The Thunderstorms

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Introduction Thunderstorms Squall lines Lightning/thunder Global distribution Tornadoes
ENV 111: Introduction to Meteorology
Lecture 6
Thunderstorms
ndettoel@2016 ENV 111: Introduction to Meteorology
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The weather storms
Convective and revolving storms

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Weather storms
The atmosphere exhibit several lifting mechanisms to generate clouds
and precipitation systems of diﬀerent scales and intensity
The instabilities include:
⇒ those that result directly in vertical mixing, such as convective instabilities
⇒ those associated with the meridional heating disparities that give rise to
baroclinic instabilities and the ubiquitous fronts
⇒ low and high pressure weather systems
So ”storm” is a generic term for all the potential disturbances that
create upward motion in the atmosphere
Many can be thunderstorms, or extratropical cyclones manifested as
rain- or snowstorms
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Weather storms
Global distribution of such events vary with climate, though exhibit
basic mechanisms for lifting air
Thunderstorms and tornadoes are mesoscale weather storms lasting
from minutes to few hours
Severe thunderstorms produce tornadoes, strong winds and large hail
Other atmospheric disturbances are associated with falling pressure,
hence grow into large weather systems
Hurricanes are macroscale weather systems at a synoptic scale
(2, 000 km) lasting for a few days to a week or more

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Thunderstorms
A thunderstorm is a convective
storm containing lightning and
thunder;
It is characterised by turbulent
vertical fluxes of heat and
momentum
Produces gusty surface winds
(> 25 m/s), heavy rain and large
hail (> 1.9 cm diameter)
The storm may be a single
cumulonimbus cloud, or several
thunderstorms clustered together
Figure 2 : Thunderstorm with a flash
of a lightning in a cloud
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Thunderstorms
Thunderstorms form with rising warm and moist air in a conditionally
unstable environment; i.e. no hydrostatic balance
Mechanisms that cause air to rise include:
⇒ random, turbulent eddies that lift small bubbles of air
⇒ unequal heating at the surface
⇒ the effect of terrain (such as small hills) or the lifting of air along shallow
boundaries of converging surface winds
⇒ large-scale uplift along mountain barriers and rising terrain
⇒ diverging upper-level winds, coupled with converging surface winds and
rising air
⇒ warm air rising along a frontal zone

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Life cycle of a thunderstorm
Figure 3 : Stages of development of an ordinary cell thunderstorm
Cumulus stage: warm, humid air rises, cools and condenses into a
single cumulus cloud or a cluster of clouds
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Ordinary cell thunderstorms
Latent heat release keeps the rising air (up-drafts) inside the cloud
warmer (less dense) than the air surrounding it
Normally, neither lightning nor thunder occurs during this stage
Mature stage: Cloud particles grow larger and heavier through
collision-coalescence process
Drier air from around the cloud is also being drawn into it through
entrainment process which causes some of the raindrops to evaporate
and chill the air
Colder and heavier air begins to descend as a down-draft

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When the cloud top reaches the stable atmosphere, the cloud takes an
anvil shape, as upper-level winds spread the cloud’s ice crystals
horizontally
Strong up-drafts and down-drafts create severe turbulence
Lightning and thunder occurs at this stage accompanied with heavy
rain (and occasionally small hail)
Often a down-rush of cooler air occurs at the surface, with the onset
of precipitation
Gust front occurs as a surface boundary separating the advancing
cooler air from the surrounding warmer air
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Dissipating stage: The up-drafts weaken as the gust front moves away
from the storm (less supply)
The down-drafts dominate much of the cloud
An ordinary cell thunderstorm does not normally last very long as the
down-drafts cut oﬀ the storm’s fuel supply by destroying the humid
up-drafts
Usually the three stages take a duration of one hour or less

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Multicell thunderstorm
Figure 4 : A model for a multi cell thunderstorm
It is a thunderstorm with many cells, each in a diﬀerent stage of
development; usually occurring in regions of moderate-to- strong
vertical wind speed shear
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Multicell thunderstorms
The up-drafts rise over the down-drafts and can generate new cells
that grow to mature thunderstorms too
Precipitation inside the storm does not fall into the up-draft (as in
ordinary cell thunderstorm), so the storm’s fuel supply is not cut oﬀ
An overshooting top occurs when convection is strong and the
up-draft intense

