Chapter 25
THE EARTH’S
ATMOSPHERE
AND WATER CYCLE
The
EARTH’S
ATMOSPHERE
The earth’s ATMOSPHERE,
which is a thin layer of gas
surrounding the planet, is
what allows life to exist on
Earth. The atmosphere is like
the earth’s blanket : It absorbs
and traps just the right amount
of heat from the sun to keep our
planet at a livable temperature. The
atmosphere also protects us against harmful radiation and
has gases, such as oxygen and carbon dioxide, which people,
animals, and plants need to survive.
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Composition
WHAT WE OFTEN REFER TO SIMPLY AS “AIR”
The atmosphere is made of both gases and AEROSOLS, which
are solid particles. The gases are:
78 percent nitrogen
21 percent oxygen
1 percent other gases
like argon, carbon dioxide, water vapor, carbon monoxide,
OZONE (a colorless, toxic gas), methane, hydrogen, etc.
Each of these gases plays an important role in the
atmosphere. WATER VAPOR is like a very fine mist that can
create clouds and weather. Ozone absorbs ultraviolet (UV)
radiation from the sun. Plants use carbon dioxide for essential
processes, and carbon dioxide is also a GREENHOUSE GAS,
which means that it traps heat from the sun, warming the
earth. Currently, the earth’s atmosphere has too much heattrapping carbon dioxide, which is causing our overall climate
to warm and change-a phenomenon called global warming,
or GLOBAL CLIMATE CHANGE.
Along with gases, there are particles in the atmosphere called
aerosols. These particles include salt evaporated from oceans,
dust from the ground, pollen from plants, ash from volcanoes,
acids, and other particles from human pollution. Aerosols can
affect the weather and climate because they reflect and
absorb sunlight.
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Atmosphere Layers
The atmosphere has five layers (listed from
closest to farthest from the ground):
THE THICKNESS
OF EACH LAYER
VARIES, SO THESE
ARE AVERAGES.
1. TROPOSPHERE (0–16 kilometers from the ground)
• The layer closest to Earth (the tallest mountains
are only about 8 kilometers high)
• Contains most of the weather
• Contains most of the air molecules
• Heated by the warmth of the earth’s surface—so the
higher you go in the troposphere, the colder it gets
-
2. STRATOSPHERE (16–50 kilometers from the ground)
• Layer above the troposphere
• Most large planes fly here.
• The ozone layer is located at the
top of the stratosphere.
• Because the ozone, which is located
high up in the stratosphere,
absorbs UV radiation from the sun,
it gets warmer as you get higher.
ozone layer
a layer of gas in the
atmosphere that
protects humans and
animals from harmful
UV rays from the sun
3. MESOSPHERE (50–80 kilometers from the ground)
• Temperatures here drop drastically
because it contains little ozone and
matter to absorb heat.
• Meteors that enter our solar system
usually burn up here. (A “shooting star”
is a meteor burning in the mesosphere.)
The prefix meso
means “middle.”
The meso sphere is
the middle layer of
the atmosphere.
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4. THERMOSPHERE (90–500 kilometers)
• The hottest layer in the atmosphere
• Filters out gamma rays and X-rays from the sun
5. EXOSPHERE (500–10,000 kilometers)
• The outermost layer of the atmosphere
• Has almost no matter, and eventually
fades into space
• Satellites orbit the earth
in this layer.
BONUS CATEGORY: IONOSPHERE
Exo is a Greek suffix that
means “outside.”
The exo sphere is the
outermost layer.
NOT TECHNICALLY A LAYER;
IT RESIDES WITHIN THE
OTHER LAYERS.
• An ion is a charged particle—the ionosphere
is made of a layer of charged particles.
• This layer absorbs the sun’s AM radio waves,
while at night, in the sun’s absence, the
ionosphere reflects radio waves from city to city.
RADIO RECEPTION
IS BETTER AT NIGHT
BECAUSE OF THE
IONOSPHERE!
Ozone Layer
The oxygen we breathe is made of two oxygen atoms
bonded together; ozone is made of three oxygen atoms
bonded together. The ozone layer, which is in the
stratosphere, protects us from the sun’s UV rays- the
same rays that cause sunburns and skin cancer.
THE OZONE HOLE IS NOT WHAT IS CAUSING
GLOBAL CLIMATE CHANGE, ALTHOUGH
MANY PEOPLE GET THESE CONFUSED.
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IONOSPHERE
-
-
Use this mnemonic to remember
the order of layers in the atmosphere:
Test Scores Make
Teachers Excited.
(Troposphere, Stratosphere, Mesosphere,
Thermosphere, Exosphere).
Save the ozone layer
Chlorofluorocarbons (CFCs), chemicals used in
some refrigerators and in aerosol spray bottles
(like hairspray), damage the ozone layer. Our
use of CFCs has caused a hole in the ozone layer
above Antarctica. You can see the ozone layer’s
development at ozonewatch.gsfc.nasa.gov.
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Pressure Changes
Because the force of gravity pulls molecules to the earth’s
surface, most of the air molecules are concentrated close to
that surface, so air pressure is greatest in the troposphere. Air
pressure decreases as you go higher in the troposphere.
It gets harder to breathe when you are in high altitudes because
air pressure decreases with altitude, so there is less oxygen
in the atmosphere. Mountain climbers spend months at
high-altitude camps in preparation for climbing a big
peak in order to get used to the different air conditions.
Temperature Changes
Like pressure, temperature changes with altitude. The earth’s
surface is heated by the sun. The troposphere, in turn,
gets most of its heat from the earth’s surface. So in the
troposphere, the temperature decreases as you travel farther
from the ground.
The stratosphere is heated by the ozone layer, which absorbs
a lot of radiation and heat from the sun. Because the ozone
layer is located at the top of the stratosphere, the temperature
increases as you travel higher.
The part of the mesosphere closest to the ozone layer gets
the most warmth. Because the ozone layer is close to the
bottom of the mesosphere, temperatures decrease as you
travel higher.
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OZONE
Temperatures in the thermosphere and exosphere also increase
as you travel farther from the earth’s surface, although there
are so few molecules it wouldn’t feel “hot.”
WATER CYCLE
Water gets cycled through the land and the atmosphere
through the WATER CYCLE. The water cycle consists of
evaporation, transpiration, condensation, precipitation, and
runoff, repeated over and over again:
EVAPORATION is when a liquid changes to vapor through
heating. The sun’s rays heat water in oceans and on the ground,
transforming it into water vapor that rises into the atmosphere.
Plants also release water vapors into
transpiration or
the air through TRANSPIRATION
evapotranspiration
a plant releasing water vapor
or EVAPOTRANSPIRATION.
into the environment
CONDENSATION is when a gas changes into a liquid through
cooling. After water has been evaporated into the air, the air cools
down and the water molecules clump together to form tiny droplets
of liquid water, forming clouds.
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PRECIPITATION is when water droplets in clouds get
large and heavy and fall from the atmosphere back to
Earth—as rain, snow, hail, and sleet.
R UN O F F
G R O U N DW
AT E R
When precipitation falls on the ground, it gets absorbed and
funneled into streams and rivers that eventually flow back into
the ocean. Water flowing above ground is called runoff, while
water flowing below the surface is called groundwater.
Warm air passes over the ground and the ocean, evaporating
water back into the atmosphere, beginning the process all
over again! So water never enters or leaves our ecosystemit just gets cycled through different forms.
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