Hurricanes - The World's Most Destructive Weather
Observing hurricanes is not just a dry academic exercise - what do you do if they head your way?
There is no doubt that hurricanes are right at the top of anyone's severe and extreme weather list. With winds in excess of 175mph (280kph) and storm surges over 20ft (6m), damage can be complete and widespread. Without adequate warning, loss of life can also be high, due to both drowning and injury.
Palm Trees and High Seas
Source; NOAA Archives
Occurrence and Formation
Hurricanes are examples of cyclones - a term for any zone
of low pressure enclosed within an area of higher pressure, in
which the system rotates anticlockwise in the northern
hemisphere, and clockwise in the south. Once the sustained
wind speed exceeds 74mph (120kph) the system is defined as
a hurricane in the Atlantic and eastern Pacific Oceans,
a typhoon in the northwestern Pacific, and a tropical
cyclone, severe tropical cyclone, or severe cyclonic storm
in the southwestern Pacific or Indian Ocean.
An excellent source of information on hurricanes can be found
National Hurricane Center
website. In particular,
check out the FAQ section.
Hurricanes occur in most parts of the tropics wherever the
ocean waters can heat up to 80°F or 26.5°C but not within
about 300 miles (500km) of the equator. They regularly
occur in seven areas.
- Atlantic Basin - North Atlantic Ocean, Caribbean Sea,
Gulf of Mexico
- Northeast Pacific Ocean - west of Mexico
- Northwest Pacific Ocean - from about the dateline to Asia,
including the South China Sea
- North Indian Ocean - Bay of Bengal and Arabian Sea
- Southwest Indian Ocean - east of Africa
- Southeast Indian Ocean - west of Australia
- Southwest Pacific Ocean - east of Australia
Apart from warm oceans, other requirements for their
- Moist air extending well into the
- Instability in the atmosphere, allowing
thunderstorms to form
- A pre-existing near surface
disturbance with some rotational component
- and not too
much wind shear, or variation in wind speed and direction with
Hurricanes usually start as a tropical depression and
intensify to become a tropical storm. With further growth,
they evolve into a well organised, rotating system
with strong sustained winds and heavy rain.
Once wind speed has reached 74mph (120kph), they've earnt
the name hurricane.
Because hurricanes vary so much in size and intensity, it
has become necessary to categorize them to give some
indication of the danger they pose to communities in their
path. So while size, internal pressure, and wind speed are
all important, other factors such as storm surge are very
The Saffir-Simpson Hurricane Scale, used in the USA and adjacent
areas, is a 1-5 rating based on the hurricane's present
intensity. This is used to give an estimate of the potential
property damage and flooding expected along the coast from
a hurricane landfall. While the hurricane is in open waters,
wind speed is the most important factor.
Wind speed is a major contributor towards damage, and also
controls the other major cause of destruction, storm surge.
However storm surge varies a lot depending on the depth
of the sea, the shape of the coast, and the position of
the center of the hurricane. As a general rule, storm
surge will be greatest to the left of the eye as it nears
and crosses the coast, viewing the hurricane from the
shoreline. Local disaster coordinators will be able to
assess the type of damage likely in their areas.
It is important to keep in mind that hurricane intensities
can change quickly. It is always better to be on the safe
side, particularly if evacuation is necessary. On the other
hand, things may not turn out as bad as they seemed. Both
Hurricanes Katrina and Rita from late 2005, devastating
as they were, decayed from Category 5 storms to Category
3 by the time they crossed the coast. But while wind
speeds were not as severe as they might have been, storm
surges generated during their most severe stages remained
Much of the section on the Saffir-Simpson scale has been
taken with little alteration from the NOAA page covering
classification. I don't like ripping off other sources,
but in this case it sppears best to include the full text
Category One Hurricane:
Winds 74-95 mph (119-153 km/hr). Storm surge generally
4-5 ft (1.2-1.5m above normal. No real damage to building
structures. Damage primarily to unanchored mobile homes,
shrubbery, and trees. Some damage to poorly constructed signs.
Also, some coastal road flooding and minor pier damage.
Hurricane Lili of 2002 made landfall on the Louisiana
coast as a Category One hurricane. Hurricane Gaston of
2004 was a Category One hurricane that made landfall along
the central South Carolina coast.
Category Two Hurricane:
Winds 96-110 mph (154-177 km/hr). Storm surge generally
6-8 feet (1.8-2.4m) above normal. Some roofing material, door,
and window damage to buildings. Considerable damage
to shrubbery and trees with some trees blown down.
Considerable damage to mobile homes, poorly constructed
signs, and piers. Coastal and low-lying escape routes
flood 2-4 hours before arrival of the hurricane center.
Small craft in unprotected anchorages break moorings.
Hurricane Frances of 2004 made landfall over the southern
end of Hutchinson Island, Florida as a Category Two
hurricane. Hurricane Isabel of 2003 made landfall near
Drum Inlet on the Outer Banks of North Carolina as a
Category 2 hurricane.
