Transcript Severe Weather Thunderstorms, Tornadoes, Hurricanes, etc…

Severe Weather
Thunderstorms, Tornadoes,
Hurricanes, etc…
Thunderstorms
• Thunderstorms are most often
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associated with a Cumulonimbus
Cloud
This is a cloud that forms when
air is forced high into the
atmosphere (against its will)
Cold fronts often create the
proper conditions for this to
occur
This produces an UNSTABLE
environment
The processes of cooling,
condensing, cloud formation,
saturation and precipitation
occur quickly
Thunderstorm Formation
• Thunderstorms have three stages…
– The Cumulous Stage: Warm air rising and condensing to form
clouds
– The Mature Stage: Strong updraft of warm, moist air, AND a
strong downdraft of wind and precipitation (hail and lightning can
occur in this stage)
– The Dissipation Stage: The surface has been cooled, cutting
off the warm, moist air that “fuels” the storm. Once the
precipitation has fallen from the sky, the storm is over
Lightning
• Lightning is created by
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friction between the rising
updraft and the falling
downdraft in a cumulonimbus
cloud
This friction creates a charge
(like rubbing your feet across
the carpet in your socks)
Eventually there is a discharge
between positive and negative
charges, creating a “spark”
(like touching a doorknob and
getting a shock)
On a larger scale, that “spark”
is a bolt of lightning
Thunder
• When lightning strikes, the
“bolt” can be as hot as 30,000
degrees Celsius.
• The air surrounding that bolt
of electricity expands rapidly
(remember… warm air
expands), and then contracts
quickly as it cools
• Because of the extremely high
temperatures involved, the air
expands and contracts rapidly
enough to break the sound
barrier (1234 Km/h or 767
mph)
• The thunder is a “sonic boom”
Wind & Downdrafts
• What goes up, must come
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down!
Thunderstorms are no
different, when warm air
rises high into the
atmosphere, it eventually
cools and comes back down
toward the surface
Cumulonimbus clouds can
be as tall as 12,000 m so…
It can fall very quickly
toward the surface,
producing strong winds
(called downdrafts)
Hail
• Moisture gets drawn up so high it freezes.
• The longer the hail is above the freezing
line the bigger it gets.
Tornadoes
Thought to be the 1st photograph of a tornado (1884)
Tornadoes
• Violent windstorms that take
the form of a rotating column
of air, or vortex, that extends
downward from a
cumulonimbus cloud.
• Because of the lower pressure
in the center vortex, air near
the ground rushes into the
tornado from all directions.
• As air streams inward, it is
spiraled upward around the
core until it eventually merges
with the airflow of the parent
thunderstorm deep in a
cumulonimbus tower.
Tornadoes
• Some tornadoes consist of a
single vortex.
• However, within many stronger
tornadoes are smaller intense
whirls known as suction
vortices, that orbit the center of
the larger tornado → Multiple
Vortex Tornadoes.
– Usually die out in less than a
minute.
Tornadoes
• Suction vortices are
responsible for most
of the narrow, short
waves of extreme
damage that
sometimes are
through tornado
tracks.
– Most reports of several
tornadoes at once
actually were multiple
vortex tornadoes.
Supercell Structure
Tornado Development
• Less than 1% of
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thunderstorms produce
tornadoes.
Most intense tornadoes
are associated with
supercells (big,
cumulonimbus cloud
structures).
• Tornado formation begins
with the development of
a mesocyclone.
– Mesocyclone - Vertical
cylinder of rotating air
that develops in the
updraft of a severe
thunderstorm.
Tornado Development
Tornado Development
• The mesocyclone within
the cloud stretches
vertically and narrows
horizontally, causing
winds speeds to
accelerate.
• Air stretches downward
until a portion of the
cloud protrudes below the
cloud base, producing a
dark, slow rotating wall
cloud.
*See supercell diagram for
location.
Tornado Classification
• Fujita Intensity Scale–
Assesses the damage produced
by a storm as it relates to wind
speed.
• F0- Moderate; 40-72 mph
• F5- Severe; 261-318 mph
– Path of Destruction
– Duration
• Problem: Doesn’t take into
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account for structural integrity of
objects.
The ENHANCED Fujita Scale (EF –
Scale) is now used in the US. It
DOES take into account structural
damage (but still uses a 0-5
numerical scale)
Watches & Warnings
• Tornado Watch
– Conditions are ideal for
a tornado to be
created.
• Tornado Warning
– An actual tornado has
been sighted in the
area or is indicated by
weather radar.
