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Thunderstorms & Tornados

Thunderstorms & Tornados. Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida. Go to Visualization. Thunderstorm Life Cycle. Thunderstorm Life Cycle.

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Thunderstorms & Tornados

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  1. Thunderstorms & Tornados Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida

  2. Go to Visualization Thunderstorm Life Cycle

  3. Thunderstorm Life Cycle • Cumulus Stage is the growth stage with the transformation of water vapor into liquid particles as warm air feeds in from below • Mature Stage starts with the appearance of a downdraft as precipitation & cold air from evaporation entrain into the cloud while the top of the cumulonimbus can reach 40,000 feet and the downdraft reaches the surface to form a gust front • Dissipating Stage occurs when updrafts weaken due to the overwhelming downdraft and the supply of warm/moist air being cut-off

  4. Severe Thunderstorms • Long-lived T-Storms have a greater chance of becoming severe • Hail to at least ¾ inch in diameter and winds gusts ≥ 50 knots (58 mph) & tops to 60,000 feet with violent updrafts • Need strong vertical wind shear to produce supercell storms lasting over 1 hour that are tilted so that the outflow of cold air from the downdraft never undercuts the updraft

  5. Severe Thunderstorms

  6. Downbursts & Microbursts • Localized downdraft that spreads like a radial burst of wind • If its small winds extending 4 km or less it is called a microburst, which can have winds up to150 mph

  7. Thunderstorm Lines & Clusters • Squall lines are usually observed near or ahead of a cold front and may extend over 600 miles • Mesoscale Convective Complexes (MCCs) are clusters that may over an entire state moving slowly and often lasting over 12 hours

  8. Thunderstorm Distribution • 50,000 T-Storms occur daily throughout the world, especially over equatorial land masses • In the U.S. thunderstorms form often in the Southeast, but also in the Midwest

  9. Lightning and Thunder • A giant spark that can take place within a cloud, from cloud to cloud, or from cloud to ground (20%) • Extreme heating causes the air to expand explosively, initiating a shock wave that becomes a booming sound wave called thunder • Sound travels 1 mile in 5 seconds, so we can count seconds to determine how far lightning has occurred

  10. Go to Visualization The Lightning Stroke • For lightning to occur, separate regions containing opposite electrical charges must exist within a cumulonimbus cloud • Negative charge at the bottom of the cloud causes the ground to have a positive charge and an electric potential between them (voltage)

  11. The Lightning Stroke • Air is not a good conductor, but as the voltage becomes sufficiently large the insulating properties of the air break down, current flows, and lightning occurs

  12. The Lightning Stroke • First a stepped leader (invisible to the human eye) of negative electrons rushes from cloud to ground • A current of positive charge starts up from the ground (from elevated objects) to meet it • After they meet, a luminous return stroke surges upward to the cloud along the path of the stepped leader • Hence, the downward flow of electrons establishes the bright channel of upward propagating current • Dart leaders allow for repeat strokes along the same established channel, making lightning appear to flicker

  13. Tornado • A rapidly rotating column of air that blows around a small circulation that reaches the ground • Most are 100 to 600 meters in diameter (extreme tornadoes can be 1 mile wide) • Most move 25-50 mph, some up to 70 mph • Most last just a few minutes with a path on the ground averaging 4 miles (extreme tornadoes last hours and travel hundreds of miles) • A funnel cloud is a tornado whose circulation has not reached the ground

  14. Tornado Occurrence • No country experiences more tornadoes than the U.S., with more than 1000 annually (most from March to July) • Tornado Alley is located in the Central Plains from Texas to Nebraska, where warm humid air is overlain by cooler drier air aloft, making for an unstable atmosphere

  15. Tornado Winds • Fujita scale classifies tornadoes based on wind damage done by the storm • Most are F0 and F1 with winds of less than 100 knots, and even the most powerful seldom exceed 220 knots • Violent tornadoes (F3 and higher) comprise only a few percent each year, but cause most deaths

  16. Tornado Formation • Speed and directional wind shear produces rotation near the surface (vortex tube) • Very strong updraft tilts the tube and draws it into the storm, forming a mesocyclone

  17. Tornado Formation

  18. Tornado Appearance • Updraft is so strong that rain and hail is absent, depicted as a weak echo region or hook echo on radar • Rotating supercell mesocyclone shows up as rotating clouds lowering at the base of the storm, or wall cloud

  19. Waterspouts • May be tornadic • Those seen in Caribbean and Florida Keys are almost always “fair weather” • Unstable air mass • Converging winds start spinning • Developing cumulus helps “lift” spinning air up into the cloud • Slow moving and less intense, with winds under50 mph

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