Transcript Air mass

Chapter 20
Atmospheric
Effects
Sections 20.1-20.5
Atmospheric Effects
• The lower atmosphere (troposphere) is
dynamic, resulting in frequent weather
changes
• The movements and interactions of large air
masses bring variations to our weather
• Large air masses can move thousands of
miles and influence a region for a considerable
time period
– The general movement of air masses depend
largely on the global circulation structure of the
Earth
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Intro
20 | 2
Condensation
• In most situations condensation occurs
within an air mass when the dew point
temperature is reached
• In some cases, though, water vapor
may be cooled below the dew point
without condensation occurring
• In this situation, the air mass is said to
be supersaturated or supercooled
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Section 20.1
20 | 3
Condensation
• Water droplets do not form randomly, but
form around microscopic foreign particles
called hygroscopic nuclei present in the air
• Hygroscopic nuclei may consist of dust,
smoke, soot, salt, or other small airborne
particles
– Since droplets form around these hygroscopic
nuclei, condensation provides a mechanism for
cleansing the atmosphere
• If the proper type/size of airborne particles
are not present, condensation may not occur
or will be retarded
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Section 20.1
20 | 4
Precipitation
• When condensation does occur, the tiny
droplets are formed in updrafts and are
easily suspended as a cloud
• Precipitation requires larger drops to form
• Two processes are thought to be
responsible for the formation of drops
large enough to fall
– Coalescence
– The Bergeron process
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Section 20.1
20 | 5
Coalescence
• Coalescence – the formation of larger
drops by the collision of droplets
– In other words, large drops form at the
expense of smaller drops
• The process of coalescence is most
efficient when the original droplets are
large
– Starting with large droplets (around 100
mm) enhances the coalescence process by
limiting the number of collisions necessary
to form raindrops
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Section 20.1
20 | 6
The Bergeron Process
• This process is named after the Swedish
meteorologist that proposed it, and is thought
to be the most important precipitation process
• The Bergeron process involves three
essential components
– Ice crystals in the upper portion of the cloud
– Supercooled vapor in the lower portion of the
cloud
– Mixing or agitation brings the ice crystals in
contact with the supercooled vapor
• The ice crystals serve as condensation nuclei
and grow larger from the vapor condensing
on them
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Section 20.1
20 | 7
The Bergeron Process
• The mixing of ice
crystals and
supercooled
water vapor lead
to the production
of large ice
crystals
• These large ice
crystals will then
melt into large
droplets of water
in the lower
portion of the
cloud, coalesce,
and fall as
precipitation
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Section 20.1
20 | 8
Rainmaking
• Modern rainmaking techniques are based on
the essentials of the Bergeron process
• In one method very small silver iodide
crystals are produced to serve as ice crystal
nuclei
– Silver iodide crystals have a mineralogic structure
similar to ice
• Another method uses dry-ice (frozen CO2)
pellets to trigger the conversion of
supercooled droplets into ice crystals
– The dry-ice sublimes (changes from solid to gas),
resulting in rapid cooling in the cloud
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Section 20.1
20 | 9
Types of Precipitation
• Precipitation occurs in the form of rain, snow,
sleet, hail, dew, or fog
• Rain is the most common form of precipitation
in the lower and middle latitudes
• Snow forms if the dew point is below 0oC
– Snowflakes are hexagonal (six-sided) due to the
hexagonal crystal pattern that ice forms
• Sleet falls directly from a cloud as ice pellets
or forms when rain freezes before it hits the
ground as it falls through a cold near-surface
layer
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Section 20.1
20 | 10
Types of Precipitation
• Hail are large pellets of ice that form from
successive vertical descents and ascents
within convection cells of thunderstorms
• Dew forms when atmospheric water vapor
condenses on various surfaces
– When the land cools quickly at night, water vapor
will condense on available surfaces, such as grass
blades
– If the dew point is below freezing, the water vapor
will change directly from water vapor into ice
crystals called frost (a process called deposition)
– Frost is not frozen dew
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Section 20.1
20 | 11
Hailstones
• If the convection cells in the thunderstorms are
powerful enough, hailstones can become quite large
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Section 20.1
20 | 12
Air Masses
• The general weather conditions at any
given place depend largely on vast air
masses that move across the country
• Air mass – a large body of air that takes
on physical characteristics that
distinguish it from the surrounding air
• The main physical characteristics the
