CHAPTER 11. Lightning, Thunder, and Tornadoes
Chapter Overview:
The atmospheric conditions that produce lightning are discussed in this chapter, as are the different
types of lightning, thunder, and lightning safety. Also included are details concerning air mass
thunderstorms and severe thunderstorms. Particular emphasis is placed upon mechanisms that
produce the latter. The chapter concludes with details concerning the evolution of tornadoes,
tornado climatology, damage, and fatalities.
Chapter at a Glance:
Thunderstorms occur about 40,000 times per day over the globe. Each of these produces a
considerable amount of lightning and a few produce tornadoes. <ME11.8>
• Processes of Lightning Formation - Cloud-to-cloud lightning, the most frequent type of
lightning, occurs when discharges of electricity occur within a particular cloud or between
clouds. Cloud-to-cloud lightning is also called sheet lightning as the sky is typically
uniformly lit while the stroke is buried within the cloud. Cloud-to-ground lightning begins
when negative charges build in a cloud base. These charges are eventually discharge onto
the positively charged ground. The ground is typically negatively charged but the cloud
charge offsets this by repelling electrons below. The sequence of a lightning stroke
follows. <Web> <ME11.1>
A. Charge Separation - Electrical charges must separate within portions of a
cloud for lightning to initiate. Positive charges typically accumulate in the upper
areas of the cloud while negatively charged particles aggregate in lower portions.
Charge separation is believed to occur in relation to ice crystals, the lighter
positively charged crystals inhabit the upper reaches of the cloud while heavier
crystals migrate to lower portions. This thermoelectric effect is still disputed to
some degree. Another process, induction, may contribute to charge separation.
Induction is based on the fact that opposite charges attract, such that the top of
falling ice pellets will be negatively charged due to the positive charge of the upper
atmosphere. Finer ice crystals or small drops, acquiring a positive charge, move to
the upper cloud area while the heavier pellets stabilize in the lower areas of the
cloud. Again, research is needed to clarify aspects of induction.
B. Leaders, Strokes, and Flashes - For cloud-to-ground lightning to occur, a
stepped-leader must emanate from the cloud base. The leader is essentially an
ionized particle chamber about 10 cm (4 in) in diameter which forks repeatedly
from a main channel. Each section travels about 50 m (165 ft) in a microsecond.
The sections continue until contact is made with an unlike charged area (the
ground). Upon connection, electrons flow resulting in an illuminated return stroke.
Although the electrical current is from the cloud to the ground, the illuminated
stroke is in the opposite direction. Air in the conducting channel heats to about
30,000 K (54,0000oF). Usually more than one stroke is needed to neutralize all
negative ions so another leader, or dart leader, is initiated and a return stroke
follows. The process is repeated about four or five times on average to neutralize
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all ions. Individual strokes are almost impossible to detect due to the speed of the
occurrence with the only evidence coming from the flicker of the lighting flash, the
combination of all strokes.
C. Types of Lightning - There are many types of lightning but probably the most
unusual is ball lightning. Ball lightning is comprised of a round mass of electrified
air about as large as a basketball. Rather bizarre behavior is typically associated
with ball lightning. St. Elmo’s fire refers to tall objects glowing as ionization
occurs in the air around them. Sprites a large short duration electrical burst, occurs
from the tops of clouds producing lightning below. Sprites are rather rare events,
occurring in only about 1% of all lightning events. Blue jets are similar to sprites in
that upward electrical bursts occur from active thunderstorm regions.
D. Thunder - The rapid expansion of air in association with a lightning stroke
causes thunder. The slower speed of sound, with reference to light, causes a lag
between the stroke and the resulting thunder. To determine distance in kilometers
of the stroke, count the seconds between the stroke and the thunder and divide by
three (five to determine distance in miles). Lightning without thunder being heard
is sometimes called heat lightning when thunder is produced but the stroke is too
far away to reach an observer. Rumbling thunder is typically caused by sound
echoing off topographic features and buildings.
