Transcript Tornadoes

THUNDERSTORMS, LIGHTNING and
TORNADOES
Thunderstorm Frequency
Average Days of Thunderstorms per Year
1500 to 2000 thunderstorms occur
Thunderstorm
Frequency
around the world
at any given
moment
FORMATION OF A CUMULONIMBUS
Thunderstorms are created when large
parcels of warm, moist air rise into
cooler air
A cold front or convection typically
causes the warm air to rise and water
vapor to condense.
All thunderstorms have 3 stages:
Ordinary Thunderstorms
• Ordinary thunderstorms
are not associated with
fronts or severe
weather….they typically
form on hot, humid
afternoons due to
convection
• These storms are like
mini-heat engines…they
need warm, moist air to
keep them alive…if that
supply is cut off, the
storm dies :(
• TS Video
Life cycle of an ordinary thunderstorm
Cumulus stage
Mature stage
Dissipating stage
Cumulus Stage
As a cumulus cloud builds,
water vapor condenses and
releases heat which keeps the
rising air inside the cloud
warmer than environment.
At this point, there is no time
for precipitation to fall because
updrafts (which may reach
100mph!!) keep water droplets
and ice crystals suspended
within the cloud
The building cumulus takes on
the appearance of a ‘tower’
Mature Stage
As the cloud builds higher than
the freezing level, cloud droplets
grow. Eventually, the rising air
can no longer keep the particles
suspended and they begin to fall.
Some raindrops evaporate on the
way down, which cools the air.
The cooler air begins to descend
as a downdraft.
The appearance of the downdraft
marks the beginning of the
mature stage.
Mature Thunderstorms
Large, mature
thunderstorms can…
• Produce precipitation at
astonishing rates, up to 14
inches/hr. This causes local
flooding.
• Create hailstones as large as
grapefruits! Ouch!
• Generate tremendous
electrical power and intense
lightning.
• Produce tornadoes with
devastating swirling winds
Thunderstorm Structure
updraft
Dissipating Stage
Occurs when updrafts weaken
and downdrafts dominate
throughout much of the cloud.
The reason ordinary T-storms
do not last very long is the
downdrafts cut off the storm’s
fuel supply by destroying the
humid updrafts.
T-storms produce summer
rainfall for a large portion of the
US, and provide momentary
cooling during a hot day.
TS Formation
Severe and Supercell Thunderstorms
Severe Thunderstorms
• If convection is strong
and updrafts are intense,
the storm can become
severe.
• Updrafts in severe
•
thunderstorms can cause
the cloud to reach into
the stratosphere and in
some cases extend up to
60,000 ft.
These storms contain
lightning, thunder, gusty
winds, and hail
Severe Thunderstorms
• Strong updrafts can keep
hailstones suspended in
the cloud long enough for
them to grow to immense
sizes!
• Once they become large
enough they either fall
out the bottom of the
cloud within a downdraft
or a strong updraft may
toss them out the side.
• Aircraft have encountered
•
hail in clear air several
miles away from a storm.
Severe TS
The Supercell Thunderstorm
• When winds become
stronger aloft, it causes
the entire storm to
rotate...
• …and the thunderstorm
may grow into a larger,
long-lasting (longer than
an hour), severe storm
called a Supercell.
• The rotation of supercells
•
can lead to the formation
of tornadoes.
Supercells
Rotating wall cloud
Extreme Low pressure
inside the Supercell
causes a lowering of
the cloud base, called a
Wall Cloud.
A wall cloud
associated with a
supercell
thunderstorm
spins
counterclockwise
The first
sign the
supercell
may form a
tornado is
the
formation of
a wall cloud.
Supercell Miami Tornado
A supercell thunderstorm with a tornado sweeps over Texas
Fig. 10-4, p. 268
A dramatic example of a shelf cloud (or arcus cloud) associated
with an intense thunderstorm
Fig. 10-11, p. 271
Mammatus clouds out ahead of a thunderstorm
Conditions for Supercells
Abrupt changes from moist to dry air, called drylines, form in
western TX, OK, and KS in the spring and summer.
