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Chapter 15
hurricanes
Tropical Weather
• Noon sun is always high, seasonal
temperature changes small
• Daily heating and humidity = cumulus
clouds and afternoon thunderstorms
• Non-squall clusters, tropical squall line,
tropical wave
• Seasons defined by precipitation as
opposed to temperature
Figure 15.1
A tropical wave (also called an easterly wave) moving off the coast of Africa over the
Atlantic. The wave is shown by the bending of streamlines—lines that show wind flow
patterns. (The heavy dashed green line is the axis of the trough.) The wave moves
slowly westward, bringing fair weather on its western side and rain showers on its
eastern side.
Tropical Disturbance
– An organized cluster of
cumulonimbus clouds over
tropical seas that has a surface
center of low pressure; usually
triggered by the ITCZ
– Easterly Wave
• A ripple in the tropical easterlies
featuring a weak trough of low
pressure
• Forms over East Africa and
propagates westward
• Precursors of ~ 65% of named
Atlantic tropical cyclones
Anatomy of a Hurricane
• Intense storm of tropical origin with winds
greater than 64kts (74 mph)
Stepped Art
Fig. 15-3, p. 414
Hurricane Formation and Dissipation
 The Right Environment
 Tropical waters with light wind
 26.5°C sea surface temperatures (June-November)
 Surface winds must converge , need trigger
 Coriolis effect zero at equator
 5-20º latitude areas of development
 Inhibited by
 Trade wind - inversion near 20deg latitude
 Strong upper level wind shear
 Strong El Nino event
Figure 15.7
Hurricanes form over warm, tropical waters. This image shows where sea surface
temperatures in the tropical Atlantic exceed 28°C (82°F)—warm enough for tropical
storm development—during May, 2002.
The total number of hurricanes and tropical storms (red shade) and hurricanes only
(yellow shade) that have formed during the past 100 years in the Atlantic Basin—the
Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. (NOAA)
Figure 15.9
The top diagram shows an intensifying tropical cyclone. As latent heat is released inside the clouds, the warming of the air aloft
creates an area of high pressure, which induces air to move outward, away from the high. The warming of the air lowers the air
density, which in turn lowers the surface air pressure. As surface winds rush in toward the surface low, they extract sensible heat,
latent heat, and moisture from the warm ocean. As the warm, moist air flows in toward the center of the storm, it is swept upward into
the clouds of the eyewall. As warming continues, surface pressure lowers even more, the storm intensifies, and the winds blow even
faster. This situation increases the transfer of heat and moisture from the ocean surface. The middle diagram illustrates how the air
pressure drops rapidly as you approach the eye of the storm. The lower diagram shows how surface winds normally reach maximum
strength in the region of the eyewall.
• The Storm Dies Out
– Cold water, land
• Hurricane Stages of Development
– Tropical Disturbance
• Mass of Tstorms
– Tropical Depression (22-34kts)
• Several closed isobars around center
– Tropical Storm (35-64kts)
• Isobars packed closer together
– Hurricane (> 65kts)
Figure 15.10
Visible satellite image showing four tropical systems, each in a different stage of its life
cycle.
Figure 15.11
Regions where tropical storms form (orange shading), the names given to storms, and
the typical paths they take (red arrows).
• Hurricanes and Mid-latitude Storms
different
– Hurricane warm core low
– Mid-latitude cold-core low
– Hurricane + upper level trough
 May become a mid-latitude cyclone
Figure 1
Surface weather map for the morning
of September 23, 2005, showing
Hurricane Rita over the Gulf of Mexico
and a middle-latitude storm system
north of New England.
 Hurricane movement
 General track: west,
northwest, northeast
 Much variation
Figure 15.13
An extremely rare tropical cyclone (with no name) near 28°S latitude spins clockwise
over the south Atlantic off the coast of Brazil during March, 2004. Due to cool water and
vertical wind shear, storms rarely form in this region of the Atlantic Ocean. In fact, this is
the only tropical storm ever officially reported there.
Naming Hurricane and Tropical
Storms
• Process has changed over the years:
– Latitude and longitude
– Letters of the alphabet around WWII
– Alphabetical female names NWS 1953
– Alphabetical, alternating female and male
names -1978- 79
– Retirement (Katrina, Camille) if a Cat3 for
10yrs
Devastating Wind, Storm Surge,
and Flooding
• Highest winds on the eastern side of storm (wind + speed of
storm)
• Swell
• Storm surge on north side of storm
– Caused most of the 1,300 fatalities associated with Hurricane Katrina
– Remains the most serious potential impact of a landfalling hurricane
• Heavy rains and inland flooding
•
•
•
•
– Freshwater flooding was responsible for 60% of deaths from 1970 – 1999
attributed to tropical cyclones or their remnants
Coastal flooding
River flooding
Hurricane spawned tornadoes
Saffir-Simpson scale
– 1 weakest, 5 strongest
A hurricane moving northward will have higher
sustained winds on its eastern side than on its western side. If the hurricane
moves from east to west, highest sustained winds will be on its
northern side.
Hurricane Hazards
• Inland Flooding
– Rains are typically 13 - 25
cm (5 - 10 in.)
