UNIT II: THE ENVIRONMENT (THE EARTH)
INTRODUCTION
I. ATMOSPHERE
The Earth is the only planet in the solar system with an atmosphere that can sustain life.
The blanket of gases not only contains the air that we breathe but also protects us from
the blasts of heat and radiation emanating from the sun. It warms the planet by day and
cools it at night.
Earth's atmosphere is about 300 miles (480 kilometers) thick, but most of it is within 10
miles (16 km) the surface. Air pressure decreases with altitude. At sea level, air pressure
is about 14.7 pounds per square inch (1 kilogram per square centimeter). At 10,000 feet
(3 km), the air pressure is 10 pounds per square inch (0.7 kg per square cm). There is
also less oxygen to breathe.
CHEMICAL COMPOSITION
By mass and by volume, more than 99% of the atmosphere is made up of nitrogen (N 2),
oxygen (O2), and argon (Ar) gases (Table 1.1). The concentrations of these atmospheric
gases, together with neon (Ne), helium (He), and krypton (Kr), have probably been
constant for many millions of years and are unlikely to change markedly, either by
natural or anthropogenic means.
Table 1.1: Constant atmospheric components
GAS Percent by volume of dry Concentrations
air
Nitrogen (N2) 78.1 780,840
Oxygen (02) 20.9 209,460
Argon (Ar) 0.9 9,340
Neon (Ne) +Helium He) + Krypton 0.002 24
(Kr)
Trace gases (as listed in Table 1.2) on the other hand are very variable, they tend to
change with time and atmospheric conditions. All these gases are affected by human
activities as well as their reactions with the soil, biosphere, and oceans.
Table 1.2: Variable gas concentrations
GAS Concentration (µLL-1 ))
Water vapour (H20) < 10,000
Carbon dioxide (C02) 380
Methane (CH4) 1.5
Hydrogen (H2) 0.50
Nitrous oxide (N20) 0.31
Ozone (03) 0.02
Carbon monoxide (C0) <0.05
Ammonia (NH3) 0.004
Nitrogen dioxide (N02) 0.001
Sulphur dioxide (S02) 0.001
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Nitric oxide (N0) 0.0005
Hydrogen sulphide (H2S) 0.00005
The atmosphere thins out in each higher layer until the gases dissipate in space.
THE KARMAN LINE
The Karman line is an attempt to define a boundary between Earth's atmosphere and
outer space. This is important for legal and regulatory measures for aircraft and
spacecraft, which fall under different jurisdictions and are subject to different treaties.
The line is defined as the altitude of 100 kilometers (62 miles; 330,000 feet) above
Earth's mean sea level.
The line is named after Theodore von Kármán (1881–1963), a Hungarian American
engineer and physicist, who was active primarily in aeronautics and astronautics. He was
the first person to calculate the altitude at which the atmosphere becomes too thin to
support aeronautical flight and arrived at 83.6 km (51.9 miles) as above this limit a
vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic
lift to support itself.
The line is approximately at the turbopause, above which atmospheric gases are not
well-mixed. The mesopause atmospheric temperature minimum has been measured to
vary from 85 to 100 km, which places the line at or near the bottom of the thermosphere.
LAYERS OF THE ATMOSPHERE
Earth's atmosphere has a series of layers, each with its own specific traits. Moving
upward from ground level, these five (5) layers are named the: (i) troposphere, (ii)
stratosphere, (iii) mesosphere, (iv) thermosphere and (v) exosphere. The exosphere
gradually fades away into the realm of interplanetary space as shown in Figure 1.
Layers of Earth's Atmosphere
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Figure 1: Layers of the atmosphere: troposphere, stratosphere, mesosphere and
thermosphere.
Temperature variation with height defines the various layers of the atmosphere. Pressure
decreases with height. The layers are divided based on temperature differences as
shown in Figure 2 and 3.