• Lightning Safety - The safest area to be in during a thunderstorm is indoors. One should
not be in contact with electrical appliances or telephones. Automobiles are also safe as
electricity will be conducted to the ground through the shell and not the interior.
• Thunderstorms: Self-Extinguishing vs. Self-Propagating - The vast majority of
thunderstorms are of the air mass variety, meaning that they are localized short-lived
phenomena. Air mass thunderstorms usually do not become severe.
A. Air Mass Thunderstorms - Air mass thunderstorms are spatially and
temporally limited. Each air mass thunderstorm is comprised of a number of
individual cells with each undergoing a particular life cycle.
1. Cumulus Stage - This first stage of thunderstorm development begins
with differential heating of the Earth’s surface leading to parcel formation
and rising air. Thus, only updrafts are present in this beginning situation.
Such rising air undergoes adiabatic cooling to initially form a cumulus
cloud. Eventually enough water vapor is present in the atmosphere to
sustain vertical cloud development. Vertical cloud development typically
occurs between 5 to 20 m/sec (10 to 45 mph).
2. Mature Stage - Precipitation and the presence of both up and downdrafts
mark the mature stage of thunderstorm development. Downdrafts are
initiated in the storm through frictional drag associated with falling
precipitation. Downdrafts are strengthened through cooling associated with
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evaporation of precipitation. The mature stage is also the time of lightning
and thunder. Cloud tops are formed where the atmosphere is stable. An
anvil head may occur as high speed winds blow ice crystals downstream.
Updrafts dominate the interior portions of the storm while downdrafts occur
toward the edges. Entrainment occurring along the cloud edges discourages
lifting in those areas, leading to well-defined cloud edges.
3. Dissipative Stage - Downdrafts dominate airflow within the
thunderstorm, which suppresses updrafts. This obstruction of additional
water vapor initiates the dissipative stage. Precipitation then ceases and the
cloud eventually evaporates. Overall, only about 20% of the available
moisture falls as precipitation, the rest evaporates. Each tower of a typical
cumulonimbus cloud represents an individual cell. Further, fuzzy cloud
edges indicate older portions of the storm which are glaciated (composed
entirely of ice crystals).
B. Severe Thunderstorms - Severe thunderstorms occur when winds exceed 93
km/hr (58 mph), have large hailstones (1.9 cm; 0.75 in), or produce tornadoes.
<Web> These systems differ from air mass thunderstorms in that the up and
downdrafts support each other to intensify the storm. Particular atmospheric
conditions must persist across the mesoscale (10-1000 km) for severe
thunderstorms to occur. For this reason, severe thunderstorms usually occur in
groups over a fairly large area. Clustered thunderstorms are deemed mesoscale
convective systems (MCSs), as squall lines, or as circular clusters called mesoscale
convective complex’s (MCCs). In these situations, individual storms develop in
concert, which propagates additional thunderstorms. Many MCSs have life spans
from up to 12 hours to several days. In the central US and Canada, these systems
may account for as much as 60% of total annual rainfall. Severe thunderstorms
may also form from individual supercells, which contain only one updraft <Web>,
but supercells may also form in association with MCSs. Atmospheric conditions
supporting severe thunderstorms include wind shear, high water vapor content in
lower portions of the troposphere, a forcing mechanism, and potential instability.
C. Mesoscale Convective Complexes - MCCs account for the greatest amount of
severe weather in the US and Canada. These roughly circular clusters of
thunderstorms are self-propagating in that individual cells create downdrafts that
interact to form new cells. Colder, denser downdrafts which spread across the
surface and help force warm, moist surface air aloft. This outflow boundary
initiates a new cell. Most often, this occurs along the southern edge of the MCC as
the downdrafts intermingle with a usual stream of warm, moist surface air from the
south. <ME11.9> However, the entire system typically propagates toward the east
along with the associated upper air wind.
