Atmospheric Pressure and Wind

Atmospheric pressure:

– force exerted by a column of air per unit area

– Normal atmospheric pressure at sea level =

1013 millibars

Air pressure patterns controlled by:

1. Temperature changes

2. Rotation of earth

1.Temperature changes:

• When air is heated:– air expands and PRESSURE DROPS

• When air is cooled:– air compresses and PRESSURE

INCREASES

Result:

• WARM surfaces develop thermal LOWS

• COLD surfaces develop thermal HIGHS

THERMAL HIGHS

THERMAL LOW

2. Rotation of earth:

• Earth’s rotation causes air to accumulate in certain latitudes and to be deflected away from certain latitudes

• accumulation : HIGH pressure• deflection: LOW pressure

Highs and Lows in cross-section:

• HIGHS:– clear skies

• rising barometer means good weather

• LOWS:– cloudy skies

• falling barometer means bad weather

Global Patterns of High and Low Pressure

Equatorial Low

• 5oN - 5oS• Intertropical Convergence

Zone (ITCZ)• thermal Low

– high sun angles, long days, available energy

– ascending air– heavy precipitation– cloud cover

Subtropical Highs• 25o - 40o N & S• rotation-induced Highs• air deflected to subtropics• descending air• clear skies• hot dry air• great deserts here

Subpolar Lows

• 55o - 70o N & S• rotation-induced Lows• warm air from low latitudes is lifted

as it meets cold polar air• ascending air• storm centers here

Polar Highs

• 90o N & S• thermal Highs• cold polar temps at high latitudes• descending air

Note: all pressure belts shift seasonally

What causes wind?

Wind is air moving from High to Low pressure.

Wind is named after direction it comes FROM.

(a “west wind” comes out of the west; flows eastward)

Two components of wind

1.Speed

2. Direction

1.Wind Speed is determined by:

a. Steepness of pressure gradient• Steep gradient: closely spaced isobars• Gradual gradient: widely spaced isobars

b. Friction• Friction from surface lowers wind speed

2. Wind Direction is determined by:

a. Direction of pressure gradient

b. Coriolis force

c. Friction

a. Direction of pressure gradient

•from High to Low

•makes wind would blow perpendicular to isobars

2. Wind Direction is determined by:

a. Direction of pressure gradient

b. Coriolis force

c. Friction

b. Coriolis force

• apparent deflection of moving things (like the wind) on a rotating surface (like the earth)

• Imagine tossing a ball across a rotating room…

Ball appears to be deflected to the right, but it has been going in the same direction all along.

the ball’sdirection

Airplanes, rockets, migrating birds, ocean currents, air are deflected from their paths of motion because the earth is rotating.

in Northern Hemisphere, deflection to RIGHT of movement

in Southern Hemisphere, deflection to LEFT of movement

Watch this animation…

Deflection increases with latitude:

• no Coriolis at equator; • greatest deflection at poles

• Imagine sitting on a chair on a platform at varying latitudes….

If you are sitting on the north

pole, how many degrees will the room rotate/spin

in one day?

YOU! 360°

If you are on the equator, how many

degrees will the room rotate/spin in one day?

0 !

If you are between the poles and the equator,

how many degrees will the room rotate/spin in one

day?

Between 0 and 360, depending on latitude

If Coriolis effect were only influence on wind direction, wind would blow parallel

to isobars

2. Wind Direction is determined by:

a. Direction of pressure gradient

b. Coriolis force

c. Friction

c. Friction

the “drag” produced by earth’s surface

– applied opposite direction of motion

– reduce angle of Coriolis deflection

Pressure gradientC

orio

lis

frict

ion

Resulting wind

direction

Northern Hemisphere

Northern Hemisphere

OUT and CLOCKWISE

Southern Hemisphere

Southern Hemisphere

OUT and COUNTERCLOCKWISE

Northern Hemisphere

Northern Hemisphere

IN and COUNTERCLOCKWISE

Southern Hemisphere

Southern Hemisphere

IN and CLOCKWISE

Winds in Upper Atmosphere

no friction

only the pressure gradient and Coriolis effect

– wind is parallel to isobars: GEOSTROPHIC WIND

Northern Hemisphere

Northern Hemisphere

CLOCKWISE

Southern Hemisphere

COUNTERCLOCKWISE

Northern Hemisphere

COUNTERCLOCKWISE

Southern Hemisphere

CLOCKWISE

Trade Winds

5o - 25o N & S

– NE, SE– steady,

persistent

Global Wind Systems (Surface Winds)

Westerlies

35o - 60o N & S

– not steady or persistent

Polar Easterlies

65o - 80o N & S

– more prevalent in Southern, variability in Northern

Equatorial Belt of Variable Winds and

Calm5oN - 5o S

ITCZ

“Doldrums”

Subtropical Belt of Variable Winds and

Calm30o - 35o N & S

“Horse Latitudes”

Polar Front Zone

60o - 65o N & S

zone of conflict between differing air masses

Polar Zone of Variable Winds and Calm

80o - 90o N & S

Hadley Cells

Winds Aloft

• Upper Level Westerlies (25o - 90o)

• Polar Low• Tropical High

Pressure Belt (15o - 20o N & S)

• Equatorial Easterlies

Jet Streams

• Narrow zones of extremely high wind speeds

• occur where there are strong temp contrasts

• Polar Jet (westerly)• Subtropical Jet

(westerly)

Summary!