Chapter 22
The Atmosphere of the Earth
The Atmosphere
• The atmosphere is a relatively thin shell of gases
that surrounds the solid Earth.
• There are rapidly moving particles with billions of
collisions every second.
• The force of gravity attracts the particles towards
the surface of the earth, so the atmosphere thins
rapidly with increasing distance above the
surface.
• Eventually it merges with the very diffuse
medium of outer space.
Air Density
• Since air density is defined by the number
of molecules in a unit of volume, the
density of the atmosphere decreases
rapidly with increasing altitude.
At greater altitudes the same volume contains fewer molecules of the gases that make up
The air. That means that the density of air decreases with increasing altitude.
The earth’s atmosphere thins rapidly with increasing altitude and is much closer to the
earth than most people realize.
Composition of the Atmosphere
• Composition by Volume:
78% N2, 21% O2, approx. 1% Ar.
• The molecules are well mixed. N2 doesn’t react
easily with rocks, so it has accumulated in the
atmosphere.
• Some bacteria in soil remove N2 from the
atmosphere and by lightning. N2 is part of the
nitrogen cycle, since plants need N2 to produce
amino acids and nucleic acids.
• The nitrogen compounds are then carried
through the food chain. Eventually the N2 returns
to the atmosphere through the decay of plant
and animal matter.
• Overall the amount of N2 in the atmosphere
remains essentially constant over time.
Earth’s atmosphere has a unique composition of gases when compared to that of
Other planets in the solar system.
Oxygen in the Atmosphere
• Oxygen is removed from the atmosphere by:
1. Living organisms use it to oxidize food to carbon
dioxide and water.
2. Rocks consume it as weathering occurs. Metals
and other elements in rocks combine with oxygen
to form oxides.
• Plants release oxygen as a result of
photosynthesis and the amount released
balances the amount removed by the organisms
and by weathering.
• Oxygen is also maintained in a state of constant
composition.
Other components of the Atmosphere
• The argon in the atmosphere is chemically inert
and doesn’t react.
• Water vapor is also present in the atmosphere.
• Water vapor is invisible and different from fog
and clouds.
• Fog and clouds are tiny droplets of liquid water.
• The amount of water vapor varies from a fraction
of a percent (cold, dry air) to about 4% (warm,
humid air) and is essential to maintaining life on
the earth.
• Water enters the atmosphere by evaporation,
mostly from the ocean, and leaves the
atmosphere as rain or snow. This is called the
hydrologic cycle.
The Atmosphere
Water vapor – fixed but highly variable (0-4%)
Supplied by evaporation
Removed by precipitation
Other components of the Atmosphere
• The remaining .03% of the atmosphere is
mostly carbon dioxide (CO2) and traces
of the inert gases neon, helium, krypton,
and xenon, along with less than 5 parts
per million of free hydrogen, methane,
and nitrous oxide.
• The CO2 content varies locally near cities
from the combustion of fossil fuels and
from the respiration and decay of
organisms and materials produced by
organisms.
Carbon Dioxide in the Atmosphere
• The overall atmospheric concentration of CO2 is regulated by:
1. the removal from the atmosphere through photosynthesis
process of green plants.
2. the massive exchanges of CO2 between the ocean and the
atmosphere.
3. chemical reactions between the atmosphere and rocks of the
surface, primarily limestone.
• The ocean contains about 50 times more CO2 than the
atmosphere in the form of carbonate ions and as dissolved CO2
gas. It serves as a buffer by releasing CO2 if the atmospheric
concentration decreases and absorbing CO2 if the atmospheric
concentration increases.
• Limestone rocks contain an amount of carbon dioxide that is
equal to about 20 times the mass of all of Earth’s present
atmosphere
Carbon Dioxide in the Atmosphere
• Limestone is CaCO3, so you can say that
each formula unit of CaCO3 contains one
molecule of CO2.
• There is a yearly increase of about 1 part
per million of CO2 in the atmosphere over
the last several decades.
• The increase is believed to be a result of
the destruction of tropical rainforests along
with increased fossil fuel combustion.
Aerosols
• The atmosphere also contains particles of dust,
smoke, salt crystals (from mist created by ocean
surf and waves) and tiny solid or liquid particles
called aerosols.
• Aerosol particles become suspended and are
dispersed among the molecules of the
atmosphere gases.
• Aerosols are produced by combustion, often
resulting in air pollution, and by volcanoes and
forest fires.
