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Saffir Simpson Scale
The Saffir–Simpson hurricane wind scale (SSHWS),
formerly the Saffir–Simpson hurricane scale (SSHS),
classifies hurricanes – Western Hemisphere tropical
cyclones – that exceed the intensities of tropical
depressions and tropical storms – into five categories
distinguished by the intensities of their sustained
winds.
To be classified as a hurricane, a tropical cyclone must
have one-minute-average maximum sustained winds at 10 m
above the surface of at least 74 mph (Category 1). The
highest classification in the scale, Category 5,
consists of storms with sustained winds of at least 157
mph. See the table to the right for all five categories
with wind speeds in various units. The classifications
can provide some indication of the potential damage and
flooding a hurricane will cause upon landfall.
The Saffir–Simpson hurricane wind scale is based on the
highest wind speed averaged over a one-minute interval
10 m above the surface. Although the scale shows wind
speeds in continuous speed ranges, the National
Hurricane Center and the Central Pacific Hurricane
Center assign tropical cyclone intensities in 5-knot (kt)
increments (e.g., 100, 105, 100, 115 kt, etc.) because
of the inherent uncertainty in estimating the strength
of tropical cyclones. Wind speeds in knots are then
converted to other units and rounded to the nearest 5
mph or 5 km/h.
The Saffir–Simpson hurricane wind scale is used
officially only to describe hurricanes that form in the
Atlantic Ocean and northern Pacific Ocean east of the
International Date Line. Other areas use different
scales to label these storms, which are called cyclones
or typhoons, depending on the area. These areas (except
the JTWC) use three-minute or ten-minute averaged winds
to determine the maximum sustained wind speed, creating
an important difference which frustrates direct
comparison between maximum wind speeds of storms
measured using the Saffir–Simpson hurricane wind scale
(usually 14% more intense) and those measured using a
ten-minute interval (usually 12% less intense).
There is some criticism of the SSHWS for not accounting
for rain, storm surge, and other important factors, but
SSHWS defenders say that part of the goal of SSHWS is to
be straightforward and simple to understand. |
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History
The scale was developed in 1971 by civil engineer
Herbert Saffir and meteorologist Robert Simpson, who at
the time was director of the U.S. National Hurricane
Center (NHC). The scale was introduced to the general
public in 1973, and saw widespread use after Neil Frank
replaced Simpson at the helm of the NHC in 1974.
The initial scale was developed by Herbert Saffir, a
structural engineer, who in 1969 went on commission for
the United Nations to study low-cost housing in
hurricane-prone areas. While conducting the study,
Saffir realized there was no simple scale for describing
the likely effects of a hurricane. Mirroring the utility
of the Richter magnitude scale for describing
earthquakes, he devised a 1–5 scale based on wind speed
that showed expected damage to structures. Saffir gave
the scale to the NHC, and Simpson added the effects of
storm surge and flooding. |
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Categories
The scale separates hurricanes into five different
categories based on wind. The U.S. National Hurricane
Center classifies hurricanes of Category 3 and above as
major hurricanes, and the Joint Typhoon Warning Center
classifies typhoons of 150 mph or greater (strong
Category 4 and Category 5) as super typhoons (although
all tropical cyclones can be very dangerous). Most
weather agencies use the definition for sustained winds
recommended by the World Meteorological Organization
(WMO), which specifies measuring winds at a height of 33
ft (10.1 m) for 10 minutes, and then taking the average.
By contrast, the U.S. National Weather Service, Central
Pacific Hurricane Center and the Joint Typhoon Warning
Center define sustained winds as average winds over a
period of one minute, measured at the same 33 ft (10.1
m) height, and that is the definition used for this
scale.
The scale is roughly logarithmic in wind speed.
The five categories are described in the following
subsections, in order of increasing intensity. Intensity
of example hurricanes is from both the time of landfall
and the maximum intensity.
Category 1
Very dangerous winds will produce
some damage
Category 1 storms usually cause no significant
structural damage to most well-constructed permanent
structures; however, they can topple unanchored mobile
homes, as well as uproot or snap weak trees. Poorly
attached roof shingles or tiles can blow off. Coastal
flooding and pier damage are often associated with
Category 1 storms. Power outages are typically
widespread to extensive, sometimes lasting several days.
