Earth’s Atmosphere:
Composition, Structure, and
Temperature
Paula Messina
Part I: The Atmosphere’s
Physical Characteristics
Earth’s Atmosphere
• A thin layer of gases that extends over 100
km (66 miles) above Earth’s surface
• Our atmosphere is held into place by
Earth’s gravitational field
• Our atmosphere drives:
– Weather: atmospheric variables that change
rapidly (I.e., hourly or faster)
– Climate: the average weather in a given region,
over a long period of time
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Atmospheric Variables
(“Elements”)
•
•
•
•
•
•
Air temperature
Humidity
Sky condition (% cloudiness)
Type/amount of precipitation
Air pressure
Wind
– Direction
– Speed
Velocity
Composition of Earth’s
Atmosphere
Carbon dioxide
• Nitrogen: 78%
• Oxygen: 21%
Argon
All other gases
Oxygen
• Argon: 1%
• Carbon dioxide:
0.036%
Nitrogen
Other Variable Components
• Water vapor (the gaseous form of H2O)
– Up to 4% by volume
– Found only at the atmosphere’s lowest levels
– When water evaporates (liquid -> gas), it needs to
absorb heat; when water vapor condenses, it releases
this (“latent”) heat
• Aerosols (suspended particles)
– Pollen, salt, smoke, dust, ash, etc.
• Ozone (O 3)
– Effective in absorbing ultraviolet rays from the Sun
2
Temperature Variability Through the
Atmosphere
Note: These diagrams are not to scale!
• Atmospheric
temperature
changes with
height above
Earth’s surface
Temperature increases
with height
Temperature decreases
with height
Temperature increases
with height
Temperature decreases
with height
Earth’s surface
Temperature Variability Defines
Atmospheric Layers
• There are four
distinct layers in
our atmosphere
•The
boundaries
between the
layers have
names, too
Mesopause
Stratopause
Tropopause
Note: These diagrams are not to scale!
Thermosphere
Mesosphere
Stratosphere
Troposphere
Earth’s surface
Atmospheric Ozone: A “Layer” that
Isn’t a Layer
Note: These diagrams are not to scale!
• The Ozone
“Layer” is a
Thermosphere
diffuse
concentration Mesopause
Mesosphere
of O3, found
predominantly Stratopause
Stratosphere
in the
Tropopause
stratosphere
Troposphere
Earth’s surface
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What is air pressure?
• Air Pressure: The force per unit area exerted
downward by the weight of air above
– Air is composed of molecules
– Molecules have mass
– Therefore, in the presence of Earth’s gravity,
these molecules have weight
– Weight is a force
– Force per unit area is known as pressure
How Much Pressure Does the Air
Exert?
• Other units for
measuring this
pressure:
• 1013.2 millibars
• 29.92 inches of
mercury (Hg)
• 1 atmosphere
• At sea level, one
square inch of a
column of air weighs
14.7 pounds (14.7 psi)
How is Air Pressure Measured?
• Barometer: An instrument that measures
air pressure
– Torricelli barometer
(uses mercury)
– Aneroid barometer
(uses no liquids at all)
Measured
in “inches
of mercury”
4
Air Pressure Variations Through
the Atmosphere
Air pressure
continuously
decreases with
height through
the
atmosphere
Thermosphere
Mesosphere
Stratosphere
Troposphere
Standard air pressure:
At sea level=
14.7 psi
(29.92” Hg)
In Denver, CO (“The
Mile High City”)=
11.6 psi (23.62” Hg)
The Atmosphere Varies From
Place to Place
• The Earth’s atmosphere
is a little thicker at the
Equator, and a little
thinner at the Poles
This is because the Earth’s spinning
motion causes the Earth to bulge
slightly at the Equator.
Part II: Earth’s Astronomical
Motions
and How They Affect Our Atmosphere
5
Earth Moves Through Space
• This motion is called Earth’s revolution
• The path the Earth takes on its journey
around the Sun is called its orbit
• The time it takes to complete one orbit is
known as Earth’s period of revolution
• Earth’s period of
revolution is 365 ¼
days (one year).
Earth Spins on its Axis
• The Earth spins (like a top) about its “axis
of rotation”
• Earth’s axis is tilted 23 ½ o
23 ½ o
• The Earth spins from west to east
• The time it takes to make one
complete 360o turn is known as
Earth’s period of rotation
• Earth’s period of rotation is 24
hours (one day)
Sun’s Rays Strike the Earth at Different
Angles in Different Locations (A)
• Where would you expect the Sun’s rays to be
stronger?
City A
City B
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Sun’s Rays Strike the Earth at Different
Angles in Different Locations (B)
• Where would you expect the temperature to be
higher?
City A
City B
Sun’s Rays Strike the Earth at Different
Angles in Different Locations (C)
• Where would you expect the Sun’s rays to be stronger?
City B
City A
Sun’s Rays Strike the Earth at Different
Angles in Different Locations (D)
• Where would you expect the temperature to be higher?
City B
City A
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Earth’s Motions are our
Reasons for Seasons (A)
City C
June 21
City D
Which city would be experiencing warmer temperatures?
Earth’s Motions are our
Reasons for Seasons (B)
City C
December 21
City D
Which city would be experiencing warmer temperatures?
Earth’s Motions are our
Reasons for Seasons (C)
Sept. 21
June 21
Dec. 21
Mar. 21
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Earth’s Motions are our
Reasons for Seasons (D): Day and Night
•On June 21 (the Summer
Solstice, in the Northern
Hemisphere), daylight
persists for about 15
hours in Northern California.
