Chapter 11
Hurricanes
Lecture PowerPoint
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Hurricanes
Natural Disasters, 8th edition, Chapter 11
Hurricanes
•
•
•
•
Large tropical cyclones
Heat engines converting heat of tropical ocean into winds, waves
Generate winds over 240 km/hr
Push massive amounts of water onshore as surges, up to 6 m over
sea level
• Heavy rains cause dangerous floods well away from coastlines
Figure 11.3
How a Hurricane Forms
• Means of transporting excess tropical heat to midlatitudes
• Requirements for development of hurricane:
– Seawater at least 27oC (80oF) in upper 60 m
– Air must be warm, humid and unstable
– Weak upper-level winds, preferably blowing in same
direction as developing storm is moving
• Begins with tropical disturbance:
– low-pressure zone drawing in cluster of thunderstorms with
weak surface winds
How a Hurricane Forms
• Becomes tropical depression (receives identifying
number):
– Surface winds strengthen and flow around and into core
– Rotates counterclockwise (in northern hemisphere)
Figure 11.5
How a Hurricane Forms
– Converging surface winds flow up central core (like chimney)
sending warm, moist air into stratosphere
– Rising moist air cools to dew point temperature, condenses and
releases latent heat
– Released heat warms surrounding air  stronger updrafts 
increased rate of upward flow of warm, moist, surface air
– When winds exceed 63 km/hr: tropical storm
– When winds exceed 119 km/hr: hurricane
How a Hurricane Works
The Eye
• As more wind blows into center of tropical storm,
difficult for all wind to reach center  spiraling upward
cylindrical wind mass in center of storm
• When surface-wind speeds reach about 119 km/hr, none
of wind reaches center  calm clear eye
• Inside eye:
– Cool, high-altitude air sinks and absorbs moisture
• Eye wall:
– Cylinder-shaped area of spiraling upward winds around eye
– Strongest winds
How a Hurricane Works
The Eye
• Eddies of storm sucked up into eye wall and stretched
vertically  caused winds on one side of 130 mph + 80
mph = 210 mph, and winds on other side of 130 mph –
80 mph = 50 mph
• Same phenomenon occurs
on different sides of
storm: one side
experiences wind speed +
travel speed of hurricane,
other side experiences
wind speed – travel speed
of hurricane
Figure 11.27
How a Hurricane Works
Hurricane Energy Release
• Transfers heat from tropical seas to core of hurricane
• Huge amounts of latent heat released as air rises
• Generates energy at 200 times greater rate than worldwide
electricity-generating capacity
Hurricane Origins
• Differences from high-latitude storms:
– Main energy source is latent heat released by water vapor
condensation
– Weaken rapidly when move onto land
– Not associated with fronts
– Weaker high-altitude winds  stronger hurricane
– Hurricane winds weaken with height
– Air in center of eye sinks downward
• Different names in different parts of the world:
– Indian Ocean: cyclones
– Western Pacific Ocean: typhoons
Hurricane Origins
• Form on west sides of oceans where warm water is
concentrated
– Also off Pacific coast of Mexico – area of warm water isolated
from cold California current by bend in coastline
– Not off coast of Brazil – Atlantic Ocean is too narrow, not
enough warm water
• Form between 5o to 20o latitude, travel to higher latitudes
• Do not form along equator – Coriolis effect is zero
• Can not cross equator once formed – lose rotation
Hurricane Origins
• Average of 84 tropical cyclones (hurricanes, typhoons,
cyclones) form each year
– About 10 in north Atlantic Ocean, Caribbean Sea, Gulf of
Mexico
Figure 11.11
Hurricane Origins
• Strength assessed by Saffir-Simpson scale
– Category 1: wind damages trees and unanchored mobile homes
– Category 2: winds blow down trees, major damage to mobile
homes, some roofs
– Category 3: winds blow down large trees, strip foliage, destroy
mobile homes, damage small buildings
– Category 4: all signs blown down, heavy damage to buildings,
major damage to coastal buildings, flooding extends inland
– Category 5: severe damage to buildings, major damage to
buildings less than 5 m above sea level and within 500 m of
shoreline, small buildings overturned and blown away
Hurricane Origins
• Strength assessed by Saffir-Simpson scale
Insert Table 11.1 here
North Atlantic Ocean Hurricanes
• From 851 to 2006: U.S. Gulf and Atlantic coastlines hit
by 279 hurricanes, three category 5 and 18 category 4
• Most form in late summer when sea surface temperatures
are warmest
– Due to high heat capacity water absorbs heat all summer
