Transcript Severe Weather

Severe Weather
Thunderstorms, Tornadoes,
Hurricanes, etc…
Thunderstorms
• Thunderstorms are most often
•
•
•
•
•
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 this
condition
This produces an UNSTABLE
environment
The process of cooling,
condensing, cloud formation,
saturation and precipitation
occurs quickly
Cold fronts often create the
proper conditions for this to occur
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 too)
– 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 believed to be
•
•
•
created by friction between
the rising updraft and the
falling downdraft in a
cumulonimbus cloud
This friction creates a
separation of charges (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 that 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
The air surrounding that
bolt of electricity expands
rapidly (remember… warm
air expands), and then
contract quickly as it cools.
Because of the extremely
high temperatures
involved, the air expands
and contracts rapidly
enough to break the sound
barrier (767 mph)
The thunder is a “sonic
boom”
Wind & Downdrafts
• What goes up, must come
•
•
•
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 40,000 feet,
so…
It can fall very quickly
toward the surface,
producing strong winds
(called downdrafts)
Hail
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.
Air streams inward, it is spiraled
upward around the core until it
eventually merges with the airflow
or 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
•
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
•
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
•
•
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
Creates a very unstable
environment
Continued Growth
• Remember that the water
•
•
over the Atlantic Ocean
near Africa is warm (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 the time
and the fuel to become
severe
Tropical Depressions, Tropical
Storms and Huricanes
• Low pressure begins as a Tropical Disturbance
• The low pressure systems that blow out over the waters of
the Atlantic Ocean begin as Tropical Depressions (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 that develop,
depending on their
location
In the Atlantic Ocean
we call them
Hurricanes
In the Pacific Ocean we
call the Typhoons
In the Indian Ocean we
call them Cyclones
They are all basically
the same type of storm
Hurricane Dangers
• The most dangerous
•
•
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
------
------
------
------
------
Tropical
Storm
35 - 63
39 - 73
------
------
------
------
------
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
•
•
•
landfall, the supply of
warm, moist air that
was fueling it is cut off
As the downdrafts and
rain cool the land, the
strength of the updraft
slows
Without an updraft,
the system will
dissipate, and
eventually die out (just
like a thunderstorm)
This may take days
and 100’s of miles