is a Hurricane?
The ingredients for a hurricane include a pre-existing weather
disturbance, warm tropical oceans, moisture, and relatively light
winds aloft. If the right conditions persist long enough, they can
combine to produce the violent winds, incredible waves, torrential
rains, and floods we associate with this phenomenon.
Hurricane Fran 1996
GOES-8 Satellite, NASA
Each year, an average of ten tropical storms develop over the Atlantic
Basin (the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.) Many
of these remain over the ocean and never impact the U.S. coastline.
On average, six of these storms become hurricanes each year. In
an average 3-year period, roughly five hurricanes strike the US
coastline, killing approximately 50 to 100 people anywhere from
Texas to Maine. Of these, two are typically "major" or
"intense" hurricanes (a category 3 or higher storm on
the Saffir-Simpson Hurricane Scale).
The official hurricane season for the Atlantic Basin is from 1
June to 30 November. The peak of the season is from mid-August to
late October, however, deadly hurricanes can occur anytime in the
||What is a Hurricane?
A hurricane is a type of tropical cyclone, which is a generic term
for a low pressure system that generally forms in the tropics. The
cyclone is accompanied by thunderstorms and, in the Northern Hemisphere,
a counterclockwise circulation of winds near the earth's surface.
Tropical cyclones are classified as follows:
An organized system of clouds and thunderstorms with a defined surface
circulation and maximum sustained winds* of 38 mph (33 kt**) or less.
An organized system of strong thunderstorms with a defined surface
circulation and maximum sustained winds of 39-73 mph (34-63 kt).
An intense tropical weather system of strong thunderstorms with a
well-defined surface circulation and maximum sustained winds of 74
mph (64 kt) or higher.
Flooding from Hurricane Floyd
Courtesy of NASA/GSFC
Hurricanes are categorized according to the strength of their winds
using the Saffir-Simpson Hurricane Scale. A
Category 1 storm has the lowest wind speeds, while a Category 5
hurricane has the strongest. These are relative terms, because lower
category storms can sometimes inflict greater damage than higher
category storms, depending on where they strike and the particular
hazards they bring. In fact, tropical storms can also produce significant
damage and loss of life, mainly due to flooding.
| *Sustained winds - A 1-minute average
wind measured at about 33 ft (10 meters) above the surface.
** 1 knot = 1 nautical mile per hour or 1.15 statute miles
per hour. Abbreviated as "kt".
|| During a hurricane
or serious tropical storm it is important to know what to do to keep
yourself and your family safe. The National
Hurricane Center has resources to help you be
Basic Hurricane Safety Actions
- Know if you live in an evacuation area. Know your home's vulnerability
to storm surge, flooding and wind.
Have a written plan based on this knowledge.
- At the beginning of hurricane season (June 1st), check the supplies
for your disaster
supply kit, replace batteries and use food stocks on a rotating
- During hurricane season, monitor the tropics.
- Monitor NOAA Weather Radio.
It is an excellent / official source for real-time weather information
- If a storm threatens, heed the advice from local authorities.
Evacuate if ordered.
- Execute your family plan
Watch Vs. Warning - Know The Difference
- A HURRICANE WATCH issued for your part of
the coast indicates the possibility that you could experience
hurricane conditions within 36 hours. This watch should trigger
your family's disaster plan, and protective measures should be
initiated, especially those actions that require extra time such
as securing a boat, leaving a barrier island, etc.
- A HURRICANE WARNING issued for your part of
the coast indicates that sustained winds of at least 74 mph are
expected within 24 hours or less. Once this warning has been
issued, your family should be in the process of completing protective
actions and deciding the safest location to be during the storm.
When the the winds from these storms reach 39 mph (34 kts), the
cyclones are given names. Years ago, an international committee
developed names for Atlantic cyclones (The
History of Naming Hurricanes). In 1979 a six year rotating list
of Atlantic storm names was adopted - alternating between male and
female hurricane names. Storm names are used to facilitate geographic
referencing, for warning services, for legal issues, and to reduce
confusion when two or more tropical cyclones occur at the same time.
Through a vote of the World Meteorological Organization Region IV
Subcommittee, Atlantic cyclone names are retired usually when hurricanes
result in substantial damage or death or for other special circumstances.
The names assigned for the period between 2004 and 2009 are shown
|Names for Atlantic Basin Tropical
|*Lili was retired after the
2002 season, replacement name to be determined.
||Saffir-Simpson Hurricane Scale
The Saffir-Simpson Hurricane Scale 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. Wind speed is the determining
factor in the scale.
Winds 39-73 mph (34-63 kt)
Category 1 Hurricane
Winds 74-95 mph (64-82 kt)
No real damage to buildings. Damage to unanchored mobile homes.
Some damage to poorly constructed signs. Also, some coastal flooding
and minor pier damage.
- Examples: Irene 1999 and Allison 1995
Category 2 Hurricane
Winds 96-110 mph (83-95 kt)
Some damage to building roofs, doors and windows. Considerable damage
to mobile homes. Flooding damages piers and small craft in unprotected
moorings may break their moorings. Some trees blown down.
- Examples: Bonnie 1998, Georges(FL & LA) 1998 and Gloria
Category 3 Hurricane
Winds 111-130 mph (96-113 kt)
Some structural damage to small residences and utility buildings.
Large trees blown down. Mobile homes and poorly built signs destroyed.
Flooding near the coast destroys smaller structures with larger
structures damaged by floating debris. Terrain may be flooded well
- Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy
Category 4 Hurricane
Winds 131-155 mph (114-135 kt)
More extensive curtainwall failures with some complete roof structure
failure on small residences. Major erosion of beach areas. Terrain
may be flooded well inland.
- Examples: Hugo 1989 and Donna 1960
Category 5 Hurricane
Winds 156 mph and up (135+ kt)
Complete roof failure on many residences and industrial buildings.
Some complete building failures with small utility buildings blown
over or away. Flooding causes major damage to lower floors of all
structures near the shoreline. Massive evacuation of residential
areas may be required.
- Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935.
||Adapted from National
Hurricane Center, September 20, 2004, Hurricane