mcknight's physical geography 11e - wordpress.com · © 2014 pearson education, inc. chapter 2...

© 2014 Pearson Education, Inc.

Chapter 2 Lecture

McKnight's

Physical Geography

11e

Lectures

Chapter 5

Atmospheric Pressure

and Wind


Michael Commons

Ohio Northern University


Atmospheric Pressure and Wind

• The Nature of Atmospheric Pressure

• The Nature of Wind

• Cyclones and Anticyclones

• The General Circulation of the Atmosphere

• Modifications of the General Circulation

• Localized Wind Systems

• El Niño-Southern Oscillation

• Other Multiyear Atmospheric and

Oceanic Cycles


The Impact of Pressure and Wind on the Landscape

• Atmospheric pressure indirectly affects the

landscape.

– Changes manifest primarily by changes in wind and

temperature.

– Wind has a visible component to its activity.

– Severe storm winds can drastically affect the landscape.


The Nature of Atmospheric Pressure

• Gas molecules are

continuously in motion.

• Force exerted by gas

molecules is called

atmospheric pressure.

• Force is exerted on every

surface the gas touches.

• Average pressure at sea

level is 1013.25mb

(approx. 14 pounds per

square inch.)



• Factors influencing

atmospheric pressure

– Density – at higher

density, particles are

closer and collide more

frequently, increasing

pressure

– Temperature – warmer

particles move faster and

collide more frequently,

increasing pressure

– Remember ideal gas law!

• P = ρRT



• Dynamic and thermodynamic influences on air

pressure are as follows:

– Strongly descending air, a dynamic high

– Very cold surface conditions, a thermal high

– Strongly ascending air, a dynamic low

– Very warm surface conditions, a thermal low

• Dynamic influences work in tandem with influences

from density to affect air pressure.

• All of these influences come back to the original

influence of temperature and are related through the

ideal gas law.


The Nature of Pressure

• Mapping pressure with

isobars

– Pressure measured with

a barometer

– Typical units are millibars

or inches of mercury

– Contour pressure values

reduced to sea level

– Shows highs and lows,

ridges and troughs

– Mapping of highs and

lows and the change in

pressure with altitude


The Nature of Wind

• Origination of wind

– Uneven heating of

Earth’s surface creates

temperature and

pressure gradients

– Direction of wind results

from pressure gradient

– Winds blow from high

pressure to low pressure


The Nature of Wind

• Forces that govern the wind

– Pressure gradient force

• Characterized by wind moving from high to low

pressure, always

• Winds blow at right angles to isobars

– Coriolis force

• Turns wind to the right in the Northern Hemisphere; left in

Southern Hemisphere

• Only affects wind direction, not speed; though faster winds

turn more

– Friction

• Wind is slowed by Earth’s surface due to friction; does not

affect upper levels


The Nature of Wind

• Force balances

– Geostrophic balance

• Balance between pressure

gradient force and Coriolis

• Winds blow parallel to

isobars

– Frictional balance

• Winds blow slightly towards

low pressure and slightly

away from high pressure

• Winds slowed by friction

weaken Coriolis, so

pressure gradient force is

stronger and turns the

winds


The Nature of Wind

• Wind speed

– Tight pressure gradients

(isobars close together)

indicate faster wind

speeds

– Wind speeds are gentle

on average


Cyclones and Anticyclones

• Wind directions around cyclones and anticyclones

dictated by force balances described previously

• Differing Coriolis force causes winds to blow

opposite in each hemisphere


Cyclones and Anticyclones

• Vertical motions

– Surface convergence

and low pressure

indicate rising motion.

– Surface divergence and

high pressure indicate

sinking motion.

– Rising motion results in

clouds and storms.

– Sinking motion results

in sunny skies.


The General Circulation of the Atmosphere

• Atmosphere – in constant motion

• Major semipermanent conditions of wind and

pressure – general circulation

• Principal mechanism for longitudinal and latitudinal

heat transfer

• Second only to insolation as a determination for

global climate



• Simple example – a

nonrotating Earth

– Strong solar heating at

equator

– Little heating at poles

– Thermal low pressure

forms over equator

– Thermal high forms over

poles

– Ascending air over equator

– Descending air over poles

– Winds blow equatorward

at surface; poleward aloft



• Observed general

circulation

– Addition of Earth’s rotation

increases complexity of

circulation

– One semipermanent

convective cell near the

equator

• Hadley cells

– Three latitudinal wind

belts per hemisphere



• Seasonal differences in the general circulation



• Components of the

general circulation

– Subtropical highs

• Persistent zones of high

pressure near 30°

latitude in both

hemispheres

• Result from descending

air in Hadley cells

• Subsidence is common

over these regions

• Regions of world’s major

deserts

• No wind, horse latitudes




general circulation (cont.)

