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Chapter 10 Lecture
Natural Hazards
Fourth Edition
Hurricanes and
Extratropical
Cyclones
Tim Frazier
The University of Idaho
© 2015 Pearson Education, Inc.
Learning Objectives
• Describe the weather conditions that create,
maintain, and dissipate cyclones.
• Explain why it can be difficult to forecast cyclone
behavior.
• Locate several geographic regions at risk for
hurricanes and extratropical cyclones.
• Give examples of the effects of cyclones in coastal
and inland areas.
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Learning Objectives, cont.
• Recognize linkages between cyclones and other
natural hazards.
• List the benefits derived from cyclones.
• Describe adjustments that can minimize damage
and personal injury from coastal cyclones.
• Propose prudent actions to take for hurricane or
extratropical cyclone watches and warnings.
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Hurricane Sandy
• Seven days from formation to landfall
– Landfall just south of New York City
– Storm had swelled to largest Atlantic hurricane on record
• “Superstorm Sandy”
– Great size, atypical path, merged with an arctic cold front
– No longer hurricane-force winds upon landfall, but was
second most expensive storm to strike the United States
• Damage in the United States
– Triggered intense snowstorms resulting in power outages
– Large waves and heavy wind and rain caused flooding and
coastal erosion
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The Effects of Sandy on New York City and
New Jersey
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10.1 Introduction to Cyclones
• An area or center of low pressure with rotating winds
– Counter-clockwise in Northern Hemisphere
– Clockwise in Southern Hemisphere
• Tropical or extratropical
– Based on origin and core temperature
• Characterized by intensity
– Sustained wind speeds and lowest atmospheric
temperature
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Hurricane Rotation
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10.1 Introduction to Cyclones, cont.
• Tropical Cyclones
– Form over warm tropical or subtropical ocean water (5°–20°
latitude)
– Have warm central cores
– Tropical depressions, tropical storms, hurricanes
– High winds, heavy rain, surges, and tornadoes
– Derive energy from warm ocean water and latent heat
• Extratropical Cyclones
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Form over land or water in temperate regions (30°–70° latitude)
Associated with fronts and cool central cores
Strong windstorms, heavy rains, surges, snowstorms, blizzards
Most do not produce severe weather
Derive energy from temperature contrasts along fronts
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Classifying Cyclones
• Scientific classification and description have roots in
regional names
– Nor’easter
• Extratropical cyclone that moves along northward along East Coast
U.S.
– Hurricanes
• Tropical cyclones in Atlantic and eastern Pacific Oceans
– Typhoons
• Tropical cyclones in Pacific Ocean west of International Dateline
and north of the equator
– Cyclones
• Tropical cyclones in Indian Ocean
• Saffir-Simpson Scale classifies hurricanes based on
wind speed
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The Saffir-Simpson Hurricane Scale
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Naming Cyclones
• Only a small percentage given names
– Extratropical storms are sometimes named after their
origins
• Tropical storms and hurricanes given names
established by international agreement through
World Meteorological Organization
– Named once winds exceed 63 km (39 mi.) per hour
– Names assigned sequentially each year from list for each
origin
– Male/female names alternated
– Names are reused every 6 years
– Names of big storms are retired (example: Katrina)
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10.2 Cyclone Development and Movement
• Tropical and extratropical cyclones differ in their
– Characteristics
– Development
• Most of both form, mature, and dissipate
independently
• Some tropical cyclones transform into extratropical
cyclones
– Weaken over land or cooler seawater at higher latitudes
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Tropical Cyclones
• General term for large thunderstorm complex
– Rotating around an area of low pressure
– Formed over warm tropical or subtropical ocean water
• Go by a variety of names based on
– Intensity
– Location
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Ocean Sea Surface Temperatures
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Tropical Cyclones, cont.
• Tropical disturbance
– Typically 200 to 600 km (120 to 370 mi.)
