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Part 1. Energy and Mass
Chapter 1.
Composition and Structure of the
Atmosphere
Meteorology
The study of the atmosphere and the processes
that cause “weather”
Climatology
Examines weather elements over long time
periods
Introduction
The Atmosphere
A mixture of gas molecules, suspended
particles, and falling precipitation
The atmosphere strongly affects our day-today lives
The Thickness of the Atmosphere
Density decreases rapidly with height
• The top of the atmosphere is undefined
• Majority of mass is compressed near the
surface
Gases and particles are exchanged
between the surface and atmosphere
A gas molecule that is input
slowly into the atmosphere has a
long average residence time.
A gas molecule that is input
rapidly into the atmosphere has a
short average residence time.
Average residence time of CO2 is about 100 years.
Evolution of the Atmosphere
Early atmosphere = hydrogen (H2) and helium
(He) -- 4 billion years ago
Secondary atmosphere formed from volcanic
outgassing (produced CO2 rich atmosphere) -- 4
to 3 billion years ago
CO2 replaced by O2 through photosynthesis (by
life!) and dissolution in water -- by 3.4 billion
years ago; significant free O2 by 2.5 billion
years ago
N2 (inert) slowly grew to present day levels
Composition of the Atmosphere
Variable Gases
Water Vapor
Most abundant variable gas
Added/ removed to air through the hydrologic
cycle
Concentrations = nearly 0% to nearly 4% (by
volume)
Important to energy balance and many
atmospheric processes (it is a greenhouse gas)
Water vapor
image showing
broader
distribution of
moisture
than the image
of actual clouds
(below)
Water
vapor
image
Dry air
Moist air
Cloud
image
Carbon Dioxide
A trace gas
• 0.038% of atmosphere’s mass
• Important to Earth’s energy balance (it is a
greenhouse gas)
Added through biologic respiration, volcanic
activity, decay, and natural and human-related
combustion
Removed through photosynthesis
Increasing at a rate of 1.8 ppm/year
Temporal increases due to human activities
Seasonal variations related to biological activity
2006 value:
about 380 ppm
Methane
A variable gas in small but recently increasing
concentrations
Increases from burning of fossil fuels, livestock
digestion, and agriculture cultivation (esp. rice)
Effective absorber of terrestrial radiation (it is
a greenhouse gas)
• Plays a role in near-surface warming
The annual increases in atmospheric methane
Aerosols (particulates)
Any solid and/or liquid particle, other than
water
Both natural (sea spray, dust, combustion) and
human (combustion) sources
Long residence times for some types
Acts as condensation nuclei
Will cool atmosphere if emitted in large
amounts (such as volcanic eruptions)
Vertical Structure of the Atmosphere
Density
Lower density
Elevation
in the
atmosphere
Mass (kg) per unit volume
(m3)
• Sea level average = 1.2
kg/m3
Near surface air is more dense
• Compressibility of air
• Mean free path (of an air
molecule)
– At surface = 0.0001 mm
– At 150 km = 10 m
Higher density
Thermal Layers of the Atmosphere
Four distinct atmospheric layers
• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere
Each has particular temperature characteristics
with height.
Troposphere
Lowest layer
Steady temperature decrease with height
• -6.5oC/km (-3.6oF/1000ft)
Virtually all weather processes
• Contains 80% of atmospheric mass
Tropopause = top of troposphere
Violent updrafts may
penetrate cloud tops
into the
stratosphere. The
flattened
top of this
cumulonimbus
cloud is in the
stratosphere.
Stratosphere
Little actual “weather”
Temperature inversion
• Caused by absorption of UV radiation by O3
(ozone) in the ozone layer
Stratopause = top of stratosphere
Ozone
Tri-atomic form of oxygen
Absorbs ultraviolet radiation
Chlorofluorocarbons (CFCs) destroy ozone
Destruction peaks over southern hemisphere
Antarctic circumpolar vortex limits latitudinal
mixing
• Leads to an O3 “hole”
The
reduction of
ozone over
Antarctica
over time.
Area in red
indicates the
“ozone
hole.”
Mesosphere
Decreasing temperatures with height
Coldest layer
Thermosphere
Slowly merges into space
Increasing temperatures with height
• “Temperature” = molecular kinetic energy
Combined Mesosphere and Thermosphere = 0.1% of
total mass of atmosphere
Ionosphere
A layer of electrically charged particles (ions)
• In the meso- and thermosphere
• D-, E-, and F-layers with increasing height
Interactions with subatomic solar particles
cause
• The aurora borealis (northern lights)
• The aurora australis (southern lights)
The
ionosphere
reflects radio
waves,
allowing radio
transmissions
over long
distances
around the
Earth.
Weather Measurements and Units
Temperature
Energy of the atmosphere, measured in degrees
(Fahrenheit, Celsius or Kelvin)
Humidity
Water content of the atmosphere, often
expressed as relative humidity
Pressure
Force of the atmosphere, measured in millibars
(mb) or kilopascals (kPa)
A Brief History of Meteorology
Galileo
Prototype thermometer in 1593
• Fahrenheit (1714) and Celsius (1736)
temperature scales
Torricelli
Barometer (1643)
Instruments to measure water vapor introduced in
late 1700s