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Chapt 8

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    • 1. Physical Geography by Alan Arbogast Chapter 8Air Masses and Cyclonic Weather Systems Lawrence McGlinn Department of Geography State University of New York - New Paltz © 2007, John Wiley and Sons, Inc.
    • 2. Air Masses & Cyclonic Weather Systems• Air Masses and Fronts• Midlatitude Cyclones• Thunderstorms• Tropical Cyclones © 2007, John Wiley and Sons, Inc.
    • 3. Air Masses & Fronts• ____________– Large body of lower atmosphere with uniform conditions of temp & moisture• Source Regions based on 2 criteria: • ____________ • c – continental – dry • m – maritime – moist • ____________ • A or AA – arctic or antarctic • P – polar – 50-60º N or S • T – tropical – 20-35º N or S • E - equatorial © 2007, John Wiley and Sons, Inc.
    • 4. Air mass source regions for North America © 2007, John Wiley and Sons, Inc.
    • 5. Fronts • __________________________________ Cold FrontWarm Front © 2007, John Wiley and Sons, Inc.
    • 6. Midlatitude Cyclone• Well-organized low pressure system that migrates across a region as it spins• Develops along ____________as it swings north and south in process called ____________• Both upper-level and surface conditions drive cyclogenesis• Southerly swing of polar jet creates an upper-level trough supporting cyclogenesis © 2007, John Wiley and Sons, Inc.
    • 7. Upper-levels and CyclogenesisUpper-level convergencepushes down on thesurface, creating highpressureUpper-level divergenceallows air to rise fromsurface, creating lowpressure © 2007, John Wiley and Sons, Inc.
    • 8. Life Cycle of Midlatitude CycloneStationary Early StageFront StageOpen-wave Closing ofStage open waveOccluded DissolvingStage Stage © 2007, John Wiley and Sons, Inc.
    • 9. Classic Midlatitude Cyclone LFormation of a Midlatitude Cyclone © 2007, John Wiley and Sons, Inc.
    • 10. Midlatitude Cyclone Migration Follows path of polar jet Migration of a Midlatitude Cyclone © 2007, John Wiley and Sons, Inc.
    • 11. Violent Weather• Thunderstorms• Tornadoes• Tropical Cyclones
    • 12. Thunderstorms• Caused by convection in ____________or by cold fronts moving into tropical air• Rapidly rising air results in ____________• Condensation adds ____________to the mix• Air becomes more unstable• Strong ____________ develop• Intense precipitation create ____________• Downdrafts cool atmosphere• Storm eventually dissipates © 2007, John Wiley and Sons, Inc.
    • 13. Thunderstorms - Stages
    • 14. Thunderstorms• General structure• Lightning & Thunder• Flash Flooding• Hail• Microbursts
    • 15. Thunderstorms - Structure
    • 16. ThunderstormsFigure 5.35
    • 17. ThunderstormsFigure 5.36
    • 18. Lightning• In cumulonimbus clouds snow crystals charge positive (+), ice pellets negative (-)• Updrafts drive snow (+) to upper part of cloud while pellets (-) collect lower• Ground gains positive charge (+)• Most lightning within clouds from + to –• In strong storms, leader (-) from cloud meets streamer (+) from ground creating “spark” i.e. cloud-to-ground lightning © 2007, John Wiley and Sons, Inc.
    • 19. Cloud-to-Ground Lightning © 2007, John Wiley and Sons, Inc.
    • 20. Flash Flooding• ____________- a sudden local flood of great volume and short duration• Of all of the phenomena that thunderstorms create, flash flooding is the deadliest in the U.S. (ave. of 140 deaths per year)
    • 21. Flash Flooding• What causes flash flooding?Two primary ways:1. ________________________2. ________________________
    • 22. Flash Flooding• ________________________: frequently occur when there is terrain (especially mountainous areas) and a lot of rainfall
    • 23. What to do when flash flooding occurs• NEVER drive across a flooded road, no matter how deep the water is• Water has tremendous momentum when it is in motion, a cubic meter of water weighs 1.1 tons• Just a few inches of water rushing across a road can be enough to sweep you away.