– Trade winds

• Diverge from subtropical

highs

• Exist between 25° N and

25° S latitude

• Easterly winds; south-

easterly in Southern

Hemisphere, northeasterly

in Northern Hemisphere

• Most reliable of winds

• “Winds of commerce”




general circulation

(cont.)

– Trade winds (cont.)

• Heavily laden with

moisture

• Do not produce rain

unless forced to rise

• If they rise, they

produce tremendous

precipitation and storm

conditions



• Components of the general

circulation (cont.)

– Intertropical Convergence

Zone (ITCZ)

• Region of convergence

of the trade winds

• Constant rising motion

and storminess in this

region

• Position seasonally shifts

(more over land than

water)

• Doldrums




general circulation (cont.)

– Westerlies

• Form on poleward

sides of subtropical

highs

• Wind system of the

midlatitudes

• Two cores of high

winds – jet streams

• Rossby waves



• Components of the general circulation (cont.)

– Polar highs

• Thermal highs that develop over poles due to

extensive cold conditions

• Winds are anticyclonic; strong subsidence

• Arctic desert

– Polar easterlies

• Regions north of 60° N and south of 60° S

• Winds blow from east

• Cold and dry



• Components of the general

circulation (cont.)

– Polar front

• Low pressure area

between polar easterlies

and westerlies

• Air mass conflict between

warm subtropics and cold

polar air

• Rising motion and

precipitation

• Polar jet stream position

typically coincident with

the polar front



• The seven components of the general circulation



• Vertical wind patterns

of the general

circulation

– Most dramatic

differences in surface

and aloft winds is in

tropics

– Antitrade winds


Modifications to the General Circulation

• Seasonal modifications

– Seven general circulation

components shift

seasonally.

– Components shift

northward during

Northern Hemisphere

summer.

– Components shift

southward during

Southern Hemisphere

summer.



• Monsoons

– Seasonal wind shift of up

to 180°

– Winds onshore during

summer

– Winds offshore during

winter

– Develop due to shifts in

positions of ITCZ and

unequal heating of land

and water



Major monsoon

systems

Minor monsoon

systems


Localized Wind Systems

• Sea breezes

– Water heats more slowly

than land during the day

– Thermal low over land;

thermal high over sea

– Wind blows from sea

to land

• Land breezes

– At night, land cools faster

– Thermal high over land;

thermal low over sea

– Wind blows from land

to sea


Localized Wind Systems

• Valley breeze

– Mountain top during the

day heats faster than

valley, creating a thermal

low at mountain top

– Upslope winds out

of valley

• Mountain breeze

– Mountain top cools faster

at night, creating thermal

high at mountain top

– Winds blow from mountain

to valley, downslope


El Niño-Southern Oscillation

• Warming of waters in

the eastern equatorial

Pacific

• Associated with numer-

ous changes in weather

patterns worldwide

• Typically occurs on time

scales of 3 to 7 years for

about 18 months



• Circulation patterns – Walker circulation (see figure)



• Patterns associated with

El Niño

• ENSO - Southern

Oscillation

• La Niña – opposite of El

Niño

• Causes of El Niño

– Atmosphere changes first

or ocean changes first?

– Weather effects of

El Niño


Other Multiyear Atmospheric and Oceanic

Cycles

• Pacific Secadal

Oscillation (PDO – see

figure)

• North Atlantic Oscillation

(NAO)

• Arctic Oscillation (AO)


Summary

• Atmospheric pressure and wind affect the

geographic landscape in several ways.

• Atmospheric pressure is the force exerted by air

molecules on all objects the air is in contact with.

• Pressure is influenced by temperature, density, and

dynamic.

• Isobars show areas of high pressure and low

pressure.

• Vertical and horizontal atmospheric motions are

called wind.

• Wind is affected by many forces.


Summary

• Geostrophic balance represents a balance between

the Coriolis force and the pressure gradient force.

• Friction slows the wind and turns it toward lower

pressure.

• Wind patterns around high and low pressure

systems are anticyclonic and cyclonic, respectively.

• Areas of divergence at the surface are associated

with sinking motion, while convergence at the

surface with rising motion.

• Close isobar spacing indicates faster winds.


Summary

• Winds increase rapidly with height; pressure

decreases rapidly with height.

• The global atmospheric circulation is called the

general circulation.

• There are seven components to the general

circulation.

• Each component has associated weather

conditions.

• Seasonal modifications to the general circulation

exist, including monsoons.

• Localized wind systems affect wind direction locally

on diurnal time scales.


Summary

• El Niño is a warming of eastern equatorial Pacific

water and subsequent switching of the high and low

air pressure patterns .

• El Niño is associated with varied weather patterns in

different locations globally.

• Other examples of teleconnections include the PDO

and the NAO/AO.

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