Chapter 11:
Hurricanes
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Tropical weather
Anatomy of a hurricane
Hurricane formation and dissipation
Some notable hurricanes
Hurricane watches, warnings and forecasts
Modifying hurricanes
Tropical Weather
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Streamlines –
lines that show
wind flow
Tropical wave –
many hurricanes
start as open
waves
• The tropics are close to the equator, where the
Coriolis force is too small to balance the pressure
gradient force. Thus winds are not geostrophic.
Tropical Weather
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Convergence on
east side of wave
(thunderstorms)
Divergence on
west side of wave
• The tropics are close to the equator, where the
Coriolis force is too small to balance the pressure
gradient force. Thus winds are not geostrophic.
Anatomy of a Hurricane
Tropical cyclone – any hurricane-like storm that
forms in the tropics
 Eye – light winds
little precipitation
lowest pressure
 Eyewall – heaviest
precipitation
strongest winds
 Spiral rainband –
precip bands that flow
toward storm center
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Stepped Art
Fig. 11-3, p. 302
The Right Environment
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Hurricanes need
convergence to form
Convergence happens in
low pressure systems
away from the equator
Trade wind inversion
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Inversion brought on by
sinking air from
subtropical highs (bad for
development)
Hurricanes need weak
wind shear to develop
The Developing Storm
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Latent heat released in thunderstorms causes
air to be warmer in thunderstorms than air
around it
This causes high pressure aloft and low
pressure at surface (surface pressure drops)
Operates like a heat engine. Warm air taken in
from ocean, energy is used, then radiation is lost
to space at top of thunderstorms
The whole thing is driven by sensible and latent
heat from where?
The Storm Dies Out
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Hurricanes could last several weeks, but
most last less than a week
Three ways for hurricane to weaken
When hurricanes travel over cold water, heat
source is cut off
 Shallow warm water/hurricane stirs it up
 Landfall. Land causes friction that tears the
hurricane apart
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• Even though making landfall cuts off a hurricane’s
energy supply, the storm is often still quite strong and
can even produce tornadoes.
Hurricane Stages of
Development
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Tropical disturbance – Mass of
thunderstorms/only slight circulation
Tropical depression – Winds between 20
and 34 knots
Tropical storm – Winds between 34 and 64
knots. Storm gets a name
Hurricane – Winds above 64 knots
• This progression of stages is followed in reverse
order as a storm weakens.
Fig. 11-9, p. 306
Hurricane Movement
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Most hurricanes move west/northwestward
Are steered by the subtropical highs
Eventually curve north/northeastward by
westerlies
No hurricanes in South Atlantic/eastern South
Pacific (why?)
Hurricanes in eastern Pacific take more west
track
• Because of the Bermuda High, westward-moving
North Atlantic hurricanes often take a turn towards the
north as they approach North America.
Fig. 11-10, p. 307
Naming Hurricanes and
Tropical Storms
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Past practices
Named by location (latitude, longitude)
 Named by radio code in WWII
 In 1953, female names by alphabet
 In 1978, eastern Pacific used male/female
 In 1979, north Atlantic used male/female
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• Whenever a hurricane has had a major impact,
any country affected by the storm can request
that the name of the hurricane be retired by the
World Meteorological Organization.
Devastating Winds and the
Storm Surge
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Where are the strongest winds?
Most destruction caused by
flooding
Storm surge – a “bubble” of
water like sucking from a straw.
Extremely dangerous
Saffir-Simpson scale
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Category 1 – 74-95 mph…4-5
foot
Category 2 – 96-110 mph…6-8
foot
Category 3 – 111-130 mph…912 foot
Category 4 – 131-155
mph…13-18 foot
Category 5 - >155 mph…>18
foot
Some Notable Hurricanes
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Camille, 1969
Hugo, 1989
Andrew, 1992
Ivan, 2004
Katrina, 2005
Other Devastating Hurricanes
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October, 1893 Gulf coast hurricane
August, 1893 Charleston hurricane
1970: killer cyclone in Bangladesh
Hurricane Watches, Warnings
and Forecasts
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Hurricane watch
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Issued when
hurricane is direct
threat (24 to 48
hours before)
Hurricane warning
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Issued when it
appears the storm
may strike an area
(comes with
probability)
Hurricane Watches, Warnings
and Forecasts
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Models use
information from
satellites, buoys,
reconnaissance
aircraft
Stepped Art
Fig. 11-25, p. 321
Modifying Hurricanes
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Cloud seeding
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Silver iodide used to seed just outside eyewall
Project STORMFURY
In 1963, NOAA seeded several hurricanes
with some success (????)
 Seeding has stopped since the 1970s
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Monomolecular films
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Place film over water to prevent evaporation