1. Chapter 10: Thunderstorms and Tornadoes
Tornado formation
Observing tornadoes and severe weather
Waterspouts

2. Thunderstorms
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

3. 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

4. Moisten environment latent heat warms downdraft cuts off
the cloud layer humid updraft;
Figure 10.1: Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.)
Watch this Active Figure on ThomsonNow website at
Q: why is the thunderstorm downdraft usually cold?
A: Entrained dry air causes raindrop evaporation, cooling the air which descends as a downdraft

5. 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
A B C
Q: which cloud in the figure is at the mature stage?
a) A, b) B, c) C

6. Gust Front
gust front: leading edge of cold air originating inside a thunderstorm
shelf cloud and roll cloud
outflow boundary: merging several gust fronts

7. Shelf cloud roll cloud
Figure 10.11: A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.

8. Microbursts
downbursts (intense downdraft) and microbursts (< 4km; caused aircraft crash)
Dust clouds due to mictoburst tail wind headwind
Microbursts present a severe hazard to aircraft, especially during
takeoff and landing.
Several airports have installed microburst detection instruments.

9. 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

10. 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).

11. 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

12. Strong wind shear in speed and direction;
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
Figure 10.5: Conditions leading to the formation of severe thunderstorms, and especially supercells. The area in green shows where severe thunderstorms are likely to form.
Q: A thunderstorm with weak wind shear is most probably
a) ordinary cell, b) multicell, c) supercell

13. A model of classical supercell

14. 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 updraft
Q: Why do wind shear (e.g., increasing wind speed with height) and upward motion would cause the updraft to rotate?
Figure 10.5: Conditions leading to the formation of severe thunderstorms, and especially supercells. The area in green shows where severe thunderstorms are likely to form.

15. Dryline Thunderstorms
These storms occur frequently in the southern Great Plains of the US.

16. 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
satellites?
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

17. 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

18. 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.

19. 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

20. Electrification of Clouds
For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere
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

21. 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

22. 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

23. 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

24. Lightning Protection Q: who invented the lightning rod?
a) Jefferson, b) Lincoln,
c) Washington, d) Franklin

25. 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
Figure 2: 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.

26. Tornadoes tornado or twister: typically 100-600 m, maybe >1 mi,
usually moves at 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.

27. 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

28. Tornado Winds multi-vortex tornadoes suction vortices
Q: what is the wind speed at A or C?
a) 100 knots, b) 112 knots, c) 150 knots

29. 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

30. 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

32. 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
lives.

33. Tornado Formation Supercell Tornadoes 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.

34. Q: What is the percentage of supercells
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 tube?
a) yes, b) no
Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.

35. Nonsupercell Tornadoes
Gustnadoes: tornado along a gust front
Landspouts: weak and short-lived, from congestus
cold air funnels: cold air aloft; short-lived; weak
Waterspouts: similar to landspouts

36. 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. (
Yellow and red:
moving away
Green and blue:
moving toward
the radar