CRAFT AND STRUCTURE
A Tour of the Atmosphere
(Genre: Explanatory Text)
The Atmosphere
Copyright © by William H. Sadlier, Inc. Permission to duplicate classroom quantities granted to users of
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atmosphere is found here, too. Nearly all
clouds and weather are contained entirely
within the troposphere.
Here’s a corny old joke: Two young fish are
swimming in a pond. An older fish swims by
and asks, “How’s the water today, boys?” One
young fish looks at the other young fish and
says, “What on earth is water?”
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Like fish in water, humans are so
thoroughly adapted to moving around in air
that we rarely acknowledge it even exists. A
box full of air is “empty.” In reality, Earth’s
atmosphere is an unbelievably complex
system that has massive effects on our daily
lives, even on our very survival. Scientists are
still exploring its composition and function.
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The atmosphere, most simply, is an
envelope of air surrounding Earth’s surface,
held in place by gravity. What we call air
is a mixture of gases: about four-fifths
nitrogen, an inert gas, and one-fifth oxygen,
the gas you breathe in. The atmosphere
also contains many trace gases, including
critical compounds such as water vapor
and carbon dioxide. The atmosphere forms
several concentric layers, each with its own
properties and functions.
The Stratosphere
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Because the troposphere absorbs heat
from Earth’s surface, it is warm near the
ground and colder at higher altitudes. You
may have felt this if you ever touched the icy
window of an airplane in flight. The higher
you go, the colder it gets, until suddenly,
the temperature stops dropping and starts
to rise. This is the tropopause, the boundary
between the troposphere and the next layer,
the stratosphere.
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The stratosphere extends to a height of
about 160,000 feet, growing warmer as the
altitude increases. Gases in the stratosphere
absorb radiation directly from the sun’s rays,
making the outer layers slightly warmer.
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Even though the stratosphere is too thin to
support human life, it is critically important
The Troposphere
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The lowest layer of the atmosphere is
the troposphere, which begins at Earth’s
surface and extends upward between 30,000
and 60,000 feet. The troposphere is by far
the densest layer, containing nearly 80
percent of the atmosphere’s entire mass.
The vast majority of the water vapor in the
Unit 7
The troposphere sustains all life on Earth.
Its oxygen fuels the metabolism of all living
cells. Its carbon dioxide, methane, and
water vapor keep Earth’s surface warm,
like a blanket, in a phenomenon known as
the greenhouse effect. The troposphere is
in constant motion. Global winds circulate
air from the equator to the poles, further
evening out Earth’s temperature. Without
the troposphere, Earth would be burning hot
during the day, freezing cold at night, and as
lifeless as the Moon.
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CRAFT AND STRUCTURE
A Tour of the Atmosphere continued
The Exosphere
to our survival. The stratosphere contains
the ozone layer, a thin layer of pure oxygen.
This layer absorbs the sun’s ultraviolet rays,
which can destroy living cells.
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The Mesosphere
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At the mesosphere, the thermometer does
another U-turn. The atmosphere gradually
drops in temperature as it climbs in altitude.
The mesosphere is the coldest place in the
atmosphere, occasionally falling to –130°F.
For most purposes, the upper edge of the
mesosphere, at about 330,000 feet, is where
The Atmosphere as a System
“Earth” ends and outer space begins. Meteors
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At almost every level, the atmosphere is a
falling to Earth start to burn up as they collide
system. It’s a constantly changing interaction
with atoms in the mesosphere. In this way,
between energy and matter, with each part
the mesosphere shields Earth from dangerous
affecting the others. The atmosphere interacts
impacts.
with the ocean and the land to create weather
and climate. It interacts with topography and
The Thermosphere
gravity to form wind patterns. It interacts
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The last organized layer of the atmosphere
with living things to move carbon, oxygen,
is the thermosphere. As its name suggests,
nitrogen, and other materials between the air,
the gases in the thermosphere can reach
the soil, and the biosphere. It also interacts
extremely high temperatures, up to 3,600°F.
with humans and the products we create.
But the thermosphere does not “feel” warm,
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In the 1970s, scientists noticed an alarming
because its molecules are so far apart they
thinning of the ozone layer, the part of the
cannot transmit much heat. Most satellites
stratosphere that shields living things from
and spacecraft orbit in the thermosphere.
ultraviolet rays. Experiments showed that
Though this layer is almost imperceptibly
compounds called CFCs, used as propellants
thin, it too provides a shield from the sun’s
in spray cans, were demolishing ozone
dangerous radiation. The thermosphere
molecules. If ozone depletion continued,
prevents highly dangerous X-rays from
cancer and other radiation-associated
reaching Earth. The thermosphere contains a
diseases would rise sharply. By the 1990s,
layer known as the ionosphere. Here, particles
every nation in the world had banned CFCs.
from the sun collide with Earth’s magnetic
Since then, the ozone layer has recovered
field, creating aurorae, glowing bands of
significantly.
reddish or greenish light that stream across
the sky.
