Chapter 10: Thunderstorms
and Tornadoes
Tornado formation
Observing tornadoes and
severe weather
•Thunderstorm: storm with lightning and thunder; they
are convective storms that form with rising air in a
conditionally unstable environment
• The trigger needed to start air moving upward may be
surface heating;
topographic lift;
convergence zone (e.g., sea breeze leading edge);
frontal lift;
divergence aloft
Q: Conditionally unstable environment means that the
environmental lapse rate is
a) > 10 K/km, b) < 6 K/km, c) between 6 – 10 K/km
Ordinary Cell Thunderstorms
Three stages: cumulus, mature, dissipating
• Ordinary cell thunderstorms are sometimes called ‘air mass
thunderstorms’ or `ordinary thunderstorm’, because they
form in conditionally unstable air masses and are not
necessarily associated with fronts or severe weather
• Shortlived (<1 hr), less than 1 km wide, low wind shear,
rarely produce strong wind or large hail
Q: Is it possible to drop air temperature by 40F in an hour?
A: Yes. On 7/13/1999 in California, a strong downdraft
from a mature thunderstorm dropped the air temperature
from 97F to a chilly 57F in one hour!
Q: Why does low wind shear produce an ordinary rather than
severe thunderstorm?
A: because downdraft would cutoff the supply of moisture
Moisten environment
latent heat warms
the cloud layer
downdraft cuts off
humid updraft;
Q: why is the thunderstorm downdraft usually cold?
A: Entrained dry air causes raindrop evaporation, cooling the air
which descends as a downdraft
Severe Thunderstorms and Supercell
Severe thunderstorm is defined as a thunderstorm with at least
one of the following:
large hail with a diameter > ¾ inch,
surface wind gusts >50 knots (58 mi/hr), and/or
produces a tornado
multicell storms
moderate wind shear;
lasting several hours;
multiple ordinary or
supercell thunderstorms
Q: which cloud in the figure is at the mature stage?
a) A, b) B, c) C
Gust Front
gust front: leading edge of cold air originating inside a
shelf cloud and roll cloud
outflow boundary: merging several gust fronts
Shelf cloud
roll cloud
downbursts (intense downdraft) and microbursts (< 4km;
caused aircraft crash)
Dust clouds due to mictoburst
tail wind
Microbursts present a severe hazard to aircraft, especially during
takeoff and landing.
Several airports have installed microburst detection instruments.
Squall Lines
squall line: multicell storms as a line
of thunderstorms extending for many
kilometers (up to 1000 km)
Pre-frontal squall line may be
initiated by gravity waves
Strong downdrafts of squall lines cause bow-shaped signal in
a Doppler radar image, called bow echo. When the damage
associated with the straight-line winds extends for a
considerable distance along the squall line’s path, the wind
storm is called a derecho (day-ray-sho).
mesoscale convective complex (MCC):
multicell storms as a large circular cluster of storms;
tend to form in summer in regions where the upper-level
winds are weak;
large size (100,000 square km);
last for several hours
Q: Who first defined MCC?
a) a UA scientist; b) a non-UA scientist
Strong wind shear in speed and direction;
Shallow inversion above warm and humid
layer acts as a lid;
Long-lasting (hours);
Larger than 1 km in diameter;
Single violently rotating
Produces tornado, large hail,
strong gusts
Q: A thunderstorm with weak wind shear is most probably
a) ordinary cell,
b) multicell, c) supercell
A model of classical supercell
Q: why is the weather pattern left is
favorable for supercell?
A: a) with wind shear, downdraft
would not cut off the updraft;
b) upper level divergence and lower
level convergence strengthen the
Q: Why do wind shear (e.g., increasing wind
speed with height) and upward motion would
cause the updraft to rotate?
Dryline Thunderstorms
• These storms occur frequently in the southern
Great Plains of the US.
Q: A circular cluster of storms is called
a) MCC,
b) squall line, c) dryline
Q: Intense downdraft is called
a) Derecho, b) gust front, c) downburst
Q: What is most damaging for aviation?
a) Derecho, b) gust front, c) microburst
Q: Which has the largest cloud area as seen from
a) dryline, b) MCC,
c) squall line, d) supercell
Q: If the vertical wind shear is weak, the thunderstorm is
a) ordinary cell, b) multicell, c) supercell
Thunderstorms and Flooding
flash floods: floods that rise rapidly with little or no
advance warning
1976 Big Thompson flash flood
(12 inch rain in 4 hours)
The Great Flood of 1993
over the upper midwest
Distribution of Thunderstorms
combination of warmth and moisture
geographical placement
Thunderstorm days
Hail days
Q: why are the Great Plains more favorable for hails than, say,
Florida (right panel above)? A: The Great Plains are more favorable
for severe thunderstorms (including larger hails); the warm subcloud
layer in Florida melts hail before reaching the ground.
Lightning and Thunder
Lightning: a discharge of electricity, a giant spark, which
usually occurs in mature thunderstorms (and may also occur
in snowstorms and dust storms);
majority of lightning strokes within clouds with only 20%
between clouds and surface;
a lightning stroke can heat the air it travels to 30,000C, 5
times as hot as the Sun’s surface
Q: why would a lightning cause a thunder?
