)
• But contour lines are
usually not straight.
– Ridges (elongated
highs) occur where
air is warm
– Troughs (elongated
lows occur where air
is cold
Surface pressure and winds
Near 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 Patterns
• At upper levels, winds blow parallel to the
pressure/height contours
Forces and winds
• Differences in pressure produce fluid movement
Forces Controlling the Wind
• Four forces act simultaneously to cause the wind
• Pressure Gradient Force
• Coriolis Force
• Centrifugal Force
• Friction Force
Pressure Gradient Force
• Magnitude
– Inversely
proportional
to distance
– Closer
together =
stronger
force
• Direction
– Always directed toward lower pressure
and perpendicular to isobars
Coriolis Force
Apparent 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 Force
• Acts to right in
northern
hemisphere
• Stronger (i.e.
more deviation)
for faster wind
Geostrophic Wind
• Geostrophic wind is flow in a straight line in
which the pressure gradient force balances
the Coriolis force. PGF=CF
994 mb
Lower Pressure
996 mb
998 mb
Higher Pressure
Geostrophic Wind
• Wind speed constant if isobars are straight
• Speed is proportional to Pressure Gradient
• Bernoulli Effect
– Same as nozzle on water hose
Geostrophic flow
• With the inclusion of the Coriolis Force, air flows
parallel to isobars of constant pressure.
Centripetal Force
• Object on a curved path has an apparent
inward force: centripetal force
• Magnitude
– depends upon the radius of curvature of the
curved path taken by the air parcel
– depends upon the speed of the air parcel
• Direction
– at right angles to the direction of movement
Friction near Earth’s surface
• Friction of the ground slows wind down
– Magnitude depends on
• Speed of the air parcel
• Roughness of the terrain
• How uniform the wind field is
– Direction
• Always opposite to air movement
– Importance of friction layer
(aka PBL = Planetary Boundary Layer)
• Approx. lowest 3,000 ft of the atmosphere
Frictional Effects
• AGAIN Friction only slows wind speed, does not
change wind direction
• Therefore, in the Northern Hemisphere
– Wind speed decreased by friction
– Coriolis force thus decreased and thus will not quite
balance the pressure gradient force
– Force imbalance (PGF > CF) pushes wind in toward
low pressure
– Angle at which wind crosses isobars depends on
surface roughness
» Average ~ 30 degrees
Frictional Effects
• Retards wind
speed near the
surface
• Lowers the
Coriolis Force
• Therefore, wind
direction is
altered from
parallel to
crossing isobars.
Cyclonic & Anticyclonic Winds
Isobar Surface Map
Winds 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 Winds
• Named for direction of origin
– North wind comes from the north
– Seabreeze comes from the sea
– Exceptions: offshore/onshore
upslope/downslope
Measuring Winds
• Instruments
– Wind vanes
– Anemometers
– Combo
• Aerovane
• Wind sock
– Profilers
– Radar
Wind Measurements
• Speeds
– Sustained: 2 minute average in past 10 minutes
– Gusts: greatest 5-second speed in past 10 minutes
– Peak: greatest 5-second speed since last observation
• Direction
– 2 minute average direction
– +/- 10 degrees
Wind Direction
• Directional
names
• (16-point
compass)
Beaufort Scale
Force
Description
Mph
Sea
Land
0
Calm
<1
Sea like a mirror
Smoke rises vertically.
1
Very Light
1-3
Ripples like scales,
Direction of wind shown by smoke drift
2
Light breeze
4-7
Wavelets, pronounced.
Wind felt on face , leaves rustle,
3
Gentle breeze
8-12
Large wavelets, crests break.
Leaves and twigs in constant motion,
4
Mod. breeze
13 - 18
Small waves becoming longer,
Wind raises dust and loose paper,
5
Fresh breeze
19 - 24
Moderate waves of long form.
Small trees in leaf start to sway,
6
Strong breeze
25 - 31
Some large waves, extensive
white foam crests, some spray.
Large branches in motion, whistling in telegraph
wires,
7
Near gale
32 - 38
Sea heaped up, white foam from
breaking waves
Whole trees in motion,
8
Gale
39 - 46
Moderately high and long waves.
Twigs break from trees, difficult to walk.
9
Strong gale
47 - 54
High waves, dense foam streaks
in wind,
Slight structural damage occurs
10
Storm
55 - 63
Very high waves with long
overhanging crests.
Trees uprooted, considerable structural damage
occurs.
Exceptionally high waves,
sometimes concealing small and
medium sized ships.
Widespread damage.
Air filled with foam and spray,
sea white with driving spray,
Widespread damage.
11
Violent storm
64 - 73
12
Hurricane
>74
NORTH
Wind Rose
20%
16%
12%
8%
4%
WEST
EAST
WIND SPEED
(Knots)
>= 22
17 - 21
SOUTH
13 - 17
9 - 13
4-9
1-4
Calms: 0.15%
Wind Rose Application
Atmospheric
Circulations
Scales of Motion
• Microscale: meters
– Turbulent eddies
• Mechanical disturbance or convection
• Minutes
• Mesoscale: km’s to 100’s of km’s
– Local winds and circulations
• Land/sea breezes, mountain/valley
winds, thunderstorms, tornadoes
• Minutes to hours
• Synoptic scale: 100’s to 1000’s of km’s
– High and low pressure circulations
• Days to weeks
• Global scale: systems ranging over entire
globe
Surface Friction and Winds
• Planetary Boundary Layer (PBL)
• Wind speeds typically increase with height but rate
depends on PBL
B) smooth
terrain =
stable
A) rough
terrain =
unstable
Eddies
• Produced by flow past a mountain range in
a stable atmosphere
Eddies
– Can form lenticular and rotor clouds
• Large gradients in wind speed over short
distances cause strong wind shear
– Clear air turbulence (CAT) can result, producing
dangerous conditions for aircraft
Eddies
Eddies
Von Karmann Eddies
Sea Breezes
• Sea breeze
– Differential
heating/
cooling of
adjacent land
and water
surfaces
• Land Breeze
– Weaker
gradients,
weaker
breeze
Florida Sea Breezes
The 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?
Valley 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 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 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 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 Winds
• High pressure over the
Great Basin
• Low pressure off Calif.
Coast
• Compressional warming
• Peak Season = Fall
• High Fire danger
Santa Ana Winds (10/23/2003)
Diablo 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’easters
• Strong low
pressure
systems moving
up Atlantic
seaboard
– Strong winds
and heavy
precipitation
Dust 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 Winds
Global 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 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 Cell
BUT THE EARTH DOES ROTATE!!
Three 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 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 Pattern
Summer Pattern
Jet 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 Streams
Jet
Stream
Map
Rossby Waves – Global Scale
The 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 Oceans
Winds and Upwelling
Ekman Spiral
Winds and Upwelling