GEOG 214 Lecture Notes - Northern Hemisphere, Solar System, Tropopause

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Published on 17 Apr 2013
School
Simon Fraser University
Department
Geography
Course
GEOG 214
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Lecture Notes GEOG 214 Lecture 1
!
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Lecture 1: The Earth’s Atmosphere
Objectives:
1. Define concepts of meteorology, weather and climate
2. Identify major permanent and variable gases in the Earth’s present-day atmosphere,
and identify major sources and sinks of these gases.
3. Describe the evolution of Earth’s atmosphere
4. Describe the average vertical structure of density, pressure, and temperature in the
atmosphere
5. Identify and describe the four atmospheric layers defined by temperature structure
6. Describe the ionosphere and its significance to radio communication
1.0 Weather and Climate
Meteorology is the study of the atmosphere and its phenomena
" From Greek meteoros, meaning “high in the atmosphere”
Weather defined as condition of atmosphere at a particular place at a particular time
" Weather comprised of following elements: air temperature, air pressure,
humidity, clouds, precipitation, visibility, winds
Climate defined as atmospheric condition at a particular place averaged over a
specific period of time (typically > 1 year)
1.1 Earth’s Atmosphere Today
Average radius of Earth ~ 6400 km
99% of the Earth’s atmosphere lies within 30 km of the Earth’s surface.
! The atmosphere is a thin layer around the solid earth, with horizontal scales much
greater than vertical scales.
There’s no upper limit to the atmosphere; it becomes thinner and thinner, eventually
merging with empty space.
1.1.1 Composition of the Atmosphere
Local composition of atmosphere determined by local production (sources) and
destruction (sinks) of atmospheric gases, as well as transport and diffusion.
Permanent gas concentrations vary only over geological timescales (i.e. timescales
> 100,000 years). Most abundant permanent gases: nitrogen (N2; 78%) and oxygen
(O2; 21%).
Variable gases vary in space and time.
Variable gases are: water vapour (H2O), carbon dioxide (CO2), methane (CH4),
nitrous oxide (N2O), ozone (O3), particles (e.g. dust, soot), chlorofluorocarbons
(CFCs).
Concentration of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons small
(< 0.1%) ! trace gases
Important class of variable gases are atmospheric pollutants, defined as
substances harmful to humans, animals and/or plants; pollutants are introduced into
the air by anthropogenic (human-produced) and natural processes (e.g. volcanoes,
forest fires)
Lecture Notes GEOG 214 Lecture 1
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Water vapour (H2O):
" highest concentration in tropical regions, generally decreases with altitude and
toward poles
" water exists in atmosphere as gas (water vapour), liquid (cloud droplets), solid
(ice crystals).
" change from gas to liquid: condensation; change from liquid to gas:
evaporation
" very important atmospheric gas (primary subject of lecture #4): condensation
releases large amounts of heat (latent heat) which is large source of
atmospheric energy; water vapour potent greenhouse gas.
Carbon dioxide (CO2):
" atmospheric sources: respiration (decay of vegetation), oceanic emissions,
volcanic outgassing, combustion of fossil fuels, deforestation, biomass burning
" sinks: photosynthesis, dissolution in ocean water; the ocean holds ~50 times as
much CO2 as the atmosphere!
" important greenhouse gas
" Varies on seasonal timescales as vegetation grows in spring/summer and
decays in autumn/winter; concentration lower in Northern Hemisphere summer
because of greater local biomass.
" Concentration now 24% higher than in 1958 (increase from 315 to 390 ppm).
Increase due to burning of fossil fuels and deforestation.
Methane (CH4):
" Sources: Anaerobic biological activity (cattle, rice fields, wetlands, landfills), fossil
fuel burning
" Sink: reaction with hydroxil (OH) radicals
" Concentrations have doubled since the beginning of the industrial era; increase
has slowed over past few years
" Potent greenhouse gas
Nitrous oxide (N2O):
" Sources: Bacterial and microbial processes in soil, biomass burning
" Sink: destruction by ultraviolet light (photolysis)
" Greenhouse gas
Chlorofluorocarbons (CFCs):
" Anthropogenic molecules
" Used in spray-can propellant, refrigerator coolant, solvent
" Only weakly reactive, therefore long-lived
" Greenhouse gases, participate in stratospheric ozone destruction
Surface Ozone (O3):
" Sources: photochemical reactions (reactions that take place in the presence of
sunlight)
" Tropospheric ozone is pollutant; irritates eyes and throat, damages vegetation
" Primary ingredient of photochemical smog (also known as Los Angeles-type
smog)
Stratospheric Ozone (O3):
" 97% of atmospheric ozone resides in upper atmosphere (stratosphere).
" Produced naturally by photochemical reactions
" Shields earth surface from ultraviolet radiation
" Concentration has been decreasing, particularly over Antarctica (ozone hole)
due to effect of chlorofluorocarbons (CFCs)
Lecture Notes GEOG 214 Lecture 1
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Aerosols:
" Suspended solid or liquid particles (e.g. saltwater drops, soot, ash, dust, pollen)
" Important in cloud formation
" Sinks: gravitational settling, adhering to surfaces (e.g. buildings), precipitation
scavenging (are rained out)
" Many aerosols are pollutants (soot, dust, pollen, lead); adversely affect
respiratory system, nervous system (lead)
Sulfur dioxide (SO2):
" Sources: Fossil fuel burning
" Reacts with water to produce sulfuric acid, contributing to acid fog or acid rain;
acid rain major environmental problem
" SO2 is pollutant; causes serious respiratory problems in humans
Other pollutants:
" Nitrogen dioxide (NO2), carbon monoxide (CO)
Concentrations of many variable gases modified by human activities
Note: Importance of atmospheric constituent not related to relative abundance
1.2. Evolution of Earth’s Atmosphere
Solar system developed ~4.6 billion years ago
Early atmosphere mostly hydrogen (H2) and helium (He); this early atmosphere
escaped to space
New atmosphere was produced by outgassing (i.e. outpouring of gases) from
Earth’s interior, mostly carbon dioxide (CO2) and water vapour
Outgassing over millions of years provided rich supply of water vapour, which
condensed, filling world’s oceans
Carbon dioxide in atmosphere decreased through dissolution in ocean and formation
of carbonate sedimentary rocks (limestone)
Slow build-up of atmospheric oxygen (O2) as sun’s energy split water (H2O) into
hydrogen and oxygen (photodissociation)
Origin of oxygen-producing plants ~2-3 billion years ago; oxygen produced by
photosynthesis, whereby carbon dioxide and water are combined in the presence of
sunlight
Gradual accumulation of N2 to present levels, partially due to action of bacteria
1.3. Vertical Structure of the Atmosphere
Mass is quantity of matter in an object
Weight is the force acting on an object due to gravity; weight is defined as mass
times the acceleration of gravity
Mass of air in rigid volume is the same everywhere in universe but weight is different
e.g. on the moon due to smaller acceleration of gravity
Air density (mass per unit volume) decreases exponentially with height, gradually
merging with empty space
Similarly, air pressure (weight of air above per unit area) decreases exponentially
with height
Common air pressure units: hectopascal (hPa), millibar (mb)
At sea level: 1013.25 mb = 1013.25 hPa
Vertical structure of air temperature more complicated; atmosphere divided into four
layers determined by temperature lapse rate (rate at which temperature decreases
with height). Note: positive lapse rate indicates temperature decrease with height,
negative lapse rate indicates temperature increase with height.