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Multicell thunderstorms
Mammatus clouds form when air spreads laterally into the anvil and
sinks in this region
The cold down-drafts at the earth’s surface, pushes outward in all
directions
This produces a strong gust front as the leading edge of the cold
out-ﬂowing air (also called straight-line winds as opposed to rotating
winds of a tornado)
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Squall lines
Figure 5 : A model for a pre-frontal squall-line thunderstorms
A squall is a strong gust of wind which may, or may not, be
accompanied by a change in direction
A squall line is a line of developing cumulus clouds with strong gusts
of wind

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Squall lines
So multicell thunderstorms may form as a line of thunderstorms
Often, it is a result of convection along cold fronts where new cells of
cumulus clouds develop constantly as the old ones die
Ordinary squall lines may form along a gust front, with a stationary
front, with a weak wave cyclone, or where no large-scale cyclonic
storms are present
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Squall lines
Squall lines are one type of convective phenomenon called a mesoscale
convective system (MCS)
Many ordinary squall lines occurring in middle latitudes exhibit a
structure similar to squall lines that form in the tropics
Mesoscale convective systems are organized thunderstorms that can
take on a variety of conﬁgurations, from the elongated squall line, to
the circular mesoscale convective vortex

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Supercell thunderstorms
Figure 6 : Model for a supercell
thunderstorm
A supercell is an intense, long-lasting
thunderstorm with a single violently
rotating up-draft
It occurs in a region with strong vertical
wind shear (speed and/or directional
shear), where the outﬂow of cold air
from the down-draft never undercuts
the up-draft
Strong wind shear create horizontal
spin, which, when tilted into the
up-draft, causes it to rotate
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Lightning and thunder
Figure 7 : Lightning and thunder
A lightning is simply a
discharge of electricity, a
giant spark,
It usually occurs in mature
thunderstorms
Lightning may take place:
⇒ within a cloud,
⇒ from one cloud to another,
⇒ from a cloud to the
surrounding air,
⇒ from a cloud to the ground

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Lightning and thunder
Thunder is shock wave with booming sound
It occurs when air is extremely heated by lightning and expand
explosively
The lightning stroke can heat the air to an incredible 30, 000 C
Questions to ponder:
Why thunder is heard afterwards?
How can we estimate the distance where lightning has just stroke?
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Geographical distribution of thunderstorms
Each day, more than 50, 000 thunderstorms occur throughout the
world
The equatorial landmasses is the most prevalent location
The most conducive factors for thunderstorms formation are:
⇒ warmth
⇒ moisture
⇒ low-level convergence especially over water along the intertropical
convergence zone to initiate uplifting of air

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Geographical distribution of thunderstorms
The heat energy liberated in these storms helps the Earth maintain
its heat balance by distributing heat poleward
Thus, thunderstorms are less prevalent in dry climates (polar regions
and the desert areas)
In many areas,they form primarily in summer during the warmest
part of the day when the surface air is most unstable
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Tornadoes
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Tornadoes
A tornado is a rapidly rotating column of air, extending down from a
cumuliform cloud, that blows around a small area of intense low
pressure with a circulation that reaches the ground
It circulation at the ground is either as a funnel-shaped cloud or as a
swirling cloud of dust and debris
Table 1 gives characteristic features of tornadoes (those occur in
North America)
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Tornadoes
Table 1 : Characteristics features of tornadoes
Characteristic feature Description
Rotation Often anticlockwise
Shapes and forms twisting rope-like funnels,
cylindrical-shaped funnels,
massive black wedge-shaped funnels,
also like an elephant’s trunk
Diameter Usually 100 − 600 m
Wind speeds Often less than 100 knots
Steering current south-westerlies for those form ahead
an advancing cold front
Duration and path length Few minutes, about 7 km
Weather heavy rain, hail, thunder, lightning