The Atmosphere
Atmosphere contains dust, smoke and salt crystals (aerosols).
Atmospheric Pressure
• At the earth’s surface the atmosphere
exerts a force of about 14.7 lbs/sq. in.. As
you go to higher altitudes above sea level
the pressure rapidly decreases with
increasing altitude.
• The decrease in pressure is due to having
less weight of the atmosphere above an
object at higher altitudes.
• A barometer is used to measure pressure.
The mercury barometer was invented in
1643 by an Italian named Torricelli.
Barometers
• The original barometer was a glass tube with
one end closed and filled with mercury (Hg).
• The tube was placed, open end down, in a bowl
of mercury.
• As the atmospheric pressure increases and
decreases, the height of the supported mercury
column moves up and down.
• Pressure is indicated according to the height of
the mercury column.
• The standard atmospheric pressure is the
atmospheric pressure at sea level and it is 76.00
cm or 29.92 in of mercury. This is also called
one atmosphere of pressure.
Warming the Atmosphere
• Radiation from the Sun must pass through the
atmosphere before reaching the earth’s surface.
• The atmosphere filters, absorbs, and reflects
incoming solar radiation. On the average, about
30% of the total radiation is reflected back into
space, most of that occurring from clouds.
• The amount reflected at any one time depends
on the extent of cloud cover, the amount of dust
in the atmosphere, and the extent of snow and
vegetation on the surface.
• Only about ½ of the incoming solar radiation
reaches the earth’s surface after deducting the
amounts reflected and absorbed by air and
clouds and scattered.
Absorbed light
• The incoming light that does reach the earth’s surface is
absorbed by rocks, soil, water, or anything else on the
ground.
• These materials then emit the absorbed energy as
infrared radiation or heat.
• Carbon dioxide and water vapor molecules absorb this
infrared radiation and their temperature increases. They
then go on to reemit this infrared radiation, or heat, in all
directions and it gets reabsorbed and reemitted by other
materials. Eventually some ends up going back to outer
space.
• The result is that the infrared radiation lingers for a
longer length of time before is released to outer space.
The temperature near the surface then increases.
Greenhouse Effect
• The greenhouse effect is the process of
heating the atmosphere in the lower parts by the
absorption of solar radiation and reemission of
infrared radiation.
• This is somewhat analogous to what happens in
a greenhouse, since the glass allows the solar
energy to enter but does not allow the heat to
leave.
• Carbon Dioxide and water vapor molecules do
not trap the infrared radiation, they just delay the
process of releasing the heat back to outer
space, resulting in increased temperatures. The
more carbon dioxide present the higher the
temperatures will tend to be.
Structure of the Atmosphere
• The greenhouse effect causes the
atmosphere to be heated from the ground
up.
• The higher altitude parts of the
atmosphere lose radiation to space more
readily than the lower altitude parts.
• The lowest part of the atmosphere is
warmer and the temperature decreases
with increasing altitude at first. The
temperature decreases on average 3.5oF
per 1000 ft.
The Troposphere
• The temperature decreases with altitude until an average
altitude of about 11 km.
• The temperature then begins to remain more or less
constant with increasing altitude.
• The lower part of the atmosphere until the point when
the temperature stops decreasing with altitude is called
the troposphere.
• Almost all weather occurs in the troposphere. The word
is derive from the Greek meaning “turning layer”.
• The air at the bottom is continually warmed by the
ground and the ocean, like a stove, and the air at the
bottom is most dense, so it absorbs heat the most.
• The warm ground-level air rises and the colder air sinks
to take its place causing weather (turbulence, clouds,
winds, rains, etc.).
The Tropopause
• The upper boundary of the troposphere is called
the tropopause and it varies with latitude and
with the seasons. It is generally one and a half
times higher than average near the equator and
about half the average height over the poles.
• It is also higher in the summer than in the winter
at any given altitude.
• The average temperature at the tropopause is
about -60oC or -80oF.
Equator
Poles
The altitude of the
tropopause is a function of
latitude. It is higher at the
equatorial areas and lower
at the poles, since the air is
warmer due to more heat
emanating from the earth at
the equator. It is at 16 km at
the equator, 10 km at the
poles.
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The Stratosphere
The stratosphere is the layer above the tropopause.
The word is from the Greek for “stratified layer”.
The temperature increases with height in this layer.
The stratosphere contains little moisture, dust or
turbulence, making it a desirable altitude to fly.