distinguish an air mass are temperature
and moisture content
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Section 20.2
20 | 13
Formation of Air Masses
• When a large mass of air remains for
some time over a particular region, the
mass of air takes on the physical
characteristics of the surface of the
region
• Source region – the region from which
an air mass derives its characteristics
• An air mass will eventually move from
its source region and will bring along its
physical characteristics
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Section 20.2
20 | 14
Classification of Air Masses
• Air masses are classified according to the
surface (land or sea) and general latitude
(warm or cold) of their source regions
– Surface: Maritime (m), Continental (c)
– Latitude: Arctic (A), Polar (P), Tropical (T),
Equatorial (E)
• The global circulation patterns greatly
influences the movement of air masses
• The conterminous U.S. is located within the
westerlies and hence the general movement
of the air masses is from west to east
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Section 20.2
20 | 15
Air-Mass Source Regions
Air masses that affect North America
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Section 20.2
20 | 16
Air Mass Boundaries
• Front – the boundary between to air
masses
• Cold front – the boundary of a cold air
mass advancing over a warmer surface
• Warm front – the boundary of a warm
air mass advancing over a colder
surface
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Section 20.2
20 | 17
Cold and Warm Fronts
• Cold fronts generally form a
sharp, steep boundary
where the lighter warm air is
displaced upward. As a
result, cold fronts are
accompanied by more
violent and sudden changes
in weather.
• Warm fronts form a more
gradual boundary because it
is more difficult for the lighter
warm air to displace the
denser cold air.
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Section 20.2
20 | 18
Graphical Symbols for Fronts
• Weather maps use standard symbols to show
the location of different types of frontal
boundaries
• Cold front –
• Warm front –
• Occluded front – when a cold front
overtakes an active warm front
• Stationary front –
• The direction of frontal advance is indicated
by the side of the line with the symbols
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Section 20.2
20 | 19
Weather Trends
• Air masses that move across the country give
rise to cyclonic low-pressure and anticyclonic
high-pressure regions
• Low-pressure systems carry rising air
currents, clouds, and possible precipitation
– Lows are generally associated with poor weather
• High-pressure systems carry falling air
currents, clear skies, and fair weather
– Highs are generally associate with good weather
• The positions of highs and lows are
monitored
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Section 20.2
20 | 20
Storms
• Storm – an atmospheric disturbance
that may develop within a single air
mass or may develop along the frontal
boundary of air masses
• Storms can be divided into local storms
and tropical storms
• Local storms include: rainstorms,
thunderstorms, ice storms, snowstorms,
and tornadoes
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Section 20.3
20 | 21
Local Storms
• Rainstorm – a heavy localized
downpour (1 to 3 inches per hour are
common)
• Thunderstorm – a rainstorm
distinguished by the occurrence of
lightning, thunder, and sometimes hail
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Section 20.3
20 | 22
Lightning
• Lightning – a discharge of electrical energy
• Lightning occurs in a thundercloud when
there is a separation of charges due to the
break up and movement of water droplets
– As the charges separate, an electric potential
arises
• Lightning can occur several ways
–
–
–
–
Entirely within a cloud (intracloud)
Between two clouds (cloud-to-cloud)
Between cloud and ground (cloud-to-ground)
Between cloud and air (air discharges)
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Section 20.3
20 | 23
Lightning
• On average, approximately 200 people are killed
each year by lightning
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Section 20.3
20 | 24
Thunder
• Thunder – the compression wave formed by
lightning’s sudden release of electrical energy
• If the lightning is very close (100m or less) the
observer will hear a single loud clap
• If the lightning is at a distance of 1 km or
more, the thunder consists of more of a
rumbling sound
– Thunder can not be heard more than about 25 km
from the lightning
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Section 20.3
20 | 25
Lightning and Thunder
• A lightning flash travels at the speed of
300,000 km/s (speed of light)
• Thunder is a sound and only travels at a
speed of approximately 1/3 km/s (1/5 mi/s)
• Therefore the lightning flash is essentially
seen immediately, but there is a delay in
hearing the sound of the thunder
• The lapse in time between seeing the
lightning and hearing the thunder can be
used to determine the distance
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Section 20.3
20 | 26
Estimating the Distance of a Thunderstorm
An Example
• Suppose some campers notice an
approaching thunderstorm in the
distance. Lightning is seen, and the
thunder is heard 5.0 s later.