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Tornado: formation
Figure 9 : A simpliﬁed view of a supercell thunderstorm illustrating formation of a
tornado
Necessary conditions:
⇒ conditionally unstable atmosphere
⇒ strong vertical wind shearndettoel@2016 ENV 111: Introduction to Meteorology
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Tornado: formation
In supercell thunderstorms, the drawn warm and humid air spins
counter-clockwise
Near the top of the storm, strong winds push the rising air to the
north-east
Up-draft and down-drafts are separate; the storm is sustained
Vertical wind shear causes the air near the surface to rotate about a
horizontal axis i.e. a vortex tube
⇒ wind directional shear in the vertical
⇒ wind speed shear in the vertical (rapid increase with height)
⇒ southerly low-level jet above weaker surface winds can cause also rotation
Up-draft strength increases as pressure in the mid-levels of the
thunderstorm drops due to up-draft rotation

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Tornado: Life cycle
Figure 10 : Tornadoes at mature (L) and decaying (R) stages in America
dust-whirl stage,
⇒ dust swirls upward from the the surface
⇒ short funnel extending downward from the thunderstorms base
⇒ damage is normally less severe
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Tornado: Life cycle
organised stage,
⇒ Increasing in intensity and downward extent of the funnel
mature stage,
⇒ the funnel reaches its greatest width and is almost vertical
⇒ circulation usually stays in contact with the ground until it dissipates
⇒ damage is normally most severe
shrinking stage
⇒ decrease in the funnel’s width but its tilt increases
⇒ damage swath narrows though it may still inﬂict damage
decay stage
⇒ tornado is stretched into the shape of a rope
⇒ damage swath narrows though it may still inﬂict damage

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Tornado observation and distribution
Figure 11 : Monthly average number
of tornadoes n US from 2000 to 2010
A doppler radar discerns what goes on
inside a tornado- generating
thunderstorm
Tornadoes occur in all regions
worldwide, with thunderstorms
From 1950s to 2000s, number of
tornadoes in US has doubled from
about 600 per year
Importance of tornado intensity
distributions:
⇒ basic climatology research
⇒ risk assessment
⇒ insurance industry
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Tornado winds and clasification
Figure 12 : Damaging effect of a tornado
The strong winds of a tornado can
destroy buildings, uproot trees, and
hurl all sorts of lethal missiles into
the air
For an approaching tornado, it is
advised to take shelter in a
basement, storm shelter, or a
dedicated safe room
If no shelter exists, lie flat on the
ground in a depression or ravine
Tornadoes are classified according to
their rotational wind speed (Dr. T.
T. Fujita)

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Tornado winds and clasiﬁcation
The winds are estimated based on the damage caused by the storm
and since 2007, a new scale ”Enhanced Fujita Scale” (EF Scale) is
being used:
.
Table 2 : Enhanced Fujita (EF) Scale for damaging Tornado Winds
Scale Category Knot Possible damage
F0 Weak 56 − 74 break tree branches
F1 75 − 95 snap trees
F2 Strong 96 − 117 uproot large trees
F3 118 − 143
F4 Violent 144 − 174 level trees, overturn cars
F5 > 174 move autos over 100 meters
damage steel-reinforced structures
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Hurricanes
Figure 13 : Tropical cyclone Giovana in Indian Ocean, February 9, 2012
A hurricane is one of the tropical revolving storms whose generic term
is Tropical cyclone

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Tropical cyclonic storms
Tropical cyclonic systems can be classified using either the Beaufort
scale of wind intensity or the pressure deficit:
System BF No. Vortex speed Pressure deficit
Depression 5 17 − 21 5 mb
” 6 22 − 27
” 7 28 − 33
Storm 8 34 − 40 15 mb
” 9 41 − 47
” 10 48 − 55
” 11 56 − 63
Cyclone 12 64− 30 mb
The local names used for tropical cyclones include:
⇒ Cyclone
⇒ Typhoon, from Chinese word ”Taifung” meaning ”big wind”ndettoel@2016 ENV 111: Introduction to Meteorology
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Tropical cyclones
Cyclones occur mostly during summer (late), though they can form at
any time of the year; often in western North Pacific
.
Table 3 : Frequency of occurrence of tropical cyclones
Region Frequency Season
%
Western North Pacific 30 June - October
North Indian Ocean 15 April - June;
October - November
South Western Indian 14 December - April
West Atlantic Ocean 12 July - October
East Pacific (north of equator) 11 July - October
South Pacific 11 December - March
North and West of Australia 7 December - February

