A tropical cyclone is a warm-core, low pressure system with an organized circulation that forms over tropical or subtropical waters. In the Atlantic/East Pacific Oceans, it is called a hurricane. In the West Pacific Ocean, it is a typhoon. In the Indian Ocean, it is known as a cyclone. (National Weather Service)

Conditions to Form

There must exists several favorable conditions for a tropical cyclone to form:

• Warm ocean water (at least 80˚F/ 27˚C) through a depth of about 150 feet
• An atmosphere that cools fast enough with ascension that it is potentially unstable for moist convection.
• Moist air near mid-level troposphere
• A minimum distance from the equator of about 300 miles (but not within 5˚ latitude)
• A disturbance near the surface
• Low vertical wind shear between surface and troposphere (of less than 23mph).

Note: Wind shear is the change in wind speed with height. Additionally, a tropical cyclone must be close to the equator to form, but, except rare cases, do not form within 5˚ latitude of the equator because of the lack of Coriolis Force. This force is what causes the cyclone to spin. The direction it spins depends on the hemisphere it which it is located. In the Northern hemisphere, it spins counter-clockwise; in the Southern hemisphere it spins clockwise. (National Weather Service)

Seven Regions of Formation

Storms form within seven regions of the earth:

• Atlantic basin
• Northeast Pacific Basin
• Northwest Pacific Basin
• North Indian Basin
• Southwest Indian Basin
• Southeast Indian/Australian basin
• Australian/Southwest Pacific Basin

Our interest lies in the Atlantic Basin, which includes the North Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea. Hurricane season in this region “officially” begins June 1 and last until November 20. The season peak is in mid-September. (National Weather Service)

Seven Regions of Tropical Cyclone Formation

National Weather Service)

Disturbances (Causes)

Several disturbances work to trigger the formation of a tropical cyclone.

Tropical Cyclone Disturbances

Easterly Waves	Also known as tropical waves; inverted trough of low pressure moving westward in the tropical easterlies. A trough is a region of relative low pressure. This is how most major storms form.
West African Disturbance Line	Line of convection the forms over West African moves into the Atlantic Ocean. Faster than tropical waves.
Tropical Upper Tropospheric Trough (TUTT)	A trough, or cold-core low in the upper atmosphere, produces convection, Sometimes forms warm-core tropical cyclone.
Old Frontal Boundary	Remnants of a polar front can become lines of convection and generate a tropical cyclone. Can form storms early or late in hurricane season in the Gulf or Caribbean Sea

Strength of a Cyclone

Once a tropical cyclone system with organized circulation forms, it can become more organized if it stays over warm waters and upper level winds (wind shear) remain weak. Key to its power is warm water and war moist air. A storm is propagated when water vapor cools and rises. It condenses into clouds, releasing heat which warms the surrounding atmosphere. The air becomes lighter and continues to rise and more air rushes in its place, which are the strong winds we feel from storms.

When the eye of the storm moves over land, it lacks the moisture and heat that power the storm and convection. It weakens and eventually diminishes.

(National Weather Service)

*Convection that Powers Tropical Cyclones

(National Weather Service)

Why do Hurricanes form?

Why is it important that they form? The tropical cyclones role is to take heat stored in the ocean and transfer it to the upper atmosphere where upper level winds carry the heat to the poles. This is important because its is what keeps the Polar Regions from being cold as can be and keeping tropics from overheating.

Tropical cyclones help regulate the earths temperature.  Therefore, it would not be wise to disrupt a cyclone’s intensity because oceans will retain that heat. Over time, a heat build-up could enhance successive storm and increase their frequency and intensity. (National Weather Service)

Tropical Cyclone Structure

“Imagine standing at the center of a Roman coliseum that is 20 miles across, with walls that soar 10 miles into the sky, towering walls with cascades of ice crystals falling along its brilliantly white surface. That's what it's like to stand in the eye of a hurricane.” Kerry Emanuel in Divine Wind

(Emanuel 2005)

Main Parts of a Hurricane

The main parts of a tropical cyclone include the rainbands, the eye and the eyewall. A cyclone is basically air that spirals to the center, up through the eyewall, and out the top in the opposite direction. In the center, air also sinks in attempt to fill the empty space that consists of the eye. The eye is a cloud-free, calm, clear region with light winds (no more than 15mph). It usually forms when the storm reaches 74mph winds or more.

