Lecture Outlines Natural Disasters, 7th edition

1
Lecture OutlinesNatural Disasters, 7th edition

• Patrick L. Abbott

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Weather Principles and TornadoesNatural
Disasters, 7th edition, Chapter 11
3
Weather Versus Climate

• Weather short-term processes
• Tornadoes, heat waves, hurricanes, floods
• Climate long-term processes
• Ice ages, droughts, atmosphere changes, ocean
  circulation shifts

4
Processes and Disasters Fueled by Sun

• Sun powers hydrologic cycle and (with gravity)
  drives agents of erosion
• Sun heats Earth unequally
• Equatorial regions receive about 2.4 times more
  solar energy than polar regions
• Earths spin and gravity set up circulation
  patterns in ocean and atmosphere to even out heat
  distribution
• Circulation patterns determine weather and climate

5
Solar Radiation Received by Earth

• Relative amounts reflected, used in hydrologic
  cycle and converted to heat are different at
  different latitudes
• Equatorial belt (38oN to 38oS) faces Sun
  directly, so massive amounts of solar radiation
  are absorbed
• Polar regions receive solar radiation at low
  angle, so much is reflected ? net cooling
• Excess heat at equator is transferred through
  mid-latitudes to polar regions

Insert new Figure 11.2 here
Figure 11.2
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Solar Radiation Received by Earth

• Climatic feedback cycle in polar regions
• Receive less solar radiation ? colder
• More snow and ice forms ? higher albedo
  (reflectivity)
• More solar radiation reflected, less absorbed
• High albedos lower Earths surface temperature

7
Solar Radiation Received by Earth

• Greenhouse effect raises Earths surface
  temperature
• Solar radiation reaches Earth at short
  wavelengths
• Absorbed solar radiation raises Earths surface
  temperature
• Excess heat is re-radiated at long wavelengths
  and absorbed by greenhouse gases (water vapor,
  CO2, methane) in atmosphere, then radiated back
  down to Earths surface ? warms Earths climate
• About 95 of long wavelength re-radiated heat is
  trapped
• Examine greenhouse effect on Earth in Chapter 12
• Runaway greenhouse effect in early Earth history
• Human-increased greenhouse effect of 20th, 21st
  centuries

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Side Note Temperature Scales

• Fahrenheit, centigrade, Kelvin
• Fahrenheit sets freezing point of water at 32oF,
  boiling point at 212oF (most common in United
  States)
• Centigrade (or Celsius) sets freezing point of
  water at 0oC and boiling point at 100oC
  (everywhere else)
• Conversion oF 9/5 oC 32 oC 5/9 (oF 32)
• Kelvin absolute zero (0K) no heat energy
  (-460oF, -273oC)
• Conversion K oC 273

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Water and Heat

• Required amount of heat to raise temperature of
  water (specific heat) is high
• Convection transmission of heat in flowing water
  or air
• Conduction direct transmission of heat through
  contact

• Beach example temperature of high heat capacity
  water changes little from day to night, but hot
  beach sand (with low heat capacity) becomes cool
  at night

Insert table 11.1
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Water and Heat

• Water vapor in atmosphere between 0 and 4 by
  volume
• Humidity
• Saturation humidity maximum amount of water an
  air mass can hold (increases with increasing
  temperature)
• Relative humidity ratio of absolute humidity to
  saturation humidity
• If temperature of air mass is lowered without
  changing absolute humidity, will reach 100
  relative humidity because at each lower
  temperature, a lower saturation humidity applies
• When relative humidity reaches 100, excess water
  vapor condenses to liquid water ? temperature
  dew point

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Water and Heat

• Water absorbs, stores and releases huge amounts
  of energy changing phases between liquid, solid
  and gas
• Ice melting to water absorbs 80 calories of heat
  per gram of water (cal/g) latent heat
• Liquid ? vapor absorbs 600 cal/g latent heat of
  vaporization

