Transcript Slide 1

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THUNDERSTORMS
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Ingredients for a thunderstorm
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Lift from
Cold/warm front
 Gust front
 Daytime heating
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Low-level moisture
Instability
Wind shear
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Severe thunderstorms
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One or more of the following:
Winds stronger than 58 mph
 Hailstones larger than 0.75 in
 Tornado
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Air mass thunderstorms
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Single Cell or “Pulse”
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Single-Cell Storms
Moves with mean environmental
wind over 5-7 km.
30- 60 minute duration.
Rainfall can be moderate to heavy,
small hail is possible.
Vertical wind shear is small.
Form in very weak shear
environments with new storms
forming in an unorganized manner
Can be severe at dissipation
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Multi Cell Clusters
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Cluster of 2-4 storms
Feed off gust front
Moderate unidirectional
shear
Moderate instability
Follow mean 700 – 500
mb flow
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Mesoscale Convective Complex
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May – Aug (86%)
Multi celled storms “gone
wild”
Oval shaped
Cloud shield
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< -32°C = 100,000km2
< -52°C = 50,000 km2
Last at least 6 hours
Usually night events
Flash floods
Hail
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Squall Lines
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Linear multi cell storms
Frontal or pre-frontal
Vertical tilt increases
severity
Moist low level S or SE
inflow
Mid level flow >25 knots
and ┴ to low level flow
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Squall Line and gust front
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Squall Line Lifespan
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Supercell
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Balanced updraft and
downdraft
Low level
mesocyclogenesis more
likely
Moderate hook echo
Tornadoes are common
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Supercell
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Hook Echo of Supercell
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Supercell
Photo by: Steven Maciejewski
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Supercell reflectivity
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Lightning and thunder
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Lightning = flashes of light caused by enormous
electrical discharges
Superheats the air abruptly resulting in
enormous shock waves
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thunder
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Lightning process
Source: Environment Canada
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Types of lightning
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Within cloud
Cloud-to-cloud
Cloud-to-air
Cloud-to-ground
Negative lightning
Positive lightning
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Positive lightning (~10%)
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Lightning strike density
Flashes
Days with flashes
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Hail
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Ice pellets
Only occur within
cumulonimbus clouds
The longer a frozen
pellet stays aloft in a
cloud, the larger the hail
can be
“Wet-bulb zero” height
important
Courtesy of: http://www.chaseday.com/hailstones.htm
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Hail frequency in the US
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Tornadoes
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As cumulonimbus clouds rise high into the
troposphere, the bottom winds move slow due
to surface friction, but the upper winds move
fast
Results in rotation of air parallel to the ground
Updrafts or downdrafts can cause the vortex to
become vertical
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Tornadoes
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Some keys to tornado formation
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Fujita scale
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F0-F1:
69% of
tornadoes;
5% of
deaths
F4-F5:
3% of
tornadoes;
70% of
deaths
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Tornado frequency in the US
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Severe wind frequency in the US
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