•North of the Arctic Circle
(66 ½ oNorth Latitude),
daylight persists for 24 hours.
•At the Equator, there are
12 hours of daylight, and 12
hours of nighttime.
•South of the Antarctic Circle (66 ½ o South
Latitude), there is no daylight at all.
Daylight
Nighttime
Earth’s Motions are our
Reasons for Seasons (E): Day and Night
Nighttime Daylight
•On June 21 (the Summer
Solstice, in the Northern
Hemisphere), daylight
persists for about 15
hours in Northern California.
•North of the Arctic Circle
(66 ½ oNorth Latitude),
daylight persists for 24 hours.
•At the Equator, there are
12 hours of daylight, and 12
hours of nighttime.
•South of the Antarctic Circle (66 ½ o South
Latitude), there is no daylight at all.
Reasons for Seasons
•
•
•
•
The Earth’s axis is tilted 23 ½ o.
The Earth revolves around the Sun.
The Earth rotates on its axis.
Temperatures on Earth are directly
proportional to the directness of Sun’s
rays.
• The duration of the daylight period changes
during one period of revolution.
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Important Dates in Earth’s Orbit
Sept. 21—Autumnal Equinox
Dec. 21
Winter
Solstice
June 21
Summer
Solstice
Mar. 21— Vernal Equinox
Earth’s Distance to the Sun
Does Not Affect Seasons!
91 million miles
Perihelion
94 million miles
(not to scale)
Aphelion
• Earth is closest to the Sun about January 3
• Earth is farthest from the Sun about July 3
Part III: How Is Heat Transferred?
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Heat and Temperature are Not
the Same Thing!
• Kinetic Energy: Energy of Motion
• As heat is added to matter, atoms move
faster (so they have more kinetic energy)
• Heat = A measure of the total kinetic
energy of the molecules in a substance.
• Temperature = A measure of the average
kinetic energy of the molecules in a
substance.
So, which of the following has
more heat?
Photo by Nelson Avidon
• A glacier . . . . . . . . . or a cup of hot coffee?
Methods of Heat Transfer
• Conduction: The transfer of heat through matter
by molecular activity
– Collisions between molecules transfer kinetic energy
from one place to another
• Convection: The transfer of heat through matter
due to differences in density
• Radiation: The transfer of heat by
electromagnetic waves
– Electromagnetic waves can travel through empty
space!
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There are many forms of
Electromagnetic Waves
Heat
Interactions of Energy with Matter
• When energy comes in contact with matter,
it may be:
–
–
–
–
Transmitted (passed directly through)
Reflected (bounced back)
Refracted (bent)
Scattered (changed in many different
directions)
– Absorbed (taken into the substance)
Energy Interactions, con’d
• Why do people wear dark colors in winter,
and light colors in summer?
• Why is cookware made of metal?
• Why are heating units (i.e., radiators, etc.)
made of metal?
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What Happens to Electromagnetic
Energy that Reaches Earth?
• Solar Constant *
*2.0 cal/cm2/min., on an average.
50% is lost
– Insolation (Incoming solar
radiation):
• 20% reflected by clouds
• 20% absorbed by atmosphere
• 4% reflected by Earth’s surface
• 6% backscattered to space
• 50% absorbed by Earth’s
surface
If the Amount of Heat Received =
the Amount of Heat Lost
• Then the Earth will be in a state of
equilibrium
• Temperatures will remain stable
Energy in
Energy out
If the Amount of Heat Received<the
Amount of Heat Lost
• Temperatures would decrease
Energy in
Energy out
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If the Amount of Heat Received>the
Amount of Heat Lost
• Temperatures would increase
Energy in
Energy out
What is the Greenhouse Effect? (A)
• How does a greenhouse work?
– Short wavelength visible light from the Sun passes
through glass
– Plants/soil absorb
short wavelength
radiation, and reradiate excess energy
at a longer wavelength
– Glass does not allow this longer wavelength (infrared)
energy to pass through it…so the energy gets
trapped inside.
What is the Greenhouse Effect? (B)
• Certain gases (CO 2,
H 2O, others) act like
glass: they allow
short wavelength
energy to pass
through the
atmosphere, but
they do not allow
longer wavelengths
through.
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Is This Happening on Earth?
• If so, what can we do about it?
Energy in
Energy out
Part IV: What Factors Affect the
General Temperature of an Area?
Temperature Controls: Latitude
• Maximum annual
insolation occurs at the
Equator
• Minimum annual
insolation occurs at the
Poles
Do you know why?
• Temperature decreases as latitude increases.
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Temperature Controls: Altitude
• Why are tall mountains, even in tropical
regions, sometimes snow-capped?
• Temperature decreases as altitude increases.
Temperature Controls: Land and
Water (A)
• Land is a good
absorber, and a
good radiator of
heat
• Water is a poor
absorber and a
poor radiator of
heat…
USA Today’s Isotherm Map, 10/10/01
Temperature Controls: Land and
Water (B)
• Since oceans’
temperatures do
not change much
during a year,
coastal land
areas stay
warmer in the
winter and cooler
in the summer
USA Today’s Isotherm Map, 10/10/01
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Temperature Controls: Cloud
Cover
• During the day, clouds reflect insolation, which
keeps temperatures on Earth’s surface lower.
• At night, clouds prevent heat from escaping
Earth’s surface, keeping temperatures higher
– clouds act like greenhouse glass since they do not
permit long wavelength energy to pass through
them
The coldest nights in winter are always associated with clear skies
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