Thunderstorms

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Thunderstorms
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Thunderstorms

• a storm containing lightning and thunder,
  towering cumulonimbus clouds. They do not
  generally rotate (non-cyclonic).
• They are associated with midlatitude cyclones,
  localized convection, orographic lifting and
  tropical cyclones.

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Thunderstorm Formation

• Ingredients
• warm, moist air (often mT)
• unstable (or conditionally unstable if lifting
  mech.)
• encouraged by diverging air aloft

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Thunderstorm Life Cycle
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The Geography of Thunderstorms
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Thunderstorm Frequency

• Probably 1500 to 2000 thunderstorms active around
  the world at any given time.

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Violent Associations

• Hail
• Downbursts(Microbursts)
• Lightning and thunder
• Tornadoes

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Hail

• Formation

• Largest? Coffeyville, KS,
  1970 (1.75 lb, 14 cm diameter)

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Downbursts

• Macrobursts - extreme downdrafts associated with
  thunderstorms.
• Microburst
• smaller
• lt 5 minute duration
• winds up to 146 mph

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Lightning

• discharge of electricity that occurs in mature
  thunderstorms
• Cause charge separation in cloud sets up
  electrical potential

• Role of lightning is to equalize these
  differences in electrical potential.
• Important fixer of Nitrogen.

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Frequency of Occurrence

• Globally, lightning flashes ? 100 times per
  second.

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Geography of Lightning
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USA Real-time Lightning
http//www.intellicast.com/Storm/Severe/Lightning.
aspx
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Thunder

• Air poor conductor of electricity
• Huge electrical potential develops
• Lightning bolt superheats the air (30,000?C,
  54,000 ?F)
• Surrounding air expands violently ? sound wave

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Distance from the Thunderstorm

• Speed of light 186,000 m/s
• Speed of sound about 1000 ft/s or 1/5 mile/s
• Distance calculation
• for each 5 seconds, about 1 mile away

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Key Points

• Thunderstorm formation and life cycle
• Thunderstorm geography and frequency
• Violent associations
• hail, downbursts, lightning/thunder
• next lecture.tornadoes

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Tornadoes

• Tornado Clip

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Tornado Formation

• Severe Super Cell Thunderstorm (1 spawn
  tornadoes)
• Often occur along cold fronts and move to NE
• Strong updrafts
• Wind shear (change in direction or strength of
  winds from top to bottom of storm)

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Funnel Cloud v. Tornado
Tornado
Funnel Cloud
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Waterspout- over water - less power- smaller
tempgradient
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World Tornado Geography

• USA 90 of all tornadoes

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USA Tornado Geography

• Every state!
• Tornado Alley (TX, OK, KS, NE)

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USA Tornado Season
Most common when temperature gradient in central
U.S. is greatest.
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St. Louis Tornadoes By Hour
Source NCDC, 2007
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USA Tornado Occurrence

• About 800-1000 tornadoes spotted each year.

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Tornado Classification

• Fujita F-scale
• F0 (winds lt 120 mph) to F5 (winds 200-300 mph)
• based on rotational wind speed and damage done
• Size Tornadoes generally lt 1/4 mile wide (damage
  path only 150 ft.)

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Deadliest Tornados

• U.S.A.
• Tri-state tornado outbreak of March 18, 1925
• Kills 689 people in Missouri, Illinois Indiana
• World
• April 26, 1989 - Bangladesh - claimed 1,300
  lives.
• Largest Tornado Outbreak
• April 3-4, 1974 Super-Outbreak
• 148 tornadoes overnight across 11 states.

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Tornado Forecasting

• NEXRAD doppler radar (WSR-88D)
• 138 stations across the USA
• Intensity of storm noted by amount of reflection
  off rain, hail, and ice crystals.
• Hook cloud is an indicator.

Orlando, Florida