D. Squall Line Thunderstorms - Squall lines differ from MCCs in that
thunderstorms are usually linear in organization. These bands of thunderstorms,
which may be as long as 500 km (300 mi), usually occur about 300-500 km
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(180-300 mi) in advance of cold fronts. Strong vertical wind shear is essential to
the development of these prefrontal waves. This wind shear ensures that the
updrafts will be positioned ahead of the downdrafts. This situation continually
feeds moisture into the system. Further, downdrafts and the associated gust front
help propagate the situation. <ME11.5>
E. Supercell Storms - Although supercells consist of a single cell, they are
typically more violent than MCCs or squall lines. Strong wind shear is responsible
for wrapping up and downdrafts around each other in these tornado producers.
This creates large-scale rotation which is typically absent from MCCs or squall
lines. However, similar to MCCs and squalls, supercell downdrafts aid the
intensification of the system through enhanced uplift of warm, moist air. Doppler
radar is used to reveal areas of rotation in thunderstorms. These hooks often signify
tornado formation. Radar may also reveal a large vacant region of the storm
located in the southeast quadrant. This vault is the location of the warm updraft and
is comprised of droplets too small to be detected by radar.
F. Microbursts - Very strong downdrafts, or downbursts, are excessively strong
blasts of air from the bottom of a thunderstorm. Winds may reach 270 km/hr (165
mph). Once reaching the surface, air spreads uniformly in all directions. Damage
is often confused with that of a tornado. Small versions of the downburst are
known as microbursts. Although small in diameter (4 km; 2.5 mi), they are
responsible for a great variety of damage, including many airplane disasters.
• Geographic and Temporal Distribution of Thunderstorms - Thunderstorms typically
develop where moist air is forced aloft. This occurs most frequently in the tropics where
thunderstorms occur nearly daily in some locations. In the US, the most frequent
thunderstorm region is the Gulf South with the absolute peak frequencies occurring in
Florida. This is due to the fact that the state is essentially a land protrusion into warm
waters.
• Tornadoes - Tornadoes are areas of rapid, rotating, lifting winds beneath cumulonimbus
clouds. Strong counterclockwise winds originate in relation to large pressure gradients
over small spatial scales. Pressure differences may be as much as 100 mb over only a few
tenths of a km. <Web> <ME11.6>
A. Tornado Characteristics and Dimensions - Tornadoes typically have
diameters of about 100 yards but may be much larger. They are usually a
short-lived phenomena lasting only a few minutes but some have lasted for hours.
Movement is generally about 50 km/hr (30 mph) over an area about 3-4 km (2-2.5
mi) long. Winds may be as low as 65 km/hr (40 mph) or as high as 450 km/hr (280
mph).
B. Tornado Formation - Tornadoes are common to frontal boundaries, squall
lines, MCCs, supercells, and tropical cyclones. The most violent tornadoes occur
in association with supercells.
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1. Supercell Tornado Development - Supercell tornado formation begins
with the development of a mesocyclone. These large rotation regions
within the cloud interior develop in the presence of vertical wind shear.
From the surface aloft, winds shift from southerly to westerly while speed
increases. Strong updrafts then tilt the rotation region to a vertical position
while the diameter decreases. With a spatial decrease comes an increase in
speed as dictated by the conservation of angular momentum. The rotating
air column will then penetrate the cloud base producing a wall cloud. From
the wall cloud, a narrow rotating region, the funnel cloud, emerges on a path
to the surface. Doppler radar detects mesocyclone development leading to
increased warning times. However, only about half of all mesocyclones
actually spawn a tornado. <ME11.10-11.15>
2. Nonsupercell Tornado Development - Nonsupercell tornadoes may be
associated with development from the interaction of outflow boundaries
between two or more thunderstorms. Another development mechanism
may be related to strong convection along a convergence zone. <ME11.7>
C. The Location and Timing of Tornadoes - The US is by far the world leader in
tornado production. This results from the regular interaction between extremely
unlike air masses. These air masses usually originate in very high latitudes and
over the Gulf of Mexico. The absence of topographic barriers ensures regular
mixing and the production of violent storm systems. The vast majority of US
tornadoes occur in Tornado Alley, a region situated from the southern Plains to the
lower Great Lakes. Within tornado alley, Texas has the highest tornado frequency.