– A organized mass of thunderstorms persisting for > 24
hours
– Associated with elongated area of low pressure (trough)
– Has a weak rotation due to Coriolis effect
– Formed by
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Lines of convection similar to squall lines
Upper-level low pressure troughs
Cold front remnants
Easterly waves of converging and diverging winds
– Atlantic Ocean hurricanes
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Easterly Waves and Formation of Tropical
Disturbances
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Tropical Cyclones, cont.
• Tropical Depression
– Tropical disturbance wind speeds increase and begins to
spin
– A low pressure center is formed
• Tropical Storm
– Winds increase to 63 km (39 mi.) per hour
– Storm is given a name
– Wind speeds are not at hurricane strength, but rainfall can
be intense
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Tropical Cyclones, cont.
• Hurricanes
– Not all tropical storms develop into hurricanes
• Classified when winds reach 119 km (74 mi.) per hour
– Environmental conditions
• Thick layer of warm ocean water
– At least 26 degrees C (~80 degrees F)
– Extend to depth of 46 m (~150 ft)
• Steep vertical temperature gradient
– Atmosphere must cool quickly with increasing altitude
• Weak vertical wind shear
– Strong winds aloft prevent hurricane development
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Tropical Cyclones, cont.
• Hurricane structure
– Rain bands
• Clouds that spiral inward around center
• Counterclockwise in Northern Hemisphere
• Increase in intensity towards the center of the hurricane
– Eyewall
• Innermost band of clouds
• Contain the greatest winds and rainfall
• Hot towers can develop within the eyewall 6 hours before a storm
intensifies
– Eye
• Area of calm at center of the hurricane
• Narrow at surface and wider at top
• Diameters range from 5 to more than 60 km (3 to more than 37 mi.)
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Stadium-Like Clouds Form a Hurricane’s
Eyewall
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“Hot Towers” in Hurricane Katrina
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Tropical Cyclones, cont.
• Typical hurricane behavior
– Warm, moist air spirals upward around eyewall
• Air rises, condenses as clouds and releases latent heat
• Latent heat encourages additional warm moist air to rise
– Upward rotation draws air from eye, causing dry air to sink
back into center
– Upward rotation also causes air to flow out the top of the
storm concentrated in exhaust jets
• Allows additional warm air to feed bottom of the storm
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Cross Section of a Hurricane
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Tropical Cyclones, cont.
• Movement of hurricanes
– Controlled by the Coriolis effect and steering winds
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In Northern Hemisphere storms deflect to the right
Track west in trade winds and curve northwest and then northeast
Hurricanes can make a loop
In North Atlantic, steered by Bermuda High
– As hurricane moves over land, it loses energy (warm
water)
• Can become extratropical cyclone
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Hurricane Paths
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Extratropical Cyclones
• Necessary conditions
– Strong temperature gradient at surface
• Strongest along cold, warm or stationary fronts
– Strong upper level winds provided by jet stream
• Polar jet stream
• Subtropical jet stream
• Relationship with jet streams
– Large high-pressure ridges and low-pressure troughs
cause jet streams to bend producing waves or meanders
– May also split around isolated high-pressures and reunite
– Cyclones often develop in curves or divergences in jet
streams
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Polar and Subtropical Jet Streams
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Extratropical Cyclones, cont.
• Bending or splitting can cause the polar jet stream
to dip south and the subtropical jet stream to flow
northeast
– The southern branch of a split polar jet stream in the
Pacific Ocean brings warm moist air out of the tropics
• West Coast forecasters refer to the flow of warm moist air as the
Pineapple Express, because of its origin near Hawai’I
– Nor’easters form when bends of the polar and subtropical
jet streams begin to merge off the southeastern coast of
the United States
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The Pineapple Express Feeding Moisture to
West Coast Extratropical Cyclone
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Extratropical Cyclones, cont.