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Figure 2: temperature differences with change in altitude. The layers are demarcated
using temperature
mbar – Millibar a Pressure Unit
Designated as millibar (mbar, mb or mbr)
Millibar is a metric unit of pressure mainly used in European countries and is derived
directly from the bar pressure unit which equals 1,000 mbar. In SI units 1 mbar equals
100 Pascals.
The millibar is most commonly used to measure barometric pressure for meteorological
purposes and low range gas pressures due to its very small value.
In recent years the mb pressure unit has been replaced by the hPa (hectopascal) which is
exactly the same value. However the millibar has been used by many people for many
years so it will be a long time before its use is completely phased out if at all.
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The black line on the figure 3 shows how temperature varies with height (the
temperature scale is given along the bottom of the diagram). The scale on the right
shows the pressure. For example, at a height of 50 km, the pressure is only about one
thousandth of the pressure at the ground.
Figure 3: temperature and pressure changes within the different layers of the
atmosphere
1. The Troposphere
Is the layer closest to Earth's surface (or is the lowest layer of the atmosphere). It is 4 to
12 miles (7 to 20 km) thick and contains half of Earth's atmosphere. Air is warmer near
the ground and gets colder higher up.
The troposphere contains about 75% of all of the air in the atmosphere, and almost all of
the water vapour (which forms clouds and rain) and dust. This is the layer where most
weather takes place. Nearly all of the water vapor and dust in the atmosphere are in this
layer and that is why clouds are found here. Most thunderstorms don't go much above
the top of the troposphere (about 10 km).
In this layer, pressure and density rapidly decrease with height, and temperature
generally decreases with height at a constant rate. . If a parcel of air moves upwards it
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expands (because of the lower pressure). When air expands it cools. So air higher up is
cooler than air lower down.
The change of temperature with height is known as the lapse rate. The standard lapse
rate for the troposphere is a decrease of about 6.5 degrees Celsius (C) per kilometer
(km) (or about 12 degrees F). Near the surface, the lapse rate changes dramatically from
hour to hour on clear days and nights. Sometimes the temperature does not decrease
with height, but increases. Such a situation is known as a temperature inversion.
Persistent temperature inversion conditions, which represent a stable layer, can lead to
air pollution episodes.
The other main characteristic of the troposphere is that it is well-mixed. The name
troposphere is derived from the Greek tropein, which means to turn or change. Air
molecules can travel to the top of the troposphere (about 10 km up) and back down
again in a just a few days. This mixing encourages changing weather.
The troposphere is bounded above by the tropopause, a boundary marked as the point
where the temperature stops decreasing with height and becomes constant with height.
Any layer where temperature is constant with height is called isothermal. The tropopause
has an average height of about 10 km (This is lowest at the poles, where it is about 7 –
10 km above the Earth's surface. It is highest (about 17 – 18 km) near the equator. This
height corresponds to about 7 miles, or at approximately the 200 mb (20.0 kPa) pressure
level. Above the troposphere is the stratosphere.
2. The Stratosphere
Is the second layer. It starts above the troposphere and ends about 31 miles (50 km)
above ground. Ozone is abundant here and it heats the atmosphere while also absorbing
harmful radiation from the sun. The air here is very dry, and it is about a thousand times
thinner here than it is at sea level. Because of that, this is where jet aircraft and weather
balloons fly. The main impact the stratosphere has on weather is that its stable air
prevents large storms from extending much beyond the tropopause.
The increase in temperature with height occurs because of absorption of ultraviolet (UV)
radiation from the sun by this ozone a triatomic form of oxygen. The interaction between
UV light, ozone, and the atmosphere at that level releases heat, warming the
atmosphere and helping to create the temperature inversion in this layer.
The maximum concentrations of ozone are at about 25 km (15 miles) above the surface,
or near the middle of the stratosphere. Temperatures in the stratosphere are highest
over the summer pole, and lowest over the winter pole.
By absorbing dangerous UV radiation, the ozone in the stratosphere protects us from skin
cancer and other health damage. However chemicals (called chlorofluorocarbon (CFC)
CFCs or freons, and halons) which were once used in refrigerators, spray cans and fire
extinguishers have reduced the amount of ozone in the stratosphere, particularly at polar
latitudes, leading to the so-called "Antarctic ozone hole".