• Ice ? vapor absorbs 680 cal/g latent heat of
  sublimation
• Liquid ? ice releases 80 cal/g latent heat of
  fusion
• Vapor ? liquid releases 600 cal/g latent heat of
  condensation
• Vapor ? ice releases 680 cal/g latent heat of
  deposition

Figure 11.5
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Vertical Movement of Air

• Air easily compressed, denser and denser closer
  to Earths surface
• Flows from higher to lower pressure, upward in
  atmosphere, if can overcome pull of gravity ? add
  heat
• As heated air rises, it is under lower pressure
  so expands
• Expansion causes adiabatic cooling (temperature
  decrease without loss of heat energy)
• Descending air is compressed and undergoes
  adiabatic warming (temperature increase without
  gain in heat energy)

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Vertical Movement of Air

• Air undergoes about 10oC adiabatic cooling per km
  of rise, 10oC adiabatic warming per km of descent
  (dry adiabatic lapse rate)
• As air cools, can hold less and less water vapor
  ? relative humidity increases
• When relative humidity 100 (altitude lifting
  condensation level), water vapor condenses and
  latent heat is released, which slows rate of
  upward cooling to about 5oC per km of rise (moist
  adiabatic lapse rate)

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Vertical Movement of Air

• Differential Heating of Land and Water
• Low heat capacity of rock ? land heats up and
  cools down quickly
• Winter
• Land cools down quickly, so cool air sinks toward
  ground ? high-pressure region
• Ocean retains warmth, so warm, moist air rises
• Cold, dry air from land flows out over ocean
• Summer
• Land heats up quickly, so hot, dry air rises ?
  low pressure
• Ocean warms more slowly, so cool, moist air sinks
  over ocean
• Cool, moist air over ocean is drawn into land,
  warms over land and rises to cool, condense and
  form rain ? summer monsoons

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Vertical Movement of Air
Figure 11.5
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Layering of the Lower Atmosphere

• Troposphere
• Lowest layer of atmosphere
• 8 km at poles and 18 km at equator
• Warmer at base, colder above ? instability as
  warm air rises and cold air sinks, constant
  mixing leads to weather

• Tropopause
• Top of troposphere
• Stratosphere
• Stable configuration of warmer air above colder
  air

Insert revised figure 11.7 here
Figure 11.6
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General Circulation of Atmosphere
Atmosphere transports heat low latitudes to high
latitudes
Insert revised figure 11.8 here
Figure 11.7
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General Circulation of Atmosphere

• Low Latitudes
• Solar radiation at equator powers circulation of
  Hadley cells
• Warm equatorial air rises at Intertropical
  Convergence Zone (ITCZ), then cools and drops
  condensed moisture in tropics
• Cooled air spreads and sinks at 30oN and 30oS,
  warming adiabatically

Figure 11.9
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General Circulation of Atmosphere

• Middle and High Latitudes
• Hadley cells create bands of high pressure air at
  30oN and 30oS
• Air flows away from high pressure zones
• Cold air flows over land from poles to collide at
  polar front around 60oN and 60oS
• Hadley, Ferrel and polar cells ? convergence at
  ITCZ (rain) and polar front (regional air masses)
• Global wind pattern modified by continental
  masses, mountain ranges, seasons, Coriolis effect

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General Circulation of Atmosphere

• Air Masses
• North America
• Cold polar air masses, warm tropical air masses
• Dry air masses form over land, wet air masses
  form over ocean
• Dominant air-mass movement direction is west to
  east
• Pacific Ocean air masses have more impact than
  Atlantic Ocean

Figure 11.10
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General Circulation of Atmosphere

• Fronts
• Sloping surface separating air masses with
  different temperature and moisture content, can
  trigger severe weather, violent storms
• Cold front cold air mass moves in and under warm
  air mass, lifting it up (tall clouds,
  thunderstorms)

• Warm front warm air flows up and over cold air
  mass (widespread clouds)

Figure 11.11
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General Circulation of Atmosphere

