Thunderstorms, Tornadoes, Hurricanes Oh My!

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Thunderstorms, Tornadoes, Hurricanes Oh My!

  1. 1. +Thunderstorms, Tornadoes, HurricanesOH MY!! Chapters 10 and 11
  2. 2. +ThunderstormsSections 10.1 to 10.6
  3. 3. + What’s in a Name? Cyclone refers to the circulation around a low-pressure center.  Hurricanes  Midlatitude cyclones  Tornadoes
  4. 4. + Thunderstorms  Thunderstorms generate lightning, thunder, winds, and hail.  There are two types of thunderstorms.  Air-mass thunderstorms are associated with warm, humid air that rises in unstable environments; cumulonimbus clouds, and mT (maritime tropical) air masses.  Severe thunderstorms may produce high winds, hail, flash floods, and tornadoes.  They are the result of uneven heating, frontal lifting, and diverging winds.
  5. 5. + Air-Mass Thunderstorms Air-mass thunderstorms—mT air masses
  6. 6. + Air-Mass Thunderstorms Cumulus stage:  Warm, humid air rises creating clouds that won’t evaporate.  A cumulonimbus tower develops as water vapor moves from the surface to greater heights.  Clouds pass freezing zone and Bergeron process starts.  Rain accumulation is too great for updraft so it falls creating a downdraft via entrainment (influx of cool dry air. Remember cold air gets heavy and sinks).
  7. 7. + Air-Mass Thunderstorms Mature stage:  Thisis the most intense phase.  This results in heavy rain and possibly small hail.  Cool downdrafts exist next to updrafts. Dissipating stage:  Thisstage is dominated by downdrafts and entrainment causing evaporation.
  8. 8. + Air-Mass Thunderstorms
  9. 9. + Air-Mass Thunderstorms Occurrence:  Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms.
  10. 10. + Severe Thunderstorms Severe thunderstorms:  Heavy downpours  Flash flooding  Straight line wind gusts  Hail, lightning  Wind shear- change in wind speed or direction at different heights  Can overshoot (enter stratosphere)  Downdraft preceding (gust front)
  11. 11. + Supercell Thunderstorms
  12. 12. + Supercell Thunderstorms  Supercells  These storms can produce extremely dangerous weather.  They consist of a single, powerful cell that can extend to heights of 20 km or more.  The clouds can measure 20–50 km in diameter.  Mesocyclone:  Verticalwinds may cause the updraft to rotate, which forms a column of cyclonically rotating air.  Tornadoes often form.
  13. 13. + Supercell Thunderstorms Squall lines:  Squall lines are narrow bands of thunderstorms.  cT (continental tropical) air is pulled into the warm sector of a midlatitude cyclone.  Mammatus skies sometimes precede squall lines. (dark cloud rolls that have downward pouches)  These can also form along a dryline, where there is an abrupt change in moisture.
  14. 14. + Supercell Thunderstorms Squall lines
  15. 15. + Lightning and Thunder  Lightning strokes:  The flash (total discharge) lasts a few tenths of a second.  It is what we see and it contains multiple strokes.  The leader is the ionized air, which forms a conductive path.  A step leader extends earthward in a short, nearly invisible burst.  The return stroke extends upward from ground to cloud.
  16. 16. + Lightning and Thunder Thunder:  The air is heated quickly to as much as 33,000°C.  It expands explosively, which produces sound waves that travels at 330 mps.  If lightning is more than 20 km away, thunder is not heard.
  17. 17. + Lightning and Thunder What causes lightning?
  18. 18. +TornadoesSection 10.7 to 10.10
  19. 19. + Tornadoes Tornadoes (twisters, cyclones):  These are violent windstorms with a rapidly rotating column of air, or vortex.  Pressures within tornadoes can be as much as 10% lower than immediately outside the storm.  It may consist of single or multiple vortices.
  20. 20. + Tornadoes
  21. 21. + The Development and Occurrence of Tornadoes
  22. 22. + The Development and Occurrence of Tornadoes Mesocyclones form  Winds are stronger aloft producing rolling motion about a horizontal axis  Stronger thunderstorm updrafts tilt the horizontal rotating air to a nearly vertical alignment
  23. 23. + The Development and Occurrence of Tornadoes The mesocyclone, vertical cylinder of air, is established. It stretches vertically and narrows horizontally causing wind speeds to accelerate inward creating a vortex.  FunnelCloud- vortex emerges from cloud  Tornado- vortex touches the ground
  24. 24. + The Development and Occurrence of Tornadoes Profile of a tornado:  Average diameter 150–600m  Travels ~45 kph  Path about 26 km long  Most travel to the NE  Exist between <3 min to >3 hours  Wind speeds between <150 kph to >500 kph  Between 90- 300mph
  25. 25. + Tornado Destruction
  26. 26. +HurricanesChapter 11
  27. 27. + Profile of a Hurricane  Hurricanes:  Hurricanes are intense centers of low pressure.  They form over tropical/subtropical oceans.  These storms have intense convective activity and rotary circulation and can have wind speeds in excess of 74mph.  They are usually 100–1500 km in diameter, form between 5° and 20° latitude and have a steep pressure gradient.  Hurricanes are called typhoons in the NW Pacific and cyclones in the SW Pacific and Indian Oceans.
  28. 28. + Profile of a HurricaneEye wall: doughnut shaped wall of intense convective activity surrounding the eye.Eye: Center of hurricane where wind and rain cease
  29. 29. + Hurricane Formation and Decay Hurricanes:  Hurricanes are fueled by the latent heat of condensed water vapor.  Most are formed from late summer to early fall.  Sea-surface temperatures of 27°C or higher provide the necessary heat and moisture.
  30. 30. + Hurricane Formation and Decay Hurricane formation:  Tropical disturbances, such as disorganized cloud arrays and thunderstorms, occasionally grow larger and develop strong cyclonic rotation.  Easterly waves gradually move from east to west.
  31. 31. + Hurricane Formation and Decay Hurricane formation:  As areas within tropical disturbances get warmer, several factors help form hurricanes.  Surface pressure drops creating a region of weak low pressure and cyclonic circulation.  Higher pressure develops at the top of the storm causing an outward flow of air from the top.  If wind speeds do not reach 63 kph, it is called a tropical depression.  When winds exceed 61 kph, it becomes a tropical storm and is given a name.
  32. 32. + Hurricane Formation and Decay Hurricane decay:  Decay occurs when latent heat is cut off.  A hurricane moves over cooler water or land.  When large scale flow aloft is unfavorable, they diminish in intensity.

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