Download File - Global Scholars

Physical Geography Script: Part 2
Hello!
Before we go into our next topic I would like to give a brief recap of what we covered in the previous
lecture. We talked about the four spheres of the Earth: atmosphere which we’ll go into greater detail
today, the biosphere which involves all living life, hydrosphere all things dealing with water, and the
lithosphere which is the Earth’s crust and upper mantle. We discussed the structure of the Earth which
you can depict with a diagram of four concentric circles: the lithosphere, the mantle (which has an upper
and lower), the outer core (which is where Earth’s magnetic field is located and at the bewilderment of
the scientific community that magnetic field reverses approximately every 1,000 years), and the inner
core which is believed to be composed of iron/silicate or iron/nickel. We talked about the Earth-Sun
relationship which without the Sun’s solar energy life could not exist. Then we covered the Annual
March of the Earth around the sun. It takes 365 days for the Earth to revolve around the Sun and the
Earth rotates on its axis one rotation in a 24 hours period. We also briefly talked about the apparent
anomaly that the Earth is actually hotter when it is further away from the sun at Aphelion then when it
is at Perihelion closer to the sun, and the reason for that is the same reason why we have the seasons
which is due to the axis tilt of the Earth. In today’s lecture we’re going to begin with the Atmosphere
and the composition of the Atmosphere. If we have time we will also touch on the controls on weather
and climate.
The composition of the Atmosphere
The two most abundant gases in the atmosphere are nitrogen and oxygen. Nitrogen makes up 78% of
the total and oxygen makes up 21%. Nitrogen is added to the atmosphere by the decay and burning of
organic matter, volcanic eruptions, and the chemical breakdown of certain rocks, and it is removed by
certain biological processes and by being washed away in rain or snow. Overall it remains constant.
Oxygen is produced by vegetation and is removed by a variety of organic and inorganic processes; its
total quantity apparently remains stable. The remaining 1 percent of the atmospheric volume consists of
the inert gas argon. These three principle atmospheric gases—nitrogen, oxygen, argon—have minimal
effect on weather and climate.
There are variable gasses that do have an impact on the atmosphere. Water vapor for one is
responsible for the formation of clouds and the precipitation that results from cloud formation. Water
vapor is more abundant in areas with warm temperatures and warm surface areas like tropical oceans;
water vapor can take up as much as 4% of the volume in those areas. In areas like deserts or polar
regions water vapor will make up a fraction of a 1% of the volume in those areas. Carbon dioxide is
another variable gas which has the ability to absorb infrared radiation which helps warm the lower
atmosphere. This concentration has been increasing steadily for the last century or so due to the
increased burning of fossil fuels. Most atmospheric scientists believe this warming of the lower layers of
the atmosphere are leading to unpredictable global climate changes. Ozone is another minor gas in the
atmosphere. It’s a molecule made up of three oxygen atoms (O₃), instead of the common two oxygen
atoms (O₂). Ozone resides primarily 15-48 kilometers (9 and 30 miles) in the atmosphere and also serves
as an excellent absorber of ultraviolet solar radiation; it filters out a number of harmful rays to protect
life from deadly effects. Other variable gases: monoxide, sulfur dioxide, nitrogen oxide, and various
hydrocarbons are increasingly being introduced into the atmosphere by emission from factories and
automobiles. All of them are hazardous to life and may possibly have some effect on climate. Methane
also absorbs some wave lengths of radiation and plays a role in regulation of our atmosphere. Another
thing you will find in the atmosphere are particulates or aerosols. They can come from innumerable
sources like volcanic ash, windblown soil and pollen grains, meteor debris, and smoke from wild fires,
and salt spray from breaking waves. These tiny particles are most numerous near their places of origin—
above cities, seacoasts, active volcanoes, and some desert regions. They may be carried great distances,
however, both horizontally and vertically by the restless atmosphere. They affect weather and climate in
two major ways:
1. Many are hygroscopic (which means they absorb water), and water vapor condenses around
such condensation nuclei. This accumulation of water molecules is a critical step in cloud
formation.
2. Some either absorb or reflect sunlight, thus decreasing the amount of solar energy that reaches
Earth’s surface.