Approximately how far away is the
storm in km and miles?
• Use equation 2.1 (d = vt)
• Given: t = 5 s, v = 1/3km/s = 1/5mi/s
• d = vt = (1/3km/s)(5.0s) = 1.6 km
• d = vt = (1/5mi/s)(5.0s) = 1.0 mi
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Section 20.3
20 | 27
Local Storms
• Ice storm – this type of storm occurs when
the temperature of the Earth’s surface a
below freezing (0oC) while it is raining
– Under these conditions the rain will freeze on
contact, building up a layer of ice
– The weight of the ice on trees and power lines can
cause significant damage
• Snowstorm – an appreciable accumulation of
snow
– When a snowstorm is accompanied by high winds
and low temperatures it is called a blizzard
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Section 20.3
20 | 28
Tornadoes
• Tornado – the most violent of storms
• Although tornadoes may not be as large
as other storms, its concentrated energy
results in great destructive potential
• Tornadoes are most common in the
U.S. and Australia
• Most tornadoes in the U.S. occur
between the Rockies and Appalachians
– April, May, and June are the peak times for
tornadoes in the U.S.
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Section 20.3
20 | 29
Tornado Destruction
• Tornado watch – issued when the
atmospheric conditions indicate that
tornadoes may form
• Tornado warning – issued when a
tornado has actually been sighted or
indicated on radar
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Section 20.3
20 | 30
Tropical Storms
• Tropical storm – a massive weather
disturbance that forms over tropical
oceanic regions
• Tropical storm are classified as
hurricanes once their wind speed
exceeds 118 km/h (74 mi/h)
• Hurricane diameters range from 480 to
960 km and their wind speeds range
from 118 to 320 km/h
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Section 20.3
20 | 31
Tropical Storm Regions of the World
• Graphs show the
average tropical storm
activity by month and
region
• Tropical storms with wind
speeds of over 118 km/h are
known as:
– Hurricanes in the Atlantic
Ocean
– Cyclones in the Indian Ocean
– Typhoons in southeast Asia
(Pacific)
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Section 20.3
20 | 32
Hurricane Formation
• Hurricanes originate over tropical ocean
areas where the sun heats enormous masses
of moist air
• As the moist air is heated a low pressure cell
is formed with accompanying rising air
pattern and counterclockwise (cyclonic)
rotation
• As the moisture in the air rises, it condenses
and releases its latent heat (Chapter 5)
• Latent heat of condensation and solar energy
are the chief sources of a hurricane’s energy
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Section 20.3
20 | 33
Hurricane Jeanne, September 25th, 2004
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Section 20.3
20 | 34
Hurricane Power
• Due to their size and intensity, hurricanes are
the most energetic storms on Earth
• Scientists have used several methods to
estimate the energy production of a hurricane
– One method uses, the amount of rain produced to
determine the amount of latent heat of
condensation that is produced – 6.0 x 1011 kWh
– In another method the kinetic energy of the winds
generated is quantified – 1.5 x 109 kWh
• The smaller value is about ½ the worldwide
electrical generating capacity
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Section 20.3
20 | 35
Hurricane Destruction and Deaths
• Although hurricane winds can do enormous
damage, drowning is the greatest cause of
death
• Most hurricane-related deaths (9 out of 10)
are attributed to the storm surge
• Storm surge – a great dome of water that
accompanies the storm as it makes landfall
• This surge of water is caused by the
hurricane winds and the low pressure of the
storm
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Section 20.3
20 | 36
Hurricane Storm Surge
• The storm surge can bring huge waves and storm
tides that may be more than 5 m above normal
• In many cases the storm surge comes suddenly,
floods broad coastal lowlands, and traps people from
leaving the area
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Section 20.3
20 | 37
Saffir-Simpson Hurricane Scale
Developed in 1969
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Section 20.3
20 | 38
Hurricanes Moving Inland
• Hurricanes rapidly diminish in strength
once they (or a portion thereof) moves
over land
• At this point the hurricane can no longer
gain energy from the warm ocean water
and is subjected to significant friction
over the land
• Even so, the remnants of a hurricane
may bring significant rainfall and
flooding for hundreds of miles inland
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Section 20.3
20 | 39
Hurricane Nomenclature
• Hurricane watch – issued for coastal
areas when there is a threat of
hurricane conditions within the next 24
to 36 hours
• Hurricane warning – issued for areas
where hurricane conditions are
expected within 24 hours
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Section 20.3
20 | 40
Atmospheric Pollution
• Pollution – any atypical contributions to
the environment resulting from the
activities of humans
• Air pollution is primarily the result of
products of combustion and industrial
processes that are released into the
atmosphere
• Releasing waste gases and particulates