Most ozone is present in the upper stratosphere and it
absorbs UV radiation from the Sun and converts it to
heat. Towards the bottom less UV radiation is left, so
less heat is produced and the temperatures are lower.
Not much turbulence. Jet aircraft mostly cruise through
here.
The temperature increases gradually to a height of about
48 km (30 mi), where it reaches a maximum of about
10oC (about 50oF).
The upper layer of the stratosphere is called the
stratopause.
In the mid-1980s, scientists identified a massive hole in the Earth's
ozone layer over Antarctica, confirming a prediction made a decade
earlier that the rampant use of refrigerants and aerosols called
chlorofluorocarbons would destroy the protective layer of the
atmosphere. In 1990, the world came together in Canada and agreed
to tackle the problem once and for all, before skin cancers and other ill
effects of unfettered ultraviolet light became a permanent part of
spending time outdoors.
A strengthening of 1987's Montreal Protocol was the result -- and it
worked. The level of CFCs detected in the lower atmosphere peaked
in 1995, and in the Antarctic stratosphere in 2001. But the story isn't
over. Though industrialized nations stopped producing most CFCs in
1996, and developing nations stopped in 2006, the ozone hole
reached its greatest ever recorded area -- the size of North America -just last year.
That's because CFCs last in the atmosphere for about 40 years, so
while the United Nations expects incremental improvement every year,
the ozone layer isn't expected to fully recover until 2036. For more
information about the state of the ozone layer, visit
ozonewatch.gsfc.nasa.gov.
Fig. 1.9
The stratopause
• The upper layer of the stratosphere is
called the stratopause.
• At the stratopause the temperature is
approximately 0oC, but the pressure is
1/1000 of the surface pressure.
The Mesosphere and the Thermosphere
• The mesosphere, from the Greek “middle layer” is
above the stratopause. The temperature in this layer
decreases again with altitude. There is little ozone
present to absorb solar radiation. The minimum
temperature reached in the atmosphere occurs here,
approximately -100oC (-148oF)
• The layer above the mesosphere is called the
thermosphere, from the Greek for “warm layer”. The
temperature goes back to increasing as you get higher
due to absorption of solar radiation. A thermometer
would not actually record these high temperatures,
however, since there are so few particles present that
there is not much heat transferred to a thermometer.
The Ionosphere and the Exosphere
• The thermosphere and upper mesosphere are
sometimes called the ionosphere.
• There are free electrons and free ions at this
altitude which are responsible for reflecting radio
waves around the earth and for the northern
lights.
• The exosphere is the outermost layer where the
molecules merge with the diffuse vacuum of
space.
• The molecules of this layer that have sufficient
kinetic energy are able to escape and move off
into space.
•The ionosphere is part of the
thermosphere.
Ionosphere
•In the ionosphere some molecules of
gas are broken apart by radiation to form
electrically charged gaseous ions.
•The aurora is caused by electrons
streaming in from the Sun combining
with ionized gases to form neutral atoms
and giving off light rays in the process.
•This layer can scatter or reflect AM
radio waves.
•Above 100 km the atmosphere is so thin
that orbiting satellites can pass through
with very little friction.
The Winds
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There is uneven heating of the Earth’s surface due to
the different abilities of materials to absorb heat due to
their specific heat capacities.
• As a local region of air becomes heated the increased
kinetic energy of the molecules expands the mass of
air, reducing its density.
• Less dense air is buoyant and is pushed upward by
nearby cooler, more dense air. This results in 3 motions
for air:
1. The upward movement of air over a region of greater
heating.
2. The sinking movement of air over a cooler region.
3. A horizontal air movement between the cooler and
warmer regions.
The Winds
• Wind is a horizontal movement of air. The
direction of a wind is defined as the direction
from which it blows.
• Clouds form over areas where the air is moving
upwards.
• The clear air between the clouds is over areas
where the air is moving downward.
• Air can be observed to move from a field of cool
grass toward an adjacent asphalt parking lot on
a calm, sunlit day and can be observed with
soap bubbles or smoke.
Local Wind Patterns
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The upward and downward movement of air
leads to:
1. The upward movement produces a lifting effect
on the surface that results in an area of lower
atmospheric pressure.
2. The downward movement produces a piling up
effect on the surface that results in an area of
higher atmospheric pressure.
• On the surface, air is seen to move from the
“piled up” area of higher pressure horizontally
to the “lifted” area of lower pressure. The
movement of air and the pressure differences
occur together. This is called convective
movement of air.