Tornado Frequency in the US
Tornado Frequency Around the
World
Cool Tornado Pictures
Hurricanes
Hurricane Formation
• The hurricanes that
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strike the Caribbean,
Gulf of Mexico and the
US originate in Africa
They begin as low
pressure storm
systems over land
The Trade Winds (one
of the systems of
global winds) blow
them out over the
warm water of the
Atlantic Ocean
Hurricane Growth
• Remember that low
pressure pulls air in at the
surface and pushes it to
higher levels in the
atmosphere
• Air temperatures at high
levels in the atmosphere
are colder, and the warm,
moist air over the ocean
begins to form clouds (BIG
Cumulonimbus clouds)
• Lots of Latent Heat
released during
condensation
• This creates a very
unstable environment
Continued Growth
• Remember that the water
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over the Atlantic Ocean
near Africa is warm (27° C
80°F) and has lots of
moisture in it (mT air
masses)
As precipitation begins, the
rain and cool downdrafts of
the Cumulonimbus system
are not enough to cut off
the updraft (it’s too warm)
The warm, moist air
continues to fuel the system
of Cumulonimbus clouds,
making them bigger, and
bigger, and bigger
Hurricane Movement
• Once the growing low
pressure system is out
over the warm water of
the Atlantic Ocean, the
Trade Winds blow it to
the west
• Because the oceans are
relatively smooth, there
isn’t much to stand in its
way and slow it down
• With 2,000 to 3,000 miles
of warm ocean water to
move across, the system
has plenty of time and
the fuel to become severe
Tropical Depressions, Tropical
Storms and Hurricanes
• A low pressure system (disturbance) that blows out over
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the ocean is a Tropical Depression (a low pressure system
is in fact a “depression” of pressure)
If winds in the system reach 39mph, the system becomes
classified as a Tropical Storm (and it is given a name)
If the system continues to gain strength and winds speeds
reach 74 mph, it is officially classified as a hurricane
Hurricanes, Typhoons and Cyclones
• We give different names to Tropical Storms depending on
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their location
Atlantic Ocean - Hurricanes
Pacific Ocean - Typhoons
Indian Ocean - Cyclones
– They are all basically the same type of storm
Hurricane Dangers
• The most dangerous
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component of a hurricane is
the “Storm Surge”
As the storm makes landfall,
a mound of ocean water
(driven by strong winds AND
the “sucking” power of the
strong low pressure system)
is pushed on shore
More people die as a result
of drowning, than do of
strong winds
Hurricane Classification
The Safir-Simpson Scale
Type
KNOTS
MPH
millibars
inches of
mercury
Feet
Meter
s
Damage
Depression
less than
34
less than
39
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Tropical
Storm
35 - 63
39 - 73
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Category 1
64 - 82
74 - 95
greater than
980
greater than
28.94
3-5
1.0 1.7
minimal
Category 2
83 - 95
96 - 110
979 - 965
28.50 - 28.91
6-8
1.8 2.6
moderate
Category 3
96 - 113
111 - 130
964 - 945
27.91 - 28.47
9 - 12
2.7 3.8
extensive
Category 4
114 - 135
131 - 155
944 - 920
27.17 - 27.88
13 18
3.9 5.6
extreme
Category 5
136+
156+
less than 920
less than 27.17
19+
5.7+
catastrophic
A Hurricane’s “Death”
• As a hurricane makes
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landfall, the supply of
warm, moist air that
was fueling it is cut off
The downdrafts and rain
cool the land,
decreasing the strength
of the updraft
Without an updraft, the
system will dissipate and
eventually die out (just
like a thunderstorm)
This may take days and
100’s of miles
Hurricane Katrina
• In August of 2005, Tropical
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Storm Katrina developed over
the Bahamas
As it approached Florida, it
developed into a Category I
hurricane
It lost some strength over the
land (turned back into a
Tropical Storm), but picked
right back up when it moved
back over the warm water of
the Gulf of Mexico
Hurricane Katrina
• As the storm moved
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across the Gulf of Mexico,
it rapidly developed
(from a Category II to a
Category V in only 9
hours)
Wind speeds reached 175
mph
A wall of water was
pushed toward Louisiana
and Mississippi
Flooding
• The storm surge
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associated with Katrina
was estimated to be 28
feet above sea level
In addition, much of
New Orleans is BELOW
sea level
A systems of levees and
dams were supposed to
protect the city, but this
was too high
85% of the city flooded
The Aftermath
• The most destructive and
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costliest natural disaster in
the history of the US
More than $81 billion
dollars in damage
More than 1800 people died
(700 still listed as missing)
More than 90,000 square
miles declared a Federal
Disaster Area
More than 3 million people
were left without power
What We’ve Learned…