Thermo Layers
Most of us have had some personal experience with temperature variables associated with altitude.
As we climb a mountain, for instance, we sense a decrease in temperature. Until about a century
ago, it was generally assumed that temperature decreased with increasing altitude throughout the
whole atmosphere, but now we know that such is not the case. If you look at the diagram,
You can see that the vertical pattern (red line) of temperature is complex, consisting of a series of
layers in which temperature alternately decreases and increases. From the surface of Earth up ,
these thermal layers are called the troposphere, stratosphere, mesosphere, thermosphere, and
exosphere. In addition to these five principal names, we also have special names for the top of the
first three layers: tropopause, stratopause, and mesopause. We use the –spehere name when
talking about an entire layer and the –pause name when our interest is either in the upper portion
of a layer or in the boundary between two layers.
Troposphere
The lowest layer of the atmosphere and the one in contact with Earth’s surface—is known as the
troposphere. The names troposphere and tropopause (the top of the troposphere) are derived from the
Greek word tropos (“turn”) and imply an overturning of the air in this zone as a result of vertical mixing
and turbulence. The depth of the troposphere varies in both time and place. It is deepest over tropical
regions and shallowest over the poles, deeper in summer than in winter, and varies with the passage of
warm and cold air masses. On the average, the top of the troposphere (including the tropopause) is
about 18 kilometers (11miles) above sea level at the equator and about 8 kilometers (5 miles) above sea
level over the poles.
Stratosphere
The names stratosphere and stratopause come from the Latin stratum (“a cover”), implying a layered or
stratified condition without vertical mixing—if we describe the air in the troposphere as being
“turbulent,” we can describe the air in the steratosphere as being “stagnant.” The stratosphere extends
from an altitude of 18 kilometers (11 miles) above sea level to about 48 kilometers (30 miles).
Upper Thermal Layers
The names mesosphere and mesopause are from the Greek meso (“middle”). The mesosphere begins at
48 kilometers (30 miles) and extends to about 80 kilometers (50 miles) above sea level. Above the
mesopause is the thermosphere (from the Greek Therm, meaning “heat”) which begins at an altitude of
80 kilometers (50 miles) above sea level and has no definite top. Instead it merges gradually into the
region called the exosphere, which in turn blends into interplanetary space. Traces of the atmosphere
extend for literally thousands of kilometers higher. Therefore, “top of the atmosphere” is a theoretical
concept rather than a reality, with no true boundary between atmosphere and outer space.
Temperature Patterns in the Atmosphere
Air temperature changes with altitude. Beginning at sea level, where the average global temperature is
about 15°C (59°F), temperature first decreases steadily with increasing altitude through the
troposphere, declining to an average temperature of about -57°C (-71°F) at the tropopause.
Temperature remains constant through the tropopause and for some distance into the stratosphere. At
an altitude of about 20 kilometers (12 miles), air temperature begins increasing with increasing altitude,
reaching a maximum at 48 kilometers (30 miles) at the bottom of the mesosphere, where the
temperature is about -2°C (28°F). Then all through the mesosphere, reaching a minimum at the top of
that layer at an altitude of 80 kilometers (50 miles). Temperature remains constant for several
kilometers into the thermosphere and then begins to increase until, at an altitude of 200 kilometers
(125) miles, it is higher than the maximum temperature in the troposphere. In the exosphere, the
normal concept of temperature no longer applies. Each “warm zone” in this temperature gradient has a
specific source of heat. In the lower troposphere, the heat source is the surface of Earth itself—solar
energy warms the surface of Earth and this energy is in turn transferred through a number of different
processes to the troposphere immediately above. The warm zone at the stratopause is near the top of
the ozone layer, where ozone is absorbing the ultraviolet portion of sunlight and thereby warming the
atmosphere. In the thermosphere, various atoms and molecules also absorb ultraviolet rays from the
Sun and are thus split and heated. The “cold zones” that separate these warm zones are cold simply
because they lack such sources of heat. Although there are many interesting physical relationships in the
stratosphere, mesosphere, thermosphere, and exosphere, our attention is directed almost entirely to
the troposphere because storms and essentially all the other phenomena we call “weather” occur here.
