Tropical Cyclone Basics
Tropical cyclones are warm core, non-frontal low pressure systems of synoptic scale that develop over tropical or subtropical
waters and have a definite organized surface circulation. Tropical depressions, tropical storms, and hurricanes are all forms
of tropical cyclones, differentiated only by the intensity of the winds associated with them.
Tropical Wave (African or Easterly Wave)
A tropical wave is a trough or cyclonic curvature maximum in the trade wind easterlies. These waves tend to reach maximum
amplitude in the lower to middle troposphere and may or may not be accompanied by thunderstorm clusters. Although there
is still some debate on the issue,these easterly waves are thought to originate or become amplified as a result of
meteorological conditions over the continent of Africa. Each hurricane season approximately 60 of these waves cross the
tropical North Atlantic. Although the majority of these waves pass through the basin without any significant tropical cyclone
development, passage of these waves is often accompanied by squally weather with brief periods of higher sustained winds.
A tropical disturbance is a discrete tropical weather system with apparently organized convection (generally 100 to 300 miles
in diameter) originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24
hours or more.
Tropical cyclones in which the maximum sustained surface wind speed (1-minute mean) is 38 mph (33 KT) or Iess. Tropical
depressions must have a closed surface circulation in order to be classified in this category.
Tropical cyclones in which the maximum sustained surface wind speed (1-minute mean) ranges from 39-73 mph (34 KT to
Tropical cyclones in which the maximum sustained surface wind speed (1-minute mean) is greater than or equal to 74mph
Hurricane Categories (Saffir-Simpson Hurricane Scale)
Hurricanes are further categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale (SSHS).
A Category 1 storm has the lowest wind speeds, while a Category 5 has the highest. These are relative terms because lower
category storms can sometimes inflict greater damage than higher category storms, depending on angle of approach,
location, and many other aspects particular to each system. Even tropical storms can produce significant damage & loss of
life, mainly due to floods.
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 96-110mph (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: Georges 1998 and Gloria 1985
Category 3 Hurricane — winds 111-130mph (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 inland. Examples: Keith 2000,Fran
1996 Opal 1995,Alicia 1983 and Betsy 1965
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 7960
Category 5 Hurricane — winds 156mph 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
1992, Camille 7969 and Labor Day 1935
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