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Announcements
• No office hours tomorrow (Wed. March 30)
• Homework 5 extra credit:
– Find and plot additional tracks of Hurricane
Ivan (after reaching its northeastmost point in
U.S)
Lightning and Tornadoes
North Dakota
Outline
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Finish lightning/thunderstorms
Tornado Facts
Formation and Conditions Needed
Damage Levels
U.S. Occurrences
Examples
Electrification of Clouds
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Generally upper level of clouds - + charge
Mid level - negative charge
Lower level - mix of charge
Why? Transfer of positive ions from warm
to cold objects in collisions between ice
droplets, water droplets
Upper level +
Mid level Lower level mix
Note positive
charge on high
objects near
cloud negative
charges
Lightning Strike
• Unlike charges attract each other
– Negative charges in cloud lead to positive
charges on ground
– Dense on high objects (trees, poles)
• Set up electric potential between cloud and
ground
• When large enough, current flows lightning
Lightning Facts
• Several strokes occur over ~1/2 second
• Travels over 6,000 miles/second
• Can produce very high temperatures
(55,000 F) briefly
– High temps cause air to expand, produce sound
wave (thunder)
Deaths
• ~143 deaths per year in U.S.
• Ways to avoid
– Stay inside house, car, truck
– Outside, move to low place
Winds
• Can be straight-line or rotating (tornadoes)
• Straight-line winds can be 80-100 mph
– Damage structures, trees
– Duration can be hours
What are tornadoes?
• Rapidly rotating winds usually descending
from thunderstorm
• Most violent storms with highest
windspeeds
• Average U.S. fatalities ~100/year
• Common in Great Plains
Tornado Facts
• Typically ~100-600 m wide
– Some up to 1 mile wide
• Speeds
– Travel up to 62 mph (100 km/hr)
– Wind speeds up to 300 mph (rare, but most
deadly)
Formation
• Some details still unknown
• Tend to form with severe thunderstorms
• Appear to require additional component to
cause shearing of thunderstorm cloud
Conditions
• Some same as needed for hailstorms
– Northward flow of warm, moist air (Gulf of
Mexico)
– Cold dry air from Canada or Western U.S.
• Jet stream - key component for spinning
the cloud
– High enough that it starts top of cloud
spinning, rest of cloud pushed by mid-level air
motion
Key
Components:
Warm moist air
Cold dry air
Jet stream
Differences between Systems
• Single cell
– Updrafts of warm air, downdrafts (rain) from
condensing, cooling air
Differences between Systems
• Supercell
thunderstorm
– Updrafts on leading
side: rain
– Downdrafts on
trailing side:
rotating air packets
that can form funnel
clouds (tornadoes
after hit ground)
– Many have centers
with rapidly
upflowing air (sucks
up material)
Importance of Rotation
• Not Coriolis force issue (feature too small
to be affected)
• Once rotation starts, pulls into tight spiral
– Speed increase
– Smaller diameter, faster it spins
Structure of tornado
• Can be horizontal or vertical
• Winds vary within cloud
– Highest a few 100 ft above ground
• Objects (trees, buildings, etc) cause friction, slows
wind speed close to surface
Destruction from Tornadoes
• High speed winds
– Damage to trees, buildings, etc
• Lifting force into funnel
– Vehicles, people, other loosely attached objects
• Explosions due to extreme changes in air
pressure
– Very low pressures inside funnel, higher
pressures outside the funnel
Measuring Damage
• Fujita Scale
– Developed in 1060s
– Based on wind speed damage
Fujita Scale
Category F0: Light Damage (<73 mph); Some damage to chimneys; branches broken off trees; shallowrooted trees pushed over; sign boards damaged.
Category F1: Moderate Damage (73-112 mph); Peels surface off roofs; mobile homes pushed off
foundations or overturned; moving autos blown off road.
Category F2: Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homes
demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated;
cars lifted off ground.
Category F3: Severe Damage (158- 206 mph); Roofs and some walls torn off well-constructed houses,
trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.
Category F4: Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure with
weak foundations blown off some distance; cars thrown and large missiles generated.
Category F5: Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations and
swept away; automobile sized missiles fly through the air in excess of 100 meters (109 yards);
trees debarked; incredible phenomena will occur.
U.S. Occurrences
• For given year (1992) there were
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696 F-0
411 F-1
129 F-2
43 F-3
13 F-4
1 F-5 ** Note that these are very rare events
Where and When are Tornadoes
Common in the U.S.?
• Where: interior of U.S. is tornado capital of
the world
– Why? Conditions there are great for formation
Thunderstorms
Tornado map doesn’t
quite match the
thunderstorm
occurrence map, but is
close to the hailstorm
map. Why?
Need cold dry air!
hailstorms
Deaths per state, 1950-1994
Where and When are Tornadoes
Common in the U.S.?
• When: late spring, early
summer
– Varies, peak moves north
during summer
– Why? Conditions right during
those months (key jet stream
positioning)
Fatalities
• Common locations
– Mobile homes
• No interior rooms for protection
• Weaker coupling to ground
– Exterior rooms with windows
• Not as much protection as solid walls
Best Places to Ride out a
Tornado
• Basement
• Interior closet, bathroom, hallway on
lowest floor of a building
• If outside, find low point (ditch, streambed)
Predictions and Warnings
• # of deaths/decade is decreasing
Why?
Prediction and Warning
• Less fatalities partially
from building design
• Partially from
development of warning
systems
– Tornado watch: conditions
are right for events
– Spotters, weather radar
look for tornadoes
– Warning issued after
touchdown spotted
• Sirens
• Radio/TV broadcasts
• Can provide minutes of
warning
Tri-State Tornado
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March 1925
Largest known single tornado (F-5)
Lasted over 3.5 hours
Traveled 219 miles through MO, IL, IN
Swath of ~1 mile wide (widest recorded)
689 fatalities
Murphysboro:
most fatalities
Why so
many
fatalities?
No
warning
system
1974 Super-Outbreak
• Weather conditions collide
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Cold front from Rockies
Low pressure moving east
Humid air over Gulf of Mexico
Polar jet stream down into TX
Dry air mass from SW moving towards low pressure
• Form inversion layer: dry air above moist air
1974 Super-Outbreak
• Moist air breaks through to form huge thunderclouds
• Set spinning by jet stream, converging air masses
• 16 hours, 147 tornadoes in 13 states
– 6 were F-5 tornadoes
– 24 were F-4
F-5 Ohio
F-4 Indiana
Ohio subdivision after 1974 Outbreak
Recent
• Tornado Activity in 2005 (from National Weather
Service)
• Jan 13, 2005 The 2005 tornado season got off to an early
start. A destructive F3 tornado moved across 20 miles of
Union County, Arkansas from Junction City to Lawson.
Two people were killed, both 83 years old, in separate
mobile homes about a mile apart. About 30 homes were
destroyed.
• January 13, 2005 In a different storm, a married couple
died at Arlington, Early County, Georgia, when their
mobile home was destroyed.
Only in rural areas?
• NO!
• Can form in cities
– Although they are small targets
• Recent (late 1990’s-2000) events include
– F-5 in Oklahoma City (42 fatalities), F-4 in
Wichita, KS; Cincinnati, OH (6-7 fatalities
each)
Outbreak in
Oklahoma had ~59
tornadoes
Remote sensing
satellite data shows
swath hit by
tornado near OK
City
Waterspout
• Rotating column of air over large body of
water
– Can be a tornado that formed over land and
then traveled over water
– Some form over water: “fair weather”
waterspouts
• Generally smaller diameter than average
tornado, move more slowly, shorter
duration
Next Time
• Hurricanes Part 1