• Jet Streams
• Relatively narrow bands of high-velocity (around
  200 km/hr) winds flowing from west to east at
  high altitudes
• Pressure decreases more slowly moving upward
  through warm air than through cold air ? warm air
  aloft has lower pressure than cold air ? warm air
  flows toward cold air (toward poles)
• Spin of Earth turns poleward air flows to
  high-speed jet stream winds from the west
  (Coriolis effect)

• Subtropical jet about 30oN
• Polar jet more powerful, about 60oN, changing
  path

Figure 11.14
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General Circulation of Atmosphere

• Rotating Air Bodies
• Northern hemisphere
• Rising warm air creates low pressure area ? air
  flows toward low pressure, in counterclockwise
  direction
• Sinking cold air creates high pressure area ? air
  flows away from high pressure, in clockwise
  direction

Figure 11.17
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General Circulation of Atmosphere

• Rotating Air Bodies
• Northern hemisphere
• Meanders in jet stream may help to create
  rotating air bodies
• Trough of lower pressure (concave northward bend)
• Forms core of cyclone (counterclockwise flow)
• Ridge of higher pressure (convex northward bend)
• Forms core of anticyclone (clockwise flow)

Figure 11.18
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General Circulation of Atmosphere

• Observed Circulation of the Atmosphere
• Significant variation of air pressure and wind
  patterns by hemisphere and season
• Seasonal changes not so great in Southern
  Hemisphere with mostly water surface
• Northern Hemisphere wind and heat flow directions
  change with seasons
• Winter has strong high-pressure air masses of
  cold air over continents
• Summer has thermal lows over continents, Pacific
  and Bermuda highs

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Coriolis Effect

• Velocity of rotation varies by latitude
• 465 m/sec at equator, 0 m/sec at poles
• Bodies moving to different latitudes follow
  curved paths
• Northern hemisphere veer to right-hand side
• Southern hemisphere veer to left-hand side
• Magnitude increases with increasing speed of
  moving body and with increasing latitude (zero at
  equator)

Insert revised figure 11.15 here
Figure 11.14
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Coriolis Effect

• Determines paths of ocean currents, large wind
  systems, hurricanes (not water draining in sinks
  or toilets)
• Merry-go-round analogy
• Looking down on counter-clockwise spinning
  merry-go-round is analogous to rotation of
  Earths northern hemisphere viewed from North
  Pole
• Outside edge of merry-go-round (equator) spins
  much faster than center of merry-go-round (North
  Pole)
• Person at center tosses ball at person on edge
  person on edge has rotated away and ball curves
  to right
• Opposite spin and direction for southern
  hemisphere

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General Circulation of the Oceans

• Surface and near-surface ocean waters absorb and
  store huge amounts of solar energy
• Some solar heat transferred deeper by tides and
  winds
• Surface- and deep-ocean circulation transfers
  heat throughout oceans, affects global climate

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General Circulation of the Oceans

• Surface Circulation
• Surface circulation mostly driven by winds
• Movement of top layer of water drags on lower
  layer, etc., moving water to depth of about 100 m
• Wind-driven flow directions are modified by
  Coriolis effect and deflection off continents
• Carries heat from low latitudes toward poles

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General Circulation of the Oceans

• Surface Circulation
• North Atlantic Ocean
• Warm surface water blown westward from Africa
  into Caribbean Sea and Gulf of Mexico
• Westward path blocked by continents, forced
  northward along eastern side of North America,
  east to Europe (warms Europe)

Figure 11.20
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General Circulation of the Oceans

• Deep-Ocean Circulation
• Oceans layered bodies of water with
  progressively denser layers going deeper
• Water density is increased by
• Lower temperature
• Increased dissolved salt content
• Deep-ocean water flow is thermohaline (from heat,
  salt) flow overturning circulation

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General Circulation of the Oceans

• Ocean water has higher density at
• High latitudes (lower temperature)
• Arctic and Antarctic (fresh water frozen in sea
  ice, remaining water made saltier)
• Warm climates (fresh water evaporated, remaining
  water made saltier)
• Densest ocean water forms in northern Atlantic
  Ocean and Southern Ocean