Insert Table 11.3 here
North Atlantic Ocean Hurricanes
Cape Verde-type Hurricanes
• Usually begin as easterly waves
• May gain tropical storm status near Cape Verde Islands
off Africa
• Blown westward with trade winds, gaining energy
• Move north by Coriolis effect when reach western
Atlantic
North Atlantic Ocean Hurricanes
Andrew, August 1992
•
•
•
•
Began as thunderstorms over West Africa, August 13
Moved out over Atlantic Ocean as rotating low-pressure air mass
Intensified into tropical storm by August 17
Reached western Atlantic on August 21 as upper-level winds died
down, allowing clouds and winds to build, and as high pressure to
the north forced Andrew over warmer water to the west
• August 22:
hurricane strength
• August 23:
northern Bahamas,
wind speeds of 240
km/hr
Figure 11.15
North Atlantic Ocean Hurricanes
Andrew, August 1992
• August 24: crossed southern Florida with 25-mile wide
path over homes of 350,000 people, with winds of 250
km/hr, gusts up to 282 km/hr, vortices up to 320 km/hr
• Killed 33 people, destroyed 80,000 buildings
• Moved across Gulf of Mexico regaining energy
• Hit Louisiana with 190 km/hr winds on August 26
• Killed additional 15 people
• Spent remaining energy in heavy rains over Mississippi
• $30 billion in damages, most destructive in U.S. history
– Much of damage was result of poor construction
North Atlantic Ocean Hurricanes
Hurricane Paths
• Main influences on hurricane paths:
– Trade winds blow cyclone west
– Coriolis effect adds curve to right, stronger with distance from
equator
Insert Figure 11.12
Figure 11.12
North Atlantic Ocean Hurricanes
Hurricane Paths
• Main influences on hurricane
paths:
– Size, position of Bermuda
High: North Atlantic high
pressure zone
• Small, to the north:
hurricanes miss
coastlines
• Strong, extensive:
guides hurricanes along
east coast of U.S.
• South: guides
hurricanes to Gulf of
Mexico
Figure 11.17
North Atlantic Ocean Hurricanes
Hurricane Paths
• Main influences on hurricane paths:
– Bermuda High part of North Atlantic Oscillation (NAO):
decade shifting of atmospheric pressures over ocean
• 1950s: east coast of North America hit by hurricanes
• 1960s, 1970s: Gulf coast hit by hurricanes
Figure 11.18
North Atlantic Ocean Hurricanes
Caribbean Sea- and Gulf of Mexico-Type Hurricanes
• Form at Intertropical Convergence Zone (ITCZ), where
trade winds meet
• Location of ITCZ moves with migration of the sun
• ITCZ located south of equator in northern hemisphere’s
winter and north of equator in northern hemisphere’s
summer
• Low-pressure area forms where air flows converge 
thunderstorms  hurricane
North Atlantic Ocean Hurricanes
Mitch, October 1998
• October 22: tropical depression 13 at ITCZ in Caribbean
• 18 hours later: Tropical storm Mitch
• 36 hours later: Hurricane Mitch
• October 26: one of strongest category 5 hurricanes on
record
– Sustained winds of 290
km/hr, gusts greater than
320 km/hr
– Winds over 250 km/hr for
15 consecutive hours
Figure 11.19
North Atlantic Ocean Hurricanes
Mitch, October 1998
• Heading toward Cuba then veered to Central America
• October 27: stalled off coast of Honduras, winds slowed
down to tropical storm strength, but absorbed enormous
amounts of water from warm ocean
• October 30: landfall in Central America, tremendous
amounts of rain in Honduras and Nicaragua
– Three-day rainfall totals up to 2 m
– About 6,500 people killed in Honduras, about 3,800 people
killed in Nicaragua, many by mudflows
– Second deadliest hurricane in history of Americas
North Atlantic Ocean Hurricanes
Hurricane Forecasts
• Increased number of storms in North Atlantic region can be forecast
based on:
– Wetter western Sahel region
– Warmer sea surface temperatures
– Low atmospheric pressure in Caribbean
– La Nina conditions in Pacific
North Atlantic Ocean Hurricanes
Hurricane Forecasts
• Increased number of storms in North Atlantic region can be forecast
based on:
– La Nina conditions in Pacific
• Westward blowing La Nina winds aid hurricane formation
• Eastward blowing El Nino winds disrupt hurricane formation
Insert figure 11.21 here
Figure 11.21
North Atlantic Ocean Hurricanes
Hurricane Forecasts
Figure 11.22
Figure 11.23
How Hurricanes get their Names
• Alternating male and female names
• Six year cycle
Insert Table 11.5
Decade of the Naughts – 2000 to
2009
• 2004: Florida hit by four hurricanes, Japan hit by 10
typhoons, first documented hurricane in south Atlantic hit
Brazil
• 2005: Hurricane records broken in North Atlantic basin
• Increase due to human activity or natural variation?
