Formation
of the
Atmosphere
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Most of the Earth’s early atmosphere
was lost due to the vigorous solar
wind from the early Sun.
Continuous volcanic eruptions built
a new atmosphere of:
water vapor
carbon dioxide
nitrogen
methane
Formation
of the Atmosphere
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Comets brought water and organic molecules.
Oxygen did not appear in the atmosphere until
after the first bacteria evolved.
Early plants released oxygen as a waste product
and helped to build the atmosphere.
Once oxygen was present in the atmosphere,
ozone could form, blocking out the Sun’s ultraviolet
rays and changing the way life evolved.
The Atmosphere
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Earth's atmosphere contains roughly:
78% nitrogen
20.95% oxygen
0.93% argon
0.038% carbon
dioxide
Trace gases
1% water vapour
The Earth’s atmosphere (where pressure becomes
negligible) is over 140 km thick. Compared to the bulk of
the planet, this is an extremely thin barrier between the
hospitable and the inhospitable.
All images: NASA
The Atmosphere
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The mixture of gases known as
air, protects life on Earth by
absorbing ultraviolet radiation and
reducing temperature extremes
between day and night.
The atmosphere is not static.
Interactions involving the amount
of sunlight, the spin of the planet
and tilt of the Earth’s axis cause
ever changing atmospheric
conditions.
Weather occurs in the troposphere. Gaseous water
molecules held together by intermolecular forces cause
the formation of clouds.
The auroras occur in the thermosphere and are caused
by interactions between the Earth’s atmosphere and
charged particles streaming from the Sun.
Atmospheric
Layers
Meteor burning up
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The atmosphere consists of
layers around the Earth,
each one defined by the way
temperature changes within
its limits.
Thermosphere
This layer extends as high
as 1000 km. Temperature
increases rapidly after
about 88 km.
Mesosphere
Temperature is constant
in the lower mesosphere,
but decreases steadily
with height above 56 km.
The layer boundaries are:
Tropopause
Stratopause
Mesopause
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Aurora, caused by collisions
between protons and
electrons from the Sun and
oxygen and nitrogen atoms in
the atmosphere.
The outermost, the
thermosphere, thins slowly,
fading into space with no
boundary.
Stratosphere
Temperature is stable to
20 km, then increases due
to absorption of UV by the
thin layer of ozone.
Troposphere
Air mixes vertically and
horizontally. All weather
occurs in this layer.
Movement of Air
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The air of the atmosphere
moves in response to heating
from the Sun.
Circulation in the
atmosphere transports
warmth from equatorial areas
to high latitudes, returning
cooler air to the tropics.
The rotation of the Earth
causes these movements to
break up into three distinct
air cells in each hemisphere.
Westerlies
High
Northeasterly trade winds
Equator
Southeasterly trade winds
High
Westerlies
Image NASA
Air Cells
60o N
Rising mid-latitude air divides, flowing to the poles
and the equator forming the Ferrel cells. These
mid-latitudinal cells produce westerly winds.
In the tropics, wind blowing towards
the equator as part of the Hadley
cells is deflected (by the Coriolis
effect) and forms the northeasterly
and southeasterly trade winds.
30o N
Air within the Hadley cells
rises moist at the equator and
subsides dry at the tropics.
Equator
0o
Image NASA
Air Cells
Image NASA
Equator
Warm air rises at the
equator and moves
poleward through the upper
troposphere before sinking
at the edge of the tropics.
The atmospheric circulation in
each hemisphere consists of
three cells (at polar, midlatitude, and equatorial
regions). These cells produce
belts of prevailing winds
around the world.
30o S
At the poles, air cools and
descends as a cold, dry high
pressure area, moving away
from the pole flowing towards
the equator.
60o S
Coriolis Effect
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Energy from the Sun is
distributed through a global
system of atmospheric and
ocean circulation that creates the
Earth's climate.
Heated air moving towards the
poles from the equator does not
flow in a single uniform
convection current.