into the atmosphere has long been
practiced
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Section 20.4
20 | 41
Historic Air Pollution
• The prolific burning of coal has plagued
England for hundreds of years
• In addition to the smoke and soot, thick fogs
are also common England with the
combination of these forming a particularly
noxious mixture know as smog (smoke and
fog)
• The presence of fog indicates that the
temperature is at the dew point
• Fog formation may lead to a temperature
inversion due to the release of latent heat
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Section 20.4
20 | 42
Temperature Inversion
• Normally the lapse
rate near the Earth’s
surface decreases
uniformly with
altitude
• A temperature
inversion exists
when near the
surface the
temperature locally
increases with
increasing altitude
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Section 20.4
20 | 43
Temperature Inversion
• Temperature inversion – a lower
atmospheric condition characterized by
an inverted lapse rate
• Under normal conditions (a decrease in
temperature with increasing altitude) hot
combustion gases will rise due to their
relative buoyancy
• When an atmospheric temperature
inversion develops, emitted hot
gases/smoke do not rise
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Section 20.4
20 | 44
Temperature Inversion Types
• Radiation temperature inversion – results
from the Earth’s radiative heat loss
– On clear nights the land surface and the adjacent
air cool quickly, however, the air some distance
above the surface remains relatively warm 
temp. inversion
• Subsidence temperature inversion – occurs
when high-pressure air mass moves into a
region and becomes stationary
– The dense air subsides, compresses, and is
heated
– The temperature of this subsiding air may become
warmer than the air below it  temp. inversion
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Section 20.4
20 | 45
Temperature Inversion Results
• When temperature inversions occur, hot
industrial and combustion gases do not
rise properly
• These waste gases are held close to
the ground and continued emission of
gases cause the air to become polluted
• Particularly hazardous breathing
conditions may develop, especially for
people with heart and lung ailments
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Section 20.4
20 | 46
Air Pollution Sources
• The major source of air pollution is due to the
combustion of fossil fuels – coal, oil, and gas
• If the fossil fuels are absolutely pure and the
combustion is complete, the products are CO2
and H2O, already natural parts of the air
– C + O2  CO2 (combustion of pure coal)
– CH4 + 2 O2  CO2 + 2 H2O (natural gas
combustion)
• Increased amounts of CO2 in the atmosphere
lead to increased acidity
– CO2 + H2O – H2CO3 (carbonic acid)
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Section 20.4
20 | 47
Decomposition
• Pollution and increased acidity of the
rain (“acid rain”) results in statues and
other structures undergoing
increased/accelerated corrosion
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Section 20.4
20 | 48
Incomplete Fossil Fuel Combustion
• When fuel combustion is incomplete,
carbon (soot) and carbon monoxide
(CO) may result
– 2 C + O2  2 CO
– CO is poisonous
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Section 20.4
20 | 49
Photochemical Smog
• Photochemical smog – results from the
reactions of hydrocarbons with other
pollutants and atmospheric oxygen in the
presence of sunlight
• Photochemical smogs have been common in
the Los Angeles basin and result in a number
of dangerous contaminants, including some
organic compounds that are carcinogens
– Ozone (O3), a lung/eye irritant at low altitudes, is
an indicator of photochemical reactions taking
place
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Section 20.4
20 | 50
Fuel Impurities
• The most common impurities in fossil fuels
are sulfur (S) and nitrogen (N)
• When fuels containing S are burned, several
different types of sulfur oxides (SOx) form
– Sulfur dioxide (SO2) is the most common
– S + O2  SO2
• SO2 emissions are most common from
burning coal and fuel oils during electricity
generation
– Most of the SO2 pollution occurs in the seven
northeastern industrial states
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Section 20.4
20 | 51
Acid Rain
• SO2 reacts in the atmosphere with O and
H2O to form sulfurous acid (H2SO3) and
sulfuric acid (H2SO4)
• N reacts in the atmosphere to form nitric acid
(HNO3)
• Under normal conditions, rain is slightly
acidic due to the natural CO2 in the
atmosphere
• With the addition of S, N, and CO2 pollutants
the precipitation from contaminated clouds is
even more acidic
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Section 20.4
20 | 52
Formation of Acid Rain
Due to the westerly direction
of the prevailing winds the
acid rains are generally to
the east of their S,N, and
CO2 sources
Most of the significant
acid rain occurs in the
most populated and
industrialized eastern
part of the country
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Section 20.4
20 | 53
Affects of Acid Rain
• Although natural buffers (rocks and
soils) in the regions of acid rain help to
neutralize the acidity, the affects of acid
rain are still a problem
• Lakes waters are particularly affected,
with some lakes in northeastern U.S.