The diagram illustrates how cool “cP” air pushes hot and dry “cT”
air over the warm moist “mT” air.
This mixing of different air masses can create supercell storms.
Lightning and Thunder
Lightning may take place within a cloud, from cloud to cloud, from a
cloud to surrounding air, from cloud to ground, or from ground to
cloud….only 20% strike the ground
Ball Lightning
A lightning stroke can heat the air to 30,000°C !!! This extreme
heating causes the air to expand explosively, initiating a booming
sound wave– called thunder.
Lightning occurs when positive and negative charges build up in
different regions of a cumulonimbus cloud or between the cloud and
ground.
The top of the cloud becomes positively charged. The bottom of the
cloud becomes negatively charged
Fig. 10-21, p. 279
The lightning stroke can travel in a number of directions
WNotice that the cloud-to-ground lightning can travel out
away from the cloud, then turn downward, striking the
ground many miles from the thunderstorm.
A cloud-to-ground
lightning flash hitting a
65-foot sycamore tree.
It should be apparent
why one should not
seek shelter under a
tree during a
thunderstorm.
6000 strikes per
minute worldwide!
Lightning is
responsible for more
deaths annually than
tornadoes and
hurricanes combined.
It causes fires and is
often responsible for
urban blackouts
Florida has the most
in the US. Why?
•The 4 marks on the road represent areas where lightning, after striking
a car, entered the roadway through the tires.
•Lightning flattened 3 of the car’s tires and damaged the radio antenna.
•The driver and a 6 year-old were taken to a nearby hospital, treated for
shock, and released.
Tornadoes
Tornadoes
• A tornado is a rapidly
rotating column of air which
rotates around a small area
of intense low pressure and
reaches the ground
• A funnel cloud, is a tornado
which has not reached the
ground.
• Sometimes called twisters
or cyclones, tornadoes can
assume a variety of shapes
and forms which range
from…..
Twisting-rope
tornadoes….ooooh!!
To Cylindrical Shaped
Tornadoes….aaahhhh!!
Tornadoes
• A majority of North American
tornadoes rotate
counterclockwise about a
central area of low pressure.
• The width of most tornadoes
is between 300-2000 ft., but
some are only…..
10 feet wide…
Aww how
cute
…while others…..
…have diameters exceeding 1 mile!....
Like the Waco, TX Tornado
of 1971….whoa!!!
The Fujita Scale
• Winds in
tornadoes are
very
destructive….yah,
no kidding….
• Most have winds
which are 150mph
or less, but the
most powerful
have winds up to
300mph!
On May 3, 1999 a total of 66
tornadoes touched down across the
state of Oklahoma
It was the most prolific tornado outbreak in
Oklahoma history
48 people perished during the outbreak
Hardest hit was Bridge Creek, Oklahoma….
….where an F5 Tornado first touched down….
before moving on to…..
…Moore, Oklahoma where winds of
301mph were recorded
36 of the 48 people who died during the
outbreak that day, perished in this tornado
Total destruction caused by an F5 tornado, which
struck Moore, Oklahoma on May 3, 1999.
Moore, OK Twister
Tornadoes occur most often in the central, Southeastern,
and Midwestern parts of the U.S. Fig. 10-32, p. 288
Tornado Incidence by State
Number of tornadoes reported by each state during a 25-year
period, and the average annual number of tornadoes per 10,000
square miles.
Fig. 10-28, p. 285
Tornadoes are composed of multiple vortices
Below is a powerful multi-vortex tornado with 3 suction vortices.
Fig. 10-31, p. 287
Average number of tornadoes during each month in the United States
Fig. 10-29, p. 285
Table 10-1, p. 287
Features associated with a tornado-breeding supercell
thunderstorm
Fig. 10-38, p. 291
A tornado-spawning supercell thunderstorm. A hook echo
in its rainfall pattern on a Doppler radar screen. The colors
red and orange represent the heaviest precipitation.
Fig. 10-36, p. 290
Graduate students from the
University of Oklahoma use
a portable Doppler radar to
probe a tornado near
Hodges, Oklahoma.
Video
Fig. 10-39, p. 292
Figure 3, p. 295