Air flowing towards, or away from,
the equator follows a curved path
that swings it to the right in the
northern hemisphere and to the
left in the southern hemisphere.
This phenomenon, known as the
Coriolis effect, is caused by the
anticlockwise rotation of the
Earth about its axis.
Friction, drag, and momentum
cause air close to the Earth's
surface to be pulled in the
direction of the Earth's rotation.
Image: NASA
Coriolis Effect
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The deflection of moving air is
called the Coriolis effect and it is
responsible for the direction of
movement of large-scale weather
systems in both hemispheres.
Air flows from high pressure to low
pressure.
Hurricane, Northern hemisphere
In the northern hemisphere,
cyclonic (low pressure) systems
rotate counterclockwise.
In the southern hemisphere,
cyclonic systems spiral in a
clockwise direction.
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All photos NASA
Cyclonic weather is usually dull,
with cloud and persistent rain.
Cyclone, Southern hemisphere
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Photo: Wiki Commons, NOAA
The Ocean
Surface
Throughout the oceans, there is a constant
circulation of water, both across the surface
and at depth.
Surface circulation, much of which is in the
form of circular gyres, is driven by winds.
The polar oceans comprise the Arctic Ocean
in the northern hemisphere and the Southern
Ocean in the south. They differ from other
oceans in having vast amounts of ice, in
various forms, floating in them.
This ice coverage has an important
stabilizing effect on global climate, insulating
large areas of the oceans from solar radiation
in summer and preventing heat loss in winter.
The Southern Ocean encircles Antarctica and is covered in ice
for much of the year. Complex currents in the Southern Ocean
produce rich upwelling zones that support abundant plankton and
complex food webs.
Satellite observations have shown the sea ice around the poles is
melting earlier and more rapidly than first thought. The loss of ice
dramatically reduces the albedo (reflectivity) of the polar regions.
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High salinity
water cools and
sinks in the
North Atlantic.
Ocean Currents
The deep-water ocean currents (the thermohaline circulation) is driven by the
cooling and sinking of water masses in polar and subpolar regions.
Cold water circulates through the Atlantic, penetrating the Indian and Pacific
oceans, before returning as warm upper ocean currents to the South Atlantic.
Deep water currents move slowly and once a body of water sinks it may spend
hundreds of years away from the surface.
Deep water returns to
the surface in the Pacific
and Indian oceans
through upwelling.
Atlantic
Ocean
Atlantic waters
are saltier and
therefore more
dense than those
in the Pacific.
Pacific
Ocean
Indian
Ocean
Cold water circulates
through the Atlantic
penetrating the Indian
and Pacific Oceans.
Cold and deep high salinity current
The polar oceans (the
Arctic and Southern
Oceans) are sources of
cold dense bodies of water
that drive the Earth’s deep
water circulation.
Gyres
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The movement of surface waters tends to form ocean-wide vortices.
At the center of these vortices, water currents are almost nonexistent
and water tends to be stagnant.
The Sargasso Sea is a well known example.
North Atlantic
Gyre
North Pacific Gyre
Sargasso Sea
South Pacific Gyre
Great Pacific
Garbage Patch
Indian Ocean Gyre
South Atlantic Gyre
Great Pacific Garbage Patch
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As with all vortices, debris is swept towards the middle of the water
currents.
The Great Pacific Garbage Patch is an accumulation of plastic and other
debris in the middle of the North Pacific Gyre.
It spans an area of approximately 1.2 million km2.
Photo: Duncan Wright
Midway Is.
Midway Island lies near the center of the GPGP.
Every tide brings in vast amounts of debris, from parts
of old fishing nets to plastic bags and bottles. It is a
poignant example of how even the most isolated
places of Earth are affected by human activities.
Nearly two million Laysan albatrosses live on
Midway. It is estimated all of them contain some
quantity of plastic. This juvenile died after
swallowing at least three bottle caps, some
fishing line and a felt tip pen.