Thunderstorms and Tornadoes

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Thunderstorms and Tornadoes

• Chapter 10

Chapter 10
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Thunderstorms and Tornadoes

• Interesting topic due to their violent nature
• Estimates - 2,000 storms occurring over earth at
  any given second!
• In this chapter
• Types
• How they form
• Life cycle, lifetime
• Lightning and thunder
• Tornadoes

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Thunderstorms

• Thunderstorms - storm which contains lightning
  and thunder
• Many variations - may contain
• Strong surface winds (non-tornadic)
• Heavy rain / no rain
• Hail
• In general, 2 things are needed to get a
  thunderstorm...

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Thunderstorms

• 1) Warm, humid air rising
• 2) Conditionally unstable environment
• The trigger to get air rising is something weve
  talked about much of the semester already
• Heating at the surface
• Fronts
• Terrain
• Convergence (surface) and/or divergence (aloft)
• Usually a combination of these to get severe
  storms

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Air Mass (Ordinary) Thunderstorms

• These are the types we get in FL often during
  summer
• Called Air Mass storms because they occur well
  away from fronts within an air mass.
• What type of air mass?? Warm/humid??
• Maritime tropical (mT) usually
• Usually weak and short lived
• Dont produce much hail or strong wind

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Air Mass Thunderstorms

• 3 distinct stages make up the life of an air mass
  t-storm
• 1) Cumulus stage
• Warm, humid air rises and condenses into a
  cumulus cloud
• May only last 10-15 minutes if the air is very
  moist where the cloud is building
• Longer if air is dry

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Air Mass Thunderstorms

• 1) Cumulus stage
• Large amounts of latent heat released
• Air in cloud is warmer and continues to rise
• Cloud continues to grow as long as air rises from
  below
• No precipitation during this stage
• Not enough time and updrafts keep drops from
  falling

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Air Mass Thunderstorms

• As the cloud grows, particles eventually get
  heavy enough to fall
• drags air downward
• Also, sides of the cloud evaporate as dry air is
  entrained
• Cooling - heavier air
• These 2 effects combine to form a downdraft

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Air Mass Thunderstorms

• 2) Mature Stage
• Marked by the appearance of a downdraft
• Now we have an updraft/downdraft couplet called a
  cell
• Usually several cells within a storm
• This is now the most intense stage of the storm

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Air Mass Thunderstorms

• 2) Mature Stage
• Up/down drafts are most intense in the middle of
  the cloud
• Heavy rain possible in this stage
• An anvil top appears as the updrafts hit the
  tropopause

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Mature Stage
Anvil top
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Air Mass Thunderstorms

• 3) Dissipating Stage
• Downdrafts choke out the updrafts
• Cuts off the warm, moist air which is the energy
  supply for the storm
• Light rain only

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Air Mass Thunderstorms

• Whole process lasts about an hour
• Ultimately, the storm kills itself with
  downdrafts
• However..these cool downdrafts may act like
  mini-cold fronts as they hit the surface and
  spread out horizontally.

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Multicell Thunderstorms

• If new thunderstorms form due to downdrafts from
  air mass t-storms, they are called multicell
  thunderstorms
• In fact, most ordinary storms are multicell
  storms (as are most severe storms)
• That is, not very often will only a singular
  storm form

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Multicell Thunderstorms
Mature Stage
Cumulus Stage
Cumulus Stage
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Severe Thunderstorms

• These are the ones which produce
• Large hail, strong surface winds, heavy rain w/
  flooding, and tornadoes
• Technically (National Weather Service)
• 3/4 inch hail and/or 50 knot wind gusts at the
  surface required to use term severe
• One big difference from ordinary t-storms is that
  many severe storms have a tilted updrafts and
  downdrafts
• Tilt keeps the downdraft from killing the storm

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Severe Thunderstorms

• Tilted updrafts are caused by vertical wind
  shear
• Change in speed or direction of the wind w/
  height
• Both cause the storm to tilt