May is the month of highest frequency while June is a close second. Many states
show tornado peaks during different months; however, late spring is the time of
greatest overall activity.
D. Tornado Damage - Winds, and not pressure change, cause the greatest amount
of damage from a tornado while flying debris causing the greatest amount of
injuries. Some tornadoes have multiple suction vortices, which may account for
rather selective damage patterns. Tornadoes are classified using the Fujita scale
which ranks tornadoes based on damage. Of the seven scale levels (0-5), nearly
three quarters of all tornadoes fall into the weak categories (0 and 1). Twenty-five
percent are classified as being strong (2-3), while only about 1% are deemed
violent (4-5). Violent tornadoes are capable of nearly catastrophic damage.
E. Fatalities - Due to their small spatial scales, tornadoes kill relatively few people.
On average 760 tornadoes occur in the US yet only 91 people are killed on average.
Therefore, 88% of all tornadoes kill no one, and this number is likely a gross
underestimation as many tornadoes go unreported. Most fatalities occur in
association with a few large tornadoes rather than with many smaller ones.
Analysis shows that only 1% of all tornadoes are responsible for over 2/3 of all
deaths. Mobile homes are frequently the site of many deaths as these dwellings
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offer little protection against strong winds. Automobiles are also the locale of high
numbers of tornado related deaths. The safest place to be in a tornado is in the
basement of a well-constructed building.
F. Watches and Warnings - A weather watch (such as a severe thunderstorm or
tornado watch) states that atmospheric conditions are favorable for the
development of a particular severe weather event. A warning states that severe
weather is imminent and precautions should be taken immediately. Because
tornadoes develop rapidly and are of short duration, many times a warning may be
disseminated only after a tornado has touched down.
G. Waterspouts - Waterspouts are similar to tornadoes except that they develop
over warm waters and are usually smaller and weaker than tornadoes. They usually
form in association with cumulus congestus clouds and although they are over
water, the spouts are composed of water vapor condensing into the low pressure
area.
Chapter Boxes:
11-1 Physical Principles: Electricity in the Atmosphere - The atmosphere is capable of
carrying an electrical charge. The upper atmosphere is normally positive due to the
presence of the ionosphere. The voltage difference between the ionosphere and the surface
is about 400,000 volts and represents the fair weather electric field. Charges are constantly
transferred between the surface and the atmosphere in order to maintain the fair weather
field. One mechanism of transfer is the cloud-to-ground lightning bolt. However,
lightning only occurs if the voltage gradient greatly exceeds that of the fair weather electric
field.
11-2 Special Interest: A Personal Account of Ball Lightning - Ball lightning is
described in a personalized account. The description is one of a tennis ball sized object
with was not very bright, nor exhibited heat and possessed irregular and rather plastic
motions and properties.
11-3 Potential Instability - Potential instability occurs when a layer of dry air rests above
a warm, humid air layer. If the air is potentially unstable, lifting of an entire air layer can
cause the temperature lapse rate to increase, making it statically unstable. Potential
instability is an important component of severe thunderstorm situations.
11-4 Special Interest: Doppler Radar - The Doppler effect refers to the bending of waves
(sound, electromagnetic) relative to position. Objects moving toward a point have closely
packed waves while objects moving away from a point have elongated waves. Doppler
radar uses this principle to detect the motion of raindrops, ice particles, and other objects,
thereby detecting wind speed and direction. Objects moving toward the radar appear as
blue or green colors on the radar scope while objects moving away from the radar appear as
red or yellow. Doppler radar can detect the development of a mesocyclone within a
thunderstorm, greatly increasing warning times for potential tornadoes. Through the
1980s, the National Weather Service largely relied on radar units produced in the 1950s. In
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1991, the first Doppler NEXRAD (NEXt generation weather RADar) radar unit was placed
into service. Today there are 160 NWS and FAA Doppler radar units across the US.