and Canada having their biota severely
impacted by lower than normal pH
• Pollutants in the form mists react with
and stain building stones
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Section 20.4
20 | 54
Air Pollutants and Their Major Sources
• Note that
transportation is the
largest contributor to
air pollution
• The particulates
emitted from
industrial processes
include a number of
harmful metals,
including Pb and As
• “Stationary sources”
refer mainly to
power generation
plants
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Section 20.4
20 | 55
Cleaner Air
• Improvements in the regulation of
emissions from transportation, industry,
and other sources over the past 40
years has lead to significant air quality
improvement
• Since 1985, Los Angeles has reported
that smog has been reduced by 75%
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Section 20.4
20 | 56
Global Climate
• Climate – the long-term average weather
condition of a region
• Geologic evidence shows that dramatic
changes in climate have occurred throughout
Earth history
• Ice sheets advanced into low latitudes over
the world’s continents until about 10,000
years ago
• Ocean sediment cores indicate that dramatic
global climate changes resulted from subtle
and regular Earth orbit variations
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Section 20.5
20 | 57
Pollution and Climate
• It is generally accepted by most scientists that
the global climate is also being affected by
atmospheric pollution
• Anthropogenic contributions to the
atmosphere that affect the radiation balance
certainly must affect the global climate
• CO2 and other “greenhouse gases,”
particulate emissions, and the resulting global
cloud cover all affect the Earth’s albedo
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Section 20.5
20 | 58
Pollution and Climate
• Scientists are trying to understand climate
changes by using various models
• Climate models give scientists the opportunity
to compare theories and use historical data to
study the interrelationships between the
Earth’s atmosphere and other factors
• For example, we know that the amount of
particulate material in the atmosphere will
decrease its transparency to insolation
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Section 20.5
20 | 59
Mount Pinatubo, 1991
• In 1991 Mount Pinatubo in the Philippines erupted
and sent volcanic debris over 15 miles into the
atmosphere
• This was probably the largest volcanic eruption of the
century and provided scientists with an excellent
opportunity to study its affects on our global climate
• Enormous quantities of particulates (volcanic ash)
and SO2 were pumped into the upper atmosphere
• In addition to the ash that immediately fell to the
surface, significant quantities of volcanic particulates
and gases were sent into the Earth’s stratosphere
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Section 20.5
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Mount Pinatubo Eruption Results
• Beautiful sunrises/sunsets occurred during the
following year due to the particulates in the upper
atmosphere
• Satellite measurements indicated that the upper
atmospheric concentration of ozone over the
South Pole dwindled, probably as a result of the
1991 Mount Pinatubo eruption
• Scientists continue to study the global
consequences of this event
• A computer map of the South Pole region
indicating a minimal concentration of total
stratospheric ozone after the 1991 Mount
Pinatubo eruption
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Section 20.5
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Global Warming and CO2
• Enormous amounts of CO2 are emitted into
the atmosphere due to fossil fuel combustion
• Most scientists agree that an increase in
atmospheric CO2 can alter the amount of
radiation absorbed from the Earth’s surface,
resulting in an increase in temperature
• The U.S. EPA reports that the global mean
surface temperature has risen 0.3 - 0.6 Co
since the late 19th century
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Section 20.5
20 | 62
Global Warming and CO2
• The EPA also reports that snow cover in the
Northern Hemisphere and floating ice in the
Arctic Ocean have decreased
• In addition sea level has risen 4-8 inches over
the past century
• As CO2 levels continue to increase, these and
other climate changes are likely to occur
• There are many unanswered questions, but
time will tell about the effects of pollution
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Section 20.5
20 | 63