• Development
– Low-pressure center develops along frontal boundary
• Cold front on southwest, warm front on east
• Conveyor belt of cold air circulates counter-clockwise
– Warm air is wedged to the east
• Conveyor belt of warm air rises on the southeast side creating a
comma
• Conveyor belt of dry air aloft feeds the cyclone from behind the cold
front
– Cold front wraps around the warm front, causing an
occluded front develop trapping warm air aloft
– Cold air completely displaces the warm air, pressure
gradient weakens and storm dissipates
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Development of an Extratropical Cyclone
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Structure of an Extratropical Cyclone
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10.3 Geographic Regions at Risk for
Cyclones
• Most serious threat in North America
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Eastern contiguous United States
Puerto Rico
Virgin Islands
U.S. territories in the Pacific Ocean
• They are a lesser threat to Hawai’i and Atlantic
Canada
• On the Pacific coast, hurricanes strike Baja
California and the west coast of the Mexican
mainland
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North American Regions at Risk for
Hurricanes
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Major Hurricane Hazard Map
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10.3 Geographic Regions at Risk for
Cyclones, cont.
• Most hurricanes that affect East and Gulf Coasts
form off the western coast of Africa
• They take one of three tracks
1. West toward East coast of Florida, sometimes passing
over Caribbean
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Move out into the Atlantic Ocean to the northeast
2. Westward over Cuba and into the Gulf of Mexico to strike
the Gulf Coast
3. Westward to the Caribbean and then northeastward
skirting the East Coast
•
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May strike the continent from central Florida to New York
Hurricane Paths
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10.3 Geographic Regions at Risk for
Cyclones, cont.
• Northwest Pacific is much more active than North
Atlantic
• Indian Ocean is also a very active hurricane zone
• South Atlantic and southeast Pacific, rarely have
hurricanes because of cold ocean water
• Hurricanes do not form close to the equator
because of the absence of the Coriolis effect
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Typical Tropical Cyclone Paths and Regions
of Formation
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10.3 Geographic Regions at Risk for
Cyclones, cont.
• Hurricanes in generally develop in summer and
early fall
– North America’s official season is June 1 to November 30
– Season is January to April in Southern Hemisphere
• Region at risk for extratropical cyclones is far
greater than for hurricanes
– Winter windstorms in Pacific Coast
– Winter snow Sierra Nevada, Rocky Mountains and east
– Spring and summer thunderstorms and tornadoes in
United States and Canada
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10.4 Effects of Cyclones Effects
• Tropical and extratropical cyclones claim many lives
and cause enormous property damage
– Both produce flooding, thunderstorms, and tornadoes
– Extratropical create snow storms and blizzards
• Three additional effects of cyclones are especially
damaging
– Storm surge
• Causes greatest damage from hurricanes
• Contributes to 90 percent of all hurricane-related fatalities
– High winds
– Heavy rains
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Storm Surge
• Local rise in sea level resulting from storm winds
• Can be > 3 m (10 ft.)
• Because of spinning, surge is greatest in right
quadrant of storm as it makes landfall
• Height is greatest near time of maximum winds
• Height is also greater if landfall coincides with high
tide
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Hazard Greatest in Right Forward Quadrant
of Atlantic Hurricanes
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Storm Surge, cont.
• Largest effect from stress exerted by wind on water
– Fetch refers to the area over which the wind blows
– Larger fetch results in larger storm surge
• Smaller effect from low atmospheric pressure in
storm pulling up on water surface
• Also depends on shape of coastline
• Continual increase in sea level as storm approaches
• Overwash can create washover channels, isolating
one area from another
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Hurricane Overwash Deposits
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High Winds
• Storm surge is generally more deadly but wind
damage is more obvious
• Described by Saffir-Simpson Scale
• Usually decrease exponentially with landfall
– Some hurricanes transition to extratropical cyclones that
maintain or increase the wind speeds
• Strongest recorded winds in United States from
extratropical cyclone
– Responsible for strong winds in blizzards and tornadoes
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Aerial View of Hurricane Wind Damage
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Heavy Rains
• Average hurricane produces trillion gallons of water
each day
• Rainfall from cyclones can cause inland flooding
• Flooding affected by:
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Storm’s speed
Land elevation over which the storm moves
Interaction with other weather systems
Amount of water in soil, streams, and lakes prior to storm
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10.5 Linkages between Cyclones and Other
Natural Hazards
• Coastal erosion
– Some of the fastest rates during the landfall of cyclones
• Some sand replaced during fair-weather conditions
• Other sand is removed entirely
• Flooding
– Saltwater from storm surge
– Freshwater from heavy rains
• Mass wasting
– Heavy rains can cause devastating landslides and debris
flows
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10.5 Linkages between Cyclones and Other
Natural Hazards, cont.
