What is a hurricane?
A hurricane is an intense
rotating storm that
forms over the tropical
oceans.
•“Hurricane” is a specific regional name.
•In general, these storms are known as
“tropical cyclones”:
• Tropical depression: winds < 39 mph
• Tropical storm: winds < 74 mph
• Hurricane: winds > 74 mph

- Atlantic and Eastern Pacific Oceans (hurricanes)
- Western North Pacific Ocean (typhoons)
- South Pacific and Indian Oceans (cyclones)
Where do tropical cyclones form?
(10)
(26)
(14)
(15)
(5)
(9)
(nn) Number of storms per year

Why are the winds so strong in a hurricane?
Winds always move from areas
of higher pressure to areas of
lower pressure. The bigger the
difference between the high
and low pressure, the stronger
the winds.
H L
The winds from
Hurricane Fabian
battering Bermuda on
September 5, 2003.

What causes the winds to rotate around a hurricane?
Hurricane Isabel making landfall
on September 18, 2003
H
H H
H
L
The Coriolis Effect: A change in the
direction of moving objects due to
those objects moving on a rotating,
spherical planet.
The Coriolis Effect caused by
Earth’s rotation.

How is a lawn mower like a hurricane?
Gas provides the
fuel for the
mower’s engine
The engine uses
the fuel to
perform work
and move the
grass-cutting
blade
The starter rope
and spark plug are
the “triggers” for
making the mower
start
Exhaust fumes
are expelled
out away from
the engine

The hurricane heat engine: how it works
3. Having lost most of its heat and
energy, the air is now expelled outward
away from the center of the storm in the
cooler hurricane outflow at high levels.
In mature hurricanes, an eye forms
in the center. Here, the air is
descending and the region is
generally cloud-free.
2. The warm, moist air rises in
thunderstorms surrounding the
eye, supplying the fuel that
helps strengthen the storm and
intensify the winds.
1. As air rushes into the hurricane’s low pressure center, the air
picks up heat and moisture (fuel) from the warm ocean surface.
H HL

Hurricane formation
• Hurricanes grow from clusters of smaller storms and become more
organized and intense as they develop.
Daily images of
Hurricane
Frances (2004),
from a weak
tropical
depression to a
Category-4
hurricane… to
landfall
Hurricanes don’t just form out of thin air. They typically grow
from clusters of smaller thunderstorms that move across the
oceans.

The triple threat of damage from a hurricane:
• Wind Damage: Wind gusts can approach 200 mph in
the strongest hurricanes.
• Storm Surge: The winds of a hurricane pile up ocean
water that can inundate coastlines with water levels up
to 30 feet higher than normal.
• Inland Flooding: Today, this is the greatest risk to life
from a hurricane. Hurricanes and tropical storms,
especially slow-moving ones, can drop huge amounts of
rain in a very short time, even inland far from where
the storm made landfall. From 1970-2000, 9 out of
every 10 fatalities in tropical cyclones were due to
drowning from inland flooding.

Wind Damage examples
A 1x4 board of
wood was
pierced
through a palm
tree by
Hurricane
Andrew’s winds
in August,
1992.
A mobile home park near Miami, FL was completely
destroyed by Hurricane Andrew in August, 1992.
An historic home along the Gulf Coast before and after the passage of
Category 5 Hurricane Camille in August, 1969.

The Saffir-Simpson Hurricane Wind Scale: A
method for categorizing hurricanes based on wind
speed
Category 1 Category 2 Category 3 Category 4 Category 5
Wind
speed (mph)
74-95 96-110 111-130 131-155 >155
Storm
surge (ft)
4-5 6-8 9-12 13-18 >18
Damage
description
Minimal Moderate Extensive Extreme Catastrophic

Storm Surge examples
Storm surge
from
Hurricane
Carol
completely
inundates a
yacht club in
August, 1954.
Apartment building in Pass Christian, Mississippi before and after the passage
of Category 5 Hurricane Camille in August, 1969. Thirty-two people ignored
warnings and stayed for a “hurricane party”. Only two survived.
Hurricane Katrina’s 30-foot storm surge obliterated
every home within several blocks of the shoreline in
Pass Christian and Waveland, Mississippi.

Storm Surge examples
Waveland, MS
David & Kimberly King’s house, 4 blocks
from the beach, on August 28
(1 day before Katrina hit)
…the same property, same view, on
September 2
(4 days after Katrina hit)

•Hurricanes, Tropical Storms, and
Tropical Depressions, form over water.
•A tropical system is also known as a
Cyclone, and a Typhoon.
•All Cyclones start as tropical
depressions then strengthen to tropical
storms, then move on to Hurricane
status.
•Hurricane strength is measured using
the Saffir-Simpson Scale.