Chapter 10
Tornadoes, Lightning, Heat and Cold
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Tornadoes, Lightning, Heat and Cold
Natural Disasters, 8th edition, Chapter 10
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 2009, U.S. had 96 weather-related disasters
causing more than $1 billion (each) in damages
• Total more than $700 billion
Cold and Nor’Easters
•
•
Hypothermia can set in if our body temperature is lowered to below 95 degrees F (normal
body temperature is 98.6 degrees F). Signs include uncontrollable shivering, slurred speech,
memory loss, incoherence and apparent exhaustion. First step in treatment: warm the body
core! Going the other way, body temperatures above 104 degrees F can be life threatening
(Hyperthermia!). Signs include that can lead to heat stroke include high body temperature,
mental confusion, staggering, strange actions, fainting and dry skin with rapid or slow pulse.
Much of severe weather in mid-latitudes of the N-hemisphere occurs via cyclones: air
masses rotating counterclockwise about a low pressure core. Some of these large-scale
cyclones are linked to troughs in the jet stream. In NE US and Canada theses cyclones can
create powerful storms called nor’easters. Moving up the eastern US and Canadian
coastline, this low pressure system (with CCW rotation) can bring different air masses into
contact: warm-moist air off the Atlantic; and on western (or landward) side, can draw cold
air down from the north. COLLISION of COLD ARCTIC AIR and WARMER OCEANIC
AIR can create powerful nor’easters.
Winter Storms
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 10.5
Winter Storms
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
Winter Storms
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
Winter Storms
Figure 10.4
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
Winter Storms
Lake-Effect Snow
• Common along eastern shores of Great Lakes
• Cold air moves over warmer waters
• Great quantities of snow along shoreline
• Most likely during late fall and early winter
• Also occurs elsewhere around the world
How Thunderstorms Work
• 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 Thunderstorms Work
Figure 10.12
How Thunderstorms Work
• 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 Thunderstorms Work
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 Thunderstorms Work
• 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 10.13
Air-Mass vs. Severe Thunderstorms
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
Figure 10.15
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
• Lifting mechanisms:
convection (air-mass Tstorms), frontal (severe Tstorms), orographic and
convergence.
Figure 10.16
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
Figure 10.19
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
How Tornadoes Form?
• 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
Figure 10.21
Tornadoes
How Tornadoes Form?
• 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
How Tornadoes Form?
• 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
Figure 10.27
Figure 10.28
Tornadoes
Tornadoes in the United States and Canada
• Enhanced Fujita wind damage scale
– EF-0, minor damage, up to EF-5, incredible damage
Insert Table 10.3 here
Tornadoes
Tornadoes in the United States
and Canada
• Three main destructive actions:
Insert
Table 10.5 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
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
The Super Outbreak, 3-4 April 1974
Figure 10.32
• 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
• 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
Lightning
Lightning
• Leading cause of forest fires, major cause of weatherrelated deaths
• Lightning distribution same as thunderstorm distribution
Insert revised figure 10.38 here
Figure 10.38
Lightning
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 10.40
Lightning
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
Lightning
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
Lightning
• 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 10.42
Heat
• Heat waves are invisible, silent killers.
• Heat waves have been the biggest killer of all
severe weather in U.S. for the past 30 years. Read
about heat wave in Chicago (1995), Europe (2003)
• Problems with heat are especially extreme in cities
due to urban heat islands.
End of Chapter 10