Lecture 10

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Pressue gradient forceCoriolis force

Geostrophic wind

Pressure gradient

force (PGF)

2

Forces

3

What creates wind?

Wind is a result of different forces acting on a parcel of air.

air moving must follows Newton’s second lawF = maF = force (a push or pull)m = mass of an object (parcel)a = acceleration (speed up or slow down)if there are many forces, the acceleration of wind will be

1 2 3 ... nF F F Fa

m

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What are these forces in the atmosphere?

Pressure gradient force Coriolis force Centripetal force Friction

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A force has two properties

MagnitudeDirection

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Pressure gradient

force (PGF)

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Pressure gradient force (PGF)a change in pressure over a

given distance, i. e.pressure differencepressure gradient = =

distance distancehigh lowP P

1020 1016 4

100 100

mb mb mb

km km

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Pressure gradient force (PGF)estimate the magnitude of PGF from a map

pressure differencepressure gradient = =

distance distancehigh lowP P

if the isobars are close together, the PGF is large

if the isobars are far apart, the PGF is small

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Pressure gradient force (PGF)magnitude and direction

PGF magnitude is calculated by formula (or assessed by spacing of isobars

pressure gradient = distancehigh lowP P

PGF direction is always directed from high pressure to low pressure and is always perpendicular to the isobars

H L

1012 mb 1004 mb

1008 mb

100 km

PGF = 8mb/100km

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Pressure gradient force (PGF)magnitude and direction

L

1008 mb

mb

1000

H1012

1008 mb

mb

Q: What is the direction of PGF surrounding an area of low pressure?

Q: What is the direction of PGF surrounding an area of high pressure?

Q: Is the magnitude of PGF larger surrounding the High or the Low?

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200mb

200mb

Q: What direction should the winds aloft at 5 km be blowing based on PGF?

500 mb

700 mb 500 mb

700 mb

5 km

Q: Actually, prevailing wind

direction is from west to east, what

does cause this discrepancy?

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PGF

Coriolis force (CF)

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Coriolis force (CF) due to earth’s rotation

Example

Nonrotating Rotating

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On nonrotating platform A, the thrown ball moves in a straight

line. On platform B, which rotates

counterclockwise, the ball continues to move

in a straight line. However, to anyone on the platform B, the ball

appears to deflect to the right of its intended path.

CF direction

Northern hemisphere Deflection to right

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60oN

30oN

0o

30oS

60oS

Northern hemisphere

Southern hemisphere

N

S

CF directionas air moves from high pressure to low pressure in the northern (southern) hemisphere, it is deflected to the right (left) by the Coriolis force

Northern hemisphereDeflection to right

Southern hemisphereDeflection to left

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17

60oN30oN

0o

30oS60oS

N

S

CF magnitude 2 sinCF V

Deflectionto right

Deflectionto left

CF =0, sin(0o)=0sin(30o)=0.5

sin(60o)=0.87CF=maximum, sin(90o)=1

CF =0 as V=0

CF is large as a parcel moves faster

The Coriolis force can only change a parcel's direction, it CAN NOT

affect its speed.

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CF acts on objects not rigidly attached to the earthCF acts on objects not rigidly attached to the earth

Q: if a high speed train travels from LA to NY, will the CF act on the train?

Q: Does the CF act on the ocean currents?

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Answer is NoAnswer is No

Answer is Yes Answer is Yes

Geostrophic Wind

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200mb

200mb

500 mb

700 mb 500 mb

700 mb

5 km

Q: Actually, prevailing wind direction is from west to east, what does cause this discrepancy?

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PGF

Parcel motion aloftQ: Where will the parcel be at later

time? Position #1 or #2?

Z= 5 km

W

N

S

E

500 mb

475 mb

450 mb

425 mb

1 2

Initially at rest

PGF

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Parcel motion aloftQ: Where will the parcel be at later time? Position #1 or #2?

Z= 5 km

W

N

S

E

500 mb

475 mb

450 mb

425 mb

1 2

Initially at rest

PGF

CF

PGFWind

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Parcel motion aloft

Q: What cause CF increase?

Z= 5 km

W

N

S

E

500 mb

475 mb

450 mb

425 mb

Initially at rest

PGF

CF

PGFWind

CF

PGFWind

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Parcel motion aloftthe parcel now is moving from west to

east, not from south to north.

Z= 5 km

W

N

S

E

500 mb

475 mb

450 mb

425 mb

Initially at rest

PGF

CF

PGFWind

CF

PGFWind

CF

PGF Wind

the PGF is equal and

opposite to the CF

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Geostrophic Wind when the isobars are straight, parallel lines, and the

only two forces acting on a parcel are the PGF and the

CF, then the wind is called geostrophic wind• PGF and CF are equal in magnitude and

opposite in direction• Geostrophic winds are always parallel to

the isobars

Surface500 mb

500 mb

475 mb

CF

PGF Wind

H

L

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