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© 2015 Pearson Education, Inc.
Composition
and Structure
of the Atmosphere
Chapter 1 Lecture
Redina L. Herman
Western Illinois University
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal, University of Oslo
© 2015 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
• Although the density of the atmosphere decreases rapidly with height, the top of the atmosphere is
undefined.
• Overall, the majority of atmospheric mass is contained in a thin layer near the surface.
• The atmosphere still contains an impressive sum of
mass.
Thickness of the Atmosphere
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• The atmosphere is a mixture of gases, suspended particles, and liquid/solid water.
• Gases and particles are readily exchanged between
the Earth’s surface and the atmosphere through physical (volcanic) and biological (photosynthesis,
respiration) processes.
• These gases are categorized in two types:
permanent and variable gases.
Factors Affecting the Composition and
Vertical Structure of the Atmosphere
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• The Permanent Gases
Composition of the Atmosphere
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• Variable Gases
Composition of the Atmosphere
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© 2016 Pearson Education, Inc.
© 2016 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
• Variable Gases: Water Vapor
– Water vapor is the most abundant variable gas, as it is
added or removed from atmosphere through the
hydrologic cycle.
– Concentrations exist from nearly 0 percent over desert and
polar regions to nearly 4 percent near tropics.
– Water vapor is a contributor to Earth’s energy balance and
many important atmospheric processes.
Composition of the Atmosphere
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• Variable Gases: Water Vapor
– Water vapor image
showing broader
distribution of moisture
than the image of actual
clouds (below).
Composition of the Atmosphere
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• Variable Gases: Carbon Dioxide
– Carbon dioxide (CO2) is a trace gas accounting for
0.039 percent of the atmosphere’s volume.
– CO2 is important to Earth’s energy balance.
– CO2 is added through biologic respiration, volcanic activity,
decay, and natural and human-related combustion.
– CO2 is removed through photosynthesis, the process of
plants converting light energy to chemical energy.
Composition of the Atmosphere
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• Variable Gases: Carbon Dioxide
Composition of the Atmosphere
The steady increase of atmospheric CO2.
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© 2016 Pearson Education, Inc.
© 2015 Pearson Education, Inc.
• Variable Gases: Ozone
Composition of the Atmosphere
– Ozone is the tri-atomic form of oxygen and is essential to life
on Earth.
– Ozone near the surface is a pollutant but in the stratosphere
is an essential absorber of ultraviolet radiation.
– Chlorofluorocarbons (CFCs), specifically chlorine atoms,
react with ozone in the stratosphere and destroy ozone.
– Ozone destruction peaks over the southern hemisphere and
persists through spring.
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• Variable Gases: Ozone
– Images showing reduction of ozone over Antarctica over time.
Composition of the Atmosphere
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• Variable Gases: Methane
– Methane is a variable gas in small but recently increasing
concentrations.
– Released to the atmosphere through fossil-fuel activities,
livestock digestion, and agriculture cultivation (esp. rice).
– Methane works as a very effective absorber of terrestrial
radiation and plays an active role in near-surface warming.
Composition of the Atmosphere
Annual increases in atmospheric methane.
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© 2015 Pearson Education, Inc.
• Aerosols
– Aerosols are any solid and/or liquid particles, other than
water, which exist in the atmosphere (synonymous with
the term particulate).
– Aerosols are both natural (sea spray, dust, combustion)
and human (combustion) produced products.
– Due to their small sizes, they can easily remain in
suspension for longer periods.
– Aerosols contribute to precipitation processes as
condensation nuclei.
Composition of the Atmosphere
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• The atmosphere is a mixture of gas molecules, small suspended particles of solid and liquid, and
falling precipitation.
• Meteorology is the study of the atmosphere and the processes that cause what we refer to as “weather.”
• Weather refers to the short-term phenomena.
• Climate refers to the long-term patterns.
• Climatology is the long-term study of the atmosphere and the processes that cause what we
refer to as “climate.”
The Atmosphere, Weather, and Climate
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• Density
– Density is mass (kg) per unit
volume (m3).
– Due to compressibility, near-
surface air is more dense than that
above.
– This may be expressed in terms of
the mean free path, or average
distance a molecule travels before
colliding with another molecule.
Atmospheric Density and Pressure
Due to compressibility, atmospheric
mass gradually “thins out” with height.
less overlying
weight
more overlying
weight
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• Pressure
– Pressure is a result of the atmosphere’s mass.
– Pressure decreases vertically but not at a constant rate.