    • 24. Hail• Hail- formed when ice crystals fall into the updraft, accumulate supercooled droplets, and are then are forced upward and completely freeze. Hail falls out of a storm, when it is heavier than the updraft can sustain in the air
    • 25. Hail• Hail forms in nearly every thunderstorm, but rarely reaches the ground. This is because hail is rarely large enough to make it to the surface before it evaporates.• What kinds of updrafts do you need for large hail? • For ____________ size hail – 55-60 mph updrafts • For ____________ size hail- 100 mph updrafts
    • 26. Hail Records• Coffeyville, Kansas, in 1970. The largest hailstone ever documented (until just recently), it weighed 1.67 pounds, and spanned 5.67 inches
    • 27. • Largest hailstone on record fell in June of 2003 in Aurora Nebraska, measuring 7 inches in diameter, weighing over a 1.3 lbs.
    • 28. Larger stones are believed to have fallen…the largest impact crater was measured to be just under 12 inches across!
    • 29. Hail can, on rare occasions, accumulate
    • 30. Microbursts• ____________- straight-line bursts of concentrated wind (diameter less than 4 km• Created by ________________________:• Dry air is pulled into the storm, and the precipitation rapidly evaporates to moisten the air…. Leading to tremendous cooling (takes energy to evaporate)
    • 31. Microbursts• Where does all of this cold air go?• The cold air very rapidly falls to the surface, greatly enhancing the downdraft, and tends to ‘splash’ when it hits the ground• Microbursts are also a threat to aviation
    • 32. Microbursts
    • 33. MicroburstsNote the nearly unidirectional snapping of trees: MICROBURST
    • 34. Microburst• Often mistaken for tornado damage: Note the cyclonic snapping of the trees, TORNADO
    • 35. Tornado Greensburg, KS 04 May 2007
    • 36. Tornadoes• Small, intense cyclone• Spawned by ________________________• Most destructive atmospheric phenomenon• Mesocyclone within the supercell• Rated on Enhanced Fujita Scale EF0 (65-85mph) – EF5 (200+ mph)• Mesocyclone rotation, not a tornado, detected by Doppler radar © 2007, John Wiley and Sons, Inc.
    • 37. Formation of a Tornado Wind Updrafts Shear TornadoTornado in aDescends supercell © 2007, John Wiley and Sons, Inc.
    • 38. Tornado
    • 39. Tornado Occurrences• Most occur between April and July• Prime region flat terrain & clash of cP & mT air masses Tornadoes © 2007, John Wiley and Sons, Inc.
    • 40. Damage from Tornadoes
    • 41. Damage from Tornadoes
    • 42. Damage from Tornadoes Enhanced Fujita Scale Wind Speed EF 0 65-85 mph EF 1 86-110 mph EF 2 111-135 mph EF 3 136-165 mph EF 4 166-200 mph EF 5 Over 200 mph
    • 43. Tropical Cyclones
    • 44. Tropical Cyclones (TCs)• Nomenclature: • __________ (Atlantic & NE Pacific) • __________ (NW Pacific) • __________ (SW Pacific & Indian) © 2007, John Wiley and Sons, Inc.
    • 45. Tropical Cyclone Structure © 2007, John Wiley and Sons, Inc.
    • 46. Tropical Cyclone Structure• Eye - descending air in the center of a tropical cyclone• Eyewall - The innermost band of thunderstorms which surround the eye (maximum latent heating)• Rainbands - the spiral bands of thunderstorms that extend outward from the hurricane center (100’s of miles)
    • 47. Tropical Cyclone StructureSpiral rainbands • From above, the eye and eyewall are very well defined features
    • 48. The Eyewall• Aircraft flown into hurricanes get an excellent view of the eyewall pictured here
    • 49. Ingredients for hurricane formation• ____________________ (SST’s)- greater than 26°C (79°F)• ____________________• __________throughout troposphere (very little vertical shear)• need __________, a trigger • tropical African wave • ITCZ• typically form between 5°-20° latitude, not on equator.... why?
    • 50. Factors inhibiting hurricane formation: • __________ • __________at low levels • strong __________ (stability) • strong _________________(vertical shear)
    • 51. Where does a hurricane get its energy?• __________• __________ (largest contributor)• Initially, need an unstable environment: cold air aloft, warm air at the surface, this leads to the growth of thunderstorms• Equivalent of 21 atomic bombs of energy used PER SECOND !!
