The Earth’s Atmosphere
Michael Beattie
Overview
• The Earth is surrounded by a blanket of air, which we
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call the atmosphere.
It reaches over 640 km from the surface of the Earth.
Almost 99% of the atmosphere lies within 64-80 km of
the earth’s surface.
This thin blanket of air shields the surface and its
inhabitants from the sun’s UV radiant energy, as well as
from material from interplanetary space.
There is no definite upper limit to the atmosphere;
rather, it becomes thinner and thinner, eventually
merging with the empty space that surrounds all the
planets.
Composition of the Atmosphere
• The atmosphere is composed of 78% nitrogen, 21% oxygen, 0.9%
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argon, 0.03% carbon dioxide, and trace amounts of other gases.
At the surface there is a balance between usage and production of
these gases.
Nitrogen is removed from the atmosphere primarily by biological
processes that involve soil bacteria.
It is returned mainly through the decaying of plant and animal
matter.
Oxygen is removed from the atmosphere during breathing, as the
lungs take in oxygen and release carbon dioxide.
It is returned during photosynthesis, as plants, in the presence of
sunlight, combine carbon dioxide and water to produce sugar and
oxygen.
Composition of the Atmosphere
• Water vapor is an extremely important gas in our atmosphere.
• Its concentration in the atmosphere varies greatly from place to
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place, and from time to time.
Close to the surface in warm, tropical locations, water vapor may
account for up to 4% of atmospheric gases, whereas in colder arctic
areas, its concentration may dwindle to a mere fraction of a percent.
It forms into cloud particles that grow in size and fall to the earth as
precipitation.
It is also provides an important source of atmospheric energy in
latent heat when it changes from vapor into liquid water or ice.
Latent heat is an important source of energy for storms, such as
thunderstorms and hurricanes.
Composition of the Atmosphere
• Carbon dioxide is present as a small but important
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percentage of air (about 0.037%).
It enters the atmosphere mainly from the decay of
vegetation, but it also comes from volcanic eruptions,
from the burning of fossil fuels, and from deforestation.
The removal of carbon dioxide from the atmosphere
takes place during photosynthesis.
Recent measurements of carbon dioxide show a 15%
increase in the atmospheric concentration since 1958.
This increase means carbon dioxide is entering the
atmosphere at a greater rate than it is being removed.
The increase appears to be due mainly to the burning of
fossil fuels; however, deforestation also plays a role.
Composition of the Atmosphere
• Carbon dioxide like water vapor is a greenhouse
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gas, meaning it traps a portion of the earth’s
outgoing energy.
As the atmospheric concentration of carbon
dioxide increases, so should the average global
surface air temperature.
Most mathematical model experiments that
predict future atmospheric conditions estimate
that increasing carbon dioxide levels, along with
other greenhouse gases, will result in a global
warming of surface air between 1 and 3.5
degrees Celsius by the year 2100.
The Early Atmosphere
• The atmosphere that originally surrounded the earth was
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probably much different from the air we breathe today.
The earth’s first atmosphere, about 4.6 billion years ago,
most likely consisted of hydrogen and helium, the two
most abundant gases found in the universe.
Most scientists feel that this early atmosphere escaped
into space from the earth’s hot surface.
A second, more dense atmosphere gradually developed
as gases from molten rock within the earth’s hot interior
escaped through volcanoes and steam vents.
The gases were mostly water vapor (about 80%),
carbon dioxide (about 10%) and up to a few percent
nitrogen.
The Early Atmosphere
• Oxygen began an extremely slow increase in
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concentration as rays from the sun split water
vapor into hydrogen and oxygen.
The hydrogen, being lighter, probably rose and
escaped into space, while the oxygen remained
in the atmosphere.
This slow increase in oxygen may have provided
enough of this gas for primitive plants to evolve,
and as plant growth increased our atmosphere
was greatly enriched with oxygen.
Air Density and Pressure
• Air density is greatest at the surface and
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decreases as we move up into the atmosphere;
rapidly at first, then more slowly as we move
farther from the surface.
The weight of all the air around the earth is
5600 trillion tons.
Atmospheric pressure always decreases with
increasing height.
Normal atmospheric pressure near sea level is
1013.25 mb or 14.7 pounds per square inch.
Layers of the Atmosphere
• Troposphere: The troposphere is the lowest region in the earth's
atmosphere. On the earth, it goes from ground level up to about 17
kilometers high. The weather and clouds occur in the troposphere.
In the troposphere, the temperature generally decreases as altitude
increases.
• Tropopause: The tropopause is the boundary zone (or transition
layer) between the troposphere and the stratosphere. The
tropopause is characterized by little or no change in temperature as
altitude increases.
• Stratosphere: The stratosphere is characterized by a slight
temperature increase with altitude and the absence of clouds. The
stratosphere extends between 17 to 50 kilometers above the earth's
surface. The earth's ozone layer is located in the stratosphere.
Ozone, a form of oxygen, is crucial to our survival; this layer
absorbs a lot of ultraviolet solar energy.
Layers of the Atmosphere
• Mesosphere: The mesosphere is characterized by temperatures that
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quickly decrease as height increases. The mesosphere extends from
between 50 and 80 kilometers above the earth's surface.
Ionosphere: The ionosphere starts at about 70-80 km high and continues
for hundreds of kilometers (about 640 km). It contains many ions and free
electrons (plasma). The ions are created when sunlight hits atoms and tears
off some electrons. Auroras occur in the ionosphere.
Thermosphere: The hot layer above the mesosphere. In the
thermosphere there are relatively few atoms and molecules. Consequently,
the absorption of a small amount of solar energy can cause a large increase
in air temperature that my exceed 500 degrees Celsius.
Exosphere: The exosphere is the outermost layer of the Earth's
atmosphere. The exosphere goes from about 640 km high to about 1,280
km. The lower boundary of the exosphere is called the critical level of
escape, where atmospheric pressure is very low (the gas atoms are very
widely spaced) and the temperature is very low.