Atmospheric Density and Pressure
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• Layering Based on Temperature Profiles
• Thermal Layers of the Atmosphere
– Four distinct layers of the atmosphere emerge from
identifiable temperature characteristics with height.
Layers of the Atmosphere
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• Troposphere
– The troposphere is the lowest layer, named so as this region
promotes atmospheric overturning.
– Layer of virtually all weather processes.
– Warmed at the surface by solar radiation.
– Identified by a steady temperature decrease with height and as
the thinnest layer (but contains 80 percent of the mass).
– Due to thermal expansion, the tropopause is roughly 16 km
over the tropics but only 8 km at poles.
Layers of the Atmosphere
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• Troposphere
Most clouds exist in the troposphere. Occasionally, violent updrafts penetrate
cloud tops into the stratosphere. The flattened top of this cumulonimbus cloud is
in the stratosphere.
Layers of the Atmosphere
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• Stratosphere
– The stratosphere is an area of little weather (“stratified”),
and the temperature increases with height.
– Inversion caused by the absorption of ultraviolet radiation
by ozone.
– Although the ozone layer exists through an altitude
between
20 and 30 km (12–18 mi), actual concentration of ozone
can be as low as 10 ppm.
Layers of the Atmosphere
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• Mesosphere and Thermosphere
– Mesosphere is characterized by decreasing temperatures with
height and is the coldest atmospheric layer.
– Thermosphere, the upper most layer, slowly merges with
interplanetary space and is characterized by increasing
temperatures with height.
– Temperatures approach 1500oC, however, this only measures
molecular kinetic energy as the sparse amount of mass produces
actual heat content.
– Combining the two layers account for only 0.1 percent of total
atmospheric mass.
Layers of the Atmosphere
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• A Layer Based on Electrical Properties: The Ionosphere
– Located within the meso- and thermospheres, the ionosphere
is supplied with ions, electrically charged particles.
– Interactions between the ionosphere and subatomic particles
emitted from the Sun excite atmospheric gases causing the
aurora borealis (northern lights) and the aurora australis
(southern lights)
Layers of the Atmosphere
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• Earth’s First Atmosphere
– The early atmosphere was likely composed of hydrogen
and helium.
– Light gases either exited the atmosphere to space through
high escape velocities or through collisions with large
celestial bodies.
Origin and Evolution of the Atmosphere
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• Earth’s Second Atmosphere
– A secondary atmosphere formed through volcanic
outgassing and by material, largely water, gained from
comets.
– These “cosmic snowballs” might be the source of most of
the world’s water vapor.
Origin and Evolution of the Atmosphere
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• Further Evolution of the Atmosphere
– Precipitation removed, and continues to remove, excess
water vapor, while high concentrations of CO2 were
replaced by oxygen through photosynthesis and
dissolution in water.
– Due to its stability, nitrogen concentrations slowly grew to
present-day levels.
Origin and Evolution of the Atmosphere
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• Atmospheric Pressure and Wind
– Gas molecules are constantly in motion.
– These molecules exert a pressure (force per unit area)
when they strike a surface.
– Molecules move in all directions, so pressure is exerted in
all directions.
– Air tends to:
• blow away from regions of high pressure to low pressure
(wind).
• rise in areas of low surface pressure and sink in high
pressure. – rising motions favor cloud formation, while sinking promotes
clear skies.
Some Weather Basics
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• Mapping Air Pressure
– Pressure equals force/area
– SI Unit: 1Pa = 1 Nm-2
– Atmospheric pressure is plotted on maps using lines,
called isobars
– Pressure often measured using hektopascals
– 1 millibar (mb) = 1 hektopascal (hPa)
Some Weather Basics
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© 2015 Pearson Education, Inc.
• Temperature
– Temperature varies in time and location.
– Drastic temperature differences are usually caused by
fronts.
• Cold fronts
• Warm fronts
• Occluded fronts
– Precipitation is common along fronts, so when a front
approaches, it is not unusual to have rain or snow in
addition to a change in temperature.
Some Weather Basics
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• Humidity
• Humidity is the term used to describe the amount of water
vapor present in the atmosphere. Relative humidity: the amount of water vapor present in
relation to the maximum for a given temperature. It is
reported in percentages.
Dew point: the higher the dew point the greater amount of
water vapor in the air.
Some Weather Basics
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• Meteorology is the science that studies the atmosphere.
• Weather forecasting is both an art and a science.
Forecasters use computer modeling (science) and their own subjective opinion and knowledge (art).
• There is no precise routine by which a weather forecast is created.
A Brief History of Meteorology
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