– Heavy rains persist as the
storm tracks inland
– Case study – Hurricane
Agnes (1972)
• Rains accounted for most
of the property damage
• Devastating floods in midAtlantic region, especially
central Pennsylvania,
when heavy rain fell on
already saturated
grounds and hilly terrain
– Hurricane Mitch (1998),
Tropical Storm Alberto
(1994), and Tropical Storm
Allison (2001) also caused
devastating flooding
Hurricane Hazards
• Wind
– Wind pressure, the force per unit area caused by air in
motion, increases with the square of the wind speed
– Debris transported by wind increases damage potential
– Small but powerful whirlwinds (spin-up vortices)
embedded in a hurricane’s circulation may be
responsible for the most severe property damage
– Winds diminish rapidly upon storm landfall
© AMS
• Hurricane over land is no longer in contact with warm ocean
water, its energy source
• Frictional resistance slows wind and shifts wind direction toward
the center; causes the storm to fill and weaken
• The system may still produce tornadoes after making landfall,
partially due to strong wind shear between the surface and aloft
21
• Storm surge
– A dome of ocean water 80 – 160 km (50 – 100 mi) wide that sweeps
over the coastline near the hurricane’s landfall
– Caused by strong winds and low barometric pressure and is most
likely on the side of the hurricane with onshore winds
– Wind-driven waves on top of the dome of water, armed with floating
debris, are responsible for much of the structural damage
– Prior to 1970, was responsible for the majority of hurricane-related
fatalities. Awareness, warnings, and evacuation have generally been
much better since then.
– From 1970 to 1999, there were only 6 storm surge deaths. Then there
was Hurricane Katrina and the death and destruction caused by its
surge.
– 1895 unnamed hurricane killed an estimated 2,000, and left 20,000 to
30,000 homeless due to the storm surge
– The most deadly U.S. natural disaster was the hurricane that
hit Galveston, TX (1900) when 8,000 people perished
– Hurricane Camille (1969) produced a 7.3 m (24.3ft) surge at Pass
Christian, MS
– East Pakistan (now Bangladesh) in 1970 – storm surge killed ~300,000
Figure 15.15 Storm Surge
When a storm surge moves in at high tide it can inundate and destroy a wide swath of
coastal lowlands.
Figure 15.16
The changing of the ocean level as different category hurricanes make landfall along
the coast. The water typically rises about 4 feet with a Category 1 hurricane, but may
rise to 22 feet (or more) with a Category 5 storm.
Saffir-Simpson Hurricane Intensity Scale
• Provides an estimate of potential coastal flooding and property
damage from a hurricane landfall
• Wind speed is the primary determining factor for a hurricane’s
rating
• Storm surge is just an estimate
– It depends on underwater topography and other factors in the
region of landfall
• Property damage rises rapidly with rating
– 100-300 times greater damage from a category 4 or 5 hurricane than
a category 1
• From 1901 to 2004, about 37% of landfalling hurricanes were
classified as major (category 3 or above)
The number of hurricanes
(by each category)
that made landfall along
the coastline of the United
States from 1900
through 2007.
All of the hurricanes struck
the Gulf or Atlantic coasts.
Categories 3, 4, and 5 are
considered major
hurricanes.
Figure 15.18 HUGO 1989
A color-enhanced infrared satellite image of Hurricane Hugo with its eye over the
coast near Charleston, South Carolina.
Figure 15.19 Andrew 1992
Color radar image of Hurricane Andrew as it moves onshore over south Florida on the
morning of August 24, 1992. The National Hurricane Center (NHC) is located about 30
km (19 mi) from the center of the eye.
Figure 15.21 IVAN 2004
Visible satellite image of Hurricane Ivan as it makes landfall near Gulf Shores, Alabama,
on September 15, 2004. Ivan is a major hurricane with winds of 105 knots (121 mi/hr)
and a surface air pressure of 945 mb (27.91 in.).
Figure 15.23
Hurricane Katrina just after making landfall along the Mississippi/Louisiana coast on the
morning of August 29, 2005. Shown here, the storm is moving north with its eye due
east of New Orleans. At landfall, Katrina had sustained winds of 110 knots, a central
pressure of 920 mb (27.17 in.), and a storm surge over 20 feet.
• Observation: Atlantic Hurricanes 2004-2005
– Abnormally warm ocean water and weak vertical
sheer allowed for high frequency of hurricanes
Tracks of three of the four hurricanes that struck Florida in 2004
Hurricane Watches, Warnings, and
Forecasts
• Watch issued 24-48 hours before
hurricane expected to make landfall
• Warning issued when storm expected to
strike coast within 24 hours and probability
of strike in a given location provided.
Hurricane Katrina over the Gulf
of Mexico with
sustained winds of 145 mi/hr on
August 28, 2005, at 1 a.m. CDT.
The current movement of the
storm is west-northwest at 8
mi/hr.
The dashed orange line shows
the hurricane’s projected path;
the solid purple line, the
hurricane’s actual path. Areas
under a hurricane warning are in
red.
Those areas under a hurricane
watch are in pink, while those
areas under a tropical storm
warning are in blue.
Hurricanes in a Warmer World
 No clear answer, need more data
 Intensity and frequency most likely to be impacted.
Figure 5
Sea-surface temperature departures from the twelve-year average (1985–1997) on
May 30, 2005. Notice that the darker the red, the warmer the surface water. (NOAA)
Figure 6
The total number of tropical storms and hurricanes (red bars), hurricanes only (yellow
bars), and Category 3 hurricanes or greater (green bars) in the Atlantic basin for the
period 1851 through 2007. (NOAA)
Modifying Hurricanes
• Operation STORMFURY: 1961- 1983
– seed clouds to create rain, weaken hurricane,
and reduce winds; no conclusive evidence it
was effective
• Oil or film on water to reduce evaporation
and latent heat available to storms
Evacuation
• Effectiveness of coastal evacuation
plans was tested in 1985 when
category 3 Hurricane Elena (bottom
figure) followed an erratic path over
the Gulf of Mexico
• The potential downside of evacuation
was illustrated by Hurricane Floyd
(1999), a very large hurricane
approaching the Southeast
– 2 million were evacuated and massive
gridlock occurred
• Greater uncertainty with forecast
track translates into a broader
evacuation zone and greater
economic losses
– Vertical evacuation may be an option