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Air Pressure and Winds
Chapter 8
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I. Atmospheric Pressure
II. Surface and Upper Level Charts
III. Newton
s Laws of MotionIV. Forces the Influence the Wind
V. Winds and Vertical Motion
Chapter 8
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Why Does the Wind Blow?
A. What is wind?
B. Wind is horizontalmovement of air
C. Why does air move?
D. Air moves in response to a
horizontal gradient in air
pressure, attempting to
equalize imbalances in
pressure
E. The wind direction is the
direction f romwhich the
wind is blowing
http://www.eo.ucar.edu/basics/wx_2_c.html
http://www.youtube.com/watch?v=1k0iYDjqJEs&feature=related
http://www.youtube.com/http://www.youtube.com/ -
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http://www.youtube.com/watch?v=QVayky_b-6U
I. Atmospheric Pressure
A. Pressure is the force exerted
on objects by the weight oftiny molecules of air
B. How heavy is the pressure?
- 1 ton per square ft
http://www.youtube.com/watch?v=QVayky_b-6Uhttp://www.youtube.com/watch?v=QVayky_b-6Uhttp://www.youtube.com/watch?v=QVayky_b-6Uhttp://www.youtube.com/watch?v=QVayky_b-6U -
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Mercury Barometer Aneroid Barometer
The instruments that measure air pressure
are called barometers
Measuring Air Pressure
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Recording Barograph
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Rainfall and Temperature Measurements
Grants Farm July 30-31, 2013
July 30 July 31
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Pressure Measurements
Grants Farm One week in August, 2013
July 30 July 31
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Pressure Measurements for Entire Month
Grants Farm August, 2013
July 30 July 31
August 1 August 31
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Pressure and Temperature Measurements for Entire Month
Grants Farm August, 2013
August 1August 31
Temperatures dominated by
daily cycles
Pressure dominated by
synoptic patterns
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August 1August 31
Pressure dominated by
synoptic patterns
Pressure and Temperature Measurements for Entire Month
Grants Farm August, 2013
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Surface and Sea Level Pressure
A. Why cant we
compare stationpressures?
B. Pressure
decreases withelevation byabout 10 mb/100m
C. We extrapolatepressure fromaltitude to sealevel a commonlevel
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Sea Level Pressure
Sample of sealevel pressuremap or chart
Thecontoursare isobars
Thenumbers atstations aresea levelpressure inmillibars(mb).
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Pressure and Temperature
A. Pressure relates to density; density relatestemperature
B. Colder air
molecules move more slowly andare closer together (more dense columncontracts)
C. Warmer air molecules move faster and are
further apart (less dense
column expands)
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Pressure Differences and Wind
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II. Surface and Upper Level Charts
A. Sea level pressure chart is a constant height chart
B. Upper level charts are constant pressure charts
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Temperature and Height
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Ridges and Troughs
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Aviation and Height
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Heights on a Weather Map
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Elevation and Height
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Summary: Pressure
A. Pressure is the weight of air above a given level
B. It decreases with height rapidly; the higherelevation; the lower pressure
C. Pressure is used to represent a verticalcoordinate, instead of altitude
- 850mb, 700mb, 500mb, 300mb, as altitude.
D. Pressure at different altitudes is converted tocommon sea level
E. Warmer (lighter) air above results in lower surfacepressure; and vice versa
F. Higher pressure typically is associated with fairweather; the opposite is true with low pressure
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III. Newtons Laws of Motion
A. 1stLaw objects in motion remain inmotion
B. 2ndLaw the force exerted on an objectequals its mass times the accelerationproduced
C. To determine wind direction we mustconsider all the forces acting on it:1. Pressure Gradient Force
2. Coriolis Force3. Centripetal Force
4. Friction
h fl i d
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B. If isobars are closer together steep (strong) pressure gradient
C. If isobars are further apart
weak pressure gradient
D. Pressure gradient force(PGF)is the force due to pressure
difference, pointing from higher tolower pressure
- Magnitude of forceproportional to pressuregradient
d
pPG
IV. Forces that Influence Wind
A. Pressure gradient the amount ofpressure change that occurs over a given
distanceExample: PG = 4 mb / 100 km
P d Ai M
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Pressure gradient
force moves airfrom high to lowpressure areas
Differential heating or cooling a column establisheshorizontal pressure gradients that cause air
movement, I.e., winds
http://admin.wadsworth.com/resour
ce_uploads/static_resources/05343
97719/1406/Ahrens9-
2SeaBreeze.swf
Pressure and Air Movement
http://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swfhttp://admin.wadsworth.com/resource_uploads/static_resources/0534397719/1406/Ahrens9-2SeaBreeze.swf -
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C i li F
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http://en.wikipedia.org/wiki/Image:Corioliskraftanimation.gif
Coriolis Force
A. Watching outside the diskball goes in straightline,
B. But to an observer on thespinning disc, the ballappears to veer to theright
C. The faster the ballmoves, the larger the
deflection
C i li F
http://en.wikipedia.org/wiki/Image:Corioliskraftanimation.gifhttp://en.wikipedia.org/wiki/Image:Corioliskraftanimation.gif -
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http://apollo.lsc.vsc.edu/classes/met130/notes/chapter8/cf_intro2.html
