Chapter 10:
Thunderstorms and
Tornadoes
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Thunderstorms
Tornadoes
Tornadic thunderstorms
Severe weather and Doppler radar
Thunderstorms
•Thunderstorm: storm with lightning and thunder
•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
Q1: 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
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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
Q2: 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!
Q3: 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;
Q4: 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
Supercell:
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
updraft;
Produces tornado, large hail,
strong gusts
Q5: A thunderstorm with weak wind shear is most probably
a) ordinary cell,
b) multicell, c) supercell
Q6: 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
strengthens the updraft
Fig. 10-5, p. 268
Squall Lines and Mesoscale
Convective Complexes
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squall line: multicell storms as a line
of thunderstorms extending for many
kilometers (up to 1000 km)
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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)
Q7: Who first defined MCC?
a) a UA scientist; b) a non-UA scientist
Dryline Thunderstorms
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dryline
• These storms occur frequently in the southern
Great Plains of the US.
Gust Fronts, Microbursts and
Derechoes
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gust front: leading edge of cold air originating inside a
thunderstorm
shelf cloud and roll cloud
outflow boundary: merging several gust fronts
downbursts (intense downdraft) and microbursts (< 4km;
caused aircraft crash)
Derecho (day-ray-sho): straight-line (i.e., not associated
with rotation) strong wind extending for 100’s km produced
by strong downdrafts;
associating with large hail and sometimes tornado
Shelf cloud
roll cloud
Dust cloud due to microburst
Microbursts present a severe hazard
to aircraft, especially during
takeoff and landing.
Several airports have installed
microburst detection instruments.
Fig. 10-13, p. 272
A squall line thunderstorms appear in the shape of a bow
(or bow echo) on a radar screen and produce a
straight-line wind (derecho)
Fig. 10-16, p. 273
Q8: A circular cluster of storms is called
a) MCC,
b) squall line, c) dryline
Q9: Intense downdraft is called
a) Derecho, b) gust front, c) downburst
Q10: What is most damaging for aviation?
a) Derecho, b) gust front, c) microburst
Q11: Which has the largest cloud area as seen from
satellites?
a) dryline, b) MCC,
c) squall line, d) supercell
Floods and Flash Floods
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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
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combination of warmth and moisture
geographical placement
Thunderstorm days
Hail days
Q12: 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; evaporation of hail surface water cools the
hailstones so that hail can reach surface (rather than melt)
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
Q13: 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)
Q14: 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
Q15: 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
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Electrification of Clouds
For normal fair weather, the atmosphere is usually
characterized by a negatively charged surface and a
positively charged upper atmosphere
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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
Q16: 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
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cloud-to-ground lightning
stepped leader
return stroke: large # 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
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Lightning Detection and Protection
lightning direction-finder
Detecting the radio waves
produced by lightning
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Lightning strikes
Q17: who created the U.S. National
Lightning Detection Network?
a) a UA scientist; b) a non-UA scientist
Q18: who invented the lightning rod?
a) Jefferson, b) Lincoln, c) Washington, d) Franklin
Q19: 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
Q20: 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
Figure 2, p. 282
Tornado Life Cycles
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tornado or twister: typically 100-600 m, may >1 mi,
usually at 20-40 knots
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funnel cloud:
not reach the ground
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dust-whirl stage
mature stage
decay stage
tornado families:
different tornados spawned
by the same thunderstorm
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tornado outbreaks:
6 or more tornados over
a particular region
Tornado Occurrence
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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
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multi-vortex
tornadoes
suction vortices
Q21: what is the wind speed at B or C?
a) 50 knots, b) 100 knots, c) 112 knots
Seeking Shelter
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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
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tornado classification based on damage
• The “F-scale” was named after Prof. Ted Fujita.
• Wind damage is proportional to the square of wind speed
Q22: How many times is the damage from F3 (160 knots)
as that from F0 (40 knots)? a) 2, b) 4, c) 8, d) 16
Tornadic Thunderstorms
Supercell Tornadoes
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mesocyclones
bounded weak echo region: inside mesocyclone (no precip)
Radar hook echo: rotating precipitation around mesocyclone
wall cloud
• A rotating wall
cloud is an
unforgettable
sight - just
ask a successful
storm chaser.
Fig. 10-35, p. 290
Nonsupercell Tornadoes
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Gustnadoes: tornado along a gust front
Landspouts: weak and short-lived, from
congestus
cold air funnels: cold air aloft; shortlived; weak
Waterspouts: similar to landspouts
Q23: Is mesocyclone caused by
the vertical tilting of the
horizontal vortex tube?
a) yes, b) no
Severe Weather and Doppler Radar
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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: go to ATMO weather site