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Severe Weather

• Causes about 75 of yearly deaths and damages
  from natural disasters
• More people killed usually by severe weather than
  by earthquakes, volcanoes, mass movements
  combined
• From 1980 to 2005, U.S. had 67 weather-related
  disasters causing more than 1 billion (each) in
  damages
• Total more than 556 billion

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Midlatitude Cyclones

• Northern Hemisphere cyclone counterclockwise air
  mass rotating around low-pressure core
• Large scale trough in jet stream juxtaposes
  northern cold front and southern warm front ?
  line of thunderstorms
• Northeastern U.S. low-pressure system moving up
  Atlantic coast draws northern cold air, moisture
  from east ? noreaster

• Medium scale individual thunderstorms
• Small scale tornado

Figure 11.22
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Midlatitude Cyclones

• The Eastern U.S. Storm of the Century of 1993
• Immense cyclone covered area from Cuba to Canada
  between March 12 to 15
• Killed 270 people, more than 8 billion in
  damages
• Large trough in jet stream caused collision of
  three air masses over Florida
• Low-pressure, warm, moist air from Gulf of Mexico
• Fast-moving frigid arctic air mass from north
• Rainy, snowy east-moving air mass from Pacific
• Rode jet stream north up coast

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In Greater Depth Doppler Radar

• Measures relative velocity between two objects
• Radar guns for police
• Velocities in sports
• Describing weather systems
• Radar detects precipitation using reflection of
  microwaves
• Reflectivity increases as precipitation increases
• Allows life-saving advance warnings

Figure 11.24
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Midlatitude Cyclones

• Blizzards
• Strong cyclone with winds at least 60 km/hr and
  below freezing temperatures, blowing/falling snow
• Cyclone may travel slowly though winds are fast

• Northeastern United States, 6-8 January 1996
• Canadian blizzard dropped record snowfalls in
  Ohio, Pennsylvania, West Virginia, New Jersey
• Wind speeds exceeding 80 km/hr
• Killed 154 people
• Followed immediately by warm weather and heavy
  rains ? destructive flooding

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Midlatitude Cyclones

• Ice Storms
• Precipitation falls as snow flakes or ice
  particles
• May pass downward through air warm enough to
  cause melting to rain
• If rain then enters below-freezing layer near
  ground, refreezes into sleet
• If rain is not in below-freezing layer long
  enough to refreeze, becomes supercooled, and then
  refreezes as soon as comes into contact with
  ground or solid object, forming coating of ice

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Midlatitude Cyclones
Insert new 11.27 here
Figure 11.27

• Canadian Ice Storm, 5-9 January 1998
• 80 hours of freezing rain
• 25 people died of hypothermia, 7 billion in
  damage
• Power system collapsed under immense damage, had
  to be rebuilt

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How a Thunderstorm Works

• Air temperature normally decreases upward from
  surface at about 6oC/km lapse rate
• If lapse rate is greater than 6-10oC/km,
  atmosphere is unstable
• Rising warm, moist air may begin condensation,
  releasing latent heat and providing energy for
  severe weather, building cloud top higher

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How a Thunderstorm Works
Insert revised figure 11.30 here
Figure 11.30
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How a Thunderstorm Works

• Early stage requires continuous supply of
  rising, warm, moist air to keep updraft and cloud
  mass growing
• Mature stage
• Upper-level precipitation begins when ice
  crystals and water drops become too heavy for
  updrafts to support
• Falling rain causes downdrafts, pulling in
  cooler, dryer air
• Updrafts and downdrafts blow side by side,
  creating gusty winds, heavy rain, thunder and
  lightning, hail
• Dissipating stage downdrafts drag in so much
  cool, dry air that updrafts necessary to fuel
  thunderstorm are cut off

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How a Thunderstorm Works

• Downbursts An Airplanes Enemy
• Violent downdrafts of cold air with rain and hail
  during mature stage of thunderstorm
• Especially dangerous to airplanes, pushing plane
  into ground before pilot can react
• Airplanes also threatened by horizontal wind
  shear wind shift from head winds (necessary to
  maintain lift) to tail winds

