1. Chapter 10: Thunderstorms and Tornadoes
Tornadic thunderstorms
Severe weather and doppler radar

2. Thunderstorms 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

3. Ordinary Cell Thunderstorms
cumulus stage
mature stage
dissipating stage
Ordinary cell thunderstorms are sometimes called ‘air mass thunderstorms’, 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
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!

4. Moisten environment latent heat warms the downdraft cuts off humid
cloud layer; updraft;
entrained dry air causes gust front is too far away
evaporative cooling and to enhance updraft
downdraft and gust front
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
Fig. 10-1, p. 265

5. Severe Thunderstorms and the 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

6. 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.
Fig. 10-5, p. 268

7. Squall Lines and Mesoscale Convective Complexes
squall line: multicell storms as a line of thunderstorms extending for many kilometers (up to 1000 km)
pre-frontal squall line
mesoscale convective complex: 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)

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

9. Gust Fronts, Microbursts and Derechoes
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 (not associated
with rotation) strong wind extending for 100’s km produced by strong downdrafts;
producing large hail and sometimes tornado

10. 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.
Fig , p. 271

11. Several airports have installed microburst detection instruments.
Microbursts present a severe hazard to aircraft, especially during takeoff and landing.
Several airports have installed microburst detection instruments.
Figure 10.13: Radar image of an outflow boundary. As cool dense air from inside the severe thunderstorms (red and orange colors) spreads outward, away from the storms, it comes in contact with the surrounding warm, humid, less-dense air forming a density boundary (blue line) called an outflow boundary between cool air and warm air. Along the outflow boundary, new thunderstorms often form.
Fig , p. 272

12. 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)
Figure 10.16: The red and orange on this Doppler radar image show a line of intense thunderstorms (a squall line) that is moving south southeastward into Kentucky. The thunderstorms are producing strong straight-line winds called a derecho. Notice that the line of storms is in the shape of a bow. Such bow echos are an indicator of strong, damaging surface winds near the center of the bow. Sometimes the left (usually northern) side of the bow will develop cyclonic rotation and produce a tornado.
Fig , p. 273

13. Floods and Flash Floods
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

14. Distribution of Thunderstorms
combination of warmth and moisture
geographical placement
Great Plains are more
favorable for hails partly
because evaporation of hail
surface water cools the dry air

15. 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 through it travels to 30,000C, 5 times hotter than the Sun’s surface
Thunder: The extreme heating causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave
Do you see the lightning or hear the thunder first?

16. Electrification of Clouds
For normal fair weather, the atmosphere is usually
characterized by a negatively charged surface and a
positively charged upper atmosphere
electrical charges in clouds
relationships of updrafts
and downdrafts to
electrical charges
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

17. The Lightning Stroke cloud-to-ground lightning stepped leader
return stroke
dart leader
-subsequent
leader

18. Types of Lightning forked lightning ribbon lightning:
hanging from clouds
due to winds
dry lightning:
cause forest fire
heat lightning:
seen but not heard
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

19. Lightning Detection and Suppression
lightning direction-finder
Detecting the radio
waves produced by
lightning

20. Where do you stay under thunderstorm? Not under trees;
Avoid elevated places;
Keep your head as low as
possible but not touch ground
Inside a building;
Inside a car
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.
Figure 2, p. 282

21. Tornado Life Cycles
tornado or twister: typically m, may >1 mi,
usually at knots
funnel cloud:
not reach the ground
dust-whirl stage
mature stage
decay stage
tornado families:
different tornados spawned
by the same thunderstorm
tornado outbreaks:
6 or more tornados over
a particular region

22. Tornado Occurrence tornado alley
time of day: most frequent at 4-6pm LT
times of year
Annual number per state;
Annual number per 100 mi by 100 mi

23. Tornado Winds
multi-vortex tornadoes
suction vortices

24. 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
Don’t stay under a highway overpass
Don’t stay near window or wall

25. 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

26. Tornadic Thunderstorms
Supercell Tornadoes
mesocyclones
bounded weak
echo region
hook echo
rotating clouds
wall cloud
A rotating wall cloud is an unforgettable sight - just ask a successful storm chaser.

27. 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.
Fig , p. 290

28. Nonsupercell Tornadoes
Gustnadoes: along a gust front
Landspouts: weak and short-lived, from congestus
cold air funnels: cold air aloft

29. Severe Weather and Doppler Radar
Doppler shift
tornado vortex signature
rapidly changing wind direction
Doppler lidar
use light beam, higher resolution
NEXRAD
waterspout