Info from Weather Channel site,
http://www.weather.com/outlook/weather-news/hurricanes/articles/hurricane-what-is-a-hurricane_2010-05-24
Hurricane: What Is a Hurricane?
If you live anywhere near the Gulf or eastern coasts of the United States, you're no doubt familiar with the telltale
sign of a hurricane from countless weather satellite and radar images on television: a big, white mass of clouds
swirling around a tightly wound eye in the center of the storm, slowly heading toward the coastline.
What you're seeing, however, could also be classified as a tropical storm depending on the speed of its winds, or
even as a typhoon depending on where in the world it forms. So what, exactly, is a hurricane?
Derived from the Spanish word "huracan" -- which was most likely inspired by Hunraken, the name for the ancient
Mayan storm god or Hurakan, the Quiche god of thunder (or any number of other Caribbean terms for evil spirits
or big winds) -- the word "hurricane" was first used to describe any localized tropical cyclone in the West Indies.
Today, a hurricane is defined as a tropical cyclone with sustained winds that have reached speeds of 74 mph or
higher. These storms reach the status of "hurricane" only after strengthening over a period of days or even weeks.
This process begins in the warm, moist air over the waters of the region known as the tropics, which includes the
Gulf of Mexico, the Caribbean Sea, the eastern North Pacific Ocean and the North Atlantic Ocean, east of the
International Dateline and north of the equator. Tropical depressions, tropical storms, hurricanes, and typhoons are
all tropical cyclones, and all may develop in this region.
Before it can become a hurricane, a tropical cyclone -- which is a low pressure system with a defined wind
circulation that develops over the tropics -- it must pass through four distinct stages: tropical disturbance, tropical
depression, tropical storm and finally, hurricane.
Stage 1: Tropical disturbance
A tropical disturbance is a discrete system of clouds, showers, and thunderstorms that originates in the tropics and
remains intact for 24 hours or more.
Tropical waves are a type of tropical disturbance that develops about every four to five days, and some of these
waves eventually strengthen to become tropical storms and hurricanes. Sometimes called easterly waves, they are
areas of low pressure that move generally from east to west, embedded in the tropical easterly winds.
Stage 2: Tropical depression
When a tropical disturbance develops a closed circulation (e.g., counter-clockwise winds blowing around a center of
low pressure in the Northern Hemisphere), it is designated as a tropical depression. Tropical depressions contain
maximum sustained one-minute winds of 38 mph (33 knots) or less, at an elevation of 10 meters.
Stage 3: Tropical storm
A tropical cyclone is given a name by the National Hurricane Center once it reaches tropical storm status. Tropical
storms have maximum sustained one-minute winds of 39-73 mph (34-63 knots), at an elevation of 10 meters.
Stage 4: Hurricane
Hurricanes have sustained one-minute winds of at least 74 mph (64 knots), at an elevation of 10 meters. Winds in
most hurricanes can become much stronger.
Hurricanes are categorized on a scale of 1 to 5 based on their wind speed, a scale known as the Saffir-Simpson
hurricane wind scale, named after its originators, Herbert Saffir and Dr. Robert Simpson.
While this satellite photo of a hurricane features a tightly formed and easily visible eye at its center, not all hurricanes appear this way.
In developing tropical cyclones, strong thunderstorms occur. Air pressure drops at the surface of these storms. This
low pressure attracts warm moist air from the ocean's surface. The Coriolis force causes the resulting low-level
winds to spiral in a counterclockwise direction around the center of the low in the Northern Hemisphere. (Winds
swirl clockwise in the Southern Hemisphere.)
Typically, an "eye" forms when the tropical cyclone reaches hurricane strength, but an eye is not necessary for a
tropical cyclone to become a hurricane.
Another way to think of a hurricane is as a large heat engine. The fuel is moisture from warm ocean water. The
moisture is converted to heat in the thunderstorms that form. Spiral rain bands that surround the tropical cyclone's
core help feed the circulation more heat energy.
As air nears the center, it rises rapidly and condenses into clouds and rain. The condensation releases tremendous
amounts of heat into the atmosphere. The result is lower surface pressure and strengthening winds.
