chapter 20 atmospheric effects sections 20.1-20.5

Chapter 20

Atmospheric EffectsSections 20.1-20.5

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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

Intro


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

Section 20.1


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

Section 20.1


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

Section 20.1


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

m) enhances the coalescence process by limiting the number of collisions necessary to form raindrops

Section 20.1


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

Section 20.1


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

Section 20.1


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

Section 20.1


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

Section 20.1


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

Section 20.1


Hailstones• If the convection cells in the thunderstorms are

powerful enough, hailstones can become quite large

Section 20.1


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

Section 20.2


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

Section 20.2


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

Section 20.2


Air-Mass Source RegionsAir masses that affect North America

Section 20.2


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

Section 20.2


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.

Section 20.2


Graphical Symbols for Fronts

• Weather maps use standard symbols to show the location of different types of frontal boundaries

• The direction of frontal advance is indicated by the side of the line with the symbols

Section 20.2

• Cold front –

• Warm front –• Occluded front – when a cold front

overtakes an active warm front

• Stationary front –


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

Section 20.2


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

Section 20.3


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

Section 20.3


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)

Section 20.3


Lightning• On average, approximately 200 people are killed

each year by lightning

Section 20.3


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

Section 20.3


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

Section 20.3


Estimating the Distance of a ThunderstormAn 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

Section 20.3


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

Section 20.3


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.

Section 20.3


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

Section 20.3


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

Section 20.3


Tropical Storm Regions of the World

• 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)

• Graphs show the average tropical storm activity by month and region

Section 20.3


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

Section 20.3


Hurricane Jeanne, September 25th, 2004

Section 20.3


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

Section 20.3


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

Section 20.3


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

Section 20.3


Saffir-Simpson Hurricane ScaleDeveloped in 1969

Section 20.3


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

Section 20.3


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

Section 20.3


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

Section 20.4


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

Section 20.4


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

Section 20.4


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

Section 20.4


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

Section 20.4


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

Section 20.4


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)

Section 20.4


Decomposition

• Pollution and increased acidity of the rain (“acid rain”) results in statues and other structures undergoing increased/accelerated corrosion

Section 20.4


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

Section 20.4


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

Section 20.4


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

Section 20.4


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

Section 20.4


Formation of Acid RainDue 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

Section 20.4


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

Section 20.4


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

Section 20.4


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%

Section 20.4


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

Section 20.5


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

Section 20.5


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

Section 20.5


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

Section 20.5


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

Section 20.5


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

Section 20.5


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

Section 20.5

chapter 20 atmospheric effects sections 20.1-20.5

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