Speed shear
Directional shear
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Severe Thunderstorms

• Bottom line is that the storm can live for many
  hours
• Updrafts can become so strong that they keep
  suspended particles (like hail) in the cloud long
  enough to become very large

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Severe Thunderstorms

• The combination of tilted up/downdrafts actually
  acts as a self feeding mechanism for the storm
• Cool air hitting the surface acts like a plow
  which feeds warm air into the updraft

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Features of Severe T-storms

• Many interesting features of severe storms as
  downdraft air hits the surface
• Remember we said the cool downdraft air acts like
  a mini-cold front?
• Actually has its own name - Gust front or
  Outflow boundary
• Cool air and gusty winds as it passes you on the
  ground
• Can be seen on RADAR occasionally
• Also causes some strange looking clouds to form
  as warm air is forced to rise over the cool air...

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Features of Severe T-storms
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Features of Severe T-storms

• Shelf cloud (arcus cloud)
• Forms at the leading edge of the gust front as
  the warm air rises, cools, and condenses

Cool Air
Warm Air
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Features of Severe T-storms

• Occasionally, downdrafts can be very localized
  and spread out evenly in all directions
• like water from a tap
• Called a downburst
• Or a microburst if winds only extend outward 4
  km or less

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Microbursts

• Big time threat to aircraft
• Have caused several airline disasters
• At a), head wind causes extra lift
• At c), tail wind causes loss of lift and
  sometimes pilot overcompensation as well

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Supercells

• Huge rotating thunderstorm
• Eerie to look at
• Updrafts over 100 mph inside
• Hail the size of grapefruit
• These are the storms that produce the most
  destructive of tornadoes

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Supercells

• Can be distinguished from other types of t-storms
  on radar by their movement
• Right movers
• Move to the right of steering winds aloft and
  other storms in the area
• Common in the plains states
• Rare in Florida but possible on occasion

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Organized Thunderstorms

• In some instances (like ahead of cold fronts),
  groups of storms can become extremely organized
• Lines of storms extending nearly 1,000 miles
• Termed squall lines
• Damaging straight line winds

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Organized Thunderstorms

• Dry lines can also cause long lines of storms
  similar to squall lines
• Dry lines are similar to fronts but represent
  only a change in moisture Sometimes called
  dew-point fronts
• Ex. Temps in the 90s on each side but dew-points
  20s one side 70s other.

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Organized Thunderstorms

• Most frequent in eastern Texas, Oklahoma, and
  Kansas during spring and early summer
• Where dry air from Mexico and the Southwest meets
  moist air from the Gulf of Mexico

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Organized Thunderstorms

• 2 things happen to form t-storms
• 1) Moist and dry air masses converge (lifting)
• 2) Warm, dry air from west side of line rides
  over the warm, humid air from the east (due to
  terrain)
• This sets up a potentially unstable situation

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Organized Thunderstorms

• Mesoscale Convective Complexes
• Large organized mass of thunderstorms
• Can last for over 12 hours
• Very important for agriculture in the plains
• Supply a large amount of annual growing season
  rainfall

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Distribution of Thunderstorms

• 40,000 storms every day worldwide!! (14 million
  per year)
• Greatest in the southeast
• Bullseye over central Florida
• Smaller bullseye in central Colorado

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Distribution of Hail Days

• Hail maximum is in the central plains -- not
  central Florida
• Huh????
• Why is this if the most t-storms are in Florida?
• The storms in the plains are generally much
  stronger -- more severe weather

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Lightning and Thunder

• Lightning - an electrical discharge from
• cloud to cloud, cloud to ground, cloud to air
• 100 flashes per second worldwide! (8.5
  million/day)

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Lightning and Thunder

• When lightning occurs, the air it travels through
  is heated
• a) 540º F ?
• b) 5,400º F ?
• c) 54,000º F ?
• 54,000º F (5x hotter than Suns surface!)

