Transcript Thunderstorms and Tornadoes

Tornadoes
Chapter 14B
Tornadoes
• Tornado - rapidly rotating
column of air that blows around a
small area of intense low
pressure with a circulation that
reaches the ground.
• Funnel cloud- tornado
beginning to form but has not
reached the ground. 30% of
funnel clouds become tornadoes
https://www.youtube.com/watch?v=7KDz6dGQ
5RE
Tornadoes
– Most rotate counterclockwise
around low pressure in North
America, with wind speeds less
than 100 knots (115mph),
– Can have speeds above 220 knots
(253mph)
– https://www.youtube.com/watch?v=
cvOut9VUqKY
Tornadoes
• Tornado life cycle
– Dust-whirl stage- dust swirling upward from
the surface marks the tornado’s circulation on
the ground, short funnel from thunderstorm
• damage is light
– Organizing stage- tornado increases in
intensity with downward extend of funnel
– Mature stage – funnel reaches greatest width
and is almost vertical
• damage is most severe
Tornadoes
• Tornado life cycle
– Shrinking stage – decrease in funnel width
and increase in tilt, narrowing of damage at
the surface (can still have intense damage)
– Decay stage – tornado shaped like a rope,
twisted/contorted, then dissipates.
• Tornadoes may not necessarily reach all stages
and some can skip stages.
• After mature stage, usually stays in contact with
ground until it dissipates.
Tornadoes
• Tornado Occurrence
– US experiences the most tornadoes in the world, 1000
annually. Can occur in all states, even Hawaii and
Alaska.
– Most occur in Tornado Alley, which stretches from
central Texas to Nebraska.
– Central Plains has warm, humid surface air and cold dry
air aloft. Large supercell thunderstorms can form with
strong wind shear that produces tornadoes
– Highest in spring, lowest in winter, because of warm,
humid surface air. Most frequent in late afternoon
because air is most unstable.
Tornado Alley, with highest incidence of tornadoes.
Figure14-43 p407
Tornadoes
• Tornado occurrence locations
– Depends on polar jet stream
– In winter, jet stream is above southern Gulf
states, where contrast between warm and
cold air masses is greatest
– In spring, humid Gulf air and jet stream
moves northward toward Great Plains.
– In summer, jet stream is near Canadian
border, more tornado activity northern plains
and New York
Tornadoes form due to cold, dry air meeting warm, humid air,
due to polar jet stream. As the jet stream moves with seasons,
the locations of tornadoes move with it.
Tornado incidence by month.
Average number of
tornadoes during each
month in the United
States from 2000 to
2010. (NOAA)
Figure14-44 p408
Tornadoes
• Tornado winds
– Measurement based
upon damage after
storm or Doppler radar
– Most studies have
shown that severe
tornadoes have 220
knots maximum speed,
with most having less
than 125 knots.
Tornadoes
– Multi-vortex
tornados have
smaller whirls rotating
within, called suction
vortices that are only
30ft in diameter but
are very fast and can
cause great damage.
Figure14-45 p408
The total wind speed of a tornado is greater on one side than
on the other. When facing an onrushing tornado, the strongest
winds will be on your left side.
Figure14-46 p409
Tornadoes
• Seeking shelter
– Basement or small, interior
room on ground floor, mattress
can reduce injury from flying
debris.
– Tornado has very low pressure
at center, which can pull the
roof and walls off of a
structure.
– Windows are often shattered
which increases pressure.
– If outside, lie in a ditch or
ravine.
Tornadoes
• Tornado watch- tornado is likely to form in the next few
hours. Often there are trained volunteer spotters who look
for tornadoes.
• Tornado warning- tornado is spotted, visually or on radar
screen.
• Tornado sirens or radio and television warnings can alert
population.
Fujita Scale
• Enhanced Fujita
Scale
– Based upon the
wind speeds and
consequent
damage of storm
– EF0 weakest, EF5
strongest
– 1-2 EF5s annually
in U.S.
Table 14-3 p411
Tornadoes
– Example of an EF5
tornado in Joplin,
Missouri on May 22,
2011, injured 1000
people and killed 159,
the greatest death toll
from a single tornado
since 1947.
– Today show coverage
of Joplin tornado:
https://www.youtube.co
m/watch?v=vn55m56tQ
6s
Tornadoes
• Yearly average of tornado deaths is
less than 100, although over 100 can
die in a single day, as in Joplin in
2011.
• 45% of all fatalities occur in mobile
homes.
• Deadliest tornadoes occur in
families, when different tornadoes
are spawned by the same
thunderstorm.
• When 6 or more tornadoes occur
over a particular region, it is termed a
tornado outbreak.
Tornado Statistics
• Super outbreak- many tornadoes in a short period of time.
