Lecture Outlines
Natural Disasters, 7th edition
Patrick L. Abbott
Weather Principles and Tornadoes
Natural Disasters, 7th edition, Chapter 11
Weather Versus Climate
• Weather: short-term processes
– Tornadoes, heat waves, hurricanes, floods
• Climate: long-term processes
– Ice ages, droughts, atmosphere changes, ocean circulation shifts
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
– Earth’s spin and gravity set up circulation patterns in ocean
and atmosphere to even out heat distribution
– Circulation patterns determine weather and climate
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 midlatitudes to polar regions
Insert new Figure 11.2 here
Figure 11.2
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 Earth’s surface temperature
Solar Radiation Received by Earth
• Greenhouse effect raises Earth’s surface temperature
– Solar radiation reaches Earth at short wavelengths
– Absorbed solar radiation raises Earth’s 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 Earth’s surface  warms Earth’s
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
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
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
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
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
Vertical Movement of Air
• Air: easily compressed, denser and denser closer to
Earth’s 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)
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)
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
Vertical Movement of Air
Figure 11.5
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
General Circulation of Atmosphere
Atmosphere transports heat: low latitudes to high latitudes
Insert revised figure 11.8 here
Figure 11.7
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
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
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
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
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
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
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
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
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
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-goround is analogous to rotation of Earth’s 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
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
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
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
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
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
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
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  nor’easter
• Medium scale: individual
thunderstorms
• Small scale: tornado
Figure 11.22
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
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
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
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
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
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
How a Thunderstorm Works
Insert revised figure 11.30 here
Figure 11.30
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
How a Thunderstorm Works
Downbursts: An Airplane’s 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
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
Thunderstorms in North America
Insert revised figure
11.33 here
Figure 11.33
• Warm, moist air
necessary for
thunderstorm
formation comes
from Gulf of
Mexico  more
thunderstorms in
central and southern
U.S., particularly
Florida
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
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
Thunderstorms in North America
Lightning
• Leading cause of forest fires, major cause of weatherrelated deaths
• Lightning distribution same as thunderstorm distribution
Insert revised figure 11.39 here
Figure 11.39
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
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
Thunderstorms in North America
• Connection of stepped leader and upward streamers completes
circuit, initiates return stroke of positive charge up to cloud
How Lightning
Works up to 55,000oF
• More lightning
strokes occur, temperatures
Figure 11.42
Thunderstorms in North America
Don’t 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; don’t touch anything (lightning can flow
through plumbing, electrical, telephone wires)
– Get inside car; don’t 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
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)
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 Earth’s 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
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
Tornadoes
What Makes Tornadoes?
• Three air masses (warm, humid, low
Gulf of Mexico air; cold, dry, midaltitude 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
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
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)
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 11.48
Figure 11.49
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
Tornadoes
Tornadoes in the United States
and Canada
• Three main destructive actions:
Insert revised
Table 11.8 here
– 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
• 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)
Tornadoes
The Super Outbreak, 3-4 April 1974
Figure 11.52
• 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
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
End of Chapter 11