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- Origins of the atmosphere
- Vertical structures of the
atmosphere
- Weather maps
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- The thickness of the atmosphere is only about 2% of Earth’s thickness
(Earth’s radius = ~6400km).
- Most of the atmospheric mass is confined in the lowest 100 km above the
sea level.
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- When the Earth was formed 4.6 billion years ago, Earth’s atmosphere was
probably mostly hydrogen (H) and helium (He) plus hydrogen compounds,
such as methane (CH4)
and ammonia (NH3).
- Those gases eventually escaped to the space.
- The release of gases from rock through volcanic eruption (so-called outgassing)
was the principal source of atmospheric gases.
- The primeval atmosphere produced by the outgassing was mostly water
vapor (H2O), with some
Nitrogen (N2) and Carbon dioxide (CO2), and trace amounts of other gases.
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- The atmosphere can only hold small fraction of the mass of water vapor
that has been injected into it during volcanic eruption, most of the
water vapor was condensed into clouds and rains and gave rise to rivers,
lakes, and oceans.
- è The
concentration of water vapor in
the atmosphere was substantially reduced.
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- Saturation vapor pressure describes how much water vapor is needed to
make the air saturated at any given temperature.
- Saturation vapor pressure depends primarily on the air temperature in
the following way:
- è
- Saturation pressure increases exponentially
with air temperature.
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- Chemical weather is the primary process to remove CO2 from the
atmosphere.
- In this process, CO2 dissolves in rainwater producing weak carbonic acid
that reacts chemically with bedrock and produces carbonate compounds.
- This biogeochemical process reduced CO2 in the atmosphere and locked
carbon in rocks and mineral.
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- Nitrogen (N2):
- (1) is inert chemically,
- (2) has molecular speeds too
slow to escape to space,
- (3) is not very soluble in
water.
- The amount of nitrogen being cycled out of the atmosphere was limited.
- Nitrogen became the most abundant gas in the atmosphere.
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- Photosynthesis was the primary
process to increase the amount of oxygen in the atmosphere.
- Primitive forms of life in oceans began to produce oxygen through
photosynthesis probably 2.5 billion years ago.
- With the concurrent decline of CO2, oxygen became the second most
abundant atmospheric as after nitrogen.
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- Radioactive decay in the planet’s
bedrock added argon (Ar) to the evolving atmosphere.
- è Argon became
the third abundant gas in the atmosphere.
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- Water vapor is supplied to the atmosphere by evaporation from the
surface and is removed from the
atmosphere by condensation (clouds and rains).
- The concentration of water vapor is maximum near the surface and the
tropics (~ 0.25% of the atmosphere) and decreases rapidly toward higher
altitudes and latitudes (~ 0% of the atmosphere).
- Water vapor is important to climate because it is a greenhouse gas that
can absorb thermal energy emitted by Earth, and can release “latent
heat” to fuel weather phenomena.
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- Carbon dioxide is supplied into the atmosphere by plant and animal
respiration, the decay of organic material, volcanic eruptions, and
natural and anthropogenic combustion.
- Carbon dioxide is removed from the atmosphere by photosynthesis.
- CO2 is an important greenhouse gas.
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- With oxygen emerging as a major component of the atmosphere, the
concentration of ozone increased in the atmosphere through a
photodissociation process.
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- Aerosols: small solid particles
and liquid droplets in the air. They serve as condensation nuclei for
cloud formation.
- Air Pollutant: a gas or aerosol produce by human activity whose
concentration threatens living organisms or the environment.
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- Weight = mass x gravity
- Density = mass / volume
- Pressure = force / area
- = weight / area
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- Pascal (Pa): a SI (Systeme
Internationale) unit for air pressure.
- 1 Pa = a force of 1 newton
acting on a surface of one square
- meter
- 1 hectopascal (hPa) = 1
millibar (mb) [hecto = one
hundred =100]
- Bar: a more popular unit for air pressure.
- 1 bar = a force of 100,000
newtons acting on a surface of one
- square meter
- = 100,000 Pa
- = 1000 hPa
- = 1000 mb
- One atmospheric pressure = standard value of atmospheric pressure at lea
level = 1013.25 mb = 1013.25 hPa.
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- Atmospheric pressure tells you how much atmospheric mass is above a
particular altitude.
- Atmospheric pressure decreases by
about 10mb for every 100 meters increase in elevation.
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- The absolute zero temperature is the temperature that the molecules do
not move at all.
- This temperature occurs at –273°C.
- The Kelvin Scale (K) is a new temperature scale that has its “zero”
temperature at this absolute temperature:
- K = °C + 273
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- The greatest production of ozone occurs in the tropics, where the solar
UV flux is the highest.
- However, the general circulation in the stratosphere transport
ozone-rich air from the tropical upper stratosphere to mid-to-high
latitudes.
- Ozone column depths are highest during springtime at mid-to-high
latitudes.
- Ozone column depths are the lowest over the equator.
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- The ionosphere is an electrified
region within the upper atmosphere where large concentration of ions and
free electrons exist.
- The ionosphere starts from about
60km above Earth’s surface and extends upward to the top of the
atmosphere. Most of the ionosphere is in the thermosphere.
- The ionosphere plays an important role in radio communication.
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- The D- and E-layers absorb AM radio, while the F-layer reflect radio
waves.
- When night comes, the D-layer disappears and the E-layer weakens. Radio
waves are able to reach the F-layer and get reflected further.
- The repeated refection of radio waves between Earth surface and the
F-layer allows them to overcome the effect of Earth’s curvature.
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- Many variables are needed to described weather conditions.
- Local weathers are affected by weather pattern.
- We need to see all the numbers describing weathers at many locations.
- We need weather maps.
- “A picture is worth a thousand
words”.
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- It is useful to examine horizontal pressure differences across space.
- Pressure maps depict isobars, lines of equal pressure.
- Through analysis of isobaric charts, pressure gradients are apparent.
- Steep (weak) pressure gradients are indicated by closely (widely) spaced
isobars.
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- Pressure decreases with height.
- Recording actual pressures may be misleading as a result.
- All recording stations are reduced to sea level pressure equivalents to
facilitate horizontal comparisons.
- Near the surface, the pressure decreases about 100mb by moving 1km
higher in elevation.
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- Saturation vapor pressure describes how much water vapor is needed to
make the air saturated at any given temperature.
- Saturation vapor pressure depends primarily on the air temperature in
the following way:
- è
- Saturation pressure increases exponentially
with air temperature.
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