Chapter 10:
Thunderstorms and
Tornadoes




Thunderstorms
Tornadoes
Tornadic thunderstorms
Severe weather and doppler radar
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
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!
Moisten environment
latent heat warms the
cloud layer;
entrained dry air causes
evaporative cooling and
downdraft and gust front
downdraft cuts off humid
updraft;
gust front is too far away
to enhance updraft
Fig. 10-1, p. 265
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
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
Fig. 10-5, p. 268
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)
Dryline Thunderstorms

dryline
• These storms occur frequently in the southern
Great Plains of the US.
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
Fig. 10-11, p. 271
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
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
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
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?
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
The Lightning Stroke




cloud-to-ground lightning
stepped leader
return stroke
dart leader
-subsequent
leader
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
Lightning Detection and
Suppression

lightning direction-finder
Detecting the radio
waves produced by
lightning
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, p. 282
Tornado Life Cycles

tornado or twister: typically 100-600 m, may >1 mi,
usually at 20-40 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
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
Tornado Winds


multi-vortex
tornadoes
suction vortices
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
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
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.
Fig. 10-35, p. 290
Nonsupercell Tornadoes

Gustnadoes: along a gust
front

Landspouts: weak and
short-lived, from congestus

cold air funnels: cold air
aloft
Severe Weather and Doppler
Radar


Doppler shift
tornado vortex signature
rapidly changing wind direction

Doppler lidar
use light beam, higher resolution


NEXRAD
waterspout