Cool air pushes the less dense, warm air upward, reducing the surface pressure. As the
Uplifted air cools and becomes more dense, it sinks, increasing the surface pressure.
Local Wind Patterns
• A local wind pattern may result from the temperature
differences between a body of water and adjacent
landmasses.
• A cool, refreshing, gentle breeze blows from the water
toward the land during the summer.
• During the day the temperature of the land increases
more rapidly than the water temperature due to
differences in specific heat capacity. The air over the
land is therefore heated more, expands, and becomes
less dense.
• Cool, dense air from over the water moves inland under
the air over the land, buoying it up. This is called a sea
breeze.
• During the night the land surface cools more rapidly than
the water and the air moves from the land to the sea.
Global Wind Patterns
• Temperature imbalances are what drive the
global circulation of the atmosphere.
• The earth receives more direct solar radiation in
the equatorial region than it does at higher
latitudes.
• The temperatures of the lower troposphere are
generally higher in the equatorial region,
decreasing with latitude toward both the poles.
• Hot air rises in the belt around the equator,
known as the intertropical convergence zone.
On a global, yearly basis, the equatorial region of the earth receives more direct
incoming solar radiation than the higher latitudes. As a result, average temperatures
are higher in the equatorial region and decrease with latitude toward the poles. This
sets the stage for worldwide patterns of prevailing winds, high and low areas of
atmospheric pressure, and climatic patterns.
Part of the generalized global circulation pattern of the earth’s atmosphere.
Water and the Atmosphere
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Water exists on Earth as solid, in the form of ice, snow, or hail, as
liquid, and as water vapor, the invisible form of water in the
gaseous state.
Over 98% of water on the earth exists in the liquid state, mostly in
the ocean. Only a small, variable amount of water vapor is in the
atmosphere at any given time.
The small amount of water vapor present in the atmosphere is
responsible for:
contributing to the greenhouse effect, which helps make the earth
a warmer planet.
serving as one of the principal agents in the weathering and
erosion of the land, which creates soils and sculptures the
landscape.
maintaining life, since almost all plants and animals cannot survive
without water.
It is the ongoing cycling of water vapor into and out of the
atmosphere that makes all this possible.
Evaporation and Condensation
Evaporation and
condensation are occurring
at all times.
If the number of liquid
molecules leaving the liquid
state exceeds the number
returning, the water is
evaporating. If the number
of molecules returning to the
liquid state exceeds the
number leaving, the water
vapor is condensing.
If both rates are equal,
the air is saturated, and
the humidity is 100%.
Evaporation and Condensation
• If the temperature is increased more water
vapor must be added to the air to maintain
the saturated condition.
• Warm air can hold more water vapor than
cooler air.
• Warm air on a typical summer day can
hold five times as much water vapor as
cold air on a cold winter day.
Humidity
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The amount of water vapor in the air is referred to as humidity.
Damp, moist air is said to have a high humidity.
Dry air is said to have low humidity.
A measure of the amount of water vapor in the air at a particular
time is called the absolute humidity.
• The relationship between the actual absolute humidity at a particular
temperature and the maximum absolute humidity that can occur
at that temperature is called the relative humidity (RH).
• A humidity of 100% (saturated air) means that the air has all the
water vapor it can hold.
• Evaporation is a cooling process since the molecules with higher
kinetic energy and therefore higher temperature are the ones that
evaporate. If the air is saturated then no further evaporation can
occur and this is why it feels hotter when the humidity is high.
Humidity
Humidity – water vapor in the air.
Absolute humidity – actual amount of water
vapor
Relative humidity – relationship between
actual water vapor and how much air can
hold.
absolute humidity
RH 
x 100
maximum humidity
The Condensation Process
• During the condensation process molecules on the
surface of an object form as dew or in the air as the
droplets of water making up fog and clouds.
• Water molecules can also join together to produce solid
water in the form of frost or snow.
• Before condensation can occur the air must be
saturated. If cooling then occurs then the capacity of the
air to hold water vapor decreases and condensation
occurs.
• This is why you can see your exhaled breath when it is
cold, since the high moisture content of your exhaled
breath is condensed into tiny water droplets by cold air
and why you see a white trail behind an airplane, since
water is one of the products of fuel combustion by
airplane engines.
• The temperature at which condensation begins is called
the dew point temperature, since it is when dew forms.
Water and the atmosphere
Dew point temperature – temperature at
which dew (or frost) will form.
How are clouds and rain formed?
Fog and Clouds
• Fog and clouds are both accumulations of tiny droplets
of water that have been condensed from the air.