Doppler units may also observe wind movements, or clear air echos, from debris or insects
in the atmosphere. <Web>
11-5 Special Interest: Tornado Research - Toto, or the Totable Tornado Observatory,
was constructed in 1980 to better study tornadoes. Storm chasers placed Toto in the path of
tornadoes in the hopes of obtaining detailed wind, pressure, and temperature data.
Although imaginative, Toto proved unreliable. Today, turtles are placed in the path of
tornadoes. These small clusters of instruments may be laid out across the potential tornado
path. Portable weather balloons with radiosondes attached are also used. Instrument
clustered rockets have also been used. The most reliable method of tornado observation is
Doppler radar with the best use coming from portable Doppler units.
Related Web Sites:
Lightning: http://covis.atmos.uiuc.edu/guide/stormspotters/html/multiline.html
Supercell Thunderstorm: http://covis.atmos.uiuc.edu/guide/stormspotters/html/index2.html
Doppler Radar: http://cirrus.sprl.umich.edu/wxnet
Tornado: www.sciam.com/explorations/052096explorations.html
Severe Thunderstorm: www.spc.noaa.gov
Media Enrichment:
ME11.1 - Movie of lightning.
ME11.2 - Cloud and 3D flow tracer movie.
ME11.3 - Movie of cloud development and vertical velocity.
ME11.4 - Vertical motion cross section movie.
ME11.5 - Cloud development and horizontal flow movie.
ME11.6 - Tornado movie.
ME11.7 - Movie of a squall line tornado over northwestern Louisiana.
ME11.8 - Image of global lightning.
ME11.9 - Shelf cloud over a gust front image.
ME11.10 - Image of mammatus.
ME11.11 - Second image of mammatus.
ME11.12 - Third image of mammatus.
ME11.13 - Tornado image.
ME11.14 - Image of a wall cloud.
ME11.15 - Second image of a wall cloud
Key Terms:
lightning
charge separation
leaders
sprites
blue jets
thunder
mesoscale convective systems
mesoscale convective complexes
air mass thunderstorms
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tornadoes
mesocyclones
squall lines
strokes
flashes
ball lightning
microbursts
mean electric field
downburst
funnel clouds
squall lines
supercells
gust front
hook
vault
severe thunderstorms
Doppler radar
St. Elmo’s fire
fair weather electric field
outflow boundary
Doppler effect
suction vortices
Fujita scale
waterspouts
sheet lightning
potential instability
wall clouds
Review Questions:
1. How common is cloud-to-ground lightning relative to cloud-to-cloud lightning?
Cloud-to-cloud lightning makes up about 80% of all lightning.
2. Describe the current theories regarding the formation of charge separation.
For lightning to strike, electrical charges must separate in different portions of a cloud.
Positive charges migrate to upper regions of the cloud while negative charges accumulate
in lower areas. Two theories account for this charge separation. The first, the
thermoelectric effect, is believed to involve ice crystals. Lighter positively charged crystals
rise to higher areas of the cloud while heavier, negatively charged crystals fall to lower
areas. Induction is the second theory concerning charge separation. Induction involves
opposite charges attracting. The top of falling ice pellets will be negatively charged due to
the positive charge of the upper atmosphere. Finer ice crystals or small drops, acquiring a
positive charge, move to the upper cloud area while the heavier pellets stabilize in the
lower cloud areas.
3. What is the difference between a lightning stroke and a lightning flash?
A stroke is comprised of a leader (stepped or dart) and a resulting return stroke. A flash is
a compilation of lightning strokes.
4. Describe the sequence by which electrical imbalances lead to lightning strokes.
For cloud-to-ground lighting to occur, a stepped-leader must emanate from the cloud base.
This ionized particle chamber forks repeatedly from a central chamber. Each section,
occurring over a single microsecond, traverses about 50 m in length. When contact is made
with an unlike charged region, electrons flow through the chamber resulting in an
illuminated return stroke. Air in the ionized chamber is rapidly heated producing a shock
wave (thunder). Typically, multiple strokes are needed to neutralize the charge
imbalances.
