Discussion

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Weather Systems and Severe Weather

(three cyclonic systems)

 

  1. Polar Front Cyclones (also called Mid-Latitude Cyclones or Traveling Wave Cyclones)

 

  1. Form and Travel at the Polar Front: Form at distinct global boundary between cold and warm air. May form under several circumstances.

 

  1. Three Critical Factors for formation (called Cyclogenesis)

    1. Surface Convergence: Fed by Polar Highs and Sub-tropical Highs

 

  1. Cyclonic Shear: Opposing air direction causes spiraling flow.

 

  1. Strong Upper Level Winds:
  • From unusually strong Polar Front Jet Stream…
  • Literally act as a vacuum to strengthen cyclone.
  • May occasionally be the initial cause.

 

  1. Frontal Development: Cold and warm fronts develop. (See Time 2 &3)

 

  1. Cyclone Migrates: Often called a Traveling Wave Cyclone; the system spirals AND travels along the Polar Front Jet Stream.

 

Mapview of Polar Front Cyclone Development

TIME 1

 

 

 

 

 

 

 

 

 

 

 

 TIME 2

TIME 3

 

 

 

 

 

Warm

Front
Cold

Front

 

  1. Tornadoes

    1. Basic Description: Extremely intense, rapidly uplifting, inward spiraling, small-scale low pressure cells.

 

  1. Basic Data

    1. Winds up to 485kmh (300mph)
    2. Pressures <900mb?? (never measured)
    3. Duration: Few minutes to half hour
    4. Size: Up to a mile across

 

 

  1. Classified using the Enhanced Fujita Scale (or EF Scale): (see text)
  2. Basic Causes

    1. Severe Instability and Rapid Uplift: Tornadoes are caused by the rapid uplift along cold fronts or within hurricanes.

 

  1. Strong Upper Level Winds: Strong upper level winds have a vacuuming effect that strengthen the system.

 

  1. Location and Timing

    1. cP and mT “Collision Zones”: Severe, unstable cold fronts and tornadoes will generally form where cold/dry air masses collide with warm/moist air masses. The most ideal collision zones are found near the Polar Front where cold land sits next to warm ocean.

 

  1. The North American Example: Tornadoes are most common in the south-central and south-eastern US, because this area is perfectly situated between two major source regions. Especially in late winter and early spring strong cP air still moves south out of Canada, while mT air is forming and strengthening over the Gulf of Mexico.  They collide in places like Texas, Oklahoma, Florida, etc.  The western states simply do not experience collisions between cP and mT air (mostly because we get mP air much more commonly than cP air.)

 

  1. The “Collision Zone” Shifts over Time: The collision zone (i.e. the Polar Front) is usually located very far south (South Texas, Alabama, Florida, etc) in the early spring, and then migrates progressively northward until it sits over Southern Canada and the Great lakes in late summer. This results in tornado likelihood “moving” from south to north as the tornado season progresses.

 

F. Impact on Humans

 

1. Wind Blown Debris:  Wind and windblown debris are by far the greatest cause of damage.

 

2. Fatalities:  Deaths from tornadoes have greatly decreased due to advancements in prediction and warning systems.

 

 

III. Tropical Cyclones (called Hurricanes when winds greater than or equal to 74mph )

 

  1. Basic Description: Extremely intense, rapidly uplifting, large-scale, inward spiraling low pressure systems. Contain hundreds of convection cells.

 

  1. Basic Data

    1. Winds up to 200kmh (125mph)
    2. Pressures in the eye down to 950mb
    3. Duration: Few days to a week
    4. Size: Hundreds of miles across

 

  1. Classified using the Safir-Simpson Scale: (see text)

 

 

  1. Basic Causes

    1. High Sea Surface Temperature (SST): Temps over 27°C are mandatory!! These conditions are most common during mid to late summer.

 

  1. Intense Evaporation and Subsequent Release of Latent Heat: Warm oceans combined with intense uplift cause much evaporation to occur. This results in intense precipitation which releases much latent heat.  This causes the storm to “heat itself” and lift faster.

 

  1. Strong Upper Level Winds: Strong upper level winds have a vacuuming effect that strengthen the system. (like tornadoes).

 

  1. Location and Timing

    1. Develop over WARM Low Latitude Oceans: They develop over tropical oceans between 8° and 15° N and S lat: (Never at the equator, as the coriolis force is not strong enough to drive a cyclonic storm).

 

  1. Generally “Dragged” by Anti-Cyclonic Winds: Tropical cyclones first get dragged by easterly winds, usually gaining some latitude. They often eventually get caught by the westerly winds and are carried back out to sea.
  2. Tropical Cyclone Season: Tropical cyclones are most common – by far – when oceans are at their warmest !!! This occurs during summer, late summer and fall.  Remember that northern hemisphere seasons occur at opposite times of year compared to southern hemisphere seasons.

 

F. Impact on Humans

  1. Wind: Causes damage.

 

  1. Sources of Flooding

    1. Heavy Precipitation (can be up to 2 feet): Standard for a hurricane; worse when they are moving slowly.

 

  1. Storm Surge (can be 20 feet or more): A combination of very high sustained wind speeds and extremely low pressures can raise the surface of the ocean as high as 30 feet in the most drastic tropical cyclones. The low pressure literally sucks it upward like a vacuum, while the wind literally pushes the entire ocean up onto the land.

 

  1. High Tides (can be up to 5 feet): Although not caused by a hurricane, the hurricane may coincide with the highest tides, causing even greater flooding.

 

  1. What causes tropical cyclones to dissipate (die)?

 

  1. Moving over land: Friction from the earth’s land surface slows winds; also, the source for water vapor (i.e. the warm ocean) is now lost.

 

  1. Moving over cold ocean water: Cold ocean water doesn’t supply the needed water vapor to fuel the storm.

                                                     See next page for sample questions >>>>>>>>
Sample Questions:  Questions similar to these will be on your exam.  As you study you should anticipate how I might use these questions to create new questions on the same concepts.

 

1  Which two air masses would be most likely to cause a tornado if they collided?

A.     mT and mP      B. mP and rQ      C. mT and cP      D. dZ and rQ        E. cP and cT
2 Which is a true statement about tropical cyclones (hurricanes)?

A.     Sea surface temperatures below 23°C are critical for formation.

B.     They form at around 40°-50° latitude.

C.     They are about 10-20 miles across.

D.     They are huge low pressure systems, formed from spiraling bands of convection cells.
3 Which one of these factors contributes most to flooding during a severe tropical cyclone (hurricane)?

A.     tornadoes    B. blocked rivers    C. heavy rain    D. storm surge     E. high tides
4 Of the locations marked on the map provided, which one is most likely to be a site where a tropical cyclone would form?
5 Which factor is not particularly necessary for cyclogenesis to occur?

A.  strong upper level winds    B. extremely dry air   C. cyclonic shear   D. converging winds
6 Which statement comparing hurricanes and tornadoes is true?

A.  The pressure at the center of a severe hurricane is lower than at the center of a severe tornado
B.  The wind speeds associated with severe hurricanes are higher than severe tornadoes

C.  Hurricanes generally last much longer than tornadoes.

D.  Hurricanes are generally much smaller than tornadoes.

 

 

GOOD NEWS!!!  The practice questions above – plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class.  You can take each quiz multiple times, and each time you will get some new questions.  Once submitted, the quizzes are graded automatically, with the correct answers provided immediately.  This is a great way to prepare for the exams!!!