Earth's
atmosphere has
a series of layers, each with its own specific traits. Moving upward from
ground level, these layers are named the troposphere, stratosphere, mesosphere,
thermosphere and exosphere. The exosphere gradually fades away into the realm
of interplanetary space.
Troposphere
The troposphere is the lowest layer of our atmosphere.
Starting at ground level, it extends upward to about 10 km (6.2 miles or about
33,000 feet) above sea level. We humans live in the troposphere, and nearly all
weather occurs in this lowest layer. Most clouds appear here, mainly because
99% of the water vapor in the atmosphere is found in the troposphere. Air
pressure drops, and temperatures get colder, as you climb higher in the
troposphere.
Stratosphere
The next layer up is called the stratosphere. The
stratosphere extends from the top of the troposphere to about 50 km (31 miles)
above the ground. The infamous ozone layer is found within the stratosphere.
Ozone molecules in this layer absorb high-energy ultraviolet (UV) light from
the Sun, converting the UV energy into heat. Unlike the troposphere, the
stratosphere actually gets warmer the higher you go! That trend of rising
temperatures with altitude means that air in the stratosphere lacks the
turbulence and updrafts of the troposphere beneath. Commercial passenger jets
fly in the lower stratosphere, partly because this less-turbulent layer
provides a smoother ride. The jet stream flows near the border between the
troposphere and the stratosphere.
Mesosphere
Above the stratosphere is the mesosphere. It extends
upward to a height of about 85 km (53 miles) above our planet. Most meteors
burn up in the mesosphere. Unlike the stratosphere, temperatures once again
grow colder as you rise up through the mesosphere. The coldest temperatures in
Earth's atmosphere, about -90° C (-130° F), are found near the top of this
layer. The air in the mesosphere is far too thin to breathe; air pressure at
the bottom of the layer is well below 1% of the pressure at sea level, and
continues dropping as you go higher.
Thermosphere
The layer of very rare air above the mesosphere is called the thermosphere. High-energy X-rays and UV radiation from the
Sun are absorbed in the thermosphere, raising its temperature to hundreds or at
times thousands of degrees. However, the air in this layer is so thin that it
would feel freezing cold to us! In many ways, the thermosphere is more like
outer space than a part of the atmosphere. Many satellites actually orbit
Earth within the thermosphere! Variations in the amount of
energy coming from the Sun exert a powerful influence on both the height of the
top of this layer and the temperature within it. Because of this, the top of
the thermosphere can be found anywhere between 500 and 1,000 km (311 to 621
miles) above the ground. Temperatures in the upper thermosphere can range from
about 500° C (932° F) to 2,000° C (3,632° F) or higher. The aurora, the
Northern Lights and Southern Lights, occur in the thermosphere.
Exosphere
Although some experts consider the thermosphere to be the uppermost
layer of our atmosphere, others consider the exosphere to be the
actual "final frontier" of Earth's gaseous envelope. As you might
imagine, the "air" in the exosphere is very, very, very thin, making
this layer even more space-like than the thermosphere. In fact, air in the
exosphere is constantly - though very gradually - "leaking" out of
Earth's atmosphere into outer space. There is no clear-cut upper boundary where
the exosphere finally fades away into space. Different definitions place the
top of the exosphere somewhere between 100,000 km (62,000 miles) and 190,000 km
(120,000 miles) above the surface of Earth. The latter value is about halfway
to the Moon!
Ionosphere
The ionosphere is not a distinct layer like the others
mentioned above. Instead, the ionosphere is a series of regions in parts of the
mesosphere and thermosphere where high-energy radiation from the Sun has
knocked electrons loose from their parent atoms and molecules. The electrically
charged atoms and molecules that are formed in this way are called ions, giving
the ionosphere its name and endowing this region with some special properties.
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