5. Briefly describe the following phenomena:
a. Ball lightning refers to a glowing round mass of electrified air that seems to roll through
the air or along a surface before dissipating or exploding. It thought to form when
lightning reduces silicon compounds in soil to tiny nanoparticles of silicon carbide,
silicon monoxide, and metallic silicon, all particles which are unstable in the
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presence of oxygen. They are ejected into air, cool rapidly and condense into filmy
chains and networks which burn brightly.
b. St. Elmo’s fire is another rare and peculiar type of electrical event. Ionization in the air,
usually just before the occurrence of cloud-to-ground lightning, can cause tall
objects to glow, and in some cases emit a hissing sound.
c. Sprites are very large but short-lived electrical bursts that rise from cloud tops as
lightning occurs below. They are usually red in color with blue or green tentacles
dangling from the reddish blob. They accompany only about 1% of all lightning
events.
d. Blue jets are upward electrical ejection from the tops of thunderstorms.
6. What causes thunder to occur?
Air heated (to approximately six times the surface temperature of the Sun) rapidly by a
lightning stroke initiates a shock wave known as thunder.
7. Why is the term heat lightning misleading?
The term implies that there is something unusual about heat lightning. However, it is just
distant lightning. Because of the distance, no thunder is heard.
8. What are the three stages of an air mass thunderstorm?
The cumulus stage, which is marked by all updrafts, feeding the storm and causing rapid
growth. The mature phase which is marked by both up- and downdrafts, and related
precipitation. And the dissipation phase, which is marked by all downdrafts.
9. How big are air mass thunderstorms and how long do they usually persist?
Air mass thunderstorms are typically highly localized but can grow to impressive heights,
topping out at the tropopause. They typically hast only about an hour.
10. Explain why some thunderstorms are self-extinguishing and others are able to develop into
severe thunderstorms.
Self-extinguishing thunderstorms largely describe air mass thunderstorms. These storms
are spatially and temporally limited. Their life cycles are comprised of a cumulus stage,
marked by updrafts, a mature stage, delineated by both up- and downdrafts, and a
dissipative stage, characterized by downdrafts. The latter stage is caused by the prevalence
of precipitation-initiated downdrafts which effectively cut off in-rushing updrafts. As the
updrafts carry water vapor needed to sustain the storm, the storm rapidly dissipates.
Severe thunderstorms occur when thunderstorms reach extraordinary proportions with
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regard to wind, precipitation, and/or produce tornadoes. Such storms are self-propagating
and typically occur in groups. The group configuration allows unusual circulation features
which support individual thunderstorms thereby increasing their spatial and temporal
extent. Groups of severe thunderstorms include Mesoscale Convective Complexes
(MCCs), and Squall Lines. Individual Supercell thunderstorms may also occur. These
storms are isolated storms which interact with environmental conditions to increase their
longevity and size. Atmospheric conditions important to virtually all severe thunderstorms
include: wind shear, high water vapor content through the lower troposphere, a forcing
(lifting) mechanism, and potential instability.
11. Describe the following types of storm systems:
a. Mesoscale convective systems - These are groups of thunderstorms which interact with
each other. There are two general types, MCSs that are also called squall lines, and
MCCs.
b. Squall lines - Linear bands of thunderstorms typically ahead of a cold front in a
mid-latitude cyclone are MCSs called squall lines.
c. Mesoscale convective complexes - Oval or roughly circular clusters of thunderstorms
are MCCs. Individual cells are part of a single system.
d. Supercells - These are intense powerful storms that contain a single updraft. They can
be isolated or as a part of an MCS.
12. How are outflow boundaries formed, and what effect do they have?
An outflow is nothing more than a thunderstorm downdraft that spreads out after hitting the
surface. They typically converge with warmer surrounding air to form an outflow
boundary.