Now humans have stopped making most of the harmful CFCs we expect the ozone hole
will eventually recover over the 21st century, but this is a slow process.
The stratosphere is bounded above by the stratopause, where the atmosphere again
becomes isothermal. The average height of the stratopause is about 50 km, or 31 miles.
This is about the 1 mb (0.1 kPa) pressure level. The layer above the stratosphere is the
mesosphere.
3. The Mesosphere
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The region above the stratosphere is called the mesosphere, which is the middle of other
layers and hence the name mesosphere. Here the temperature again decreases with
height, reaching a minimum of about -90°C at the "mesopause", where the atmosphere
again becomes isothermal. This is around the 0.005 mb (0.0005 kPa) pressure level.
This layer starts at 31 miles (50 km) and extends to 53 miles (85 km) high. The top of the
mesosphere, called the mesopause, is the coldest part of Earth's atmosphere, with
temperatures averaging about minus 130 degrees F (minus 90 C). This layer is hard to
study. Jets and balloons don't go high enough, and satellites and space shuttles orbit too
high. Scientists do know that meteors burn up in this layer.
The air is extremely thin at this level. Over 99.9 percent of the atmosphere's mass lies
below the mesosphere. However, the proportion of nitrogen and oxygen at these levels is
about the same as at sea level.
Above the mesosphere is the thermosphere.
4. The Thermosphere
This layer extends from about 56 miles (90 km) to between 310 and 620 miles (500 and
1,000 km).
Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. It is a warm layer
and there is a significant temperature inversion. The few molecules that are present in
the thermosphere receive extraordinary amounts of energy from the sun, causing the
layer to warm. Though the measured temperature is very hot, if you exposed your skin to
the thermosphere, the perceived temperature would be very cold. Because there are so
few molecules present, there would not be enough molecules bombarding your body to
transfer heat to your skin. Temperature is a measurement of the mean kinetic energy, or
average speed of motion, of a molecule. So although there are only a few molecules,
each has a huge amount of kinetic energy.
The thermosphere is considered part of Earth's atmosphere, but air density is so low that
most of this layer is what is normally thought of as outer space. In fact, this is where
the space shuttles flew and where the International Space Station orbits Earth.
This is also the layer where the auroras occur. Charged particles from space collide with
atoms and molecules in the thermosphere, exciting them into higher states of energy.
The atoms shed this excess energy by emitting photons of light, which we see as the
colorful Aurora Borealis and Aurora Australis.
The thermosphere lies above the mesopause, and is a region in which temperatures
again increase with height. This temperature increase is caused by the absorption of
energetic ultraviolet and X-Ray radiation from the sun.
The region of the atmosphere above about 80 km is also caused the "ionosphere", since
the energetic solar radiation knocks electrons off molecules and atoms, turning them into
"ions" with a positive charge. The temperature of the thermosphere varies between night
and day and between the seasons, as do the numbers of ions and electrons which are
present. The ionosphere reflects and absorbs radio waves, allowing us to receive
shortwave radio broadcasts in New Zealand from other parts of the world.
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Above the thermosphere is the exosphere. Unlike the layers discussed previously, there
is no well-defined boundary between the thermosphere and the exosphere (i.e., there is
no boundary layer called the thermopause)
5. The Exosphere
This is the highest layer, is extremely thin and is where the atmosphere merges into
outer space. It is composed of very widely dispersed particles of hydrogen and helium.
The region above about 500 km is called the exosphere. It contains mainly oxygen and
hydrogen atoms, but there are so few of them that they rarely collide - they follow
"ballistic" trajectories under the influence of gravity, and some of them escape right out
into space. The exosphere is the region where molecules from the atmosphere can
overcome the pull of gravity and escape into outer space. The atmosphere slowly
diffuses into the void of space.
The Magnetosphere
The earth behaves like a huge magnet. It traps electrons (negative charge) and protons
(positive), concentrating them in two bands about 3,000 and 16,000 km above the globe
- the Van Allen "radiation" belts. This outer region surrounding the earth, where charged
particles spiral along the magnetic field lines, is called the magnetosphere
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