Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

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Understanding Weather and Climate 3rd
EditionEdward Aguado and James E. Burt

• Anthony J. Vega

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Part 4. Disturbances

• Chapter 11
• Lightning, Thunder, and Tornadoes

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Introduction

• 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
• Processes of Lightning Formation
• Cloud-to-cloud lightning, the most frequent type,
  occurs when discharges of electricity occur
  within a particular cloud or between clouds
• This 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 eventually discharge onto the
  positively charged ground
• The ground is typically negatively charged but
  the cloud charge offsets this by repelling
  electrons below

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• 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 occurs 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 being
  researched
• 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

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Charge separation in a cloud leading to lightning
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A strong electrical field occurs prior to
lightning
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• 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 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 return stroke is in the
  opposite direction
• Air in the conducting channel heats to about
  30,000K (54,000oF)
• Usually more than one stroke is needed to
  neutralize all negative ions
• Another leader, or dart leader, is initiated and
  a return stroke follows
• The process is repeated about 4-5 times on
  average
• Individual strokes are almost impossible to
  detect but the entire lightning flash, a
  combination of all strokes, typically flickers

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Development of lightning
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• Types of Lightning
• There are many types of lightning
• Ball lightning may be the most unusual
• A round mass of electrified air about as large as
  a basketball
• Displays rather bizarre behavior
• St. Elmos fire refers to tall objects glowing as
  ionization occurs in the air around them
• Sprites are large, short duration electrical
  bursts from the tops of clouds producing
  lightning below
• 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
• Thunder
• The rapid expansion of air associated 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

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Above A sprite Left A blue jet
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• To determine the distance in km, count the
  seconds between the stroke and thunder and divide
  by 3 (by 5 to determine distance in miles)
• Lightning without thunder being heard is
  sometimes called heat lightning
• 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 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

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• 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
• Spatially and temporally limited
• Each is comprised of a number of individual cells
  with each undergoing a particular life cycle
• The life cycle begins with the cumulus stage
• This begins with differential heating of the
  Earths surface leading to parcel formation and
  rising air
• Only updrafts are present as air rises and
  adiabatically cools
• Eventually, enough water vapor will be present to
  sustain vertical cloud development which occurs
  between 5-20 m/sec (10-45 mph)
• The mature stage is marked by precipitation and
  the presence of both up and down drafts
• Downdrafts are initiated through frictional drag
  associated with falling precipitation
• This is also a time of lightning and thunder

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• 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
• The dissipative stage occurs when downdrafts
  dominate airflow within the thunderstorms
• This suppresses updrafts and the addition of
  water vapor
• 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
• Fuzzy cloud edges indicate older portions of the
  storm which are glaciated (composed of ice
  crystals)

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Air mass thunderstorm lifecycle
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• Severe Thunderstorms
• Occur when winds exceed 93 km/hr (58 mph), have
  large hailstones (1.9 cm 0.75 in) or produce
  tornadoes
• 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 develop
• Usually occur in groups over fairly large areas
• Clusters are deemed mesoscale convective systems
  (MCSs), as squall lines, or as circular clusters
  called mesoscale convective complexs (MCCs)
• Individual storms develop in concert in a
  situation which propagates additional
  thunderstorms
• Many MCSs have life spans from up to 12 hrs to
  several days
• Systems may account for as much as 60 of the
  total annual rainfall
• Severe thunderstorms may also form from
  individual supercells which contain only one
  updraft (supercells may also be a part of an MCS)
  
• Atmospheric conditions supporting severe
  thunderstorms include wind shear, high water
  vapor content in lower portions of the
  troposphere, a forcing mechanism, and potential
  instability

