GLOBAL TEMPERATURE PATTERNS - Discovering Physical Geography - page 87

at any altitude within the troposphere, from near ground level
to thousands of meters up. Temperature inversions are usually
associated with local weather phenomena, such as when the
ground cools quickly on a clear night or along coastlines when
cold ocean water cools the air close to the ground. Inversions
are important meteorologically because they act to cap air from
freely moving upward (Figure 5.3b, bottom). These conditions
can result in the development of fog, as will be discussed in
Chapter 7. They also cause smog in many of the world's largest
cities, such as Mexico City (Figure 5.3c), Los Angeles, and
London, because urban pollutants are trapped under the inver-
sion layer. If the inversion layer persists, smog can build up to
become a serious health hazard.

In addition to its filtering effects, the stratosphere is also
important for humans because it is the portion of the atmosphere
through which commercial jets fly. The stratosphere is a perfect
medium for flight because it contains very little water vapor and
few impurities; thus, pilots find few clouds and good visibility.
In addition, the air is relatively calm compared to the turbulent
troposphere below because air in the stratosphere usually flows
parallel to the surface of Earth.

The Mesosphere
The mesosphere is a layer of decreasing temperature that occurs
from about 50 km to 80 km (30 mi to 50 mi) in altitude (see Figure
5.1); it is the coldest of the atmospheric layers. Its name comes
from the Greek word mesos , which means “middle.” Vertical
temperature trends in the mesosphere have a positive lapse rate
because temperature decreases with increasing distance from the
ozone layer, which is located in the stratosphere below. The alti-
tude at which temperature stops decreasing is known as the me-
sopause and is the upper boundary of the mesosphere. At this
altitude, air temperature is about Ľ 100°C ( Ľ 148°F). In the meso-
sphere, solar radiation reduces gas molecules to individual elec-
trically charged particles called ions. This deep layer of charged
particles can disrupt communications between astronauts and
ground control and interfere with various satellite communica-
tions, such as transmission of television signals.
Another interesting fact about the mesosphere is that it is the
layer where most meteors burn up as they fall through the atmo-
sphere. Did you know that most of these shooting stars are about
the size of one sand grain? They are destroyed because they col-
lide with billions of ions and gas particles as they zip through the
mesosphere. These collisions create sufficient heat to burn the tiny
rock fragments, creating a short-lived streaking path, long before
they strike the ground. Occasionally, the largest rock fragments
reach the Earth's surface. If they do, they are called meteorites .

The Stratosphere
The layer of the atmosphere that lies immediately above
the troposphere is the stratosphere (see Figure 5.1), which
ranges in altitude between about 12 km and 50 km (between
~7.5 mi and 30 mi). This portion of the atmosphere derives its
name from the Greek word stratos , which means “layer.” The
stratosphere is critically important to life on Earth because it
contains the ozone layer, which occurs at an altitude between
20 km and 50 km (12 mi and 31 mi). The concentration of
ozone in this portion of the atmosphere is about 10 parts per
million by volume (ppmv), whereas it is only 0.04 ppmv in
the troposphere.
As discussed in Chapter 4, the ozone layer filters UV
radiation from the Sun and re-radiates it as infrared energy.
Stratospheric temperature trends reflect this filtering and
the overall thickness of the ozone layer. From the top of the
tropopause to the base of the ozone layer, temperatures are
consistently about Ľ 57°C ( Ľ 70°F) (see Figure 5.1b). Above
that altitude, however, temperatures increase with altitude
because of the way ozone absorbs UV energy from the Sun.
At the lower altitudes in the stratosphere, ozone absorbs UV
radiation at wavelengths between 44 nm and 80 nm. It ab-
sorbs these wavelengths inefficiently, however. At higher al-
titudes in the stratosphere, ozone very efficiently absorbs UV
at wavelengths between 200 nm and 350 nm. This difference
in absorption efficiency explains why temperature increases
with altitude in the stratosphere. The top of the stratosphere,
or stratopause , is marked by the altitude where temperature
stops increasing. At this altitude, the average temperature is
about Ľ 5°C ( Ľ 23°F).

The Thermosphere
The thermosphere is the upper layer of the atmosphere and
occurs between about 80 km and 480 km (50 mi to 300 mi) in
altitude. Atmospheric gases in this portion of the atmosphere
are sorted into a variety of sublayers based on their molecular
mass. Oxygen molecules are few and literally many kilome-
ters (miles) apart from one another. In fact, they are so widely
spaced that the boundary with space is very diffuse and thus
difficult to precisely determine.

Stratosphere The layer of the atmosphere, between the
troposphere and mesosphere, that ranges between about 12 km
and 50 km (between ~7.5 mi and 31 mi) in altitude.

Mesosphere A layer of decreasing temperature in the atmo-
sphere that occurs from about 50 km to 80 km (~30 mi to 50 mi)
in altitude.

Stratopause The upper boundary of the stratosphere where
temperature reaches its highest point.

Mesopause The upper boundary of the mesosphere where
temperature reaches its lowest point.