Thunderstorms and Lightning
Review of last lecture
• Bergeron process: happens with coexistence of ice and supercooled water. Key: Saturation vapor pressure of ice < that of
super-cooled water at the same temperature.
• Further growth of ice crystals (riming and
aggregation)Extensive riming in strong updrafts (graupel, hail)
• Definition of airmasses. Bergeron classification of air masses
(3 letters).
• Fronts: 6 types (cold, warm, stationary, occluded, dry line,
squall line)
• Cold front (narrow, fast, heavy precipitation), Warm front (wide,
slow, light precipitation)
• The developmental stages and vertical structure of middle
latitude cyclones (boundary between northern cold air and
southern warm air, upper level low to the west of surface low)
• The three regions of cyclogenesis and typical tracks
The most common atmospheric
circulation structure
H
L
Radiation
Cooling
or No
Heating
Convection
Heating
Latent/Sensible
Conduction
H
L
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Global Climate System
Spatial
Scale
Globe
Global warming
Multi-decadal Oscillation
Ice age
Glacial cycle
El Nino
Biennial Oscillation
Continent
Monsoon
Madden-Julian Oscillation
Tropical waves
Annular modes
Diurnal variation
Heat wave
Midlatitude cyclone
Tropical cyclone
State
City
Football
field
1 mm
1 m
10-4 m
Mesoscale convective system
Thunderstorm
Tornado
Shallow convection
Boundary layer turbulence
Cloud/precipitation
Radiation
Composition
10-15sec 1sec 1min
1day
1mon
1yr
10yr
100yr
100,000yr
Time Scale
Convective systems
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Tornadoes: about 100-600 m, last 1 minute to 1 hour
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Thunderstorms: about 10 Km, last 10 minutes to a
couple of hours. 3 types: ordinary, multicell, supercell
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Mesoscale convective systems (MCSs): A cloud
system that occurs in connection with an ensemble
of thunderstorms and produces a contiguous
precipitation area on the order of 100 Km or more in
at least one direction, and often last for several hours
to a couple of days.
Thunderstorms
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A storm containing lightning & thunder
Convective; form when warm, humid air
rises in conditionally unstable environment
The warmer the rising air parcel is relative
to environment, the more buoyant force is
driving it upwards (stronger convection)
Trigger to start uplift: warming sfc, terrain
(orography), converging sfc winds, frontal
zones, divergence aloft (or combination)
Thunderstorms I. Ordinary Storms
Three stages have been identified in ordinary thunderstorms:
a) an unstable atmosphere and vertical updrafts keep precipitation suspended
b) MATURE: entrainment of dry air that causes cooler air from evaporation,
triggering downdrafts and falling precipitation and gust fronts
c) DISSIPATING: weakening updrafts and loss of the fuel source after 15 to 30
minutes.
Thunderstorms II. Multicell Storm
Cool downdrafts leaving a
mature and dissipating storm
may offer relief from summer
heat, but they may also force
surrounding, low-level moist air
upward.
Hence, dying storms often
trigger new storms, and the
successive stages may be
viewed in the sky.
A Multicell Thunderstorm
Thunderstorm III. Supercell Storm
Storms producing a minimum of
a) 3/4 inch hail and/or
b) wind gusts of 50 knots and/or
c) tornado winds, classify as severe.
Formation of supercell thunderstorms
1. Before thunderstorms develop, a
change in wind direction and an increase
in wind speed with increasing height
creates an invisible, horizontal spinning
effect in the lower atmosphere.
2. Spinning horizontal vortex tubes created
by surface wind shear may be tilted and
forced in a vertical path by updrafts. This
rising, spinning, and often stretching
rotating air may then turn into a
mesocyclone.
3. Most strong and violent tornadoes form
within this area of strong rotation.
Vertical structure of a supercell thunderstorm
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In ordinary storms, the downdraft and falling precipitation cut
off the updraft. But in supercell storms, winds aloft push the
rain away and the updraft is not weakened and the storm
can continue maturing and maintain its structure for hours.
Cloud structure of a supercell thunderstorm
Video: All Alone in the Night Time-lapse footage of the Earth
as seen from the ISS
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http://www.youtube.com/watch?v=FG0fTK
AqZ5g
Lightning: Introduction
• Lightning is a discharge of
electricity, a giant spark
• 80% of lightning occurs when
clouds discharge electricity. This is
referred to as cloud-to-cloud
lightning; occurring when voltage
gradient overcomes the electrical
resistance of the air.
• 20% of lightning occurs when
electrical discharge travels
between the base of the cloud and
the surface. This is referred to as
cloud-to-ground lightning.
Video: Lightning in super slow
motion
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http://www.youtube.com/watch?v=RLWIBr
weSU8
Processes of Lightning Formation
1. Charge separation. Charge layers in the
cloud are formed by the transfer of positive
ions from warmer graupel to colder ice crystal
when they collide with each other.
2. Stepped leader. When the negative charge
near the bottom of the cloud is large enough to
overcome the air's resistance, a stepped
leader forms.
3. Return stroke. A region of positive ions
move from the ground toward this charge,
which then forms a return stroke into the cloud.
4. Dart leader. Not all of the first stroke
neutralizes the negatively charged ions and
results in another leader in 1/10 of a second
Thunder
• Charge differences between the thunderstorm and ground can
cause lightning strokes of 30,000°C, and this rapid heating of air
will creates an explosive shock wave called thunder.
• It takes about 3 seconds for thunder to travel 1 kilometer (5 sec
per mile). A lag in lightning strike and thunder occurs due to
sound traveling slower than light.
• When thunder is farther away, the echoing of sound waves off of
objects (like buildings and hills) causes thunder to sound
rumbling.
Global distribution of lightning strikes
Significant difference between continent and ocean
Lightning Safety
Practice the following when lightning is present:
• Always take cover in a building.
• Do not stand under a tree or other tall object that might
serve as a lightning rod.
• Avoid standing on mountain summits, ridges, rooftops,
or other high areas.
• Avoid caves.
• Avoid open water (pool, lake, or hot tub).
Summary
1. The general size and lifetime of mesoscale convective
systems, thunderstorms and tornadoes. 3 types of
thunderstorms.
2. 3 stages of the ordinary thunderstorms. Downdraft and
falling precipitation cut off the updraft.
3. Formation of multi-cell thunderstorms. Downdrafts initiate
new thunderstorms in nearby regions.
4. 3 stages of the supercell thunderstorms. Winds aloft push
downdraft/precipitation away and the updraft is not
weakened.
5. Two types of lightning (cloud-to-cloud 80%, cloud-toground 20%)
6. 4 steps of lightning development. How fast does thunder
travel?
Works cited
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http://www.nps.gov/romo/index.htm
http://www.rockymountainhikingtrails.com/lightninghiking-rocky-mountains.htm
http://en.wikibooks.org/wiki/Engineering_Acoustics/Thun
der_acoustics