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• Mesoscale Convective Complexes
• MCCs account for the greatest amount of severe
  weather in the U.S. and Canada
• Roughly circular clusters of thunderstorms which
  are self propagating in that individual cells
  create downdrafts which interact to form new
  cells
• Colder, denser downdrafts spread across the
  surface and help force warm, moist surface air
  aloft
• This outflow boundary initiates a new cell, most
  often along the southern edge of the MCC as the
  downdrafts intermingle with a usual stream of
  warm, moist surface air from the south
• The entire system typically propagates toward the
  east along with the associated upper air wind
• Squall Line Thunderstorms
• Differ from MCCs in that thunderstorms are
  usually linear
• Bands may be as long as 500 km (300 mi) usually
  about 300-500 km (180-300 mi) in advance of cold
  fronts
• Strong vertical wind shear is essential to the
  development of these prefrontal waves as it
  ensures that updrafts will be positioned ahead of
  the downdrafts
• This feeds moisture into the system which is also
  aided by gust front propagation ahead of the
  situation

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A squall line (MCS)
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An MCC over South Dakota
A radar image of outflow boundaries
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Thunderstorm movement in an MCC
Wind shear and vertical motions in a squall
line thunderstorm
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• 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 and squall lines
• 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
  
• Radar hooks 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

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Right Internal structure of a supercell Below
Organization of a supercell as seen on a radar
image (actual radar, below right)
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• Geographic and Temporal Distribution of
  Thunderstorms
• Thunderstorms develop where moist air is forced
  aloft
• Occurs frequently in the tropics, nearly daily in
  some locations
• In the U.S., most frequent region is the Gulf
  South
• Absolute peak in Florida as the state is a land
  protrusion into warm waters

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• Tornadoes
• 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 a few tenths of km
• Tornado Characteristics and Dimensions
• Typically have diameters of about 100 yards but
  may be much larger
• Usually a short lived phenomena lasting only a
  few minutes, but some have lasted for hours
• Movement is generally about 50km/hr (30 mph) over
  an areas 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)
• Tornado Formation
• Common to frontal boundaries, squall lines, MCCs,
  supercells and tropical cyclones
• Most violent tornadoes are associated with
  supercells

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• Supercell Tornado Development
• Begins with the development of a mesocyclone
• A large rotation region within the cloud interior
  which develops in the presence of vertical wind
  shear
• From the surface aloft, winds shift from
  southerly to westerly while speed increases
• Strong updrafts 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
• Only about 1/2 of all mesocyclones actually spawn
  a tornado

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A supercell with tornado
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Mesocyclone formation from a tilted horizontal
vortex
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A tornado producing supercell
A non-supercell tornado develops where the
outflows from separate storm downdrafts cause
convergence
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Tornado development along a convergence boundary
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• Nonsupercell Tornado Development
• 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
• The Location and Timing of Tornadoes
• The U.S. is the world leader in tornado
  production
• This results from the regular interaction between
  extremely unlike air masses which 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 occur in Tornado Alley, a
  region from the southern Plains to the lower
  Great Lakes
• Texas has the highest tornado frequency of any
  state
• 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

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Global tornado frequencies
Annual U.S. tornado frequencies
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Monthly tornado frequencies for the U.S.
(left) and by state (below)
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• Tornado Damage
• Winds, not pressure change, cause the greatest
  amount of damage
• Flying debris causes 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 scale levels (0-5), nearly 3/4 of all
  tornadoes fall into the weak categories (0-1)
  while 25 are classified as being strong (2-3),
  and only 1 are deemed violent (4-5)
• Violent tornadoes are capable of nearly
  catastrophic damage
• Fatalities
• due to their small spatial scales, tornadoes kill
  relatively few people
• On average 760 tornadoes occur in the U.S. yet
  only 91 people are killed
• 88 of all tornadoes kill no one
• Most fatalities occur in associated with a few
  large tornadoes rather than with many smaller
  ones
• Only about 1 of all tornadoes are responsible
  for over 2/3 of all deaths
• Mobile homes and autos are the sites of many
  deaths
• The safest place to be during a tornado is in a
  building basement

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• Watches and Warnings
• A weather watch (severe thunderstorm, tornado,
  etc.) 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, may times a warning may be
  disseminated only after a tornado has touched
  down
• Waterspouts
• Similar to tornadoes except that they develop
  over warm waters and are usually smaller and
  weaker than tornadoes
• 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

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End of Chapter 11 Understanding Weather and
Climate 3rd EditionEdward Aguado and James E.
Burt