• One of the most violent super outbreaks occurred on April
3-4, 1974 in the U.S.
• During a 16-hour period, 148 tornadoes cut through 13 states,
killing 307 people, injuring 6000, and causing $600m in
damage.
• Thirty were F4-F5 (six were F5), with a total destruction path
of 4181 km.
• From this outbreak, severe weather safety training in schools
began to be implemented across the U.S., and the
government began a ‘natural disaster reduction initiative’ to
increase preparedness and awareness. Radar technologies
like Doppler radar were not around at the time.
Tornado Statistics
• Deadliest tornado was March 18,
1925 when 695 people died in
Missouri, Illinois, and Indiana.
• More recent super outbreak was
in 2011 across 3 days where 336
tornadoes broke out across the
southern U.S., hardest hit was
Tuscaloosa, Alabama.
• Despite new technology like
Doppler Radar and improved
warning systems since 1974
super outbreak, tornadoes can
still be very deadly.
Tornado Formation
• Basic requirements for tornado formation are an
intense thunderstorm, conditional instability, and
strong vertical wind shear.
• Supercell Tornadoes
– Supercell thunderstorm: single rotating updraft that
can last for hours.
– Warm humid air is drawn in and spins
counterclockwise as it rises, mixes with dry air and
some evaporates and cools and makes a downdraft,
called a forward-flank downdraft.
Supercell Tornado Formation
• Wind shear
causes spinning
vortex tube that
is pulled into
thunderstorm by
the updraft;
Supercell Tornado Formation
• The tube can
be tilted from
horizontal to
vertical.
Supercell Tornado Formation
• The rising, spinning air is called a
mesocyclone, which lowers the pressure in the
mid-level of the thunderstorm. It can be 5-10km
across.
• Precipitation can also be spun inside
counterclockwise around the updraft, which can
be seen by radar, although there is no
precipitation in the mesocyclone.
• As the precipitation is drawn into a spiral around
the mesocyclone, it can look like a hook, called a
hook echo.
Figure14-54 p414
Tornado
Formation
• Tornadogenesis- the
formation of a tornado
• As the rain-chilled air of the
rear-flanked downdraft
wraps around the updraft,
there is additional spin in the
mesocyclone, and the
updraft at the base rises
more slowly than the updraft
aloft.
Supercell Tornado Formation
• The rising updraft
stretches vertically and
thins horizontally, which
causes the air to spin
faster (centripetal forcelike the skater pulling her
arms in spins faster).
• If it keeps stretching, it
can shrink into a narrow
column of rapidly rising
air – a tornado vortex.
Supercell Tornado Formation
• As the air spins around
the low-pressure core of
the vortex, the air
expands, cools, and
condenses into a visible
cloud- a funnel cloud.
• As the air is drawn into
the core, the air cools and
condenses and the funnel
cloud descends.
• The cloud can then reach
the ground and pull in dirt
and debris.
Supercell Tornado Formation
• The air outside the
tornado is spiraling
upward, but the air inside
the tornado descends
down and warms, causing
evaporation.
RFD: rear-flank downdraft
FFD: forward-flank downdraft
Tornado Formation
• Supercell Tornadoes
– Less than 15% of supercells produce tornadoes.
– First sign of tornado is the sight of rotating clouds.
– The rotating cloud can lower and become a wall
cloud.
– Sometimes the air is dry so the funnel remains
invisible until it hits the ground and picks up dust.
– Some tornadoes sound like a freight train, while
others can be silent.
Figure14-55 p415
Non-supercell Tornadoes
• Nonsupercell tornadoes
– Tornadoes not associated with a pre-existing wall
cloud in a supercell are called nonsupercell
tornadoes.
– Some extend from a thunderstorm and others can
build upward from the ground.
– When formed along a gust front it is called a
gustnado.
– Waterspouts are connected to a cumuliform cloud
over a large body of water.
– Weak, short-lived tornadoes that rapidly build with
cumulous congestus clouds are landspouts.
Figure14-57 p418
Figure14-58 p418
Figure14-59 p418
Observing Tornadoes and Severe
Weather
• Doppler radar measures the speed of
precipitation toward and away radar unit
– Mesocyclones and tornadoes have a unique
image on the radar display- a tornado vortex
signature.
• NEXRAD
– 150 Doppler radar units within the continental
U.S.
– Algorithms to give warning of tornado.
Figure14.60
Doppler radar
display of winds.
The close packing
of the horizontal
winds blowing
toward the radar
(green and blue
shades), and those
blowing away from
the radar (yellow
and red shades),
indicate strong
cyclonic rotation
and the presence of
a tornado.
Figure14-60 p419
Figure14.61
Doppler
radar display
showing
precipitation
inside a large
supercell that
takes on the
shape of a
hook.
Figure14-61 p419