• Total energy release by hurricanes is higher, probably due
to increasing sea-surface temperatures and increased water
vapor in lower atmosphere, quite possibly due to global
warming
Decade of the Naughts – 2000 to
2009
Insert Table 11.6 here
North Atlantic Ocean Hurricanes
Hurricane Damages
• Hurricane deaths are down in recent decades
• Hurricane damages are up
– Larger, more expensive homes on coastlines
• Destruction and deaths caused by:
– Winds
– Storm surges
– Heavy rains and inland flooding
North Atlantic Ocean Hurricanes
Storm Surge Hazards
• Rise in sea level under storm due to:
– Winds push water ashore to pile up above normal levels,
especially on right-hand side of storm – highest wind velocities
• In northern hemisphere, highest storm surge occurs 15-30 km
to right of path of eye
– Low atmospheric pressure causes water to mound up under
eye of hurricane
– Also large waves blown
by hurricane winds
– Also sometimes already
high astronomical tides
Figure 11.28
North Atlantic Ocean Hurricanes
Heavy Rains and Inland Flooding
• After moving on land, no more water vapor is absorbed
into hurricane, loses strength
• Precipitation of massive volume of water in dissipating
hurricane can cause massive flooding
• Causes majority of deaths
Insert table 11.10
Hurricanes and the Gulf of Mexico
Coastline
Galveston, Texas, September 1900
• Deadliest natural disaster in U.S. history
• Galveston – wealthiest Texas city – 38,000 residents
given warning of hurricane but many did not evacuate
• Category 4 hurricane – at high tide, with 200 km/hr winds
• Highest point on island flooded to 0.3 m
• 6,000 people killed, but many survived 10 m waves by
crowding in lighthouse
• City constructed sea wall, brought in sand to elevate land
• 1915 hurricane killed another 275 people
Hurricane Katrina
• August 24, 2005: Tropical air mass over Bahamas first catalogued
as Katrina
• August 25, 2005: Grew to hurricane two hours before crossing
Florida, lost strength but grew again rapidly over Gulf of Mexico
 double size, wind speeds 280 mph
• August 26, 2005: models forecast 17% chance of direct hit to New
Orleans
• August 27, 2005: Warnings issued, residents began to flee
• August 28, 2005: Mandatory evacuation ordered for 1.2 million
residents, all roads exit New Orleans
Hurricane Katrina
• August 29, 2005:
– Came ashore east of New Orleans, wind speeds of 195 mph
– Pushed in enough water to breach levees, overflow canals and
flood 80% of low-lying New Orleans
– 1,500 of 100,000 residents who did not evacuate were killed
– Deadliest natural disaster since 1928 Okeechobee hurricane
Insert figure 11.35
Figure 11.34
Figure 11.35
Hurricane Katrina
Were the Katrina-Caused Deaths and Destruction a
Surprise?
