Lecture Outlines Natural Disasters, 7 th edition

Lecture OutlinesNatural 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 mid-latitudes to polar regions

Figure 11.2

Insert new Figure 11.2 here

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 ScalesFahrenheit, 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

• 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

Water and Heat

– 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 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 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 AirDifferential 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 AtmosphereTroposphere: • 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 weatherTropopause:• Top of troposphere

Stratosphere:• Stable configuration

of warmer air above colder air

Figure 11.6

Insert revised figure 11.7 here

General Circulation of AtmosphereAtmosphere transports heat: low latitudes to high latitudes

Figure 11.7


General Circulation of AtmosphereLow 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

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

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

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

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

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

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)

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-go-

round 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

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


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

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


• 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

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

Midlatitude Cyclones

• 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

In Greater Depth: Doppler Radar

Figure 11.24

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


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


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


Figure 11.27

Insert new 11.27 here

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

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

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

How a Thunderstorm Works

Thunderstorms in North AmericaAir Mass Thunderstorms: Most common type , result from convectionCommon in low latitudes all yearCommon in mid-latitudes in summer, especially late afternoon

Severe Thunderstorms:Mid-latitude frontal collisions


Figure 11.32

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

Figure 11.33



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

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


Figure 11.36

Insert revised figure11.36 here

Lightning• Leading cause of forest fires, major cause of weather-

related deaths• Lightning distribution same as thunderstorm distribution


Figure 11.39


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

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


How Lightning Works


• 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

Figure 11.42

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


Destructive Winds• Straight-line winds can be as damaging as tornadoes• Widespread, powerful wind storm: derechoDerechos• Advancing thunderstorms form line of ferocious winds

with hurricane-force gusts, lasting 10 to 15 minutesOntario 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

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, 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

Tornadoes

Figure 11.45

Insert revised Figure 11.45 here

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

Tornadoes

Figure 11.49Figure 11.48

Insert Figure 11.48

Tornadoes in the United States and Canada • Enhanced Fujita wind damage scale

– EF-0, minor damage, up to EF-5, incredible damage

Tornadoes

Insert new table 11.6 here

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

Tornadoes

• 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)

Insert revisedTable 11.8 here

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

Tornadoes

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

Figure 11.52

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

Tornadoes

End of Chapter 11

Lecture Outlines Natural Disasters, 7 th edition

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