The eye is formed by a combination of angular momentum and centrifugal force. Because of conservation of momentum, as air approaches the center, its speed increases. The increased speed of spiraling air creates a centrifugalforce that wants to pull it outward, thus forming an clear eye with strong winds in the eyewall. Rising eye in the eye create the eyewalls.

(National Weather Service)

The eyewall consists of bands of clouds, strong winds and heavy rains.  It is the strongest of the storm with rings of tall thunderstorms. 

The rainbands, or feederbands, are bands of showers, gusty winds, and often heavy rain with gaps into bands. They become more pronounced as a storm intensifies. These bands are curved and trail away for the center in a spiral-like fashion.  Rainbands often include bursts of rain and wind and tornadoes.

(Accuweather.com Hurricane Center)

(National Weather Service)

Structure of a Tropical Cyclone

(National Weather Service)

The size of a hurricane averages near 300 miles wide with great variations. The size of the storm is not an indication of the hurricane’s intensity. Hurricane force winds are felt out to about 150 miles from the center. Tropical-force winds extend out 300 miles from the center.

Tropical Cyclone Classification

A tropical cyclone is classified according to wind speed values. A system with organized circulation with wind speeds less than or equal to 38 mph is called a tropical depression. Once it reached 39 mph, it is called tropical storm and given a name. Wind speeds of 74mph or higher is classified as a hurricane.

Classification	Wind speeds
Tropical Depression	Less than or equal to 38mph
Tropical Storm	39mph or greater
Hurricane	74mph or greater

Hurricanes are classified according to the Saffir-Simpson scale (Category 1-5) based on the wind speeds of the hurricanes. The scale estimated the potential property damage and flooding along the coast from hurricane landfall. Storm surge values are highly dependent on the slope of the continental shelf in the landfall region.

Cat.	Wind		Water		Cat.
	Speed	Damage	Storm Surge	Effect
1	74-95 mph 64-82 kts 119-153 kph Minimal Hurricane	No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs.	Generally 4-5 ft above normal	Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorage torn from moorings.	1
	Examples: Hurricanes Allison of 1995 and Danny of 1997 were Category One hurricanes at peak intensity.
2	96-110 mph 83-95 kts 154-177 kph Moderate Hurricane	Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers.	Generally 6-8 feet above normal	Coast roads and low-lying escape routes inland cut by rising water 2 to 4 hours before arrival of hurricane center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying areas required.	2
	Examples: Hurricane Bonnie of 1998 was a Category Two hurricane when it hit the North Carolina coast, while Hurricane Georges of 1998 was a Category Two Hurricane when it hit the Florida Keys and the Mississippi Gulf Coast.
3	111-130 mph 96-113 kts 178-209 kp Extensive Hurricane	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 tress blown down. Mobile homes and poorly constructed signs are destroyed.	Generally 9-12 ft above normal.	Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. Flooding near the coast destroys smaller structures with larger structures damaged by battering of 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 within several blocks of the shoreline may be required.	3
	Examples: Hurricanes Roxanne of 1995 and Fran of 1996 were Category Three hurricanes at landfall on the Yucatan Peninsula of Mexico and in North Carolina, respectively.
4	131-155 mph 114-135 kts 210-249 kph Extreme Hurricane	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.	Generally 13-18 ft above normal	Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the hurricane center. 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).	4
	Examples: Hurricane Luis of 1995 was a Category Four hurricane while moving over the Leeward Islands. Hurricanes Felix and Opal of 1995 also reached Category Four status at peak intensity.
5	Greater than 155 mph 135 kts 249 kph Catastrophic Hurricane	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.	Generally >18 ft above normal	Low-lying escape routes are cut by rising water 3-5 hours before arrival of the hurricane center. 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.	5
	Examples: Hurricane Mitch of 1998 was a Category Five hurricane at peak intensity over the western Caribbean. Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone of record.
Cat.	Speed	Damage	Storm Surge	Effect	Cat.
	Wind		Water

(National Weather Service)

Hurricane Names

Once a tropical cyclone is considered a hurricane, it is given a name. There are six lists of names that rotate every hurricane season. There are no names that begin with the letter Q, U, X, Y, and Z because of the scarcity of the names. Once the list of names is used up for the season, the Greek alphabet is used. The names of major land falling storms that may have a major economic impact are retired. Examples include Katrina, Rita, Wilma, Andrew and Camille.