• Other types of severe weather
– Tornadoes, severe thunderstorms, snowstorms, and
blizzards
– Hurricanes may generate downbursts and tornadoes
• Directly linked to climate change in coastal areas
– Global sea rise
• Effect of cyclones making landfall will become more severe
• Storm surges will be able to penetrate farther inland
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10.6 Natural Service Functions of Cyclones
• Primary source of precipitation
• Redistribute warm air from tropics
• Maintain ecosystems
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Winds carry plants, animals, and microorganisms
Waves stir up deeper, nutrient-rich waters
Winds topple weak and diseased trees in forests
Waves break apart some corals
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10.7 Human Interaction with Cyclones
• Human interaction with cyclones has increased markedly
in the past four decades
– Population growth greatest in coastal areas
– About 53 percent of United States population live in coastal
counties
• Urban development in coastal areas
– Urbanization of vulnerable coastlines increases magnitude of the
effect of cyclones
– Destruction of sand dunes makes areas more susceptible to
hurricane winds
– Construction of seawalls and bulkheads reflect waves and
contribute to beach erosion
– Poor building materials and practices can make hurricanes more
dangerous to people
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10.7 Human Interaction with Cyclones, cont.
• Global warming may contribute to higher intensity
and frequency of hurricanes in the future
– Raising temperatures of the seas surface
• Possible that warmer ocean water will increase hurricane intensity
– Contributing to rising sea level
• Increase the reach of large waves that ride the surge
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10.8 Minimizing the Effects of Cyclones
• Cannot prevent the cyclone hazard
• Primary way to reduce property damage and avoid
loss of life is to
– Accurately forecast the storms
– Issue advisories to warn people in their path
• Other ways to minimize the effects
– Adjustments to living with hazard
– Enforcing building codes and evacuation procedures
• Examples are for hurricanes but also apply to
extratropical cyclones
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Forecasts and Warnings
• Forecast includes:
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If it will make landfall
Where and when it will strike
Wind strength
Width of affected area
Rainfall amount
Storm surge
• Monitored by U.S. Hurricane Center, Canadian
Hurricane Center
• Hurricane watch means likely hurricane in 36 hours
• Hurricane warning given when hurricane is likely
within 24 hours or less
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Forecasts and Warnings, cont.
• Hurricane forecasting tools
– Weather satellites
• Detect early warning signs
• Can not show wind speed
– Aircraft
• U.S. Air Force, NOAA airplanes fly into the storm to collect data
– Doppler radar
• Give information on rainfall, wind speed, and direction of the storm
– Weather buoys
• Continuously record weather conditions
– Computer models
• Make predictions about storm tracks
• Global Forecast System (GFS) model runs four times a day
• Still not completely accurate in predicting storm intensity
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Radar Image of Hurricane Charley
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Hurricane Track Map
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Forecasts and Warnings, cont.
• Storm surge predictions
– Predict the time and elevation of surge
• Forecasters use wind speed, fetch and average water depth
– Need detailed information on topography
• Different elevations on land affect the storm surge
• LIDAR (Light Detection and Ranging) used to prepare detailed
digital elevation models
– Computer models use central pressure, size, forward
speed, track, wind speed, and seafloor topography
• Overall are within 20 percent of forecast values
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Hurricane Storm Surge Flooding of
Galveston, Texas
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Forecasts and Warnings, cont.