In this way, the tropical cyclone's engine refuels itself, concentrating its power in a donut-shaped area, called the
eye wall, surrounding the center. The eye wall typically contains the strongest surface winds.
Sinking air at the center clears the tropical cyclone of clouds and forms the "eye." Falling surface pressure can
occur only if air mass is removed from the circulation center. This is accomplished by wind flowing away from the
circulation in the upper atmosphere.
Hurricane: Saffir-Simpson Hurricane Wind Scale
When you hear terms like "category one," "category three" or even the rare "category five" mentioned in a news
report on hurricanes, what is being discussed is the classification system for hurricanes that has now become
familiar, even if the name for the system (and its inventors) is still largely unknown.
The Saffir-Simpson Hurricane Wind Scale was first developed in the early 1970s by Herbert Saffir, a consulting
engineer who lived in Florida, and Dr. Robert Simpson, who was then director of the National Hurricane Center.
The current version is strictly a wind scale. Previous versions listed central pressures typically associated with each
category due to a relationship that exists between pressure and wind, but the details can vary quite a bit
depending on the nature of each particular hurricane. Also, storm surge was quantified by category.
However, hurricanes with wind fields which are very large in size can produce storm surge heights that are much
higher than is average for a give category, such as was the case with Category 2 Hurricane Ike in 2008.
Conversely, very compact hurricanes, even if extremely strong wind-wise and with very low central pressures as
was the case with Hurricane Charley in 2004, can produce surges substantially lower than what was included in the
original scale.
Today, the Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the
indicated time. The scale provides examples of the type of damage and impacts in the United States associated
with winds of the indicated intensity. In general, damage rises by about a factor of four for every category
increase.
The scale was modified slightly in 2012. Category 3 hurricanes now have a wind speed range of 111-129 mph
(previously 111-130 mph). Category 4 hurricanes now have a wind speed range of 130-156 mph (previously 131155 mph). Category 5 hurricanes now have winds of 157+ mph (previously 156+ mph).
This modification was done in order to help resolve rounding issues from knots to mph in the advisories The
National Hurricane Center issues. An example of this is when a hurricane has an intensity of 115 knots. Although
115 knots is within the Category 4 range, it converts to 132.3 mph, which rounds down to 130 mph. This would
classify a hurricane as a Category 3 in the old scale when using mph. The National Hurricane Center would then
have to incorrectly covert 115 knots to 135 mph in their advisory products to work around this issue.
Hurricane: Watches & Warnings
When hurricanes or tropical storms approach the U.S. coastline, the National Hurricane Center issues one of four
key alert messages for those living in the affected areas.
Tropical storms
Tropical storm watch
Tropical storm conditions, with sustained winds from 39 to 73 mph, are possible in your
area within the next 48 hours.
Tropical storm warning
Tropical storm conditions are expected in your area within the next 36 hours.
Hurricanes
Hurricane watch
Hurricane conditions, with sustained winds greater than 73 mph, are possible in your area
within 48 hours.
Hurricane warning
Hurricane conditions are expected in your area in 36 hours or less.
Coastal floods
Coastal flood watch
The possibility exists for the inundation of land areas along the coast within the next 12 to
36 hours.
Coastal flood warning
Land areas along the coast are expected to become, or have become, inundated by sea
water above the typical tide action.
Hurricane: Damage & Effects of Hurricanes
When making landfall along the coastline, a hurricane brings more than its intensely powerful and destructive
winds ashore. Hurricanes also are responsible for a range of weather impacts, from storm surge and flooding
caused by heavy rains to tornadoes.
Rainfall induced flooding
The heavy rains associated with a tropical weather system are responsible not only for major flooding in areas
where the storm initially strikes, but also can affect areas hundreds of miles from where the storm originally made
landfall.
During landfall, it is not uncommon for 5-10 inches of rain to fall. If the storm is large and moving slowly, rainfall
could be even more excessive. As the storm moves inland, and is downgraded to a tropical depression, the
continued circulation, tropical moisture, and topography can contribute to copious amounts of rainfall.
