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7/25/2019 Air Pressure New

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General Meteorology

December 2001

CHAPTER 4

AIR PRESSURE & WINDS


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Pressure

Gravity

But, the airdoesnt fall

down!!!

Balanced by

air pressure.

Pressure: Force due to gravity that the mass of

the atmosphere eerts on a unit area of surface.


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Pressure

Barometers

"ercury barometer

#neroid barometer $deformation%

&sed in altimeters

' mb()** ft

"easuring the height of the +nergyenter


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Pressure

#ir pressure can be thought of as the weight of a

column of air etending from the +arths surface

to the top of the atmosphere.

"ercury

Pressure

-acuum

"ercury Barometer:

eight of the mercury

is balanced by the force of the air $pressure% on

the mercury in the dish.

h


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/ecords $sea0level%:

)*12.' mb in 3iberia211.4 mb in a Pacific 5yphoon

Pressure

#verage sea0level pressure: )*)'.4 mb )*).' 6Pa

47.74 inches 8g )9.1 pounds per in4

1* mm of 8g '9 feet of water


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Pressure decreases

with altitude.

Gravity Pressure increases nearly

mb ta6ing the elevator from

the )9thfloor to the )stfloor.

71.4 mb on the )9thFloor

71'.; mb on the )stFloor

Pressure


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5 = 9*F

P = )*)' mb

5 = ;F

P = )*)' mb

"ore >ense ?ess >ense

But: "olecules are moving faster

Pressure

Pressure acts e@ually in all directions

#tmospheric pressure is proportional to

temperature and density.


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Pressure

#tmospheric pressure proportional to

temperatureand density.

5wo identicalcolumns.

ooled armed

3till: 3ame

Pressure, "ass


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8owever, smaller horiAontal changes produce

the winds.


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Pressure

3ea ?evel PressureC 00 hats up with thatDDDD

5he earth is not flat.

Pressure is recorded at various

altitudes.

Eeed to have a level playing fieldC to

get an idea of the horiAontal pressure

structure.


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Pressure

/eduction to sea0level

+ach station has its own set of

e@uations to reduceC station pressure

to sea0level pressure.

5he fictitious column is assumed to

have a 6nown temperature profile.


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Pressure

3ea ?evel

"easured

Pressure:

7;* mb

Weight per unit area of

a fictitious column between

the station elevation and

sea level. (60 mb)

orrectedC 3ea0level pressure = )*)* mb


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Pressure

5he added correctedC pressure dependson the temperature of the fictitious column

of air. Recall p = RT

5 = average of 5 and 50)4 hrs


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Pressure

+rrors are greatest for the mountain

stations 00

5he greater the altitude, the larger the error

Few number of stations in the mountains

Be careful analyAing pressure over themountains!


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Forces of Nature -- Air in Motion

8oriAontal pressure differences are

caused by /adiation!

5he daily variation in the air temperaturenear the surface of the earth is controlled

by:


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Horizontal Pressure Differences

onsider two columns of air

Ps Ps

Pressure is the weight of the

air above you.

Ps Ps

Cool Warm

arm one side and cool

the other side. "aintain

a constant surface pressure.


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Ps Ps

Cool Warm

onsider an airplane flying at a

constant height. hat are the

pressuresD

5here are more molecules in the column above the plane

in the warm column than in the cold column.


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3o we see a high pressure, at the altitude of the plane, in thewarm column and a low pressure in the cool air.

Ps Ps

Cool Warm

?ow 8igh

5he horiAontal difference in temperature creates

a horiAontal difference in pressure.


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Newtons First Law of Motion

# body at rest will remain at rest and a body in motion

will remain in motion, travelling in a straight line at

constant speed, unless acted upon by an unbalanced

force.

Consider an automobile on flat land

3tart at rest 00 5he car will remain at rest.

8it the gas $force% 00 5he car will accelerate.


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#cceleration 00 5he change in velocity$speed or direction% over a period of time.

Force >iagram

Friction

Gravity

+ngine

5o move forward:

+ngine H Friction


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oasting >ownhill Friction

Gravity

Gv Gh

5he gravity force can be

bro6en into two components:

one horiAontal to the groundand one vertical to the

ground.


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/eview

ith a balance of forces:

/est

"otion in a straight line at constant speed

ith &nbalanced forces:

5he speed, direction, or both will change

hat does this have to do with the windD


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Forces Actin! on t"e At#os$"ere

5he wind is a result of forces acting on the

atmosphere.

Pressure Gradient Force $PGF%

Gravity $G% Friction $Fr%

oriolis $o%

entrifugal $e%

Eet force = PGF I G I Fr I o I e


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Forces Actin! on t"e At#os$"ere

Pressure Gradient Force

8igh

Pressure

?ow

Pressure

hen the valve is opened, the water will flow from tan6

# to tan6 B ==H 8igh to low pressure.

#B


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Pressure %ra&ient Force


8as application to weather maps..............

1012

1014

PGF = $)(% $p(n%

is the density of the air

p is the pressure difference

n is the distance between the

stations


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5he closer the isobars on a weather map,

the greater the pressure gradient force.

"eteorologists measure pressure and

draw isobars to get the wind.


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-ertical pressure gradient

force is larger than the

horiAontal pressure

gradient force.

-ertical pressure gradient

force

miles)4*

mb)***PK

PZ

P

PH

3o why dont winds blow

straight upD

-ertical pressure gradientforce is much $)**L% larger

than the horiAontal pressure

gradient force.


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Gravity: #lways acts downward $eathward%M

6eeps air molecules attracted to the center of

the earth.

PK5wo huge

forces...

G

Gravity Nust balances PK$almost%.

Hydrostatic

Balance


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Measure#ents -- 'in&

ind: 5he motion of air. ind is a vectorM ithas both magnitude and direction.

direction: wind or weather vane, wind soc6

speed: anemometer 0 0 most common typesare a rotating cup or a propeller

ind direction is the direction from which the

wind is coming.

Boo6 /ecord: 4') mph at the surface at "t.ashington, E8.


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Measure#ents -- 'in&

esterly

indEortherly

ind

+asterly

ind

3outherly

ind

Eew 3peed /ecord: /ed /oc6, O 00 4 #pril )77)

$5ornado!% )4* 0 )4; m s0)$42 0 42* mph%

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