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Regions and tracks
Figure 14 : Tracks of cyclones in regions of occurrence
Cyclones originate over eastern part of tropical oceans where trade winds
had long passage. The tracks are guided by basic ﬂow in upper
troposphere, Coriolis force, outﬂow jet, and terrain feature like an island
or mountain ndettoel@2016 ENV 111: Introduction to Meteorology
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Structure of a Hurricane
Dimension: 200 − 1000 km radius; 10 − 15 km high
The diameter is smallest when the cyclone is closest to the equator,
and it increases as the cyclone recurves poleward
The cyclones originate in the doldrums over the ocean between 6 ◦
and 20 ◦
North and South, when the doldrums is farthest away from
the equator
However, no cyclones occur in the South Atlantic Ocean since in this
ocean, the doldrums (ITCZ) never cross the equator southwards.

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Figure 15 : Hurricane Elena over the Gulf of Mexico
Hurricane Elena, as one of the average storm had its central pressure
of 955 mb, and sustained winds of 105 knots
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The storm’s thickest clouds covered an area of about 500 in diameter
An eye is the centre of the storm with a relatively clear area:
⇒ Winds are light and clouds are mainly broken
⇒ Surface air pressure is lowest, around 955 mb
⇒ The width of the eye is almost 40 km
The eye-wall is a ring of intense thunderstorms that whirl around the
storm’s centre
⇒ It may extend upward to almost 18 km above sea level
⇒ Heaviest precipitation and the strongest winds exist here

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Vertical structure
Figure 16 : A model depicting a vertical structure of a cyclone
The vertical view of a hurricane shows that the storm is composed of
an organized mass of thunderstorms
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Formation and life cycles of cyclones
The Conditional Instability of the Second Kind (CISK) summarize the
processes responsible for the formation of a tropical cyclone, that:
Depression with falling pressure leads to horizontal inﬂow which attract
vertical motion. The vertical motion leads condensation that cause the
formation of cumulonimbus clouds and release of latent heat. This leads to
further drop of pressure and the cycle continues
The life cycles of a tropical cyclone are characterized as:
⇒ Formative stage: disorganised squall type of clouds and rainfall; low winds
(about 34 kt); surface pressure of 1000 mb
⇒ Immature stage: wind speed at hurricane force (64 kt); clouds and rainfall
spiralling inward

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Formation and life cycles of cyclones
⇒ Mature stage: Low pressure (900 mb or less); more steady hurricane wind
force; torrential weather
⇒ Decaying stage: frictional dissipation; disruption of vortes; low moisture;
move to colder areas
Conditions necessary for the formation
⇒ SST of about 26.5 ◦
C. Warm ocean water favours convection instability
⇒ Enough moisture to provide latent heat of condensation
⇒ Pre-existing region of low level convergence e.g. monsoon lows, easterly
waves and ITCZ
⇒ Coriolis force large enough to provide vortex
⇒ Vertical instability aloft for formation of Cbs
⇒ Weak wind shears to maintain warm air and Cbs
⇒ Upper level divergence to enhance vertical motion and low level convergence
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Hurricane naming and Impacts (Homework)
Meteorologists have devised a means to name the hurricane whenever
they occur. Your are advise to read and find out why hurricanes are
named and what is the system used for naming. Further find out, at
which stage will a hurricane be given a name.
Hurricane can also be classified as we have seen in the case for
tornadoes. Then find out the system used for classifying hurricanes.
Hurricane is one of the most severe weather systems hence it can
cause a lot of damage. However, it is also possible that a hurricane
can be of benefit somehow. therefore, discuss the possible impacts
(positive and negative) of hurricanes in different sectors. Support your
discussion with vivid examples.

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