CH08 Air Pressure Winds Jf Mods Sp2014(1)

8/12/2019 CH08 Air Pressure Winds Jf Mods Sp2014(1)

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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


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



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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)


16/45http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter05/

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/

CH08 Air Pressure Winds Jf Mods Sp2014(1)

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