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Earth’s atmosphere looks like a thin blue halo of light from outer space. This thin layer
of gases provides us with the air we breathe, regulates global temperatures, and filters
out dangerous solar radiation. The atmosphere is comprised of about 78% nitrogen,
about 21% oxygen, and the remaining amount is made up of trace gases such as
carbon dioxide and argon.
There are four main layers to the atmosphere:
1. Troposphere – the closest layer; gets colder with altitude; where weather occurs;
densest layer
2. Stratosphere – gets warmer with altitude; very little water vapor; few clouds, no
storms; contains the ozone layer.
3. Mesosphere – gets colder with altitude; temperature reaches as low as -80◦C.
4. Thermosphere – gets warmer with altitude; absorbs intense solar radiation and
reaches temperatures of 980◦C; outermost layer called the exosphere (gases both
escape the atmosphere and are captured from space here)
The layers are separated by boundaries called pauses. Between the troposphere and
stratosphere is the tropopause. Between the stratosphere and mesosphere is the
stratopause. Between the mesosphere and thermosphere is the mesopause.
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The ionosphere exists in the lower thermosphere and upper mesosphere.
Solar energy is absorbed in the ionosphere. This can cause the northern (aurora
borealis) and southern (aurora australis) lights.
Electrons in the ionosphere reflect radio waves, allowing them to be received over long
distances. Sometimes you can get radio stations from hundreds of miles away because
of this effect. This effect is strongest at night, because the ions require solar radiation
to form. At night the radio signals will travel farther up into the ionosphere before
reflecting, causing them to “bounce” further from their source.
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When Earth began to solidify about 4.4 billion years ago, volcanic eruptions released a
variety of toxic gases. This process of creating an atmosphere is called outgassing.
Earth’s early atmosphere contained hydrogen, water vapor, ammonia, methane, carbon
monoxide, carbon dioxide, and nitrogen, but NO OXYGEN.
As photosynthetic organisms evolved, they were able to remove some carbon dioxide
from the atmosphere and create oxygen gas as a waste product. As aerobic, or oxygenbreathing, organisms evolved, they joined plants in a balance that led to our present
atmosphere.
Ozone , O3, is a fragile compound found in the stratosphere, which absorbs much of
the sun’s damaging ultraviolet radiation. Without the ozone layer, ultraviolet rays
would cause serious damage to the cells of living things. Man-made chemicals, called
chlorofluorocarbons (CFCs) react with ozone have depleted the ozone layer. As a result
holes in the ozone layer have formed. In 1996 most industrialized countries stopped
the production of CFCs.
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The greenhouse effect results in heat being trapped in Earth’s atmosphere. Some
energy from the sun is absorbed by Earth’s surface. Some is also radiated back out
toward space. Some gases in Earth’s atmosphere absorb this energy before it can
escape Earth’s atmosphere.
Gases such as carbon dioxide, water vapor, and methane can cause the atmosphere to
heat up. Without the greenhouse effect, Earth would have a colder average
temperature than it does. But too much of the greenhouse effect can cause problems.
Global temperatures will rise and icecaps could melt, causing ocean levels to rise and
droughts in some areas.
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