FREQUENTLY
ASKED QUESTIONS ABOUT HURRICANES AND STORMS
The
Hurricane Risk of the Hawaiian Islands
George
Pararas-Carayannis
How Frequent are Hurricanes?
Fortunately, major
destructive hurricanes are relatively rare events at any location.
What is a Tropical Depression?
Tropical cyclone is
the general term that descrives a low pressure system that originates
over the tropical oceans. By international agreement, tropical
cyclones are classified according to their intensity. Tropical
Depression is an area of developing counterclockwise (in the
northern hemisphere, clockwise in the southern hemisphere) wind
circulation that may include localized rain and thunderstorms.
Maximum sustained winds up to 38 MPH (33 Knots). Tropical Storm:
A well defined area of counterclockwise rotating wind of 39-73
MPH (34-63 Knots). Usually includes rain and thunderstorms. It
is assigned a name.
How are Tropical Storms
created?
The great storms are
driven by the heat released by condesing water vapor, and by
external mechanical forces. Once cut off from the warm ocean,
the storm begins to die, starved for water and heat energy, and
dragged apart by friction as it moves over the land.
What are Hurricanes?
Hurricanes are severe
tropical cyclones with winds reaching sustained speeds of 74
miles per hour or more. Hurricane winds spiraling toward a a
relatively calm center or eye of low pressure at speeds which
may reach more than 150 miles per hour (130 knots). Near the
center, hurricane winds may gust to more than 200 miles per hour.
Although usually erratic and unpredictable, hurricanes generally
follow a westerly to northwesterly path. In the Atlantic, they
move toward the Gulf of Mexico or the Eastern U.S. coast causing
abnormal water level fluctuations known as hurricane surges.
In the Pacific, hurricanes or cyclones (as often called) generally
follow the same westerly to northwesterly path. Major hurricane
hazards inclulde high winds, heavy rainfall, flooding, storm
surge and high surf. If a hurricane has developed from a tropical
storm, it retains the same name as the storm. Every year, these
violent storms bring destruction toc oastlines and islands in
their erratic path.
Stated very simply,
hurricanes are giant whirlwinds in which air moves in a large
tightening spiral around a center of extreme low pressure, reaching
maximum velocity in a circular band extending outward 20 or 30
miles from the rim of the eye. This is called the area of maximum
winds. This ci rculation is counterclockwise in the Northern
Hemisphere, and clockwise in the Southern Hemisphere. The entire
storm dominates the ocean surface and lower atmosphere over tens
of thousands of square miles.
The storms move forward
very slowly in the tropics, and may remain almost stationary
for short periods of time.The initial forward speed is usually
15 miles per hour or less. This is called the translational speed
of the hurricane. Then, as the hurricane moves farther from the
Equator, its forward speed tends to increase; at middle latitudes
it may exceed 50 miles per hour in extreme cases.
NOAA
GRAPHIC OF FOUR HURRICANES IN 1998
BE PREPARED
What makes Hurricanes
dangerous?
What makes hurricanes
the dangerous storms they are is that they combine the triple
hazard of violent winds, torrential rains, and abnormally high
waves and storm tides. Each of these by itself can pose a serious
threat to life and property. Taken together they are capable
of causing widespread destruction. For example, the hurricane
that hit Galveston in 1900 resulted in 6,000 deaths and the alsmost
complete destruction of the city. Hurricane Camille, which struck
the Mississippi coast in 1969, killed 262 persons and caused
damages of nearly $1 billion. Recent hurricanes have caused similar
damages, but fortunately with a reduced toll on lives, as Warning
systems have been implemented.
What is the eye of a Hurricane?
The eye, like the
spiral structure of the storm, is unique to hurricanes. Here,
winds are light and skies are clear or partly cloudy. But this
calm is deceptive, bordered as it is by maximum force winds and
torrential rains. Many persons have been killed or injured when
the calm eye lured them out of shelter, only to be caught in
the maximum winds at the far side of the eye, where the winds
blow from a direction opposite to that in the leading half of
the storm.
Hurricane Hugo
What is the Hurricane
Surge?
Hurricane or storm
surge is an oceanographic phenomenon of water level fluctuations
caused by the atmospheric pressure field and wind stress on the
water surface, accompanying the moving hurricane or storm systems.
Specific factors which can combine to produce extreme water fluctuations
at a coast during the passage of a storm or a hurricane include:
storm intensity, size, path, and duration over water; atmospheric
pressure variation; speed of translation; winds and rainfall;
bathymetry of the offshore region; astronomical tides; initial
water level rise; surface waves and associated wave setup and
runup.
Hurricane Surge constitutes
a greater hazard to lives and coastal property than hurricane
winds. Hurricane surges have been estimated to account for 75
to 90 percent of all deaths resulting from a hurricane. Surge
inundation is also responsible for extensive damage to coastal
property. Since 1900, hurricane damages to coastal property have
averaged more than $50 million per year. The per year average
has been far greater for the last twenty years.
