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Severe Weather

Severe Weather. Severe Thunderstorms. Cold fronts come with low pressure in upper levels Increases temp difference between top and bottom of storm Increases instability Increases strength of up and down drafts Can develop into supercells Intense, rotating drafts. Lightning. Electricity

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Severe Weather

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  1. Severe Weather

  2. Severe Thunderstorms • Cold fronts come with low pressure in upper levels • Increases temp difference between top and bottom of storm • Increases instability • Increases strength of up and down drafts • Can develop into supercells • Intense, rotating drafts

  3. Lightning • Electricity • Rapid rush of air in cumulonimbus • Friction between up and downdrafts • Separates electrons • From atoms in cloud or ground • Atoms are positively or negatively charged • Creates regions of oppositely charged air

  4. Stepped Leader – channel of neg. charged air • Moves cloud to ground • Return Stroke – channel of positive ions • moves ground to cloud • Meets stepped leader • Illuminates channel with 100 mill volts • Heats air to ~ 30,000°C • Thunder – sound produced when superheated air expands and contracts

  5. Tornadoes • Associated with supercells • Violent, whirling column of air • Must be in contact with ground • Air visible from dust/debris

  6. Tornado Formation • Wind speed & direction change suddenly • Called wind shear • Produces horizontal rotation near surface • Near updrafts – column of wind becomes vertical • Updrafts accelerate rotation • Air removed from center of column • Lowers air pressure • High winds • BBC NEWS | Special Reports | Animated guide: Tornadoes

  7. Tornado Classification • Fujita Intensity Scale • Ranks tornadoes according to: • Path of destruction • Wind speed • Duration • Range: F0 – F5 • F0 – winds 118 km/h • F5 – winds 500 km/h • Most do not exceed F1 status

  8. Distribution • Form in spring • Late afternoon or evening • Temp contrasts between polar and tropical air • Spark development of supercells • Most frequent: central US • Tornado Alley – N. Texas, Oklahoma, Kansas, Missouri • cP (from Canada) meets mT (from Gulf)

  9. Safety • National Weather Service issues warnings • Signs of approaching tornado • Dark, greenish skies • Towering clouds • Hail • Roaring noise • Not all tornadoes can be predicted

  10. Tips for Safety • Move to lowest point possible • Stay away from cars and windows • Do not try to outrun tornado in vehicle • Move to designated shelter • If outside – find a ditch or depression to lie in

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