chapter 7 atmospheric motions chapter 7 atmospheric motions

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CHAPTER 7 ATMOSPHERIC MOTIONS

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Page 1: CHAPTER 7 ATMOSPHERIC MOTIONS CHAPTER 7 ATMOSPHERIC MOTIONS

CHAPTER 7

ATMOSPHERIC MOTIONS

CHAPTER 7

ATMOSPHERIC MOTIONS

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Atmospheric Pressure is the force per unit area of a column of air above you

In other words, pressure is the weight of the column of air above you - a measure of how hard this column of air is pushing down

More fundamentally - atmospheric pressure arises from gravity acting on a column of air

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1000 mb 1000 mb

500 mb level

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1000 mb

500 mb

500 mb

1000 mb

The heated columnexpands

The cooledcolumn contracts

original 500 mb level

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1000 mb

new 500 mblevel in warmair

new 500 mblevel in coldair

1000 mb

The 500 mb surface isdisplaced upward in the warmer column

The level corresponding to 500 mb is displaced downward in the cooler column

original 500 mb level

The surface pressure remainsthe same since both columnsstill contain the same mass of air.

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1000 mb

new 500 mblevel in warmair

new 500 mblevel in coldair

1000 mb

The 500 mb surface isdisplaced upward in the warmer column

The 500 mb surface isdisplaced downward inthe cooler column

original 500 mb level

The surface pressure remainsthe same since both columnsstill contain the same mass of air.

A pressure difference in the horizontal direction A pressure difference in the horizontal direction develops above the surfacedevelops above the surface

HighLow

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1003 mb 997 mb

original 500 mb level

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

HighLowWarm air aloft = high pressure

Cold air aloft = low pressure

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1003 mb 997 mb

original 500 mb level

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

HighLow

High Low

Where would we haverising motion?

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1003 mb 997 mb

original 500 mb level HighLow

High LowAir diverges around the surface high

Air converges around the surface low

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Rising air above the surface low leads to clouds and storms◦ Low pressure centers

aka “cyclones” Sinking air above

the surface high leads to fair weather◦ High pressure centers

aka “anticyclones”

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Cold air aloft means low pressure (heights), warm air aloft means high pressure (heights).

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Above the ground, we typically look at maps showing the height of a given pressure level

If there are no horizontal variations in pressure, the pressure at a constant height level, or the height at a constant pressure level, are the same thing

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When there are horizontal variations, we see high heights in warm air aloft; low heights in cold air aloft

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Steeper slope means the contour lines are closer together!

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Table 7.1, p. 181

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The cause of the wind! Horizontal pressure gradients lead to winds PGF always directed from high to low

pressure The stronger the pressure gradient, the

stronger the wind ◦ Or, in other words, the closer the isobars are

together, the stronger the wind will be

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The length of the red arrows indicate the strength of the PGF

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We don’t actually see the wind blow straight across from high pressure to low pressure

There must be other force(s) at work…

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“Apparent” force due to rotation◦An outside (nonrotating) observer

doesn’t experience it◦An observer on the rotating body (like

the earth, or the turntable) does experience it

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Since we (and the atmosphere) are rotating with the earth, we are affected by this force

Coriolis force turns moving objects/air parcels to the right in the northern hemisphere, to the left in the southern hemisphere

The faster the motion, the stronger the Coriolis force

Coriolis force is zero at the equator, relatively strong at the poles

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Pressure gradient force (PGF)◦ Always from high pressure to low pressure

Coriolis force◦ Always toward the right (in the northern

hemisphere) When these two are in balance, it is called

the geostrophic wind◦ Geostrophic = “earth turning”

If you’re traveling with the geostrophic wind, low pressure is always on your left!◦ “when the wind is at your back, lower pressure is

to your left (NH)”

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The geostrophic wind blows parallel to straight isobars

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But what if the isobars (or isoheights) aren’t straight?◦ (They’re usually curved – troughs and ridges)

When there is curvature, an observer (or an air parcel) in the rotating frame of reference experiences a force directed outward – the centrifugal force – think of being in a car going around a curve

Magnitude of centrifugal force is related to the velocity and the radius of curvature◦ Faster speeds = greater centrifugal force◦ Tight curves = greater centrifugal force

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Involves the PGF, Coriolis, and Centrifugal forces – flow is parallel to curved isobars

This is a good estimate of the winds, except right near the ground

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Stepped Art

Fig. 8-29, p. 214

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There’s one more force that’s important for winds near the ground

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Near the surface, the wind is slowed by drag from the ground, trees, buildings, etc.

What happens to force balance of geostrophic wind when the wind slows down?

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When wind speed slows down, Coriolis force also is reduced

Therefore, PGF is stronger than Coriolis, and wind blows across isobars toward lower pressure

Wind blows in toward a surface low, and away from a surface high

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Aloft – flow parallel to isobars or isoheights

Near surface – Near surface – flow in toward flow in toward low, away low, away

from highfrom high Cyclonic flow Cyclonic flow

(counterclockwise (counterclockwise in NH)in NH)

Anticyclonic flow Anticyclonic flow (clockwise in (clockwise in NH)NH)

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Fig. 7.17, p. 189

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Like a tornado, or your water in your bathtub

In these situations, the balance is between the PGF and centrifugal forces (Coriolis is unimportant)◦ This is called a

cyclostrophic wind The water flowing out

of your bathtub doesn’t change directions in different hemispheres!

