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Thunderstorms & Tornadoes
This chapter discusses:
1. The different types and development stages of
thunderstorms
2. The life-threatening storm components of lightning
and tornadoes
Ordinary Thunderstorms
Three stages have been identified in ordinary
thunderstorms:
a) an unstable atmosphere and vertical
updrafts keep precipitation suspended
b) entrainment of dry air that causes cooler
air from evaporation, triggering
downdrafts and falling precipitation and
gust fronts
c) weakening updrafts and loss of the fuel
source after 15 to 30 minutes.
Figure 14.2A
Mature Stage Thunderstorm
Figure 15.2
During the mature stage, updrafts may stop at the troposphere
where the cloud ice crystals are pushed horizontally by winds and
form an anvil top, or they may overshoot further into the
tropopause.
Dissipating Stage of Thunderstorm
Once downdrafts
dominate
updrafts, the
storm ends as
precipitation
leaves the cloud
faster than it is
replenished by
rising, condensing
air.
Figure 15.3
Often, lower level
cloud particles
will evaporate
leaving an isolate
cirrus anvil top
section.
Multicell Storms
Figure 15.4
Cool downdrafts leaving a mature and dissipating storm may offer
relief from summer heat, but they may also force surrounding, lowlevel moist air upward.
Hence, dying storms often trigger new storms, and the successive
stages may be viewed in the sky.
Severe Thunderstorms
Figure 15.5
Storms producing a minimum of
a) 3/4 inch hail and/or
b) wind gusts of 50 knots and/or
c) tornado winds, classify as severe.
In ordinary storms, the downdraft and falling
precipitation cut off the updraft.
In severe storms, winds aloft push the rain ahead and
the updraft is not weakened and the storm can continue
maturing.
The single supercell storm shown here maintained its
structure for hours.
Gust Front & Microburst
Turbulent air
forms along the
leading edge of the
gust front, which
can generate
tumbling dust
clouds.
Such gust fronts
and associated
cold dense air
often feel like a
passing cold front,
and may cause a 1
to 3 mb local rise
in pressure, called
a mesohigh.
Figure 15.6
Gust Front Shelf Cloud
When unstable air
is prevalent near
the base of the
thunderstorm, the
warm rising air
along the forward
edge of the gust
front is likely to
generate a shelf,
or arcus, cloud.
Figure 15.6
Gust Front Roll Clouds
Figure 15.7
Turbulence in the fast moving gust front will spawn eddies and
possibly roll clouds beneath the shelf cloud.
These clouds spin about a horizontal axis near the ground.
Microbursts from Dense Air
Figure 15.9
Dry air entrained into the thunderstorm will evaporate and cool the
falling mix of precipitation and air, which may create dry, and in
humid areas wet, microbursts of strong winds.
Flying into a Microburst
Figure 15.10
A pilot flying into a microburst must anticipate sudden and strong
changes in wind direction and speed.
Initially a headwind is encountered that lifts the plane, followed by a
strong downdraft, and when leaving the storm a tailwind causes a
loss of altitude.
Storm Radar Bow Echo
Derecho, or straight-line
winds, may form ahead
of a several hundred
kilometer cluster of
storms, known as a
squall line or mesoscale
convective system, often
formed a few hundred
kilometers ahead of a
cold front.
This image shows a
squall line within the
bow shaped radar echo.
Figure 15.11
Pre-Frontal Squall Line Storms
Pre-frontal squall lines
identify major storms
triggered by a cold
front that may contain
several severe
thunderstorms, some
possibly supercells,
extending for more
than 1000 kilometers.
This 1989 storm
spawned 25 tornadoes,
the worst killing 25
people.
Figure 15.12
Gravity Waves
Figure 15.13
Pre-frontal squall line formation is not fully understood.
One theory suggests that a surging cold front may initiate "gravity
waves" aloft, where the rising motion of the wave causes cumulus
cloud development.
Trailing Stratified Clouds
Figure 15.14
An extensive region of stratified clouds may follow behind a squall
line.
This figure shows a loop of rising and falling air that supplies the
moisture to the stratiform clouds and associated light precipitation.
Mesoscale Convective Complex
Figure 15.15
An organized mass, or collection, of thunderstorms that
extends across a large region is called a mesoscale
convective complex (MCC).