• Hurricane prediction and the future
– Deaths have decreased dramatically because of better
forecasting, improved education, and greater public
awareness
– However, coastal populations are increasing, increasing
risk
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Property Damage Costs Associated with
Hurricanes—U.S. Atlantic and Gulf Coasts
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10.9 Perception of and Adjustment to
Cyclones: Perception of Cyclones
• Residents in areas at risk for cyclones have
significant experience, yet do not always percieve
the danger
• Perception of hazard depends on personal
experience
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New residents may underestimate the hazard
More experienced people may take hazard more seriously
More seasoned people may also take less precautions
Incorrect predictions may lower the risk perception
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Adjustment to Cyclones Hurricanes and
Extratropical Cyclones
• Community adjustments to cyclone hazard
– Warning systems
• Give public maximum possible advance notice
• Media broadcasts, local use of sirens
– Evacuation plans and shelters
• Developed prior to hurricane season
• Public transportation provided during hazard
– Insurance
– Building design
• Withstand hurricane-force winds
• Allow passage of storm surge
• Recommendations available from Partnerships for Advancing
Technology in Housing (PATH)
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Hurricane-Resistant House
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Adjustment to Cyclones Hurricanes and
Extratropical Cyclones, cont.
• Personal adjustments to cyclone hazard
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Be aware of hurricane season
Prepare homes and property for hazard
Obtain flood insurance
Install heavy shutters that can be latched
Learn evacuation route
Make a family emergency plan
Collect emergency supplies
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Hurricane Sandy – Applying the 5
Fundamental Concepts
• Developed in late October 2012
– Impacted much of the eastern seaboard of the United
States and Canada
– Began as a tropical wave in the southern Caribbean and
quickly upgraded to tropical storm
– Category 1 west of Haiti and the Dominican Republic
• Residents still living in poorly constructed shantytowns
• Flooding and mudslides
• More 50 killed
– Category 3 when striking Cuba
• Storm surge caused extensive flooding and coastal erosion
• Completely destroyed more than 15,000 homes and killed 11
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Flooding in Haiti
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Hurricane Sandy – Applying the 5
Fundamental Concepts, cont.
• Computer models did not agree on path
– Most trusted NHS global model predicted Sandy would
head east into the Atlantic and dissipate
• Local media downplayed the potential impact on the United States
– European Centre for Medium-Range Weather Forecasts
(ECMWF) showed Sandy taking a completely different
path and making landfall near New York City
• High-resolution forecast showed it joining an arctic front
• The ECMWF prediction was closest to actual
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Modeled and Actual Path of Hurricane
Sandy
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Hurricane Sandy – Applying the 5
Fundamental Concepts, cont.
• Atypical intense high-pressure ridge over Greenland
– Blocked jet stream’s westerly winds
– Caused Hurricane Sandy (at Category 1) to turn west
– Also caused arctic cold front to stall over northeastern
United States
• Sandy would merge with this front
• Formed hybrid “superstorm”
• Wind diameter was three times larger than Katrina
• Predicted effects of the “superstorm”
– Dump heavy widespread snow
– Massing flooding
– Massive power outage
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Hurricane Sandy’s Unusual Path
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Hurricane Sandy – Applying the 5
Fundamental Concepts, cont.
• Landfall
– Predicted 24-hours before to come ashore within 25 miles
of Atlantic City, New Jersey
– New York City would be in right front quadrant of hurricane
– Track meant huge storm surge could be expected and
arrival was timed with full moon and high tide
– Evacuation of New York City’s residents in Evacuation
Zone A
– Storm no longer had hurricane-strength wind velocities
when it came ashore in New Jersey
• However, huge size still meant high winds, rain, and storm surge
battered the coasts of New York and New Jersey
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New York City Flood Evacuation Plan and
Sandy Inundation
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Hurricane Sandy – Applying the 5
Fundamental Concepts, cont.
• Effects
– Storm surge highest recorded
• Topped seawalls
• Widespread power outages
• Flooding of subway system and tunnel connecting Manhattan and
Brooklyn
– Damage in over 16 states
• Power outages for more than 8.5 million residences
• About 380,000 homes destroyed or damaged
• Heavy snow blanketed 5 states
– Flooding
• Caused the greatest monetary damage and loss of life
• Flooding greater than expected due to winds
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Widespread Blackouts along the Eastern
Seaboard
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Hurricane Sandy – Applying the 5
Fundamental Concepts, cont.
