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EOS 340 Atmospheric Science
EOS 340 Atmospheric Science
Variable Gases
Vertical Structure of Atmosphere
While the dominant gases are permanent, others “cycle”
through the system (earth-ocean-atmosphere), and their
concentration is variable.
This variability exists both in space and time.
The residence time (τ) is the “typical” time an individual gas
may persist in the atmosphere, and tells us the scale (temporal)
of processes which are important in influencing it’s
concentration and distribution.
τ=
The atmosphere can be divided into distinct layers associated
with variations in the content and related processes. The layers
are “spheres”, while the interfaces between layers are “pauses”.
Troposphere
The lowest (8-16 km) layer of the atmosphere, containing most of
the weather. Heating of the Earth’s surface and the greenhouse
effect cause the surface to be warm. The temperature decreases with
altitude at the lapse rates between 4-10°C per 1 km of altitude gain.
Well mixed, except for water vapour. Capped by Tropopause,
where air temperature is uniform with height.
concentration
consumption _ rate
Gas
N2
O2
H2O
CO2
CH4
O3
Aerosols
CFCs
Stratosphere
Residence Time
4,000,000 years
5000 years
10 days
150 years
10 years
5-30 days
?
100 years
Layer with maximum Ozone concentration, which absorbs UV
radiation and warms the air. Temperature increases with altitude
throughout the stratosphere. Capped by the Stratopause, where the
temperature is uniform with height (again). Maximum height is
about 50 km.
Mesosphere
Air is low in density, and temperatures decrease with altitude.
Radiation readily passes through this layer, capped at 80 km by the
Mesopause.
Thermosphere
Consumption Examples:
CO2: Produced by respiration, combustion, eruption; consumed
by photosynthesis and dissolution.
Photosynthesis: Radiation+6H2O+6CO2à C6H12O6+6O2
Respiration: 6O2+C6H12O6+enzymesà6CO2+6H2O+energy
CH4: (Methane) Produced by biology (cattle, rice fields,
wetlands), consumed by reactions with hydroxyl radicals.
Air is extremely “thin”, tapering eventually to almost nothing at
altitudes of >120km. However, since the air is so rarified, the small
amount of radiation that does “hit” the rare air molecule, imparts
significant kinetic energy. A long free-path means the kinetic
energy is not distributed among many molecules. High molecular
kinetic energy is high temperature, so the temperature rises
throughout the thermosphere.
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EOS 340 Atmospheric Science
EOS 340 Atmospheric Science
Ionosphere
In the upper reaches of the
atmosphere, solar radiation can strip
off electrons from the air molecules
(mostly N2 , O, and O 2), introducing
free electrons and ion molecules
and atoms. Between 80 and 100 km
altitude (D), the ion concentration
increases during the day (radiation),
but at night, the air density is
sufficient that electrons and ions
will recombine to form neutrally
charged molecules and atoms. At
higher altitudes (>100km), the recombination rate is slow, and a near
permanent ion layer exist (F). AM radio waves are absorbed and
damped within the “D” layer of the ionosphere, limiting their
effective range. However, at night, AM radio waves can
propagate higher, and reflect off the “F” layer of the ionosphere,
thus reaching listening sites at long ranges.
Magnetosphere
Still reaching further into outer space (~10,000 km), is the
Earth’s magnetic field. In addition to radiation, the sun emits
solar particles (the solar wind). The ions within the solar wind
will be funneled by the Earth’s magnetic field. These energetic
particles can excite molecules in the upper atmosphere, which
then give off light upon returning to their normal energy state.
The light thus given off can be seen at night as the aurora
borealis and aurora australis.
Some Fundamental Physics
Energy is defined as “the ability to do work” and is measured in
Joules (J). Ultimately, energy cannot be made or destroyed, but
it can be transferred from one form to another. Power is the rate
at which energy is transferred, in J per s or Watts [=Js -1]. There
are three primary forms of energy:
1) Potential Energy (e.g. chemical and gravitational)
2) Kinetic Energy (e.g. heat and motion)
3) Radiation and Mass (Em=mc2, E r=hν, h=Planck constant)
Heat: The total kinetic energy of a volume of molecules.
Temperature: The average kinetic energy of the molecules in a
region.
Heat ≠ Temperature
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EOS 340 Atmospheric Science
EOS 340 Atmospheric Science
Energy Transfer
Solar and Terrestrial Radiation
There are three primary ways to transfer energy:
1) Conduction: the diffusion of heat energy by direct molecular
collision.
2) Advection/Convection: the transfer of energy by the
movement of air (advection=horizontal,
convection=vertical).
3) Radiation: the transfer of energy by electromagnetic (EM)
radiation (light).
The first is a local change, enhanced where large temperature
differences between surfaces/air masses exists. Advection and
convection are larger, both in net heat flux and physical scale, as
large volumes of air move throughout the atmosphere. The third,
em radiation, is important on the largest scales (up to global
scales), and ultimately determines our net heating and cooling
rates.
Later we will discuss sensible heat and latent heat. These terms
relate to heat exchanged by two different processes. Sensible
heat exchange is another way of stating conduction (direct
molecular contact). Latent heat exchange is associated with the
evaporation of liquid water and the subsequent condensation of
water vapour. Advection and convection are essential
mechanisms for moving moist and dry air, aiding the exchange
of latent heat.
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The Sun is our (atmosphere, ocean, land) primary source of
energy. This energy drives both the circulations in the
atmosphere and oceans, as well as the life growing on Earth. We
receive that energy in the form of electromagnetic radiation.
Light has both wave and particle characteristics (duality). As a
wave, light can diffract; as a particle (photon), light can impart
momentum. Waves are characterized by a frequency (ν, the
number of wave crests passing by per second), and a
wavelength (λ, the distance between wave crests).
The wave speed is given by:
c=νλ
[m/s]
For light in a vacuum: c = 3 × 108 m/s
The energy of a Photon: E=hν=hc/λ,
Type of Radiation
Gamma ray
X ray
Ultraviolet (UV)
Visible Light
Infrared (IR)
Microwave
Radio
h=6.63×10-34 Js
Wavelength (λ)
<0.0001 µm
0.0001 – 0.01 µm
0.01 – 0.4 µm
0.4 – 0.7 µm
0.7 – 100 µm
100 µm – 1 m
>1m
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