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Lightning and Thunder

• We know heating causes air to expand
• Lightning heats the air so rapidly and to such a
  high temperature that it causes a shock wave to
  form - Thunder
• Travels all directions from the flash at the
  speed of sound - 1,100 feet / second or 1 mile
  in 5 seconds
• So you can estimate how far away lightning is by
  counting the seconds between the flash and the
  thunder (flash gets to your eyes instantly)
• 10 seconds 2 miles away

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Lightning and Thunder

• The type of sound you hear also tells you
  something about the distance
• Sharp clap very close
• Rumbling further away
• This type of noise is due to sound from different
  parts of the stroke reaching your ears
• Includes sound which has been bounced off of
  objects (buildings, etc.) before it reaches you
• So, what causes lightning in clouds?
• Its and electrical process so there must be some
  kind of charge separation ( somewhere, -
  elsewhere)

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Lightning

• How clouds become electrified is not completely
  understood.
• The most widely accepted theory is that
  electrification of clouds occurs due to
• interaction of ice super-cooled water drops
• Recall from earlier in the semester, a
  cumulonimbus (t-storm) cloud has 3 regions within
  itself...

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Lightning

• This setup is ideal for the cloud to become
  electrified
• How it works
• Mature storm - hail, ice, graupel particles being
  moved around by the up/down drafts
• In the middle of the cloud, these particles come
  in contact w/ super-cooled water which freezes
  upon them..

Ice
Ice super-cooled water
Water
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Lightning

• As the freezing of water occurs (liquid to solid
  phase change), what happens in terms of latent
  heat?
• Latent heat released
• Result is that hail/graupel particles are a
  little warmer than surrounding ice crystals

Ice
Ice super-cooled water
Water
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Lightning

• Now heres the kicker
• There is a net transfer of positive ions to the
  ice
• So the hail/graupel is negatively (-) charged and
  the ice crystal positively ()
• charge separation
• Ice is light weight so it gets blown to the top
• Hail/graupel is heavier so it stays in the
  mid/lower part

Ice
Ice super-cooled water
Water
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Lightning

• So now the cloud has
• charge on top
• - charge in the middle
• -/ on the bottom
• mostly - on bottom except where precipitation is
  falling
• All of the negative charge in the mid/lower part
  causes objects below to become positively charged
• As the cloud moves along, so does the charge at
  the surface (like a shadow)

Ice
Ice super-cooled water
Water
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Lightning

• The strength of the charge separation is called
  the electrical potential gradient
• Greatest over tall objects
• Trees, buildings, etc.
• So dont run for the cover of a tree outside!
• When the electrical potential gradient becomes
  great enough, a lightning stroke forms

Ice
Ice super-cooled water
Water
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Lightning

• From the cloud base, lightning travels in a
  series of steps
• About 50-100 meters each
• 5/100,000 of a second pause in between each step
• Initial step is called the stepped leader
• Not usually visible
• As it approaches the ground, a charge from the
  surface rushes up to meet it.

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Lightning

• As the charges meet, a very bright return
  stroke appears from the ground up to the cloud
  base
• follows path of the stepped leader
• the return stroke is what you see
• Ever hear someone say lightning travels from the
  ground up?

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Lightning

• What about multiple flashes in the same place?
• Often the leader-return stroke process happens
  several times in the same channel
• However, after the first stroke, the leader is
  now called a dart leader
• So, theres only one stepped leader
• Every leader thereafter is a dart leader
• A 3 flash sequence would be
• Stepped leader, return stroke, dart leader,
  return stroke, dart leader, return stoke

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Lightning

• How do we keep lightning from destroying things?
• Like on the roof of the Love building, we have
  all kinds of expensive stuff
• What we do is actually try to attract the
  lightning to some other object.