13. Describe the processes that lead to tornado development in supercell and nonsupercell storms.
Supercell tornadoes begin with mesocyclone development in the presence of vertical wind
shear. Winds shift from southerly to westerly with height and strong updrafts tilt he
rotation region to a vertical position. The diameter of the rotating column decreases,
initiating an increase in wind speed through the conservation of angular momentum. The
rotating air column penetrates the cloud base producing a wall cloud. From this, a narrow
rotation region, the funnel, emerges and continues to the surface.
Nonsupercell tornadoes initiate from the interaction of outflow boundaries between two or
more thunderstorms. They may also generate from strong convection along a convergence
zone.
14. What features of Doppler radar make it an effective tool for severe storm forecasting?
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Doppler radar detects mesocyclone development. Conventional radar only detects hook
echoes associated with a mature tornado. By detecting the developing mesocyclone,
adequate lead times are available to administer warnings through the potential tornado
path.
15. What are hook echoes and vaults, and why are they important?
A hook echo is a small appendage attached to the main body of the storm on a radar image.
Hooks are significant because their appearance usually means tornado formation is
imminent. A vault is where the inflow of warm surface air enters a supercell. Air entering
the vault rises and water vapor condenses to form a dense concentration of water droplets.
On radar, the vault is typically found in the southeast quadrant, an area that appears
“missing” on the radar image.
16. Explain how microbursts form, and why they present a serious threat to aviation?
A microburst is a localized downburst. They create strong wind shear upon reaching the
surface. Planes flying into the microburst first encounter a strong headwind which
provides lift. Pilots respond by turning the aircraft downward. After the plane passes
through the core of the downdraft, the headwind disappears and is replaced by a tailwind
which causes the plane to abruptly lose altitude and crash.
17. Describe the location and timing of tornadoes in North America.
The regular mixing of vastly different air masses in the central US leads to a global high
frequency of these phenomena. The absence of topographic barriers and the presence of
the warm Gulf of Mexico allow for regular and repeated incursions of cold, dry
air-streaming equatorward to mix with warm, moist air flowing poleward. The Great
Plains, therefore, sees the highest frequency of tornadoes with Texas being the state with
highest tornado numbers. May is the time of greatest overall frequencies followed closely
by June.
18. Describe the process of tornado formation from supercell storms.
In a supercell, the first step of tornado production is the slow, horizontal rotation of a large
segment of cloud. Rotation begins deep within the cloud interior several kilometers above
the surface. These mesocyclones often preceded a tornado by as much as 30 minutes.
Mesocyclones depend on strong wind shear which results as winds shift from southerly to
westerly with altitude along with an increase in speed. Strong updrafts in the storm tilt the
horizontally rotating air so that the axis of rotation becomes vertical. Intensification of the
mesocyclone requires a decrease in rotational area which leads to an increase in wind
speed. The narrowing column of rotating air stretches downward and a portion of the cloud
base protrudes downward to form a wall cloud. A funnel cloud forms when a narrow,
rapidly rotating vortex emerges from the base of the wall cloud. If the funnel cloud touches
the surface, a tornado has officially occurred.
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19. What are wall clouds, and why is their appearance a cause for concern?
The narrowing of the mesocyclone causes air to stretch downward with a portion of the
cloud base protruding downward. This is the wall cloud. It indicates mesocyclone
strengthening and the likelihood of tornado development.
20. What is the leading threat to human safety when tornadoes hit?
Flying debris is the leading threat to humans.
21. Describe the Fujita scale for the classification of tornadoes. Which category is most common?
What is the highest F-value that can actually occur in nature?
The Fujita scale is a scale from 0 to 5 with a category 5 being the strongest tornado
possible. In the US, 69% of all tornadoes are classified as being weak (F0-F1), 29% are
strong (F2-F3) and only 2% are violent (F4-F5).
22. What is the difference between a tornado watch and a tornado warning?
A watch is issues by the SPC and signifies that a part of the country appears vulnerable to
impending severe storm activity.
A warning alerts the public to the observation of an actual tornado or the detection of
tornado precursors on Doppler radar.
23. How do waterspouts compare to tornadoes on average in terms of intensity?
They are usually smaller and weaker, but they can have wind speeds of up to 150 km/hr.
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