Hurricanes and Extratropical Cyclones

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Hurricanes and Extratropical Cyclones

• Chapter 9

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• Traits common to tropical cyclones
• Form between latitudes of 5 and 20 N or S
• Most common in summer months of respective
  hemisphere
• Form in E or SW Pacific, E N Atlantic Ocean, and
  N Indian Ocean
• Rotational winds produce low pressure
• Warm water of 25 - 26 C 77-80 F
• Extratropical cyclones
• Form over land or water in higher latitudes,
  usually 30 - 70 latitude
• Cool central cores driven by jet stream

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Location of cyclonic storms
Typhoons
Hurricanes
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15
30
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Cyclones
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Due to Coriolis forces, cyclonic storms must form
at least 300 mi from the Equator (about 5
degrees N or S)
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Two jet streams
33,000 ft alt Stronger 110 mph winter 55 mph
summer
43,000 ft alt weaker
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• Definitions of Tropical, Low-Pressure Weather
  Systems. Storms begin as Disturbances and can
  Become Hurricanes
• Tropical disturbance
• Organized mass of convectional air and
  thunderstorms with partial rotation present
  generally 100 to 300 nautical miles in diameter
  that originate in the subtropics or tropics
• Tropical depression
• Closed circulation with sustained winds of 38 mph
  (33 kt or 62 kph) or less
• Tropical storm NAME ASSIGNED NOW
• Sustained winds (1-minute measurement, 10 m above
  water) of 39 mph (63 kph or 34 kts) up to 73 mph
  (63 kts)
• Hurricane
• Sustained winds of 74 mph (119 kph or 64 kts) or
  more

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Consider quadrants Wheres the worst one?
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• Naming of storms
• Done when tropical storm forms winds gt 39 mph
• Helps keep track of storm
• Six lists are used and they repeat in 6 yrs
• Mens names added in 1979 and alternate with
  womens names each year
• This yearAna, Bill, Claudette, Danny, Erika,
  Fred
• Names retired for major storms
• Hugo, Carla, Floyd, Katrina, Rita, Wilma
• 67 retired, 22 in past 10 years!

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• For United States, between 1851 and 2004,
  tropical storm occurrence rate
• On average
• 8.5 storms per year
• 5.2 hurricanes
• 1.78 hurricanes hit land in U.S.
• Worst months
• September, August, October

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• Thermal energy drives cyclone
• Rotational forces extract more water and heat
  from ocean surface
• Unstable conditions aloft
• Concentration of energy toward center
• Spinning ice skater effect
• Conditions worsen
• In northern hemisphere
• Storms curve to right
• Prevailing trade winds, Bermuda High and Coriolis
  forces affect storm tracks

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• Year 2005 worst in terms of number of named
  storms
• 21 formed, all names were used, so Greek letters
  were assigned to the last six
• Letter not used for names
• Q, U, X, Y, Z

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Cape Verde source area
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Hurricane season in 2005
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2006
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2008
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Effects of Cyclones

• Storm surge
• Wind-pushed waves fetch in NE quadrant
• Lower pressure raises water level
• Effects amplified in bays and lagoons seiches
• Worst at high tide
• Record surge 42 ft in Australia!
• Flooding
• Greatest killer often freshwater rainfall
  flooding
• Avg hurricane produces the flow of the St.
  Lawrence River in THREE SECONDS!

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Effects of Cyclones

• Heavy rain
• Avg storm produces about one trillion gallons of
  rainwater daily, 3x annual usage in U.S.
• T. S. Allison 2001 dumped 37 in rain on Houston
• Hilly topography affects rainfall H. Agnes 1972
  in Appalachian Mtns of eastern United States
• High winds
• Direct damage
• Storm surge

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Hatteras Island, NC, breached by Hurricane Isabel
Sept 2003
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Beach erosion
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Beach erosion in H. Dennis, 9/1/99NC Outer Banks
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Categories of Cyclones

• Wind damage
• Category 1 74 95 mph
• Category 2 96 110 mph
• Category 3 111 130 mph
• Category 4 131 155 mph
• Category 5 gt 155 mph
• Highest recorded winds 190 mph
• Eyewall has highest velocity landfall location
  important

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H. Andrew, Aug 23-25,1992
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Damage from Hurricane Andrew 1992
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Risk factor for United States
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Major hurricane hazard map
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Most costly storms in U.S.