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Thunderstorms in North America
Air Mass Thunderstorms Most common type ,
result from convection Common in low latitudes
all year Common in mid-latitudes in summer,
especially late afternoon Severe
Thunderstorms Mid-latitude frontal collisions
Insert revised figure 11.32 here
Figure 11.32
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Thunderstorms in North America

• Warm, moist air necessary for thunderstorm
  formation comes from Gulf of Mexico ? more
  thunderstorms in central and southern U.S.,
  particularly Florida

Insert revised figure 11.33 here
Figure 11.33
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Thunderstorms in North America

• Heavy Rains and Flash Floods
• Thunderstorms can be major supplier of water to
  area
• Central Texas
• Warm, moist air from Gulf of Mexico meets warm,
  dry air from west, forming dry line (thunderstorm
  trigger)
• Air flow turned upward at escarpment of Balcones
  fault zone eroded fault scarp 30 to 150 m high,
  545 km long
• Torrential thunderstorm downpours

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Thunderstorms in North America

• Hail
• Layered ice balls dropped from storms with
• Buoyant hot air rising from heated ground
• Upper-level cold air creating large temperature
  contrasts
• Strong updrafts keeping hailstones aloft while
  adding layers
• Most common in late spring and summer, along jet
  stream in colder midcontinent

Insert revised figure 11.36 here
Figure 11.36
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Thunderstorms in North America

• Lightning
• Leading cause of forest fires, major cause of
  weather-related deaths
• Lightning distribution same as thunderstorm
  distribution

Insert revised figure 11.39 here
Figure 11.39
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Thunderstorms in North America

• How Lightning Works
• Flow of electric current top of clouds excess
  positive charge seeks balance with bottom of
  clouds excess negative charge

• Speeds up to 6,000 miles/second, in several
  strokes within few seconds

Figure 11.41
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Thunderstorms in North America

• How Lightning Works
• Charge imbalance from freezing and shattering of
  super-cooled water drops charge separations
  distributed by updrafts and downdrafts during
  early cloud buildup
• Negative charge at bottom of cloud induces
  buildup of positive charge in ground below
• Discharge begins within cloud, initiates downward
  stream of electrons ? stepped leader
• As stepped leader nears ground, ground electric
  field increases greatly, sending streamers of
  positive sparks upward, connecting with stepped
  leader about 50 m above ground

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Thunderstorms in North America

• Connection of stepped leader and upward streamers
  completes circuit, initiates return stroke of
  positive charge up to cloud
• More lightning strokes occur, temperatures up to
  55,000oF

How Lightning Works
Figure 11.42
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Thunderstorms in North America

• Dont Get Struck
• Lightning can strike up to 16 km from
  thundercloud
• Area of risk extends wherever thunder can be
  heard
• Avoid lightning
• Get inside house dont touch anything (lightning
  can flow through plumbing, electrical, telephone
  wires)
• Get inside car dont touch anything (lightning
  usually flows along outside metal surface of
  vehicle, jumps to ground through air or tire)
• If outside, move to low place, away from anything
  tall assume lightning crouch on balls of feet
  with hands over ears

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Thunderstorms in North America

• Destructive Winds
• Straight-line winds can be as damaging as
  tornadoes
• Widespread, powerful wind storm derecho
• Derechos
• Advancing thunderstorms form line of ferocious
  winds with hurricane-force gusts, lasting 10 to
  15 minutes
• Ontario to New York Derecho, 15 July 1995
• Thunderstorms moving 80 mph with 106 mph gusts
  blew from Ontario to New York for two hours, in
  early morning (hot, humid air supplied energy to
  thunderstorms through night)