Intense flooding also can occur from tropical depressions and storms that do not reach hurricane strength.
Storm surge
Storm surge is a rapid rise in the level of water that moves onto land as the eye of the storm makes landfall.
Generally speaking, the stronger the hurricane, the greater the storm surge.
As a hurricane approaches the coast, its winds drive water toward the shore. Once the edge of the storm reaches
the shallow waters of the continental shelf, water piles up. Winds of hurricane strength force the water onto shore.
At first, the water level climbs slowly, but as the eye of the storm approaches, water rises rapidly. Wave after wave
hits the coast as tons of moving water hammer away at any structure on the coastline. A cubic yard of water
weighs about 1,700 pounds.
The surge is greater if a hurricane's track is perpendicular to the coastline, allowing the surge to build higher. The
storm surge is also greater if the storm affects a bay or if it makes landfall at high tide. The greatest storm surge
occurs to the right of where the eye makes landfall.
Winds
The winds of a hurricane range from 74 mph (65 knots) in a minimal storm to greater than 155 mph (136 knots) in
a catastrophic one. Accurate readings of high wind gusts during landfall are difficult to obtain because
anemometers (wind-speed measuring devices) at reporting stations can be ripped from their foundations.
Wind is responsible for much of the structural damage caused by hurricanes. High winds uproot trees and tear
down power lines. The maximum winds from fast moving and powerful storms may remain high, even when the
storm is well inland. Often this is actual wind speed combined with the speed of the storm.
Tornadoes
Tropical cyclones also can trigger tornadoes. Each storm has a unique pattern of tornadoes whose frequency and
occurrence is highly variable from one storm to the next.
Tornadoes spawned from hurricanes are more likely during an intense hurricane or one that is intensifying at or
near landfall.
Hurricane: Hurricane Season
Although hurricanes can form as early as late May and continue into December in the Caribbean Sea or the Gulf of
Mexico, the official Atlantic hurricane season starts June 1st and ends November 30th. In the eastern North Pacific,
the official season begins May 15th and ends November 30th. Both official seasons cover more than 99 percent of
all tropical cyclones in any given year.
During this time, the National Hurricane Center (NHC) maintains a continuous watch on tropical cyclones over the
Atlantic Ocean, Caribbean Sea, and Gulf of Mexico, as well as the eastern North Pacific. They also issue public
watches and warnings.
In the Atlantic Basin (the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea), hurricane season is
typically most active between mid-August through October. Height of the season is in mid-September, during
which about as many major hurricane landfalls have occurred in the past as in October and August combined.
While it is possible for a strong hurricane to develop in November, it is infrequent. Hurricanes like Kate in 1985 are
an exception. Generally, warm water needed to generate hurricanes is beginning to cool in late autumn, and
weather patterns become unfavorable for tropical cyclone development.
Tropical forecast parameters
In addition to the seasonal likelihood for hurricanes, a number of parameters are also known to be associated with
the occurrence of hurricanes in the Atlantic, including El Nino. To help determine whether a season will be an active
one, forecasters focus on atmospheric variables like these:
Stratospheric Quasi-Biennial Oscillation (QBO)
QBO refers to east-west (or west-east) winds circling the earth near the equator, from 60,000 to 70,000 feet above
the surface. It takes about two years for these winds to complete a cycle from east-west through west-east back to
east-west.
El Nino-Southern Oscillation
El Nino years are characterized by warm sea-surface temperatures over the equatorial eastern Pacific. This
anomaly is related to 200-millibar westerly winds and surface pressure over the Caribbean and the western
Atlantic. During El Nino years, stronger 200-millibar Westerlies produce shearing over the hurricane-generating
areas of the Caribbean and the western Atlantic, as well as higher surface pressure, which can suppress hurricane
development.
African rainfall
The incidence of Atlantic hurricane activity is related to current year June-July Western Sahel rainfall and previous
year August-November Gulf of Guinea rainfall.
Influence of West Africa west-to-east surface pressure and temperature gradients
Atlantic hurricane activity is enhanced when the February-May east-west pressure gradient is higher than normal
or when the west-east temperature gradient is lower than normal.