• Fog is sometimes described as a cloud that forms near
the surface.
• These water droplets are very small and a very slight
upward movement of the air will keep them from falling.
If they do fall they usually evaporate immediately.
• Fog and clouds form because air containing water vapor
has been cooled to the dew point. There must be a
condensation nucleus present, which can be a particle
of salt or dust, in order for the water droplets to gather
and gain size. Since salt attracts water, it is particularly
effective.
Water and the atmosphere
For condensation, air must be
saturated and condensation
nuclei present.
Water and the atmosphere
Particles of salt, dust, smoke, soot
or other aerosols act as
condensation nuclei.
Water and the atmosphere
As condensation begins, process
continues to attract more moisture
adding to its size.
Water and the atmosphere
Other factors are involved to
increase the cloud size to large
raindrops within a short time
frame.
Review Exercises
• P. 537-538 Applying the Concepts:
#2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 16, 17, 19, 20,
21
New Book:
p. 594-597 # 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
14, 15, 16, 17, 18, 19, 20, 22, 23, 24, 25,
27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,
38, 39, 40, 41, 42, 43, 44, 45, 47, 48
Summary
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The density of the atmosphere decreases rapidly with
increasing altitude.
Composition of the atmosphere by volume:
78% N2, 21% O2, 1% Ar. Also CO2 and H2O vapor.
N2 is relatively inert and involved in the nitrogen cycle
with plants. O2 is used by rocks for weathering and by
animals for breathing and released by plants during
photosynthesis. Ar is inert.
Water enters the atmosphere by evaporation, mostly
from the ocean, and leaves the atmosphere as rain or
snow. This is called the hydrologic cycle.
CO2 is used for photosynthesis by green plants. There
are massive exchanges of CO2 between the ocean and
the atmosphere. There are chemical reactions involving
CO2 between the atmosphere and rocks of the surface,
primarily limestone.
There are also aerosols in the atmosphere:, dust,
smoke, and salt crystals which result in pollution.
Mercury barometer, atmospheric pressure.
The atmosphere filters, absorbs, and reflects incoming
solar radiation. 30% of the total radiation is reflected
back into space, most of that occurring from clouds.
The amount reflected at any one time depends on the
extent of cloud cover, the amount of dust in the
atmosphere, and the extent of snow and vegetation on
the surface. Only about ½ of the incoming solar
radiation reaches the earth’s surface after deducting the
amounts reflected and absorbed by air and clouds and
scattered.
The result of absorption of infrared radiation by CO2 and
H2O vapor is that the infrared radiation lingers for a
longer length of time before is released to outer space.
The temperature near the surface then increases.
Greenhouse effect.
Layers of the atmosphere: Troposphere (T decreases
upwards, where weather occurs), stratosphere (T
increases upwards due to ozone layer, very calm),
mesosphere (temperature decreases with altitude), and
thermosphere (temperature increases with altitude but
not felt because of very few particles).
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The tropopause (its height varies, higher at the equator) is
the layer between the troposphere and the stratosphere.
The thermosphere and upper mesosphere are the
ionosphere-radio waves travel here due to free ions. The
exosphere is the outermost layer.
Freon and other refrigerants can destroy the ozone layer,
which normally protects us vs. harmful UV radiation by
absorbing it.
Because warm air is less dense than cool air warm air
rises, cool air sinks, and air moves horizontally (wind).
Convective movement of air from area of high pressure
(air moving downwards) to area of low pressure (air
moving upwards).
During the day sea breeze is movement of air from cooler
air above sea to warmer air above land. At night the air
moves from the land to the sea, since air above the sea is
warmer at night.
Hot air rises in the belt around the equator, known as the
intertropical convergence zone. This leads to wind
patterns.
Most water on earth is liquid. It maintains life on earth. The
small amount of water vapor present in the atmosphere is
responsible for contributing to the greenhouse effect,
which helps make the earth a warmer planet, weathers
and erodes the land, which creates soils and sculptures
the landscape, and maintains life, since almost all plants
and animals cannot survive without water.
Warm air on a typical summer day can hold five times as
much water vapor as cold air on a cold winter day. Air with
maximum humidity (water vapor content) is saturated.
Formula for relative humidity.
For condensation to occur air must be saturated and the
temperature must be low (dew point temperature). Forms
dew and clouds. If colder temperatures forms frost and
snow.
For clouds and fog to form there must be condensation
nuclei (dust or salt particles).