Straight Line Winds
• are responsible for most thunderstorm wind
damage
• winds can exceed 100 mph
• one type of straight line wind is called the
downburst; it can cause damage equivalent to a
strong tornado and can be extremely dangerous
to aviation
• damage from straight line winds is often
incorrectly attributed to a tornado
As thunderstorm downdrafts increase, winds are
pushed along the ground.
Straight line winds in a thunderstorm.
Straight line wind damage to forested land
Analysis of straight line wind damage
Tornado!
What is a tornado?
• A tornado is a violently rotating column of air
extending from a supercell thunderstorm to the
ground. (Only 1 in 1000 thunderstorms becomes
a supercell and only 1 in 6 supercells spawns a
tornado.)
• Tornadoes can stay still, hop/skip places, or
move forward at speeds up to 70 mph. Their
average speed is 30 mph.
• Damage paths can be in excess of 1 mile wide
and 50 miles long. (Longest known path was
219 miles in 1925.)
Where do tornadoes form?
• Tornadoes occur in many parts of the world but
are most frequent in the United States east of
the Rocky Mountains during the spring and
summer months.
• In the Central Plains, thunderstorms develop
along a "dryline" as it moves east during the
afternoon. This area is called Tornado Alley.
• Tornadoes may form as air near the ground
flows "upslope" toward higher terrain.
• Tornadoes occasionally accompany tropical
storms and hurricanes that move over land.
40% of all US tornadoes occur in
Tornado Alley
The sun heats dry air more. Winds push the dry air
eastward into the humid air during the afternoon. A dry line
separates the two air masses. Most tornadoes occur
between 3 and 9 PM
A tornado usually forms in the wall cloud,
the dark base cloud, of a supercell.
Tornado Formation: Step 1
Cumulonimbus clouds often
grow to over 40,000 feet. Some
even pop through the
tropospause to the
stratosphere! A column of warm,
humid air rises very quickly.
How the column of air begins to
rotate is not completely
understood by scientists, but
one way the rotation appears to
happen is that winds at two
different altitudes blow at two
different speeds.
Tornado Formation: Step 2
If this column gets
caught in an updraft,
the column’s spin
tightens and it
speeds up (much
like a skater spins
faster when arms are
pulled close to the
body).
A funnel cloud is
created.
Tornado Formation: Step 3
The rain and hail in
the thunderstorm
cause the funnel to
touch down, creating
a tornado.
Radar Hook Echo
Tornado!
Tornado Strength
• Weak Tornadoes
– 69% of all tornadoes
– Less than 5% of tornado deaths
– Lifetime 1-10+ minutes
– Winds less than 110 mph
Tornado Strength
• Strong Tornadoes
– 29% of all tornadoes
– Nearly 30% of all tornado deaths
– May last 20 minutes or longer
– Winds 110-205 mph
Tornado Strength
• Violent Tornadoes
– Only 2% of all tornadoes
– 70% of all tornado deaths
– Lifetime can exceed 1 hour
– Winds greater than 205 mph
• New computer models suggest that the winds at
the surface could exceed the speed of sound.
• One probe sent into a tornado registered an air
pressure drop of 100 mb!
Damages
• cause an average of 80 deaths and 1500
injuries each year
• most fatalities occur because people do
not leave mobile homes and automobiles
when told to do so
• cause $400 million in property damages
each year
Why you should leave when told to.
Why you should leave
your car!
National Weather Service
Announcements
• TORNADO WATCH: Tornadoes are
possible in your area. Remain alert for
approaching storms.
• TORNADO WARNING: A tornado has
been sighted or indicated by weather
radar. (Warning time averages only 13
minutes.)
Tornado Safety
• Move to a basement.
• Get in the bathtub with mattress or cushions on
top.
• If an underground shelter is not available, move
to an interior room or hallway on the lowest floor
and get under a sturdy piece of furniture.
• Stay away from windows.
• Get out of automobiles.
• Do not try to outrun a tornado.
• Abandon mobile homes.
Tornado Safety in School
• Have drills for your severe weather action plan.
• Schools without basements should use interior
rooms and hallways on the lowest floor and away
from windows as tornado shelters.
• Use air horn or megaphone in case of power
failure.
• Turn off electricity and gas if school is damaged.
• Move students out of gymnasiums, cafeterias,
and auditoriums.
Tornado Safety in School
• Students should assume the tornado protection position.
This map shows the number of days of thunderstorm
activity. NJ has 10 – 20 such days.
This map shows the average number of tornadoes. NJ has
one or fewer.
Waterspouts
• Waterspouts are weak tornadoes that form
over warm water.
• Waterspouts are most common along the
Gulf Coast and southeastern states.
• Waterspouts occasionally move inland
becoming tornadoes causing damage and
injuries.
Waterspout
May 9, 2012
Grand Isle, LA
• http://www.weather.com/weather/videos/ne
ws-41/top-stories-169/funnel-cloud-scaresgraduation-ceremony-29066
Top 10 Twisters in History
•
•
•
•
•
•
•
•
•
•
March 18, 1925
May 7, 1840
May 27, 1896
April 5, 1936
April 6, 1936
April 9, 1947
April 24, 1908
June 12, 1899
June 8, 1953
May 11, 1953
Update: On May 23,
2011, the Joplin,
Missouri tornado killed
117 people, making it
the 8th worst in US
history.
Joplin, MO tornado May 23, 2011
One Year Later
Tornado Tidbits
• A tornado lifted a child and put him down
unharmed in a tree.
• A plane was carried 35 miles. This is the
heaviest piece of debris ever carried.
• A check was found 305 miles away, the longest
distance debris has been carried.
• A baby was found unhurt on her mattress under
a wall of her wrecked home.
• Horses hitched to a rail were found ¼ mile away
unhurt and still hitched to the rail.
• Sometimes it rains fish, frogs, snails or
salamanders.
I’ll Get You!