Chapter 8 continued…..
Three types of cyclones:
1. Wave cyclones: cyclone/low
2. Tropical cyclone: hurricane/typhoon
3. Tornado
Local Severe Weather
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Flash Flooding
Lightning
High Winds
Hail
Tornadoes
Air Mass Thunderstorm Life Cycle
Air Mass Thunderstorm Life Cycle
Air Mass Thunderstorm Life Cycle
•Downdrafts spread outward when they hit the ground and converge with
the warmer surrounding air to form the outflow boundary.
Life-Cycle of a Thunderstorm
Total time can be less than one hour
Supercell Thunderstorms
Supercell Thunderstorm over LaPlata, MD April 2002
Inside a Supercell
Radar Image of a Supercell – Hook Echo
Thunderstorms may group together as a
Mesoscale Convective Complex (MCC)
Downbursts
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Deadly Downdrafts
Strong downdrafts may create downbursts
Wind speeds can exceed 270 km/hr (170 mph).
When strong downdrafts reach the surface, they can spread
outward in all directions to form intense horizontal winds, capable
of causing severe damage.
Downbursts with diameters of less than 4 km are called
microbursts
Dangerous problem when they occur near airports.
Geographic Distribution of Thunderstorms
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Thunderstorms develop where moist air is forced aloft
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Occurs frequently in the tropics, nearly daily in some
locations
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In the United States, most frequent region is the Gulf South
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Highest incidence in Florida because it is almost completely
surrounded by warm waters
Average annual number of days with thunderstorms
Flash Flooding
FLASH FLOODING
More persons are killed by flash flooding than
any other form of severe weather.
An average of 135 persons die each year.
Lightning
On average, 44,000 thunderstorms occur
around the world every day.
At any given time, an average of nearly
2000 thunderstorms exist.
Since each thunderstorm will generate an
average of 100 cloud-to-ground lightning
strokes, the total daily number of lightning
discharges around the world is
approximately 4 – 5 MILLION!!
Much of the energy of a lightning
discharge is dissipated through the air
as shock waves that we call thunder
Air heated to a
temperature of
50,000°F in the
conducting channel
Light travels at 300,000
km/sec but sound waves
travel at only 0.33 km/sec
Thus the thunder is heard
after the lightning stroke
Thunder
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Caused by the rapid expansion of air from tremendous increase in
temperature during a lightning stroke
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Lightning without thunder being heard called heat lightning
Thunder is produced but stroke is too far away to reach an observer
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Rumbling thunder is typically caused by sound echoing off topographic
features and buildings
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Slower speed of sound (vs. speed of light) causes a lag between the
stroke and the resulting thunder
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To determine the distance in km, count the seconds between the stroke
and thunder and divide by 3 (divide by 5 to determine distance in miles)
Charge Separation – Initiating Lightning
Leaders, Strokes, and Flashes
Development of Cloud-to-Ground Lightning
Processes of Lightning Formation
(Source: http://thunder.msfc.nasa.gov/primer)
Types of Lightning – Many types exist
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St. Elmo’s Fire
tall objects (e.g., church steeples) glow as
ionization occurs in the air around them
Blue-green tint, hissing sound
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Ball lightning
may be the most unusual type
A round mass of electrified air about as large as a basketball
Rolls through the air or along a surface for 15 seconds or so before
either dissipating or exploding
St. Elmo’s Fire
(Source: http://www.physics.northwestern.edu/classes/2001Fall/Phyx135-2/17/relatedequations.html)
Ball Lightning
(Source: http://www.pharmachip.hu/zyx/images/ball.jpg)
A strong electrical field occurs prior to lightning
Winds
Roll cloud with gust front
Microburst and Wind Shear
HAIL
Tornadoes
Tornadoes Around The World.
The Location and Timing of Tornadoes
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The United States is the world leader in tornadoes!
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Results from the regular interaction between extremely
unlike air masses which originate in very high latitudes and
over the Gulf of Mexico
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Absence of topographic barriers (especially East-West
barriers) ensures regular mixing and the production of
violent storm systems
Average Tornado Incidence per 10,000 mi2
Tornado Alley
The Location and Timing of Tornadoes
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Texas has the highest annual tornado frequency of any state
Oklahoma and Florida have the highest concentrations
(annually) of tornadoes (and Delaware! ☺)
Tornadoes can happen in any month of the year in the United
States
Late spring is the time of greatest overall activity in the United
States
May is the month of highest tornado frequency
June is the month of next highest tornado frequency
Many states show tornado peaks during different months
The distribution
of tornadoes
during the
year by state.
Monthly Tornado Frequencies for the U.S.
A Perfect Geography!!
Tornado-producing Supercell
Tornado Development along a Convergence Boundary
where strong convection exists
A wall cloud may descend from the rotating mesocyclone
Wall cloud
Wall cloud
The life cycle of a tornado….
The College Park, MD Tornado
A huge Great Plains Supercell and Tornado
Toward the end of their life cycle, many
tornadoes become “rope tornadoes”
Mammatus Clouds Over Mexico
Credit & Copyright: Raymundo Aguirre (2004)
Tornado Damage
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Winds, not pressure change, cause the greatest amount of
damage
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Flying debris causes the greatest amount of injuries
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Although most tornadoes rotate around a single, central core,
some of the most violent ones have small zones of intense
rotations called suction vortices
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The existence of multiple suction vortices (≈10 m in diameter)
within a single tornado account for selective damage patterns
Tornado Damage
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Tornadoes are classified using the Fujita Scale which
ranks tornadoes based on damage
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Roughly 69% of all tornadoes fall into the weak categories
(0-1) while 29% are classified as being strong (2-3), and
only 2% are deemed violent (4-5)
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Violent tornadoes are capable of nearly catastrophic
damage
The Fujita (F) Scale
Tornado Fatalities
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Due to their small spatial scales, tornadoes kill few people
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On average 760 tornadoes occur in the United States annually
Only 91 people are killed annually
88% of all tornadoes kill no one
Most fatalities occur in associated with a few large tornadoes
rather than with many smaller ones
Only about 1% of all tornadoes are responsible for over 2/3 of all
deaths
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Mobile homes and cars are the sites of many deaths
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Safest place to be during a tornado is in a building basement
Cars are the worst place to be during a tornado!
Over 600
fatalities
and
thousands
of injuries!
The “super
outbreak” of
tornadoes on
April 3-4, 1974
spawned 148
tornadoes in 24hours!
Aerial View of Damage from Moore, Oklahoma Tornado (May 3, 1999)
(Source: http://www.k5kj.net/news_archives.htm)