Even though it is the least intense type of hurricane,
they can still produce widespread damage and can be
life-threatening storms.
Category 2
Extremely dangerous winds will
cause extensive damage
Storms of Category 2 intensity often damage roofing
material (sometimes exposing the roof) and inflict
damage upon poorly constructed doors and windows. Poorly
constructed signs and piers can receive considerable
damage and many trees are uprooted or snapped. Mobile
homes, whether anchored or not, are typically damaged
and sometimes destroyed, and many manufactured homes
also suffer structural damage. Small craft in
unprotected anchorages may break their moorings.
Extensive to near-total power outages and scattered loss
of potable water are likely, possibly lasting many days.
Category 3
Devastating damage will occur
Tropical cyclones of Category 3 and higher are described
as major hurricanes in the Atlantic or Eastern Pacific
basins. These storms can cause some structural damage to
small residences and utility buildings, particularly
those of wood frame or manufactured materials with minor
curtain wall failures. Buildings that lack a solid
foundation, such as mobile homes, are usually destroyed,
and gable-end roofs are peeled off. Manufactured homes
usually sustain severe and irreparable damage. Flooding
near the coast destroys smaller structures, while larger
structures are struck by floating debris. A large number
of trees are uprooted or snapped, isolating many areas.
Additionally, terrain may be flooded well inland.
Near-total to total power loss is likely for up to
several weeks and water will likely also be lost or
contaminated.
Category 4
Catastrophic damage will occur
Category 4 hurricanes tend to produce more extensive
curtainwall failures, with some complete structural
failure on small residences. Heavy, irreparable damage
and near-complete destruction of gas station canopies
and other wide span overhang type structures are common.
Mobile and manufactured homes are often flattened. Most
trees, except for the hardiest, are uprooted or snapped,
isolating many areas. These storms cause extensive beach
erosion, while terrain may be flooded far inland. Total
and long-lived electrical and water losses are to be
expected, possibly for many weeks.
Category 5
Catastrophic damage will occur
Category 5 is the highest category of the Saffir–Simpson
scale. These storms cause complete roof failure on many
residences and industrial buildings, and some complete
building failures with small utility buildings blown
over or away. Collapse of many wide-span roofs and
walls, especially those with no interior supports, is
common. Very heavy and irreparable damage to many
wood-frame structures and total destruction to
mobile/manufactured homes is prevalent. Only a few types
of structures are capable of surviving intact, and only
if located at least 3 to 5 miles (5 to 8 km) inland.
They include office, condominium and apartment buildings
and hotels that are of solid concrete or steel frame
construction, multi-story concrete parking garages, and
residences that are made of either reinforced brick or
concrete/cement block and have hipped roofs with slopes
of no less than 35 degrees from horizontal and no
overhangs of any kind, and if the windows are either
made of hurricane-resistant safety glass or covered with
shutters. Unless most of these requirements are met, the
catastrophic destruction of a structure may occur.
The storm's flooding causes major damage to the lower
floors of all structures near the shoreline, and many
coastal structures can be completely flattened or washed
away by the storm surge. Virtually all trees are
uprooted or snapped and some may be debarked, isolating
most affected communities. Massive evacuation of
residential areas may be required if the hurricane
threatens populated areas. Total and extremely
long-lived power outages and water losses are to be
expected, possibly for up to several months. |
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Criticism
Some scientists, including Kerry Emanuel and Lakshmi
Kantha, have criticized the scale as being simplistic,
indicating that the scale takes into account neither the
physical size of a storm nor the amount of precipitation
it produces. Additionally, they and others point out
that the Saffir–Simpson scale, unlike the Richter scale
used to measure earthquakes, is not continuous, and is
quantized into a small number of categories. Proposed
replacement classifications include the Hurricane
Intensity Index, which is based on the dynamic pressure
caused by a storm's winds, and the Hurricane Hazard
Index, which is based on surface wind speeds, the radius
of maximum winds of the storm, and its translational
velocity. Both of these scales are continuous, akin to
the Richter scale; however, neither of these scales have
been used by officials. |
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|
 Kiddle: Saffir Simpson Scale
Wikipedia: Saffir Simpson Scale |
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