NOAA GRAPHIC
Are Hurricanes assigned
magnitudes?
Yes. Hurricanes are
categorized from1 through 5, by the Saffir/Simpson Scale. These
magnitudes are assigned in accordance to the amount of potential
damage and wind speed.
Description of Damage
Wind Speeds (MPH) Storm Surge (feet) Examples
Category 1 : Minimal Damage, Wind speeds
of 74 - 95 (MPH), Storm Surge 4 - 5 (feet)
Examples: Hurricane
Iwa (Hawaii) , Winds 92 MPH, Nov. 1982
Category 2 : Moderate Damage, Wind speeds
96 - 110 (MPH), Storm surge 6 - 8 (feet). None
Category 3 : Extensive Damage, Winds 110 -
130 (MPH). Storm surge 9 - 12 (feet).
Example: Hurricane
Uleki, 128 MPH, Sep. 1992
Category 4 : Extreme damage, Wind speeds
131 - 155 (MPH), Storm surge 13 - 18 (feet),
Example: Horricane
Iniki, 145 MPH, Sep. 1992
Category 5 : Catastrophic Wind speeds greater
than 155 MPH, Storm surge 18 or more feet.
Examples: Hurricanes Emilia & Gilma,
161 MPH, Jul 94, Hurricane John,/173 MPH Aug. 1994
NOAA
GRAPHIC
What
is the Hurricane and Tropical Storm risks for Hawaii?
In Hawaii, hurricane
winds, especially where augmented by local terrain, have been
very damaging to trees, vegetation, and crops, as well as to
lightly built dwellings and other structures. Heavy and prolonged
hurricane rains falling over steep hillsides can cause landslides
and severe flash flooding. Large swell moving out ahead of the
hurricane may begin to reach island shores while the storm itself
is still several hundred miles away. As the hurricane nears the
coastline, rapidly rising water levels from above-normal storm
tides and high wind-driven waves will inundate coastal areas,
erode beaches, and pound and undermine waterfront structures,
highways, and other facilities.
During the last 50
years many hurricanes and tropical storms have come close to
the Hawaiian Islands, but only three have had direct impact.
In all three cases, Kauai was the hardest hit, although Oahu
suffered significant damages as well. Hurricane Iniki was by
far the most destructive storm to strike Hawaii in recorded history,
with widespread wind and water damage exceeding 2.2 billion dollars.
Losses in Hurricane Dot, August of 1959 were about 6 milliion
dollars. Hurricane Iwa, in Novenber of 1982 caused over 250 million
dollars in damages.
Other hurricanes have
occasionally come close enough to cause relatively minor damage,
mainly in coastal areas vulnerable to high waves. Thus, Hurricane
NINA, in late November 1957, brought surf of 35 feet to Kauai's
southern coast, while waves from Hurricane FICO in July 1978
, damaged homes and roads on the Big Island's Ka'u coast when
the storm itself was more than 400 miles to the southeast.
Tropical cyclones
of less than hurricane strength also have been destructive. For
example, in August 1958, flooding rains and high winds from a
storm that crossed Hawaii Island caused more than $500 thousand
in damage. Most Central Pacific hurricanes originate near the
coasts of Central America or southern Mexico. Long before reaching
the Hawaiian area, however, many of these storms die off when
they move northwestward over cooler water or encounter unfavorable
atmo spheric conditions. Of those that survive, most remain far
enough away to spare Hawaii their effects. Some hurricanes form
nearer the Hawaiian Islands, while a few, like NINA and IWA,
orginate far to the southwest.
What is the Hurricane
Season for the Hawaiian Islands?
Hurricane season begins
in June and lasts through November in the Hawaiian Islands. In
some hurricane seasons, many Central Pacific tropical cyclones
occur; in others, few or none. In 1978, for example, there were
13, three of them full-fledged hurricanes, while the following
year had none. There is no way of telling in advance how active
a hurricane season is likely to be. Hurricanes begin as relatively
small tropical cyclones, generally off the southwest coast of
Mexico or west coast of Central America. Some have, however,
slowly formed sout of the state of Hawaii. They then drift to
the west-northwest, imbedded in the westward-blowing tradewinds
of the tropics. Under certain conditions these disturbances increase
in size, speed, and intensity until they become full-fledged
hurricanes.
BE PREPARED
(Sources of
Information)
1. "Verification Study of a Bathystrophic Strom
Surge Model"
by George Pararas-Carayannis, Technical Memorandum No. 50, U.S.
Army, Corps of Emgineers, Coastal Engineering Research Center,
May 1975, a study supported by the U.S. Atomic Energy Commission
(AEC)(now, U.S. Nuclear Regulatory Commission)
2. Pamphlets published
by the Federal Emergency Management Agency, Hawaii State Civil
Defense, and National Oceanic and Atmospheric Administration
(NOAA).
3. NOAA Images