pressure and wind

8/3/2019 Pressure and Wind

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Pressure in the AtmosphereUnlike temperature, pressure decreases

exponentially with altitude.

80 percent of the atmospheres mass iscontained within the 18 km closest to the

surface.Atmospheric pressure is generally measured in

millibars (mb);

this unit of measurement is equivalent to 1 gramper centimeter squared (1 g/cm2).

Other units are occasionally used, such as bars,atmospheres, or millimeters of mercury.
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Bars Millibars Atmospheres Millimeters

of mercury

1.013 bar 1013 mb 1 atm 760 mm Hg


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At sea level, pressure ranges from about 960 to

1,050 mb, with an average of 1,013 mb.

At the top of Mt. Everest, pressure is as low as300 mb.

Because gas pressure is related to density,

it means that there are approximately one-third

as many gas molecules inhaled per breath on

top of Mt. Everest as at sea level

That is why climbers experience ever moresevere shortness of breath the higher they go, as

less oxygen is inhaled with every breath.
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Atmospheric Pressure

Units of atmospheric Pressure General circulation

Vertical profile of atmospheric pressure

Horizontal variation of atmospheric pressure

Isobars

Diurnal variation in pressure

Cyclones and anticyclones

Characteristics of cyclones and anticyclones


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Definition

It is defined as the weight exerted by air

column on unit surface of earth.

Therefore, Pressure (P) =

P =

ForceArea

H ga

a= H g


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Units of Pressure

1 Atmosphere 1 bar

76 cm x 13.6 g/cm2 1033.3 g/cm2

29.92 inches or

76 cm or 760 nm1013.250 mb

101.325 KPa

P = hg 76 cm x 13.6 x 980.6

= 1013250 dynes/ cm2

= 1.014 x 106 dynes/ cm2


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Factors affecting Pressure at a place

Temperature

Altitude

Water vapour in air Revolution of earth

Gravitation of earth


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General Circulation

Equator absorb more heat and loses lessheat

Polar region give off more heat than they

receive This regional variation in temperature

cause difference in pressure

The atmospheric motion or wind when

summarised over earths surface is known

as general circulation of atmosphere


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Vertical profile

The amount of pressure

exerted by air at aparticular place isdetermined by height ofthe air column, density

of air and gravitationalforce exerted by earth.

The rate of decrease inpressure with increase

in height can be givenby

dpdh

Altitude (km) Pressure (mb)

0 1013.00

2 795.00

4 616.006 472.00

8 356.00

10 264.00

15 120.0020 55.21

30 11.52

40 2.78

50 0.9360 0.35

70 0.12

80 0.03

90 0.008100 0.003

= -g


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Horizontal variation

Source: Aggarwal et al. 1994

The horizontal distribution of pressure depends ontemperature, extent of water vapour, latitude and land

water relationship.

The uneven heating of earths surface at different

latitudes result in pressure variation

Pressure and temperature act inverse to each other.

Presence of water vapor in air make the air lighter.

Thus saturated air is lighter than dry air


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Horizontal variation

Source: Aggarwal et al. 1994

There are seven alternating low and high pressurebelts on earths surface:

Equatorial trough

Subtropical high pressure belt (Northern

hemisphere)Subtropical high pressure belt (Southern

hemisphere)

Sub polar low (Northern hemisphere)

Sub polar low (Southern hemisphere)

Polar high (Northern hemisphere)

Polar high (Southern hemisphere)


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Horizontal pressure variation

Equatorial low

Sub polar low

Sub polar low

Sub tropical high

Sub tropical high

Polar high

Polar high N

5

5

7060

35

25

7060

3525


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Isobars

Any line joining places of equalatmospheric pressure on weather maps iscalled isobar.

Where isobars are closely spaced a rapid

or steep change in pressure

When isobars are widely spaced slowchange in pressure

Two isobars never cross each other


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Isobars

Isobars assume different shapes as followsDepression

Secondary depression

Trough

Anticylones

Ridge or wedge

COL

Straight isobars


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Diurnal variation in pressure

Shows two highs two lows

Highs at 10 am and 10 pm

Lows 4 pm and 4 am

Pressure Gradient

The rate of change of pressure per unit distance

between two points at same elevation is known

as pressure gradient or isobaric slope.

Expressed in terms if decrease in pressure per

unit horizontal distance as mb/100 m


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Cyclones

A low pressure centre enclosed by circularor oval shaped isobars is known as cyclone.

Pressure decreases from its outer rim to itscentre

Air rushes spirally towards centre

Velocity of air > 34 knots

Air circulates in anticlockwise in N.H. andvice versa


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Characteristics of cyclone The foremost characteristic is that tropical cyclones are most

violent, most awesome and most disastrous of all theatmospheric disturbances.