Coriolis Force
A. Because the Earth is rotating, air motions will
appear to turn or deflect, similar to merry-go-round
B. This deflection is an apparentforce meaning it
would not exist if it were not for the rotation of
the Earth
C. This apparent force is called the Coriolis force
(CF)
C i li F
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter8/cf_intro2.htmlhttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter8/cf_intro2.html -
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Coriolis Force
A. As an air parcel intends to move from south tonorth along a meridian, it actually moves towards
northeastB. Similarly, a south-moving parcel would be
deflected towards southwest
C. In both cases, the parcel is deflected toward rightin the Northern Hemisphere
F t I ti D fl ti
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Factors Impacting Deflection
A. The amount of Coriolis deflection depends
on:1. The rotation of earth
2. The latitude
3. The objects speed
S F
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Summary: Forces
A. Two main forces act on air aloft where the surface
friction is negligibleB. Pressure gradient force (PGF) as an actual force;Coriolis force (CF) is caused by earth rotation andair movement
C. PGF is proportional to gradient; steep gradient
produces stronger forceD. CF depend are wind speed and direction
1. Direction: always perpendicular to wind deflection tothe right in the northern hemisphere
2. Magnitude: increases with wind speed; If air is still,CF deflection is zero
E. Near ground surface, friction slows down airmovement and deflects toward low pressure
St i ht Li Fl G t hi Wi d
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Air flowing parallelto straight isobarsis known as the
geostrophic wind
Straight Line FlowGeostrophic Wind
A. Air aloft is away from the influence of friction due
to surface objects
B. As soon as the PGF initiates movement, the CF
deflects the parcel to the right
C. As wind speed increases, so does CF
D. Eventually CF balances PGF: parcel no longer
accelerates then it moves at constant speed
H t T ll Wi d Di ti
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PGF
CF
PGF
CFFriction
How to Tell Wind Direction
A. By knowing the isobar pattern, we can
determine the direction and speed of thegeostrophic wind using Buys-Ballot Law
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C d Fl G di t Wi d
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Curved FlowGradient Wind
A. Real isobaric contours are seldom straight,
and winds rarely brow straight trajectoriesB. Thus, we study circular wind pattern -
gradient wind
Gradient Winds
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Gradient WindsA. Unlike geostrophic winds blowing parallel to
straight lines, gradient winds blow along curved
isobars
1. For cyclonic flow, PGF>CF
2. For anticyclonic flow, PGF
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Surface Friction
A. Both geostrophic and gradient winds ignore
frictionB. The surface friction decreases the wind
speed and increases the angle between thewind and isobars
C. Decreasing wind speed also reduces CF; CFis smaller than PGF. The wind would blowtoward low pressure side
D. Because of friction, winds seldom blow along
straight lines or perfect circlesE. They are only good approximation to real
winds aloft away from ground surface
Real Wind Patterns
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Real Wind Patterns
A. Winds flow in northern hemisphere
1. counterclockwise and inward into a low (L)
2. clockwise and outward from a high (H)
Real Wind Patterns
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Low (L) low center
High: H
high center
Trough elongated low
Ridge elongated high
Isobar
Isotherm
Isoheight
Wind barbs
1 knot=1.15 mi/hr
Trough
Ridge
Real Wind Patterns
V Winds and Vertical Motions
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V. Winds and Vertical Motions
A. As air converges to a low-pressure center, it
must go somewhere up!
B. The air aloft diverges to compensate for the
converging air; opposite is true for a high-
pressure center at the surface
Convergence(Confluence) vs Divergence(Difluence)
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Note: confluence is not the same as convergence! Confluence refers to air flowing
towards an axis parallel to the airflow; convergence is the net inflow of air into a
given area.Looks more like a confluent
process to me!
http://www.pragmatek.com/images/convergence.gif
Difluence
Confluence
http://docs.lib.noaa.gov/rescue/mwr/085/mwr-085-01-0028.pdf
http://www.islandnet.com/~see/weather/elements/whatgoesup3.htm
Convergence(Confluence) vs. Divergence(Difluence)
Hydrostatic Equilibrium
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Hydrostatic Equilibrium
A. The upward directed PGF is balanced by
the downward force of gravity
B. The atmosphere is normally in hydrostatic
balance except within violent storms
Winds and Vertical Motions
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Winds and Vertical Motions
A. Recall that the vertical motion is very
important for weather1. Rising air cools down and expands to form
clouds and precipitation and that air mustcontinue to rise to maintain this precipitation
2. The stronger the vertical motion the moreintense the precipitation (as a general rule)
B. The vertical motion is supported by pressuregradients that in turn are related to
temperature distributionC. Next chapter will study air masses and fronts
which characterize horizontal temperaturepatterns
Summary: Wind Patterns
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Summary: Wind Patterns
A. Two main wind patterns are discussed:
geostrophic wind and gradient windB. Geostrophic wind results when pressure gradient
force (PGF) and and Coriolis force (CF) balance1. It blows along straight isobars; its direction is such
that when you stand with your back against wind, thehigh pressure in on your right in northernhemisphere.
C. Gradient wind describes circular flow when PGFislarger or smaller than CF
D. The surface friction, another force, causes flow tocross isobarstowards the lower pressure side1. Convergencegenerated by cross-isobars flow
generates upward motion and thus weather
http://www.youtube.com/