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Lightning Rods

• They are attached to an insulated conducting wire
• Wire is attached to another metal rod buried in
  the ground
• So, the charge goes into the ground, not into the
  equipment

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Lightning

• Final notes about lightning
• Florida has the most lightning deaths annually
• Makes sense due to the fact that central Florida
  has the most thunderstorms (and tourists) annually

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Tornadoes

• Probably the most awesome of all weather
  phenomena
• Rapidly rotating wind around a small area of
  intense low pressure
• Come in all shapes/sizes
• Skinny, rope-like
• Huge - over 1 mile wide
• Only occur in cumulonimbus (t-storm) clouds

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Tornadoes

• On average
• Roughly 600 meters in diameter
• Last only a few minutes
• Travel about 4 miles
• Just averages though
• Can be much larger
• Last for many hours
• Travel 100s of miles
• So, there is a great deal of variability w/ them

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Tornado Occurance

• U.S. is the tornado capitol of the world!
• Over 900 annually
• Most occur in the plains and southeast
• just like t-storms
• The maximum in the plains is sometimes referred
  to as Tornado Alley
• Clashing of cool/dry and warm/moist air masses
  here

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Tornado Facts

• 75 of all tornadoes occur in Spring/Summer
• March to July
• What time of day (based on what you know about
  t-storms in Tallahassee)?
• 4-6 p.m. mostly
• Most of the strongest ones occur in the plains
• Weaker ones elsewhere

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Fujita Scale

• Scale which categorizes tornadoes based on wind
  speed
• Ranges from F0 (weakest) to F5 (strongest)
• Table 10.1 (page 277)
• Almost all tornadoes are F0, F1, or F2
• Only 1 F5 per year
• Its the small of strong tornadoes that account
  for the most deaths

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Tornadoes

• Many of the strongest tornadoes actually contain
  several smaller circulations within them -
  multiple vortex tornadoes
• Each smaller circulation is called a suction
  vortex

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Tornado Formation

• How do they form?
• Some debate but.
• 1) Most are in Spring
• 2) Most occur in severe t-storms
• 3) Usually in a conditionally unstable
  atmosphere
• Based on studies, meteorologists have come up
  with this diagram of the ideal setup for tornado
  formation

Divergence
Unstable
Rising air
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Tornado Formation

• Recall that all t-storms have updrafts
• But to get a tornado, the updraft must rotate
• Due to wind shear
• Mesocyclone - rotating updraft and precursor of
  tornadoes
• Doppler radar can detect mesos before they reach
  the ground

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Tornado Formation

• Process
• Mesocyclone appears at low/mid-levels
• As it rises, it is stretched vertically and
  shrinks horizontally
• This causes the speed to increase
• Like a skater pulling arms in
• Air rushes in toward lower pressure, expands, and
  cools
• Wall cloud forms

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Tornado Formation

• Process
• If air contains enough moisture, this process
  causes a funnel cloud to form
• As the funnel approaches the surface, it picks up
  dirt/debris and forms the typical tornado
• The more debris that gets dragged in, the more
  ominous the storm appears

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Tornadoes

• Watches vs Warnings
• Whats the difference??
• Watch - conditions are favorable for development
• Thats why you may get a watch when its bright
  and sunny outside
• Warning - Tornado has been visually identified by
  someone OR Doppler radar sees a mesocyclone

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Tornadoes on Radar

• Appear as a hook echos on radar
• Usually on the edge of the storm
• Rain is being wrapped around the tornado by
  strong winds

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Tornadoes on Radar

• Doppler radar gets its name from being able to
  detect speed/direction of rain
• Not just intensity
• Phase shift in the radar beam (Doppler principle)
• So, it can detect areas of rapidly changing wind
  direction
• Mesocyclones tornadoes

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Tornadoes on Radar

• Rotations above the surface are termed mesos
• mesocyclones
• Smaller scale rotations at or near the ground are
  called tornado vortex signatures (TVS)

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Tornadoes on Radar

• Detection of mesos is important since it gives
  forecasters time to warn of tornadoes before they
  touch down
• 30 of all mesos produce tornadoes
• 20 minutes after initiation