    • 52. Tropical Cyclones (TCs)• Stages of development: • First __________– trough in tropical easterlies • May become ____________________– rotation starts with one closed isobar • Next ____________________– sustained winds over 39 mph; receives a name • Finally, __________ – sustained winds over 74 mph © 2007, John Wiley and Sons, Inc.
    • 53. Hurricane Development1. Intense latent heating from convection heats the column of air near the center of the storm • lowers the pressure at the surface (low pressure) • generates divergence (high pressure) aloft2. lower surface pressure increases the pressure gradient at low levels - generates stronger surface winds.3. stronger convergence into storms center4. enhance convection - back to #1• the above is a positive feedback loop - enhancing the
    • 54. Tropical CyclonesFigure 5.39
    • 55. Hurricane Size• Typical hurricanes are about __________although they can vary considerably, as shown in the two enhanced satellite images below. Size is not necessarily an indication of hurricane intensity. Hurricane Andrew (1992), one of the most devastating hurricane of DANNY FRAN
    • 56. Hurricane Strength- Saffir-Simpson Scale• Category 1 Hurricane — winds 74-95 mph (64-82 kt) No real damage to buildings. Damage to unanchored mobile homes. Some damage to poorly constructed signs. Also, some coastal flooding and minor pier damage. - Examples: Irene 1999 and Allison 1995• Category 2 Hurricane — winds 96-110 mph (83-95 kt) Some damage to building roofs, doors and windows. Considerable damage to mobile homes. Flooding damages piers and small craft in unprotected moorings may break their moorings. Some trees blown down. - Examples: Bonnie 1998, Georges(FL & LA) 1998 and Gloria 1985• Category 3 Hurricane — winds 111-130 mph (96-113 kt) Some structural damage to small residences and utility buildings. Large trees blown down. Mobile homes and poorly built signs destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland. - Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy 1965• Category 4 Hurricane — winds 131-155 mph (114-135 kt) More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland. - Examples: Hugo 1989 and Donna 1960• Category 5 Hurricane — winds 156 mph and up (135+ kt) Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required. - Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935
    • 57. Hazards of Hurricanes1. Storm Surge2. High Winds3. Flooding4. Tornadoes
    • 58. Storm Surge• Storm Surge- water that is pushed toward the shore by the force of the winds swirling around the storm This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more.• In addition, wind waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides.
    • 59. Storm Surge • Primarily determined by the slope of the beach gradual slope – steep slope – • The primary cause of death in non-U.S. locationsGradual Slope Steep Slope
    • 60. Storm Surge• Power of Hurricane Camilles storm surge seen on the next two slides: Three-story apartment building directly across the highway from the beach at Pass Christian, MississippiOnly category 5 hurricane to make direct landfall in the U.S.
    • 61. Storm Surge
    • 62. Storm Surge
    • 63. High Winds• Understanding the category of storm greatly helps understand the wind damage• Example: A cat 4 storm is likely to do 100 times the damage as a cat 1
    • 64. Fresh Water Flooding• The ____________________from hurricanes in the U.S. in the past 30 yrs.• Rains are generally heaviest with slower moving storms (less than 10 mph).• To estimate the total rainfall in inches, one rule of thumb is to divide 100 by the forward speed of the hurricane in miles per hour (100/forward speed = estimated inches of rain).• North Carolina receives __________of annual rainfall from tropical cyclone systems
    • 65. Tornadoes• Because hurricanes have tremendous rotation associated with them, it is not surprising that tornadoes are often spawned in the rainbands and can cause additional damage and fatalities• Tornadoes can make even weak hurricanes very deadly
    • 66. Where is the worst location to be?• The right-front quadrant of the storm
    • 67. Atlantic Hurricane Season Official season June 1-Nov 30 Peak seasonMid-Aug – Mid-Oct Typical August hurricane tracks Migration of Hurricane Katrina © 2007, John Wiley and Sons, Inc.
    • 68. Hurricane Movement Tropical storm and hurricane tracks during the 2005 season shows a wide range of tracks Because they are so powerful, hurricanes are very difficult to forecast
    • 69. Hurricane Movement• Hurricane Gordon (1994) • A very erratic track• Storms are very hard to predict when the steering winds (winds around the hurricane) are very weak
    • 70. Hurricane Katrina

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