• Mitigation of future flood damage
– Insurance cost increase in restricted areas
– Buyouts at full prestorm market value in New York
• Never build on land again
• Turn into parks or wetlands, dunes, and other natural buffers
– Construction of moveable storm surge barriers
• Built at three key locations
• Stop flooding of parts of New York and New Jersey
• Price tag of $10 billion – is the cost too great?
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Storm Surge Barriers
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Chapter 10 Summary
• Cyclones, large areas of low atmospheric pressure
with winds converging toward the center, are
associated with most severe weather.
• Tropical cyclones have warm cores, are not
associated with weather fronts, and form over
tropical and subtropical oceans between 5 and 20
degrees latitude.
• Extratropical cyclones have cool cores, develop
along weather fronts, and form between 30 and 70
degrees latitude over either the land or ocean.
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Chapter 10 Summary, cont.
• Tropical cyclones are classified as tropical
depressions, tropical storms, and hurricanes with
increasing wind speed.
• Tropical storms and hurricanes are given names
from lists for the region where they form, whereas
exgtratropical cyclones are sometimes named for
their geographic area of origin or prevailing wind
direction.
• Most tropical cyclones start out as a tropical
disturbance, al rage thunderstorm complex
associated with a low-pressure trough.
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Chapter 10 Summary, cont.
• Hurricanes develop spiraling rain bands of clouds
around a nearly calm central eye.
• Most hurricanes are steered by winds in the middle
and upper troposphere and these storms generally
lose intensity over land or cooler water, whereas
extratropical cyclones typically form along fronts
where there are strong, diverging winds in the upper
troposphere.
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Chapter 10 Summary, cont.
• In North America, the Gulf and Atlantic Coasts are at
highest risk for tropical cyclones, with hurricanestrike probabilities being the greatest in southern
Florida, the northern Gulf Coast, and Cape Hatteras.
• Extratropical cyclones are the primary severe
weather hazard on the Pacific Coast and cause
storms on the Great Lakes.
• Cyclones produce coastal storm surges, high winds,
and heavy rains with major hurricanes and intense
extratropical cyclones generating storm surges on
more than 3 m (~10 ft).
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Chapter 10 Summary, cont.
• In the Northern Hemisphere, the greatest storm
surge and highest winds are typically in the right
front quadrant of tropical cyclones.
• Strong currents from storm surges cut channels
through islands and peninsulas and deposit sand as
overwash.
• Wind damage from hurricanes is more widespread
but less deadly than the storm surge.
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Chapter 10 Summary, cont.
• Although hurricanes hold most rainfall records,
tropical storms also produce intense rains and
flooding.
• Cyclones are closely linked with other severe
weather, flooding, landslide, and debris flow
hazards.
• The hazard posed by storm surges and erosion from
coastal cyclones will increase as sea level rises
worldwide from global warming.
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Chapter 10 Summary, cont.
• Both hurricanes and coastal extratropical cyclones
will become more destructive to our society as
coastal populations and per capita wealth grow.
• Accurate forecasts, effective storm warnings, strict
building codes, and well-planned evacuations can
minimize the effects of coastal cyclones.
• Hurricane forecasting relies on weather satellites,
aircraft flights, Doppler radar, and automated
weather buoys. Computer models predict hurricane
tracks more accurately than their intensity.
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Chapter 10 Summary, cont.
• Perception of the coastal cyclone hazard depends
on individual experience and proximity to the
hazard.
• Community adjustments to hurricanes in developed
countries involve building protective structures or
modifying people’s behavior through land-use
zoning, evacuation procedures, and warning
systems.
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Chapter 10 Summary, cont.
• Individual adjustments to hurricanes include having
emergency supplies on hand; preparing once a
storm prediction is made; and if required, evacuating
before the storm hits.
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