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Copyright © by William H. Sadlier, Inc. Permission to duplicate classroom quantities granted to users of
The exosphere is an incredibly thin, almost
nonexistent collection of molecules. Earth’s
gravity holds these molecules in place,
but they are not dense enough to form an
organized layer. There is no real boundary
between the exosphere and true outer space.
Molecules just become fewer and fewer until
they disappear. If the atmosphere surrounds
the planet the way your clothing surrounds
your body, the exosphere is the invisible trail
of tiny hairs and dust that trail off your skin.
Unit 7
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CRAFT AND STRUCTURE
A Tour of the Atmosphere continued
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The products of human activities harm
the atmosphere in many ways, from
smog particles that cause lung disease to
greenhouse gases—most notably carbon
dioxide and methane—that trap heat and
threaten to raise global temperatures. Our
response to CFCs shows that humans can
reverse the damage if we act swiftly and
decisively. The atmosphere protects human
beings every day; human beings should
return the favor.
Comprehension Check
1. Why does the author include the information in the section titled “The
Copyright © by William H. Sadlier, Inc. Permission to duplicate classroom quantities granted to users of
Atmosphere” before discussing the layers of the atmosphere individually?
2A. How does the author organize the
2B. Which detail from the text does NOT
middle sections of the text?
support the answer to Part A?
a. in order from the highest layer of
a. The troposphere is the densest
the atmosphere to the lowest
layer of the atmosphere.
b. in order from the most important
b. The exosphere blends with
layer of the atmosphere to the
least important
outer space.
c. The mesosphere is the unofficial
c. in order from the warmest layer of
boundary between the atmosphere
and space.
the atmosphere to the coldest
d. in order from the lowest layer of
d. The troposphere begins at
the atmosphere to the highest
Earth’s surface.
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CRAFT AND STRUCTURE
3A. What is the meaning of the phrase
4B. Which sentence from the text best
trace gases in paragraph 3?
supports your answer to Part A?
a. gases in very small amounts
a. “The products of human activities
harm the atmosphere in many
ways, from smog particles that
cause lung disease to greenhouse
gases—most notably carbon
dioxide and methane—that trap
heat and threaten to raise global
temperatures.”
b. very light or thin gases
c. unimportant gases
d. gases that do not react with
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other substances
3B. What context provides the best clue
for the answer to Part A?
b. “The atmosphere also contains
many trace gases, including critical
compounds such as water vapor and
carbon dioxide.”
a. The author gives examples of
trace gases.
b. The author lists trace gases after
c. “Its carbon dioxide, methane,
Copyright © by William H. Sadlier, Inc. Permission to duplicate classroom quantities granted to users of
listing the major gases.
and water vapor keep Earth’s
surface warm, like a blanket, in
a phenomenon known as the
greenhouse effect.”
c. The author contrasts trace gases
with more important gases.
d. The author defines the term
trace gases.
d. “Global winds circulate air from
the equator to the poles, further
evening out Earth’s temperature.”
4A. What does the technical term
greenhouse effect refer to?
a. global warming
b. the range of temperatures on
Earth that make it suitable for life
c. the tendency of gases to trap
heat close to Earth’s surface
d. the ability of gases to reflect or
absorb dangerous radiation
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CRAFT AND STRUCTURE
5. Explain the author’s point of view about how humans interact with the
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atmosphere. Use details from the text to support your answer.
6. Find the word tropopause in the text. State its definition, and tell how
Copyright © by William H. Sadlier, Inc. Permission to duplicate classroom quantities granted to users of
you used context to figure out the meaning.
7A. What is the technical definition of
7B. Which evidence best supports the
system as used in this passage?
answer to Part A?
a. a method of doing a task
a. “The atmosphere, most simply, is an
envelope of air surrounding Earth’s
surface, held in place by gravity.”
b. a dynamic interaction between
many parts
b. “At almost every level, the
c. the sun and all its orbiting planets
atmosphere is a system.”
and bodies
c. “It’s a constantly changing
d. a collection of organs and tissues
interaction between energy and
matter, with each part affecting
the others.”
with a single function
d. “Our response to CFCs shows that
humans can reverse the damage if
we act swiftly and decisively.”
Unit 7
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