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Tornadoes

• Rapidly rotating column of air from large
  thunderstorm
• Highest wind speeds of any weather phenomenon
  (more intense and more localized than hurricanes)
• About 70 of Earths tornadoes occur in Great
  Plains of central U.S.
• Move from southwest to northeast
• Travel up to 100 km/hr, wind speeds up to 500
  km/hr
• Core of vortex less than 1 km wide, sucks up
  objects
• Form hundreds of meters high in atmosphere, may
  never touch ground

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Tornadoes

• Tri-State Tornado, 18 March 1925
• Largest known tornado moved at about 100 km/hr,
  through Missouri, Illinois and Indiana, leaving
  nearly 2 km wide path of destruction
• Destroyed 23 towns and killed 689 people on 353
  km long path

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Tornadoes

• What Makes Tornadoes?
• Three air masses (warm, humid, low Gulf of Mexico
  air cold, dry, mid-altitude Canadian or Rocky
  Mountain air fast, high-altitude jet stream
  winds) moving in different directions give shear
  to thunderstorm
• Rising Gulf air is spun one way by mid-altitude
  cold air then spun another way by jet stream ?
  corkscrew effect
• Warm air rising on leading side
• Cold air descending on trailing side

Insert revised Figure 11.45 here
Figure 11.45
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Tornadoes

• What Makes Tornadoes?
• Thundercloud tilted by wind shear may grow into
  supercell thunderstorm
• Rain falls with downdrafts in forward flank of
  storm
• Warm air rises (updrafts) in middle of storm
• Downdrafts of cool, drier air in trailing side of
  storm

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Tornadoes

• What Makes Tornadoes?
• Tornadoes form between middle updraft and rear
  downdraft
• Rotation develops in wide zone
• Core pulls into tighter spiral ? speed increases
  dramatically (angular momentum is preserved)
• Downward-moving air in center surrounded by
  upward-spiraling funnel
• Wind speeds highest few hundred meters above
  ground (slowed by friction at ground level)

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Tornadoes

• Tornadoes in the United States and Canada
• Air masses collide in interior U.S. (world
  tornado capital), head NE
• Occur any time, most common in late spring, early
  summer

Insert Figure 11.48
Figure 11.49
Figure 11.48
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Tornadoes

• Tornadoes in the United States and Canada
• Enhanced Fujita wind damage scale
• EF-0, minor damage, up to EF-5, incredible damage

Insert new table 11.6 here
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Tornadoes

• Tornadoes in the United States and Canada
• Three main destructive actions
• High-speed winds
• Winds throw debris like bullets or shrapnel
• Fast winds blowing into building rapidly increase
  air pressure inside, sometimes blowing roof up
  and walls out

Insert revised Table 11.8 here

• Declining numbers of tornado deaths in recent
  decades
• More risk to old people, mobile-home residents,
  occupants of exterior rooms with windows, those
  unaware of alerts
• Safer in car than mobile home (lower center of
  gravity)

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Tornadoes
Figure 11.52

• The Super Outbreak, 3-4 April 1974
• Five weather fronts
• Cold front in Rocky Mountains
• Low-pressure system moving east
• Strong polar jet stream with bend to south
• Warm, humid air from Gulf of Mexico
• Dry air from southwest, overriding Gulf of Mexico
  air, forming unstable inversion layer (cold air
  over warm air)
• All weather fronts came together, as Gulf of

• Mexico air burst up through inversion layer,
  creating thunderclouds set spinning by other
  converging air masses
• In 16 hours, 147 tornadoes in 13 states, six
  tornadoes of F-5 force
• Overwhelming destruction

63
Tornadoes

• Tornadoes and Cities
• Urban heat islands up to 10oC warmer than
  surrounding
• Warm air rising above city creates low-pressure,
  convecting cell that can form thunderstorms
• 10 of 3,600 tornadoes between 1997 and 2000
  struck cities, including Nashville, Salt Lake
  City, Fort Worth, Oklahoma City
• Safe Rooms
• Traditional cellar rarely built in modern homes
• Interior closet or bathroom built with concrete
  walls and roof, steel door
• Safe even when rest of house destroyed

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End of Chapter 11