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Part 4. Disturbances

Part 4. Disturbances. Chapter 11 Lightning, Thunder, and Tornadoes. Annual distribution of lightning strikes. There are about 40,000 thunderstorms daily on the Earth that produce lightning. A few produce tornadoes. Development of lightning and thunder.

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Part 4. Disturbances

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  1. Part 4. Disturbances Chapter 11 Lightning, Thunder, and Tornadoes

  2. Annual distribution of lightning strikes There are about 40,000 thunderstorms daily on the Earth that produce lightning. A few produce tornadoes.

  3. Development of lightning and thunder (a) Positive and negative charges separate in the cloud. (b) The step leader is a flow of negative charges (electrons) toward positive charges. (c) A flow of positive charges moves toward the step leader. As the positive and negative charges combine, the lightning stroke is seen. A dart leader is a secondary stroke just after the first. Thunder arises from the rapid heating of the air by the lightning stroke, which sends out a sound wave at 5 miles/sec.

  4. Charge separation in a cloud • Lightning only forms in clouds that extend into air that is below freezing • Collisions between ice crystals and graupel may help transfer charge from solid ice to liquid films on some ice crystals • Positive charges tend to accumulate at the top of a cloud, negative charges in the lower part of a cloud • Runaway discharge -- electrons accelerated to a very high speed, colliding with air molecules and creating more free electrons. High-speed moving electrons radiate light as lightning.

  5. Strong electrical fields occur prior to lightning Safest areas from lightning -- indoors or in an automobile. Outdoors, do not stand under tall objects. Do not touch telephones or electrical appliances. Lightning can strike in the same place twice!

  6. A positive stroke can occur when thunderstorms become tilted

  7. A blue jet • Types of lightning • Forked • Sheet (heat) • Ball • St. Elmo’s fire • Sprites • Blue jets

  8. Types of Thunderstorms • Air Mass • Frontal • Squall Line • Mesoscale Convective Complexes (MCC)

  9. Air Mass thunderstorm lifecycle Air Mass thunderstorms form in humid, unstable air. Each cell lasts no more than a few hours from development to dissipation.

  10. Severe Thunderstorms • Winds exceed 93 km/hr (58 mph), have large hailstones (1.9 cm; 0.75 in) or produce tornadoes • Mesoscale convective complex’s (MCCs) • Self-propagating thunderstorm systems • Mesoscale convective systems (MCSs) include MCCs and squall lines • Squall line -- linear band of thunderstorms usually out ahead of a cold front (a type of MCS)

  11. An MCC over South Dakota

  12. A radar image of outflow boundaries Outflow boundaries -- front edge of cold air flowing out away from a thunderstorm; a gust front occurs at an outflow boundary

  13. Direction of movement of the individual thunderstorm cells Thunderstorm movement in an MCC Cells dissipating Movement and lifecycle of individual thunderstorm cells (A, B, C, D, E, F, and G) and how they change with time Direction of movement of the line of thunderstorms Cells forming

  14. A squall line (MCS) This squall line is probably along or ahead of an advancing dry line

  15. A radar image of a squall line

  16. Wind shear and vertical motions in a squall line thunderstorm Vertical wind profile in (a) here

  17. Gust front-induced shelf and roll cloud

  18. Internal structure of a supercell A supercell thunderstorm is an extremely powerful thunderstorm cell. Supercell thunderstorms can spawn tornadoes.

  19. Organization of a supercell and actual radar signature Hook echo Outflow boundary The above are map views of a supercell thunderstorm.

  20. Downbursts, Derechos, and Microbursts • Downbursts -- strong downdrafts in a mature thunderstorm • Derecho -- MCS-induced strong downdraft that can last for hours • Microburst -- small diameter downburst that usually lasts only a few minutes

  21. Microbursts create aviation hazards

  22. Tornadoes • Tornado characteristics and dimensions • 100-yard average diameter • Movement = 50km/hr (30 mph) over 3-4 km (2-2.5 mi) • Winds = 65 km/hr (40 mph) to 450 km/hr (280 mph) • Tornado formation • Squall lines, MCCs, supercells, tropical cyclones

  23. Tornado-producing supercell Tornadoes typically drop out of the wall cloud on the southwest side of a supercell

  24. A possible mechanism of tornado formation

  25. Tornado development along a convergence boundary

  26. A non-supercell tornado development along outflow zone Circular areas shows places where thunderstorm inflow and outflow circulation resulted in vorticity and tornado development

  27. Global tornado frequency Most of Earth’s tornadoes occur in the lower elevation areas of North America

  28. “Tornado Alley”

  29. Most tornadoes occur in the springtime, when the contrast between warm and cold air in the atmosphere is the greatest

  30. -- Most injuries and deaths in tornadoes are in automobiles and mobile homes and are caused by flying debris -- US averages 91 tornado deaths each year -- Safest area in a tornado is in a basement or an interior room, away from windows

  31. Multiple suction vortices greatly increase damage

  32. Linear tornado damage path

  33. Tornado outbreaks • A single weather system producing a large number of tornadoes

  34. Waterspouts • Similar to tornadoes • Develop over warm waters • Smaller and weaker than tornadoes

  35. End of Chapter 11Understanding Weather and Climate4th EditionEdward Aguado and James E. Burt

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