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ForceForce DirectionDirection MagnitudeMagnitude Important Important when…when…

Pressure Pressure Gradient Gradient (PGF)(PGF)

From high to low From high to low pressurepressure

Stronger when Stronger when pressure pressure differences are differences are greatergreater

Pressure Pressure varies varies horizontallyhorizontally

CoriolisCoriolis

To the right of To the right of wind in NH, to the wind in NH, to the left of motion in left of motion in SH – always at SH – always at 90º angle to wind90º angle to wind

Increases from Increases from equator toward equator toward pole, increases pole, increases with increasing with increasing wind speedwind speed

Earth is Earth is rotating, rotating, system is system is large and large and lasts a long lasts a long timetime

CentrifugaCentrifugall

Outward from Outward from center of center of curvaturecurvature

Increases with Increases with increasing increasing speed, increases speed, increases with sharper with sharper curvecurve

Flow/motion Flow/motion is curvedis curved

FrictionFrictionIn the opposite In the opposite direction of the direction of the wind wind

Increases with Increases with increasing increasing speed, increases speed, increases for rough for rough surfacessurfaces

Near the Near the earth’s earth’s surface surface (lowest 1000 (lowest 1000 m)m)

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NameName Forces Forces involvedinvolved ResultResult Valid when…Valid when… ExampleExample

GeostrophiGeostrophicc

PGF & PGF & CoriolisCoriolis

Flow Flow parallel to parallel to straight straight isobarsisobars

Isobars are Isobars are straight, no straight, no frictionfriction

Upper-Upper-level zonal level zonal windwind

GradientGradient

PGF, PGF, Coriolis, Coriolis, CentrifugCentrifugalal

Flow Flow parallel to parallel to curved curved isobarsisobars

Isobars are Isobars are curved, no curved, no friction, friction, system is largesystem is large

Upper-Upper-level low level low pressure pressure centercenter

Surface / Surface / Boundary Boundary layerlayer

PGF, PGF, Coriolis, Coriolis, FrictionFriction

Flow Flow toward low, toward low, away from away from highhigh

Isobars are Isobars are straight, straight, friction friction importantimportant

Wind near Wind near surfacesurface

CyclostropCyclostrophichic

PGF & PGF & CentrifugCentrifugalal

Flow Flow parallel to parallel to curved curved isobars isobars (can be (can be either either direction direction around a around a low)low)

Isobars are Isobars are curved, system curved, system is small is small (Coriolis (Coriolis unimportant)unimportant)

Tornado, Tornado, draining draining sinksink

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Fig. 7.13, p. 185

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Mercurial (Fortin) Aneroid

◦ Recording: Barograph Electronic (Pressure

Transducer)

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◦ Wind vane◦ Cup anemometer◦ Aerovane (Wind

Monitor by R.M. Young)◦ Sonic◦ Rawinsonde (lifted by

Weather Balloon) Wind soundings Wind Profiler

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A small increase in wind speed can greatly increase the wind force on an object◦ F ~ V2

◦ Turbulent whirls (eddies) pound against the car’s side as the air moves past obstructions, such as guard railings and posts

◦ Similar effect occurs where the wind moves over low hills paralleling a highway

◦ Weird Stuff: Wind erosion, desert pavements, sand ripples, snow ripples, snow dunes, snow rollers, snow fences, windbreak, shelter belt

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Table 7.2, p. 196

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Fig. 7.24, p. 196

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Fig. 7.25, p. 197

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Fig. 7.26, p. 197

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Fig. 7.27, p. 198

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Fig. 7.28, p. 198

Winds which are more likely to come from a general direction can have a large influence on climate.

Wind sculptured trees (even in BCS, my backyard)

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Wind also influences water Waves forming by wind blowing over the

surface of the water In general, the greater the wind speed, the

greater the amount of energy added, and the higher the waves will be◦ Wind speed◦ Length of time wind blows ◦ Fetch (distance of straight wind over water)

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As waves travel across the open ocean into areas of weak winds, their crests become lower and more rounded, forming swells

https://www.fnmoc.navy.mil/wxmap_cgi/index.html

https://www.fnmoc.navy.mil/ww3_cgi/index.html

Seiches◦ Sloshing back and forth of a semi-enclosed body

of water (Great Lakes, bays)

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Fig. 7.29, p. 199

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The windiest region in the US is the Central Plains

Other windy spots include Alaska, Hawaii, and Atlantic and Pacific coasts

Mountaintops and passes tend to be windy

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Fig. 7.30, p. 201

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Table 7.3, p. 201

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The estimated maximum speed at which wind can blow at sea level is 200 to 225mi/hr

Above this speed, friction with the earth’s surface creates such a drag on the wind that it cannot blow any faster

Wind speeds in excess of 225 mi/hr are possible on mountaintops, narrow valleys, and tornadoes.

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Few locations in the world that have in place anemometers capable of measuring wind speeds over 200 mi/hr

Many instruments are simply blown away by winds of this magnitude

National Hurricane Center – anemometer died at 164 mph.

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Fig. 7.31, p. 203