Understanding Hurricane Numbers & Nomenclature

The Saffir-Simpson Hurricane Scale is a 1-5 rating based a hurricane’s intensity; 5 being the most destructive. The scale is used to 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, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline, in the landfall region. The barometric pressure also contributes to a storm’s Saffir-Simpson ranking.

The Saffir-Simpson Scale
Category One Hurricane: Winds 74-95 mph. Storm surge of 4-5 feet. No real damage to building structures but some damage to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Some coastal road flooding and minor pier damage can be expected. In 2002 Hurricane Lili made landfall on the Louisiana coast as a Category One hurricane. Other Examples: Irene 1999 and Allison 1995.

Category Two Hurricane: Winds 96-110 mph. Storm surge of 6-8 feet. Some roofing material, door, and window damage of buildings will occur with considerable damage to shrubbery and trees, some trees blown down. There will be considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes will flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages will break moorings. Examples: Bonnie 1998, Georges (FL & LA) 1998 and Gloria 1985.

Category Three Hurricane: Winds 111-130 mph. Storm surge 9-12 feet. Some structural damage to small residences and utility buildings is expected with a minor amount of curtain wall failures. Damage to shrubbery and trees and large trees will be blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut off by rising water 3-5 hours before arrival of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain lower than 5 feet above sea level may be flooded inland 8 miles or more. Evacuation of low-lying residences will be required. Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy 1965.

Category Four Hurricane: Winds 131-155 mph. Storm surge 13-18 feet above normal. More extensive building damage with some complete roof structure failures on small residences will occur. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Low-lying escape routes will cut off by rising water 3-5 hours before arrival of the hurricane. Major damage will occur to lower floors of structures near the shore. Terrain lower than 10 feet above sea level will be flooded requiring massive evacuation of residential areas as far inland as 6 miles. Examples: Hugo 1989 and Donna 1960.

Category Five Hurricane: Winds greater than 156 mph. Storm surge will generally be greater than 18 feet. Complete roof failure on many residences and industrial buildings. Some complete building failures. All shrubs, trees, and signs will be blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Low-lying escape routes will be cut off by rising water 3-5 hours before arrival of the hurricane. Major damage to lower floors of all structures located less than 15 feet above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles of the shoreline required. Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935

The Beaufort Wind Scale: Wind Force – Knots to Miles Per Hour

The Beaufort Winds Scale is a tool used by meteorologists to measure the effects of winds. The wind speeds below show winds in MPH and conditions observed at those speeds.
0 - Calm smoke rises vertically
1 - 4 Light air direction of wind shown by smoke but not by wind vanes
4 - 7 Light breeze wind felt on face; leaves rustle; ordinary wind vane moved by wind
8 - 12 Gentle breeze leaves and small twigs in constant motion; wind extends light flag
13 - 18 Moderate breeze raises dust and loose paper; small branches are moved
19 - 24 Fresh breeze small trees in leaf begin to sway; crested wavelets form on inland water
25 - 31 Strong breeze large branches in motion; telephone wires whistle; umbrellas used with difficulty
32 - 38 Moderate gale whole trees in motion; inconvenience in walking against wind
39 - 46 Fresh gale breaks twigs off trees; generally impedes progress
47 - 54 Strong gale slight structural damage occurs; chimney pots and slates removed
55 - 63 Whole gale trees uprooted; considerable structural damage occurs
64 - 72 Storm very rarely experienced; accompanied by widespread damage
73+ Hurricane devastation occurs

Since tropical storm and hurricane winds strengths are often referred to in knots, the following Conversion table for knots to miles per hour will aid in understanding the severity of storm winds.
Knots to Miles Per Hour Winds
5 Knots = 5.8 MPH
10 Knots = 11.5 MPH
15 Knots = 17.3 MPH
20 Knots = 23.0 MPH
25 Knots = 28.8 MPH
30 Knots = 34.6 MPH
35 Knots = 40.3 MPH
40 Knots = 46.1 MPH
45 Knots = 51.8 MPH
50 Knots = 57.6 MPH
55 Knots = 63.4 MPH
60 Knots = 69.1 MPH
65 Knots = 74.9 MPH
70 Knots = 80.6 MPH
75 Knots = 86.4 MPH
80 Knots = 92.2 MPH
85 Knots = 97.9 MPH
90 Knots = 103.7 MPH
95 Knots = 109.4 MPH
100 Knots = 115.2 MPH
105 Knots = 121.0 MPH
110 Knots = 126.7 MPH
115 Knots = 132.5 MPH
120 Knots = 138.2 MPH
125 Knots = 144.0 MPH
130 Knots = 149.8 MPH
135 Knots = 155.5 MPH
140 Knots = 161.3 MPH
145 Knots = 167.0 MPH
150 Knots = 172.8 MPH


The Millibar Factor -- Reading the “mb”
Measured in inches and millibars (mb), barometric pressure is key to understanding hurricane forecasts. At sea level during calm weather, the normal barometric pressure is roughly 30.00 inches, or about 1,000 millibars. The barometric pressure reading in the eye of a hurricane drops as the storm intensifies. Extremely powerful hurricanes have very low barometric pressure readings in their centers. For example, in October 2005 Hurricane Wilma's barometric pressure dropped to 26.04 inches, or 882 millibars—the lowest ever recorded for a hurricane in the Atlantic Ocean.
A falling barometric pressure in a hurricane of any category is always a cause for concern. Barometric pressure readings are part of the Saffir-Simpson scale and yet are routinely left off published charts of the scale. Note that the stronger the storm, the lower the barometric pressure.
Category One Hurricane -- Barometric pressure: No lower than 28.94 inches, or 980 millibars (mb).
Category Two Hurricane -- Barometric pressure: 28.50 to 28.92 inches, or 965 to 979 mb.
Category Three Hurricane -- Barometric pressure: 27.91 to 28.47 inches, or 945 to 964 mb.
Category Four Hurricane -- Barometric pressure: 27.17 to 27.88 inches, or 920 to 944 mb.
Category Five Hurricane -- Barometric pressure: Lower than 27.17 inches, or 920 mb.

The central pressure in Hurricane Katrina fell to 902 mb on August 28. This pressure was (at the time) the fourth lowest on record in the Atlantic basin, behind in Gilbert (1988) with 888 mb. The Labor Day Hurricane of 1935 registered a low pressure of 892 mb, and Hurricane Allen registered 899 mb in 1980. However, it has since fallen to sixth lowest, following an observation of 897 mb in Hurricane Rita (September 2005) and the new record of 882 mb for Hurricane Wilma in October 2005.

Tornadoes – Using the Power of Hurricanes
Tornadoes can form when hurricanes make landfall – even thousands of miles away -- when their winds at ground level slow down while the winds near the top keep their momentum. Recent data show this tendency seems to have increased. Residents of hurricane-prone areas should always prepare for the eventuality of tornadoes.

A tornado is simply a violently rotating column of air that extends from a storm in the atmosphere to the ground. The pressure inside can be 10 percent lower than the surrounding air, and this causes that air to rush towards the low-pressure center from all directions. As it streams inward, the air spirals upward around the core until it merges with the airflow of the storm that gave rise to the tornado. Researchers often don't realize a tornado has formed until they survey the hurricane's damage and recognize the tell-tale signs of a tornado.