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© 2014 Pearson Education, Inc.
Chapter 2 Lecture
McKnight's
Physical Geography
11e
Lectures
Chapter 5
Atmospheric Pressure
and Wind
© 2014 Pearson Education, Inc.
Michael Commons
Ohio Northern University
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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.)
© 2014 Pearson Education, Inc.
The Nature of Atmospheric Pressure
• 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
© 2014 Pearson Education, Inc.
The Nature of Atmospheric Pressure
• 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.
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
The Nature of Wind
• Wind speed
– Tight pressure gradients
(isobars close together)
indicate faster wind
speeds
– Wind speeds are gentle
on average
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Seasonal differences in the general circulation
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Components of the
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”
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Components of the
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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Components of the
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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• 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
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• The seven components of the general circulation
© 2014 Pearson Education, Inc.
The General Circulation of the Atmosphere
• Vertical wind patterns
of the general
circulation
– Most dramatic
differences in surface
and aloft winds is in
tropics
– Antitrade winds
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
Modifications to the General Circulation
• 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
© 2014 Pearson Education, Inc.
Modifications to the General Circulation
Major monsoon
systems
Minor monsoon
systems
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
El Niño-Southern Oscillation
• Circulation patterns – Walker circulation (see figure)
© 2014 Pearson Education, Inc.
El Niño-Southern Oscillation
• 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
© 2014 Pearson Education, Inc.
Other Multiyear Atmospheric and Oceanic
Cycles
• Pacific Secadal
Oscillation (PDO – see
figure)
• North Atlantic Oscillation
(NAO)
• Arctic Oscillation (AO)
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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.
© 2014 Pearson Education, Inc.
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.