Tropical
Cyclones
Also: Hurricanes,
Typhoons
Tropical Cyclone Ingredients
• Light winds  instability along I.T.C.Z.
• High humidity (oceans)  fuel (latent heat)
• Warm surface waters over a vast area
(at least 26ºC / 76ºF  tropics, < 25 ºN or S)
• Will not form within 5o of equator
(no Coriolis)
Tropical Cyclone Features
• Spiral bands
• No Fronts
• Eye wall
cloud
• Eye
• Size: 100-600
miles, but can
be larger
Death Toll: 12,000
Tropical Cyclone Formation
• Easterly waves (fig. 8-24)
– vertical perturbations within the trade winds
– about 10% intensify and rotate
• Tropical depression (winds < 63 kmph, 39 mph)
• Tropical storm (63-118 kmph, 39-73 mph)
– naming of the storm
• Cyclone (winds > 119 kmph, 74 mph)
– Saffir-Simpson scale (Table 8-1)
– ranks 1 to 5 (5 is “catastrophic”)
Saffir-Simpson Scale
Category
Winds
Effects
One
74-95 mph
Two
96-110 mph
No real damage to building structures. Damage primarly to unanchored mobile
homes, shrubbery, and trees. Also, some coastal road flooding and minor pier
damage
Some roofing material, door, and window damage to buildings. Considerable
damage to vegetation, mobile homes, and piers. Coastal and low-lying escape
routes flood 2-4 hours before arrival of center. Small craft in unprotected
anchorages break moorings.
111-130 mph
Some structural damage to small residences and utility buildings with a minor
amount of curtainwall failures. Mobile homes are destroyed. Flooding near the
coast destroys smaller structures with larger structures damaged by floating
debris. Terrain contin
131-155 mph
More extensive curtainwall failures with some complete roof strucutre failure on
small residences. Major erosion of beach. Major damage to lower floors of
structures near the shore. Terrain continuously lower than 10 feet ASL may be
flooded requiring mass
Three
Four
Five
Complete roof failure on many residences and industrial buildings. Some
complete building failures with small utility buildings blown over or away. Major
damage to lower floors of all structures located less than 15 feet ASL and within
greater than 155 mph 500 yards of the sh
Hurricanes, Typhoons & Cyclones
World Map shows locations of Hurricanes,
Typhoons &
Cyclones
Names
Reflect
Locations
All are the
Same type
Of storm
Paths of Hurricanes:
North Atlantic
North Atlantic hurricanes develop 5 to 10 degrees north of
The equator & travel to the west, yet as they move farther
North they
change
directions
& move
eastward
Most do
not make
landfall
Tropical Cyclone Timing
(Northern Hemisphere)
Hurricanes:Coastal Damage/Death
Coastal damage can be extensive as high winds, high
Tides And ‘storm surges’ push ocean water far inland
Galveston, Texas
Was struck in 1900
& 6,000 were killed
In 1970 a Hurricane
Hit Bangladesh &
Killed 300,000 in
1991 200,000 died
(Note that many people in Australia, Mexico, and S.E. Asia
depend on the hurricanes for live-giving moisture each year.)
Tropical Cyclone Hazards
• Storm surge (up to 20 feet)
– pressure (1 cm rise per 1 mb drop in pressure)
– winds / friction
• rain / flooding
causes most
damage!
• winds
• tornadoes
• Power
Tropical Cyclone Power
– one hour = total USA electricity for one year
– one day = nuclear arsenals of USA and Russia
• Highest U.S.death toll: Galveston, TX, 1900 20 ft. storm surge killed 6,000 (1/6 the
population)
2005 Atlantic Hurricane Season
• Most named tropical storms in history (28); including
most intense ever (Wilma: 882 millibars; 185 mph
sustained winds) and most Category 5 storms (4) ever
• New Orleans destroyed; worst U.S. hurricane damage
ever. Cost estimated at: $100,000,000,000
• Death toll: at least 2048 confirmed dead as of 4/2006;
most were among poorest elderly people.
Forecasting and Safety
• Tracking
– satellites
– coastal radar
• Prediction
– computer modeling
• City evacuation plans
Extratropical v. tropical cyclones
Midlatitude cyclone
• Forms outside the
tropics.
• Largest diameter of all
storms
• No eye
• Has distinct fronts.
Tropical cyclone
• Forms over a tropical ocean.
• Eye and dramatic pressure drop (900 mb or lower)
• Has no fronts.
• Smaller average size, but much higher winds.
Hurricane Links
Hurricane & Storm Tracking for the Atlantic & Pacific Oceans - Shows
all current hurricanes and their tracks
US National Hurricane Center - North Atlantic, Eastern Pacific
The Hurricane Hut - Information on all past storms to 1950, along with
images and individual storm summaries.
Create-a-Cane Interactive fun site from NOAA, allows to specify
conditions and see how they impact storm formation
NASA Hurricane Web Page - Data, research, science & multimedia
resources from NASA
WMO guide on cyclone terminology
NOVA scienceNOW: Hurricanes