The average speed is 120 kmph. Although it may vary from 32kmph to 200 kmph or more. At times it reaches 400 kmph also.

They have closed isobars. The pressure gradient is very sharp.More closely spaced isobars represents greater velocity of thestorm and vice-versa. The pressure at the center is extremelylow. The winds from the surrounding area are drawn towardsthis low-pressure core called the "eye" of the cyclone.

Tropical cyclones develop over oceans and seas only. They aremost violent and vigorous over water. On landfall, their velocitydecreases due to friction, and as the source of energy is cut off,

they dissipate soon. Thus they affect the coastal areas only. The movement oftropical cyclone is affected by the prevailing

wind system. Normally they move from east to west under theinfluence of trade winds.

They are seasonal in nature and occur during a specific period

of the year only.
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Anticylones

A high pressure centre enclosed by isobarsdecreasing pressure in oval or elliptical shape

is known as anticyclones Pressure decreases

from its outer rim to its centreAir moves spirally outwards in clock wise

direction N.H. and vice versa

Scanty weatherSubsidence and divergent wind system do not

favour condensation and cloud formation


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Characteristics of anticyclones

Pressure distribution

Temperature distribution

Distribution of moisture and cloudsDistribution of rainfall

Wind velocity and direction

Eye of the cyclone


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Instruments used for measuring

atmospheric pressure

Fortins barometer

Kew pattern barometer

Aneroid barometer

Barograph


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Wind : Air in Motion

Horizontal flow of air is called wind

Wind results from the pressure gradient

The flow of wind is from high pressure to

low pressure

Wind plays important role in making of

weather


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Causes of wind generation

Differences in temperature over theearths surface

Temperature pressure and wind arerelated to each other

High temperature = Low pressure & viceversa

At low temperature density of air is high& vice versa


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Forces responsible for wind

generationPressure gradient force

Geostrophic or coriolis force

Cyclostrophic or centrifugal force

Frictional force


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Pressure gradient force

The pressure decrease per unit horizontaldistance is called pressure gradient.

It acts in a perpendicular direction to isobars

PG =1/ x dp/dn

Where, = air density,

dp/dn = rate of change in pressure with

distancedp = change in pressure

dn = horizontal distance between the isobars


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Coriolis force

This force comes in to play due to rotation of earth

The air particles cannot move in straight line but

are deflected and move in curvilinear path

In N.H. wind move towards right due to C.F. and in

S.H. they move towards leftAt equator C.F. = 0, but increases regularly towards

poles.

CF = 2VSin

Where, V= velocity of wind, = constant equal to

angular velocity of earth, = latitude where the

motion occurs


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Centrifugal force

When there is marked curvature in the isobars a

third force i.e. centrifugal force is introduced

It acts outwards from the centre of any curved

motion

Frictional forceThe frictional force is maximum at surface

Gradually decreases with height until it becomes

insignificant.

This decrease with height leads to clockwise change

in wind direction, which is sometimes called Ekman

spiral


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Geostrophic wind When the wind flows in a straight line with no

acceleration or frictional force acting on it, the onlyforces acting are coriolis force and pressure gradientforce.

The wind flowing under these conditions is called

geostrophic wind. It blows parallel to the isobars with high pressure to

right in N.H.

Its velocity is inversely proportional to the distance

between the isobars. Thus in geostrophic wind,

CF = Pressure gradient force

2VSin = 1/ x dp/dn


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General Circulation

Equator absorb more heat and loses lessheat

Polar region give off more heat than they

receive This regional variation in temperature

cause difference in pressure

The atmospheric motion or wind whensummarised over earths surface is known

as general circulation of atmosphere


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Wind pattern can be divided as

Doldrums

Trade wind belts

Westerlies

Polar easterlies


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Classification of Wind

Wind

Vertical current Horizontal

1. Divergence

2. Convergence

3. Eddies

4. Convection

Periodic

Monsoon

winds

General

circulation

Local

winds

1. S.E. Monsoon

2. N.E. Monsoon

1. Trade winds

2. Westerlies

3. Polar easterlies

1. Land /Sea breeze

2. Mountain/valley breeze

3. Fohn/Chinkoo winds

4. Tornadoes


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Horizontal pressure variation

Equatorial low

Sub polar low

Sub polar low

Sub tropical high

Sub tropical high

Polar high

Polar high N

5

5

7060

35

25

7060

3525


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Wind Rose

Wind rose is pictorial representation of

wind direction and wind speed.

It gives overall idea of distribution of W.D.and W.S. at a given location for given

period of time.

It is an useful tool in wind break and shelterbelt studies.


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Construction of Wind Rose

pressure and wind

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