With weak upper level winds, such MCC's can
regenerate new storms and last for upwards of 12 hours
and may bring hail, tornadoes, and flash floods.
They often form beneath a ridge of high pressure.
Dryline Thunderstorms
Figure 15.16
Abrupt geographic changes from moist to dry dewpoint temperature, called drylines, form in western TX,
OK, and KS in the spring and summer.
The diagram illustrates how cool cP air pushes hot and
dry cT air, at the height of the central plains, over the
warm moist mT air.
Such mixing causes large scale instabilities and the
birth of many supercell storms.
Thunderstorm Movement
Figure 15.17
Middle troposphere winds often direct individual cells of a
thunderstorm movement, but due to dying storm downdrafts
spawning new storms, the storm system tends to be right-moving
relative to the upper level winds.
In this figure, upper level winds move storms to the northeast, but
downdrafts generate new cells to the south, which eventually cuts
off moisture to the old cell.
Flash & Great Floods
Figure 15.18
Figure 15.19
Thunderstorms frequently generate severe local flooding, but in
the summer of 1993 a stationary front beneath the unusually
southerly polar jet triggered several days of thunderstorms and
rain.
The jet caused weak surface waves and provided uplift of warm,
moist Gulf air for thunderstorm growth throughout the northern
Mississippi region.
Floods took 45 human lives and 74,000 were evacuated.
Average Thunderstorm & Hail Days
Figure 15.20
Figure 15.21
Observed frequency in the pattern and occurrence of
thunderstorms does not overlap with hail frequency, possibly
because hail falling into the thick layer of warm Gulf air will melt
before reaching the ground.
Lightning & Thunder
Charge differences between
the thunderstorm and
ground can cause lightning
strokes of 30,000°C, and
this rapid heating of air will
creates an explosive shock
wave called thunder, which
requires approximately 3
seconds to travel 1 kilometer.
Figure 15.22
Lightning Stroke Development
Charge layers in the cloud are formed
by the transfer of positive ions from
warmer hailstones to colder ice
crystals.
When the negative charge near the
bottom of the cloud is large enough to
overcome the air's resistance, a
stepped leader forms.
A region of positive ions move from the
ground toward this charge, which then
forms a return stroke into the cloud.
Figure 15.23A
Types of Lightning
Figure 15.24
Nearly 90% of lightning is the negative cloud-toground type described earlier, but positive cloud-toground lightning can generate more current and more
damage.
Several names, such as forked, bead, ball, and sheet
lightning describe forms of the flash.
Distant, lightning with unheard thunder is often called
heat lightning.
Lightning Rods & Fulgurite
Figure 15.26
Figure 15.25
Metal rods that are grounded by wires provide a low resistance
path for lightning into the earth, which is a poor conductor.
The fusion of sand particles into root like tubes, called fulgurite,
may result.
Lightning Detection & Suppression
Figure 15.27
Figure 15.28
When lightning is nearby, trees are not safe because they may generate a
return stroke, but a car may provide protection by transferring the charge
through its body to the tires.
Lightning is more often the cause for forest fires, triggering nearly 10,000
yearly in the U.S.
A National Lightning Detection Network helps monitor this storm activity.
Tornado
A rapidly
rotating column
of air often
evolve through a
series of stages,
from dust-whirl,
to organizing
and mature
stages, and
ending with the
shrinking and
decay stages.
Figure 15.29
Winds in this
southern Illinois
twister exceeded
150 knots.
Tornado Occurrence
Figure 15.30
Tornadoes from all 50 states of the U.S. add up to more than 1000
tornadoes annually, but the highest frequency is observed in
tornado alley of the Central Plains.
Nearly 75% of tornadoes form from March to July, and are more
likely when warm humid air is overlain by cooler dryer air to cause
strong vertical lift.
Tornado Wind Speed
As the tornado moves
along a path, the
circular tornado winds
blowing opposite the
path of movement will
have less speed.
For example, if the
storm rotational speed
is 100 knots, and its
path is 50 knots, it will
have a maximum wind
of 150 knots on its
forward rotation side.
Figure 15.31
Suction Vortices & Damage
A system of tornadoes with smaller
whirls, or suction vortices,
contained within the tornado is
called a multi-vortex tornado.
