Part 4. Disturbances

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Part 4. Disturbances

• Chapter 11
• Lightning, Thunder, and Tornadoes

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Annual distribution of lightning strikes
There are about 40,000 thunderstorms daily on the
Earth that produce lightning. A few produce
tornadoes.
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Development of lightning and thunder

• (a) Positive and negative charges separate in the
  cloud.
• (b) The step leader is a flow of negative charges
  (electrons) toward positive charges.
• (c) A flow of positive charges moves toward the
  step leader.
• As the positive and negative charges combine, the
  lightning stroke is seen.
• A dart leader is a secondary stroke just after
  the first.

Thunder arises from the rapid heating of the air
by the lightning stroke, which sends out a sound
wave at 5 miles/sec.
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• Charge separation in a cloud
• Lightning only forms in clouds that extend into
  air that is below freezing
• Collisions between ice crystals and graupel may
  help transfer charge from solid ice to liquid
  films on some ice crystals
• Positive charges tend to accumulate at the top of
  a cloud, negative charges in the lower part of a
  cloud
• Runaway discharge -- electrons accelerated to a
  very high speed, colliding with air molecules and
  creating more free electrons. High-speed moving
  electrons radiate light as lightning.

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Strong electrical fields occur prior to lightning
Safest areas from lightning -- indoors or in an
automobile. Outdoors, do not stand under tall
objects. Do not touch telephones or electrical
appliances. Lightning can strike in the same
place twice!
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A positive stroke can occur when thunderstorms
become tilted
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A blue jet

• Types of lightning
• Forked
• Sheet (heat)
• Ball
• St. Elmos fire
• Sprites
• Blue jets

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• Types of Thunderstorms
• Air Mass
• Frontal
• Squall Line
• Mesoscale Convective Complexes (MCC)

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Air Mass thunderstorm lifecycle
Air Mass thunderstorms form in humid, unstable
air. Each cell lasts no more than a few hours
from development to dissipation.
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• Severe Thunderstorms
• Winds exceed 93 km/hr (58 mph), have large
  hailstones (1.9 cm 0.75 in) or produce tornadoes
• Mesoscale convective complexs (MCCs)
• Self-propagating thunderstorm systems
• Mesoscale convective systems (MCSs) include MCCs
  and squall lines
• Squall line -- linear band of thunderstorms
  usually out ahead of a cold front (a type of MCS)

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An MCC over South Dakota
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A radar image of outflow boundaries
Outflow boundaries -- front edge of cold air
flowing out away from a thunderstorm a gust
front occurs at an outflow boundary
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Direction of movement of the individual
thunderstorm cells
Thunderstorm movement in an MCC
Cells dissipating
Movement and lifecycle of individual thunderstorm
cells (A, B, C, D, E, F, and G) and how they
change with time
Direction of movement of the line of thunderstorms
Cells forming
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A squall line (MCS)
This squall line is probably along or ahead of an
advancing dry line
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A radar image of a squall line
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Wind shear and vertical motions in a squall line
thunderstorm
Vertical wind profile in (a) here
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Gust front-induced shelf and roll cloud
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Internal structure of a supercell
A supercell thunderstorm is an extremely powerful
thunderstorm cell. Supercell thunderstorms can
spawn tornadoes.
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Organization of a supercell and actual radar
signature
Hook echo
Outflow boundary
The above are map views of a supercell
thunderstorm.
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• Downbursts, Derechos, and Microbursts
• Downbursts -- strong downdrafts in a mature
  thunderstorm
• Derecho -- MCS-induced strong downdraft that can
  last for hours
• Microburst -- small diameter downburst that
  usually lasts only a few minutes

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Microbursts create aviation hazards
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• Tornadoes
• Tornado characteristics and dimensions
• 100-yard average diameter
• Movement 50km/hr (30 mph) over 3-4 km (2-2.5
  mi)
• Winds 65 km/hr (40 mph) to 450 km/hr (280 mph)
• Tornado formation
• Squall lines, MCCs, supercells, tropical cyclones

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Tornado-producing supercell
Tornadoes typically drop out of the wall cloud on
the southwest side of a supercell
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A possible mechanism of tornado formation
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Tornado development along a convergence boundary
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A non-supercell tornado development along outflow
zone
Circular areas shows places where thunderstorm
inflow and outflow circulation resulted in
vorticity and tornado development
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Global tornado frequency
Most of Earths tornadoes occur in the lower
elevation areas of North America
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Tornado Alley
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Most tornadoes occur in the springtime, when the
contrast between warm and cold air in the
atmosphere is the greatest
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-- Most injuries and deaths in tornadoes are in
automobiles and mobile homes and are caused by
flying debris -- US averages 91 tornado deaths
each year -- Safest area in a tornado is in a
basement or an interior room, away from windows
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Multiple suction vortices greatly increase damage
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Linear tornado damage path
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• Tornado outbreaks
• A single weather system producing a large number
  of tornadoes

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• Waterspouts
• Similar to tornadoes
• Develop over warm waters
• Smaller and weaker than tornadoes

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End of Chapter 11 Understanding Weather and
Climate 4th EditionEdward Aguado and James E.
Burt