A: the extreme heating from a lightning causes the air to expand
explosively, thus initiating a shock wave that becomes a
booming sound wave (or a thunder)
Q: Do you see the lightning or hear the thunder first?
a) see the lightning first, b) hear the thunder first,
c) at the same time
Q: Assuming sound speed is 330 m/s and you hear the thunder 5
seconds after seeing the lightning, what is the distance of the
lightning stroke?
a) 330 m, b) 1 km, c) 1 mile
Electrification of Clouds
For normal fair weather, the atmosphere is usually characterized
by a negatively charged surface and a positively charged upper
relationships of updrafts and downdrafts
to electrical charges in clouds
There is a net transfer of positive ions
(charged molecules) from the warmer (and
larger) hailstone to the colder (and smaller)
ice crystal or supercooled droplets which
are lifted to the upper layer of clouds
Q: Why are hailstones warmer than
smaller droplets?
a) because latent heat release;
b) because hailstones are bigger;
c) because smaller droplets are supercool
The Lightning Stroke
cloud-to-ground lightning
stepped leader
return stroke: large number of electrons flow to the ground and
a much larger, more luminous return stroke (current) to cloud
dart leader – subsequent leader
Types of Lightning
forked lightning
 ribbon lightning:
hanging from clouds
due to winds
 dry lightning:
not producing rain;
cause forest fire
 heat lightning (in
summer): seen but
not heard because
sound wave
propagation is affected by air
 St. Elmo’s fire: a corona discharge or sparks,
can cause the top of a ship’s mast to glow;
also seen over power lines and aircraft wings
Lightning Detection and Protection
lightning direction-finder: detecting the radio waves
produced by lightning
Satellites can also monitor global lightning activities
Q: who created the U.S. National Lightning Detection
Network? a) a UA scientist; b) a non-UA scientist
Lightning Protection
Q: who invented the lightning rod?
a) Jefferson,
b) Lincoln,
c) Washington, d) Franklin
Q: Where do you stay under
thunderstorm? A:
• Not under trees;
• Avoid elevated places;
• Keep your head as low as
possible but not touch ground
• Inside a building;
• Inside a car;
• Not in a golf cart
Q: why don’t you want to lie
down on the ground?
A: lightning channels usually
emanate outward at a point of
lightning strike, a surface
current may travel through your
body and injure or kill you
tornado or twister: typically 100-600 m, maybe >1 mi,
usually moves at 20-40 knots
funnel cloud:
not reach the ground
dust-whirl stage
mature stage
decay stage
Tornadoes ripped through the
Dallas region on 4/3/2012 and
a truck became airborne.
Tornado Occurrence
tornado alley: Great Plains
time of day: most frequent at 4-6pm LT
times of year: May and June
Annual number per state;
Annual number per 100 mi by 100 mi
Tornado Winds
suction vortices
Q: what is the wind speed at A or C?
a) 100 knots, b) 112 knots, c) 150 knots
Seeking Shelter
tornado watch: likely to form
tornado warning: spotted visually or by radar
• It’s always a good idea to know what to do if a
tornado watch or warning is issued for your area.
• Take shelter in the basement or small room in the
middle of the house at a lower level and cover your head
• Lie flat on the ground in a ditch (but not lie down on a
flat surface)
• Don’t stay under a highway overpass
• Don’t stay near window or wall
• Don’t stay in a mobile home
The Fujita Scale
tornado classification based on damage
• The “F-scale” was named after Prof. Ted Fujita.
• Wind damage is proportional to the square of wind speed
Q: How many times is the damage from F3 (160 knots) as
that from F0 (40 knots)? a) 2, b) 4, c) 8, d) 16
• Tornado outbreaks
Tornado families: different tornados
spawned by the same thunderstorm
Tornado outbreaks: 6 or more
tornados over a particular region
Tri-state tornado outbreak on
3/18/1925: at least 7 tornadoes
traveled a total of 437 miles
across portions of Missouri,
Illinois, and Indiana, causing
747 human casulties.
Meteorologists are doing a
better job now in protecting
Tornado Formation
Supercell Tornadoes
bounded weak echo region: inside
mesocyclone (no precip)
Radar hook echo: rotating precipitation around mesocyclone
wall cloud
• A rotating wall
cloud is an
sight - just
ask a successful
storm chaser.
hook echo
Q: What is the percentage of supercells
producing tornadoes: a) 100%, b) 70%,
c) 40%, d) 15%
Q: Is mesocyclone
caused by the
vertical tilting of the
horizontal vortex
a) yes, b) no
Nonsupercell Tornadoes
Gustnadoes: tornado along a gust front
Landspouts: weak and short-lived, from
cold air funnels: cold air aloft; shortlived; weak
Waterspouts: similar to landspouts
Observing Tornadoes and Severe Weather
Doppler shift: similar to change of sound frequency as a train
approaches the observer
tornado vortex signature: rapidly changing wind direction
Doppler lidar: use light beam (instead of microwave in
radar), higher spatial resolution
NEXRAD: >150 Doppler radars over
continental U.S. (www.weather.gov)
Yellow and red:
moving away
Green and blue:
moving toward
the radar
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