Damage from tornadoes may
include its low pressure centers
causing buildings to explode out
and the lifting of structures.
Human protection may be greatest
in internal and basement rooms of
a house.
Figure 15.32
Fujita Tornado Scale
Figure 15.33
Tornado watches are issued when tornadoes are likely, while a
warning is issued when a tornado has been spotted.
Once the storm is observed, or has passed, the Fujita scale is used
to classify tornadoes according to their rotational speed based on
damage done by the storm.
Atmospheric Conditions for Tornadoes
A specific pattern of events often
coincide during the formation of
tornadoes and severe thunderstorms.
This may include when an open-wave
mid-latitude cyclone mixes together
cold dry air with warm moist air at the
surface, and 850 mb warm moist and
700 mb cold dry air aloft flow north
and north east, as shown in this figure.
Further, at the 500 mb level a trough of
low pressure pressure forms to the west
of the surface low, and the 300 mb polar
jet swings over the region.
Figure 15.34
Thunderstorm Sounding
Figure 15.35
Temperature and dew point have typical vertical profile in the
warm sector before a tornado occurs, including the shallow
inversion at 800 mb that acts like a cap on the moist air below.
The cold dry air above warm humid air produces convective
instability and lifting.
Vorticity from Horizontal to Vertical
Figure 15.36
Figure 15.37
Spinning horizontal vortex tubes created by surface wind shear
may be tilted and forced in a vertical path by updrafts. This rising,
spinning, and often stretching rotating air may then turn into a
tornado.
Tornado Breeding Supercell Storm
Figure 15.38
Supercell thunderstorms may have many of the features illustrated
here, including a mesocyclone of rotating winds formed when
horizontal vorticity was tilted upwards.
Radar Image of Supercell
The area of
precipitation and
winds in the
mesocyclone is
known as the
bounded weak echo
region (BWER)
which the radar is
unable to detect and
displays as a black
core to this storm.
Figure 15.39
The cyclonic flow of
precipitation on the
radar screen is often
shaped like a hook
echo.
Rear Flank Downdraft
Supercell thunderstorm
development may create an area
where the updraft and
counterclockwise swirl of upper
winds converge into a rear flank
downdraft.
This downdraft can then interact
with lower level inflow winds and
spawn a tornado.
Figure 15.40
Rotating Clouds as Tornado Signal
Figure 15.41
The first
sign that a
supercell
may form a
tornado is
the sight of
rotating
clouds at the
base of the
storm, which
may lower
and form a
wall cloud,
shown in this
picture.
NonSupercell Tornadoes
If a preexisting wall
cloud was not
present, than
any tornado
formed is not
from a
supercell
storm, and is
often called a
funnel cloud,
or may be a
gustnado if
the form
along a gust
front.
Figure 15.42
Landspout Formation
Figure 15.43A
Landspouts, which form over land but look like
waterspouts, form when surface winds converge along
a boundary where opposite blowing wind creates a
horizontal rotational spin.
If a storm passes above, its updraft may lift and
stretch the horizontal spinning air, causing it to
narrow and increase in rotational speed due to the
conservation of angular momentum.
Doppler Radar Analysis
A single Doppler radar unit can uncover
many features of thunderstorm rotation
and movement, but cannot detect winds
parallel to the antenna.
As such, data from two or more units
might be combined to provide a complete
view of the storm.
Dopplar lidar (light beam rather than
microwave beam) provides more details
on the storm features, and will help
measure wind speeds in smaller
tornadoes.
Figure 15.44
NEXRAD Wind Analysis
NEXt Generation
Weather RADar
(NEXRAD) is
operated by the
National Weather
Service and uses
Doppler
measurement to
detect winds
moving toward
(green) and away
(blue) from the
antenna, which
indicates areas of
rotation and
strong shear.
Figure 15.45
Portable Radar Units
Thunderstorm chasers may carry
portable radar to image finer
details of a storm as it moves along
the flat lands of Tornado Alley.
Figure 15.46
Waterspout Funnel
Warm, shallow coastal water is
often home to waterspouts,
which are much smaller than
an average tornado, but similar
in shape and appearance.
The waterspout does not draw
water into its core, but is a
condensed cloud of vapor.
A waterspout may, however, lift
swirling spray from the water
as it touches the water surface.
Figure 15.47