• Hurricane like Katrina was anticipated for years
– Special reports in New Orleans Times Picayune in 2002,
among others
• September 2004: Hurricane Ivan narrowly missed hitting New
Orleans, after massive evacuation (practice run) of city
Hurricane Katrina
New Orleans: Can This Setting Be Protected?
• Water and floods have always been part of city life, three centuries
of floods from river and hurricanes
• Billions of dollars spent on levees
• City built on delta: loose mixture of sand, mud and water deposited
by Mississippi River
Figure 11.36
Hurricane Katrina
New Orleans: Can This Setting Be Protected?
• Deltas naturally subside  city has subsided to 6 m below sea
level and continues to subside
• Levees prevent flooding, therefore also prevent deposition of new
layers of sand and mud that would build up land level
• Creates bowl, with lake and river water levels higher than city land
• Subsidence will continue  no easy solutions
Figure 11.37
Hurricanes and the Atlantic Coastline
Hugo, September 1989
• Charleston, South Carolina ruined by Civil War, severely
damaged by 1886 earthquake, experienced tenth strongest
U.S. hurricane in 1989
• 5.2 m storm surge, 220 km/hr winds at Fort Sumter,
Charleston
• Passed Charlotte, North Carolina with tropical storm
intensity, into Appalachians as losing status, rains from
New York into Canada
The Evacuation Dilemma
• Satellite photos allow warning of hurricane making
landfall, people can evacuate out of way
• Almost 50 million people live in Atlantic Ocean or Gulf
of Mexico coastline counties, population growth faster
than construction of new roads or bridges
– Aided by assurance of federal disaster assistance
• Warning of 1999 Hurricane Floyd hitting South Carolina
created massive gridlock leaving people exposed on roads
– Hurricane Floyd hit North Carolina instead
• Evacuation dilemma: estimated 72 hours to evacuate
most cities
• Hurricane path not well known 72 hours in advance
Reduction of Hurricane Damages
Building Codes
• After many manufactured homes destroyed by 1992 Hurricane
Andrew, tougher building codes enacted – new manufactured
homes survived four hurricanes of 2004
Roofs
• First step in destroying building is often to lift off roof
• Prevention by:
– Elimination or strengthening of eaves
– Strap roofs to walls
– Ban stapled asphalt roofing sheets on plywood
Impact of Wind-borne Debris
• Windows of shatter-proof glass or protected by shutters or plywood
• Remove loose objects outside
Reduction of Hurricane Damages
Land-Use Planning
• Low-lying coastal land used for parks, farm fields, golf courses,
nature preserves, etc., where flooding is not damaging
Coastal Development Restrictions
• Current building boom on shorelines
• Thousands of new homes built since last hurricane strike
• FEMA estimates next 60 years: 25% houses within 150 m of
shoreline will fall into water during some hurricane, without
mitigating actions
Global Rise in Sea Level
• Global rise of about 1 ft in 20th century, probably 2
ft or more in 21st century.
• Can move beaches inland by 1,000 ft in low-lying
areas
• Continuing buildup of housing along the coastline is
just asking for trouble.
Insert figure 11.41
Hurricanes and the Pacific Coastline
• 15% of Earth’s tropical cyclones: offshore southern Mexico,
Guatemala, El Salvador
• Why is Pacific coastline hit by fewer hurricanes?
– Trade winds blow hurricanes west out to sea
– Cold California current from Alaska drains hurricane energy
Iniki, September 1992
• Hawaiian Islands at northern edge of hurricane-generating warm
waters, hit by storms formed to the south
• Iniki: category 4 storm tore across Kauai with 210 km/hr sustained
winds and gusts of 260 km/hr
• Damaged all buildings, cost island economy $2 billion
Cyclones and Bangladesh
• Seven of world’s nine most deadly weather events in 20th
century were cyclones hitting densely populated
Bangladesh – low-lying sediments of deltas of Ganges
and Brahmaputra Rivers, 1/3 less than 6 m elevation
• In average year, 20% of country submerged in floods
• 1970 cyclone during full Moon high tides brought surge
of 7 m, winds of 235 km/hr  400,000 people killed
• 1991 cyclone with 6 m surge, 235 km/hr winds killed
140,000 people
• Population of Bangladesh is expected to double in next
30 years
End of Chapter 11