Transcript Tornadosx

Tornados
Understanding severe weather is critical for pilots
and people who live in tornado prone areas
Ted Spitzmiller
Tornados (twisters)
• A violent, dangerous, rotating column of
air in contact with both the surface of
the earth and a cumulonimbus cloud
• Come in many shapes and sizes
– Typically a form of visible condensation
funnel
– Narrow end touches the earth and
encircled by debris cloud and dust
• Most tornadoes wind speeds <110 mph
– Approximately 250 feet across
– Travel a few miles before dissipating
• Some attain wind speeds of 300 mph
– Stretch more than two miles across
– Stay on the ground for dozens of miles
Tornados Winds Destroy
Size
• 500 feet across on average
• On ground for 5 miles
• Weak tornadoes, can be narrow,
sometimes only a few feet
• One reported a damage path only
7 feet wide
• Wedge tornadoes can have a
damage path a mile wide or more.
• Hallam, Nebraska 2004, was
2.5 wide at the ground
• Path length—Tri-State Tornado,
(Missouri, Illinois, and Indiana) 1925,
on ground 219 miles
• Many which appear to have path
lengths of 100 miles or longer are
composed of a family of tornadoes
formed in quick succession
Types of Tornadoes
• Landspout, Multiple vortex ,
and Waterspout.
• Waterspouts are a spiraling
funnel-shaped wind current
develop over bodies of
water
• Frequently in tropical areas
close to equator, less
common at high latitudes
• Other tornado-like
phenomena include the
gustnado, dust devil, fire
whirls, and steam devil.
Tornadoes On Every Continent Except
Antarctica.
• Vast majority occur
in Tornado Alley
region of US
• Can be detected
before or as they
occur using PulseDoppler radar
• Recognized patterns
in velocity and
reflectivity data
• Hook echoes
Scales for Rating Strength
• Fujita scale rates tornadoes by damage
caused
• Some countries use Enhanced Fujita
Scale
• F0 or EF0 tornado, weakest category,
damages trees, but not substantial
structures
• F5 or EF5 tornado, strongest category,
rips buildings off foundations and can
deform large skyscrapers.
• Doppler radar data, photogrammetry,
and ground swirl patterns (cycloidal
marks) may also be analyzed to
determine intensity and assign a rating
Etymology (Origin of words)
• Word tornado altered form of Spanish word tronada—
"thunderstorm"
• Taken from Latin tonare, meaning "to thunder“
• Commonly a "twister", or old-fashioned colloquial term cyclone
• “Cyclone" a synonym for "tornado" in 1939 film, The Wizard of Oz.
Definitions
• For a vortex to be classified as a tornado,
it must be in contact with both the ground
and the cloud base
• Tornado refers to the vortex of wind, not
the condensation cloud
Funnel Cloud
• A tornado is not necessarily visible
– intense low pressure causes high
wind speeds (Bernoulli's principle)
and rapid rotation (due to
cyclostrophic balance)
– Water vapor in air to become visible
as a funnel cloud or condensation
funnel
• Funnel cloud term defines any
rotating cloud below a cumuliform
cloud
• Tornadoes often begin as funnel
clouds
• Difficult to discern the difference
between a funnel cloud and a
tornado from a distance
Outbreaks and Families
• Occasionally, a single
storm will produce
more than one
tornado
• Multiple tornadoes
produced by same
storm cell are a
"tornado family"
• Tornadoes look
different depending
on viewing angle
Life cycle
• A sequence of images showing
the birth of a tornado.
• First, the rotating cloud base
lowers.
• Lowering becomes a funnel,
which continues descending
while winds build near the
surface, kicking up dust and
other debris.
• Finally, visible funnel extends
to the ground, and the tornado
begins causing major damage.
Often Develop From Supercells
• Most intense tornadoes (EF3 to
EF5) develop from supercells.
• Very heavy rain, frequent
lightning, strong wind gusts,
and hail are common
• Most tornadoes from supercells
follow a recognizable life cycle.
• Begins when increasing rainfall
drags an area of quickly
descending air known as the
rear flank downdraft (RFD)
• Downdraft accelerates as it
approaches the ground
• Drags the supercell's rotating
mesocyclone towards the
ground with it
Formation
• As mesocyclone
approaches the ground, a
visible condensation
funnel appears to descend
from the base of storm,
often from a rotating wall
cloud
• As the funnel descends,
the RFD also reaches the
ground, creating a gust
front that can cause
damage a good distance
from the tornado
• Usually, funnel cloud
becomes a tornado within
minutes of the RFD
reaching the ground
Maturity
• Initially, tornado has a good source
of warm, moist inflow to power it
• It grows until it reaches the
"mature stage“
• Can last anywhere from a few
minutes to more than an hour
• During that time a tornado often
causes the most damage,
• Meanwhile, the RFD, now an area
of cool surface winds, begins to
wrap around the tornado, cutting
off inflow of warm air which feeds
the tornado
Demise
• As RFD completely wraps around and chokes off
tornado's air supply, vortex weakens, becomes
thin and rope-like
• “Dissipating stage"; no more than a few
minutes, tornado fizzles.
• During this stage the shape can be blown into
fantastic patterns
• Even though dissipating, it is still causing
damage.
• Storm contracts into a rope-like tube and, like
the ice skater who pulls her arms in to spin
faster, winds can increase at this point.
• Its associated mesocyclone weakens, as the rear
flank downdraft cuts off inflow powering it.
• If a new mesocyclone develops, the cycle may
start again, producing one or more new
tornadoes.
Keeping Safe
How to Avoid Injury
• Be aware of approaching bad weather
(radio/TV)
• Evacuate the warning area
• Go to local public shelters
• Take shelter in a basement or first floor
bathroom.
• Crouch in the bathtub with a blanket over you