Category Three Hurricane:
Winds 111-130 mph (178-209 km/hr). Storm surge
generally 9-12 ft (2.7-3.7m) above normal. Some structural
damage to small residences and utility buildings with a
minor amount of curtainwall failures. Damage to shrubbery
and trees with foliage blown off trees and large trees
blown down. Mobile homes and poorly constructed signs are
destroyed. Low-lying escape routes are cut by rising water
3-5 hours before arrival of the center of the hurricane.
Flooding near the coast destroys smaller structures with
larger structures damaged by battering from floating debris.
Terrain continuously lower than 5 ft above mean sea level
may be flooded inland 8 miles (13 km) or more. Evacuation
of low-lying residences with several blocks of the
shoreline may be required. Hurricanes Jeanne and Ivan of
2004 were Category Three hurricanes when they made
landfall in Florida and in Alabama, respectively.
Category Four Hurricane:
Winds 131-155 mph (210-249 km/hr). Storm surge generally
13-18 ft (4-5.5m) above normal. More extensive curtainwall
failures with some complete roof structure failures on small
residences. Shrubs, trees, and all signs are blown down.
Complete destruction of mobile homes. Extensive damage to
doors and windows. Low-lying escape routes may be cut by
rising water 3-5 hours before arrival of the center of the
hurricane. Major damage to lower floors of structures near
the shore. Terrain lower than 10 ft above sea level may be
flooded requiring massive evacuation of residential areas
as far inland as 6 miles (10 km). Hurricane Charley of
2004, a Category Four hurricane, made landfall in Charlotte
County, Florida with winds of 150 mph. Hurricane Dennis of
2005 struck the island of Cuba as a Category Four hurricane.
Category Five Hurricane:
Winds greater than 155 mph (249 km/hr). Storm surge
generally greater than 18 ft (5.5m) above normal. Complete
roof failure on many residences and industrial buildings.
Some complete building failures with small utility
buildings blown over or away. All shrubs, trees, and signs
blown down. Complete destruction of mobile homes. Severe
and extensive window and door damage. Low-lying escape
routes are cut by rising water 3-5 hours before arrival
of the center of the hurricane. Major damage to lower
floors of all structures located less than 15 ft above
sea level and within 500 yards of the shoreline. Massive
evacuation of residential areas on low ground within
5-10 miles (8-16 km) of the shoreline may be required.
Only 3 Category Five Hurricanes have made landfall in
the United States since records began: The Labor Day
Hurricane of 1935, Hurricane Camille (1969), and
Hurricane Andrew in August, 1992. The 1935 Labor Day
Hurricane struck the Florida Keys. Hurricane Camille
struck the Mississippi Gulf Coast causing a 25-foot
(7.6m) storm surge, which inundated Pass Christian.
Hurricane Andrew of 1992 made landfall over southern
Miami-Dade County, Florida causing 26.5 billion dollars
in losses--the costliest hurricane on record. In
addition, Hurricane Gilbert of 1988, plus Hurricanes
Katrina, Rita and Wilma were all Category Five hurricanes
at peak intensity.
Evacuation of residential areas near the shore is likely
with Category 3 Hurricanes, and mandatory for Category
4 and 5. But if you are worried, don't wait for the
Other areas affected by hurricanes, typhoons or cyclones
use similar intensity scales.
Hurricanes and their equivalents in other parts of the world
are given names for several reasons. Firstly, a simple
naming system makes the warning process much more effective.
Secondly there may be more than one active hurricane in an
area, and individual names allows better reporting of their
positions and threats.
Hurricanes Emmy and Frances interacting in 1976
Source; NOAA Archives
Names are also given to tropical storms in the North American
region, although not elsewhere in the world. This is sensible, as tropical storms can evolve into hurricanes,
degenerate back to storms, and even reform to hurricane grade.
Regardless of their classification they are a single weather
event, and need to be watched until their final decay.
New names are assigned at the beginning of each hurricane
season and dealt out in alphabetical order as each storm or
hurricane appears. Names may be reused, but names of
notable destructive hurricanes (Andrew, Camille, Gilbert,
for example) are withdrawn.
The letters Q, U, X, Y, and Z are not used (not very many
names start with these letters, giving an annual reserve
of 21 names. In most years, thankfully, this is more than
enough, but in the record year 2005 it wasn't. Plan B
was to use the Greek alphabet, so Alpha, Beta, Gamma, Delta and
Epsilon were used late in the season. Even Zeta got
the nod for a late storm right at the end of 2005, well after the official end of the
OK, take a break before going on to Part 2 -
Hurricane Tracking & Recording
. Or, for something a bit lighter, try Part 3 -
Tracking Hurricane Rita
. Nothing too heavy there - just a few notes on what I was able to
see using available resources (links are included on the page)
as Rita exploded from Category 1 to Category 5 before calming
to Category 3 before crossing the Gulf Coast on the Texas
- Louisiana border.
More information on Hurricanes can be found among the
articles, while you'll find heaps of other great resources -DVDs, videos, books, posters etc - atMore Hurricane Resources
Ever wondered what it's like to stay on as a hurricane passes overhead? Check out Hurricane Safety for an account of sitting through Hurricane Wilma as it struck Southern Florida in October 2005.
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Last update 05/24/2011