Pressure
Atmospheric pressure can be thought of, for simplicity’s sake, as the “weight” of the overlying air. The
taller the “column of air” above an object, the greater the air pressure exerted on the object. Because
air is highly compressible, the lower layers of the atmosphere are compressed by the air above, and this
compression increases both the pressure exerted by the lower layers and the density of these layers. Air
in the upper layers is subjected to less compression and therefore has a lower density and exerts a lower
pressure. Air pressure is normally highest at sea level and decreases rapidly with increasing altitude. The
change of pressure with altitude is not constant, however. As a generalization, pressure decreases
upward at a decreasing rate.
Composition
The principal gases of the atmosphere have a remarkably uniform vertical distribution throughout the
lower 80kilometers (50 miles) or so of the atmosphere. This zone of homogenous composition is
referred to as the homosphere. The sparser atmosphere above this zone does not display such
uniformity; rather, the gases tend to be layered in accordance with their weights—molecular nitrogen
(N₂) below, with atomic oxygen (O), helium (He), and hydrogen (H) successively above. This higher zone
is called heterosphere. Water vapor also varies in its vertical distribution. Most is found near Earth’s
surface, and generally diminishes with increasing altitude. Over 16 kilometers (10 miles) above sea level,
the temperature is so low that any moisture formerly present in the air has already frozen into ice. At
these altitudes, therefore, there is rarely enough moisture to provide the raw material to make even a
wisp of a cloud. If you have done any flying, you may recall the remarkable sight of a cloudless sky
overhead once the plane breaks through the top of a solid cloud layer below. Two other vertical
compositional patterns are worthy of mention here.
1. The ozone layer, which, as stated above, lies between 15 and 48 kilometers (9 and 30 miles) up,
is sometimes called the ozonosphere. Despite its name, the ozone layer is not composed
primarily of ozone. It gets its name because that is where the concentration of ozone relative to
other gases is at its maximum. Even in the section of the ozone layer where the ozone attains its
greatest concentration, at about 25 kilometers (15 miles) above sea level, this gas only accounts
for no more than about 15 parts per million of the atmosphere.
2. The ionosphere is a deep layer of electrically charged molecules and atoms (which are called
ions) in the middle and upper mesosphere and the lower thermosphere, between about 60 and
400 kilometers (40 and 250 miles). The ionosphere is significant because it aids long-distance
communication by reflecting radio waves back to Earth. It is also known for its auroral displays,
such as the “northern lights” that develop when charged atomic particles from the Sun are
trapped by the magnetic field of Earth near the poles. In the ionosphere, these particles “excite”
the nitrogen molecules and oxygen atoms, causing them to emit light, not unlike a neon light
bulb.
Alright, it’s time for a brief recap. In today’s lecture we covered the atmospheric composition of the
Earth. It is composed primarily of 3 gases, nitrogen, oxygen, and argon. These three are constants in
our atmosphere. There are also variable gases that have an effect on our atmosphere like water
vapor, and depending where you are at, the effect is greater. For example, water vapor has a
greater effect on the atmosphere in the tropics where it can make up as much as 4% of the
atmosphere, whereas in deserts and polar areas water vapor may make up a fraction of 1%.
Atmospheric pressure can be thought of as weight in a given area, for example, the atmospheric
weight is greater at sea level, and as a general rule, the higher we elevate in altitude, the
atmospheric pressure decreases or atmospheric weight decreases. We also covered the thermal
layers: the troposphere lies at the bottom of the atmosphere and peaking in the tropopause about
11 miles, the next thermal layer up which is the stratosphere that extends up to the stratopause is
between 11miles to 30 miles, from 30 miles to 50 miles is the thermal layers of the mesosphere and
at the top is the mesopause, above that thermosphere, and exosphere.
Texes Essential Skills and Knowledge (TEKS)
(3) Geography. The student understands how physical processes shape patterns in the
physical environment. The student is expected to:
(A) explain weather conditions and climate in relation to annual changes in
Earth-Sun relationships;
(C) examine the physical processes that affect the lithosphere, atmosphere,
hydrosphere, and biosphere.