Hurricane Names

2006	2007	2008	2009	2010	2011
Alberto	Andrea	Arthur	Ana	Alex	Arlene
Beryl	Barry	Bertha	Bill	Bonnie	Bret
Chris	Chantal	Cristobal	Claudette	Colin	Cindy
Debby	Dean	Dolly	Danny	Danielle	Don
Ernesto	Erin	Edouard	Erika	Earl	Emily
Florence	Felix	Fay	Fred	Fiona	Franklin
Gordon	Gabrielle	Gustav	Grace	Gaston	Gert
Helene	Humberto	Hanna	Henri	Hermine	Harvey
Isaac	Ingrid	Ike	Ida	Igor	Irene
Joyce	Jerry	Josephine	Joaquin	Julia	Jose
Kirk	Karen	Kyle	Kate	Karl	Katia
Leslie	Lorenzo	Laura	Larry	Lisa	Lee
Michael	Melissa	Marco	Mindy	Matthew	Maria
Nadine	Noel	Nana	Nicholas	Nicole	Nate
Oscar	Olga	Omar	Odette	Otto	Ophelia
Patty	Pablo	Paloma	Peter	Paula	Philippe
Rafael	Rebekah	Rene	Rose	Richard	Rina
Sandy	Sebastien	Sally	Sam	Shary	Sean
Tony	Tanya	Teddy	Teresa	Tomas	Tammy
Valerie	Van	Vicky	Victor	Virginie	Vince
William	Wendy	Wilfred	Wanda	Walter	Whitney
Greek Alphabet: Alpha, Beta, Gamma, Delta, Epsilon, Zeta,
Eta,Theta, Iota, Kappa, Lambda, Mu, Nu, Xi, Omicron, Pi, Rho,
Sigma, Tau, Upsilon, Phi, Chi, Psi, Omega

(National Weather Service)

Hurricane Hazards

Hurricanes cause several hazards, directly and indirectly. They cause storm surge, wind and squalls, inlands floods, flash floods, urban/area floods, river floods, and tornadoes.

Storm surge is basically water pushed towards the shore by the force of winds swirling around the storm. Gusts are short but rapid bursts of wind speed in winds. Squalls, often associated with bands of thunder storms which make up the spiral bands of a storm, are longer period of increased wind speeds.

A weak slow moving hurricane can cause more damage due to flooding than a more powerful, fast-moving hurricane.

Tornadoes usually form in the right-front quadrant, relative to the storms motion, or can be found embedded in rainbands.

(National Weather Service)

ENSO refers to the warming of ocean waters along the equator in the Eastern Pacific. It results in more rainfall in the Eastern Pacific while the West Pacific is relatively dry. Normally, however, sea surface temperatures are 14˚F higher in the Western Pacific, with cool temperature off of South America due to upwellings of cold, nutrient-rich water from the deeper level of water off the northwest coast of South America.

During El Niño years, trade winds decrease in the central and western Pacific which leads to a reduced upwelling cooler water. This cuts of the e supply of nutrient-rich water. Consequently, there is a rise in sea surface temperature and a large decline in the fishing industry.

El Niño has a significant impact on tropical cyclones in the North Atlantic. During El Niño years there is a:

• Reduction in the number of cyclones
• 60% reduction in the number of hurricane days
• Overall reduction in the system intensity

This occurs because of stronger than normal westerly winds that develop in the western North Atlantic and Caribbean during El Niño.

(National Weather Service)

Global Warming

Global warming is defined as the progressive gradual rise of the earth's surface temperature thought to be caused by the greenhouse effect and responsible for changes in global climate patterns. It results in an increase in the near surface temperature of the Earth and has occurred in the distant past as the result of natural influences, but the term is most often used to refer to the warming predicted to occur as a result of increased emissions of greenhouse gases.

(Weather and Climate Glossary)

Many scientists recently believe that hurricanes are becoming more intense because of rising sea surface temperature.

Leading hurricane authority Kerry Emanuel explains that an upswing in the tropical North Atlantic sea surface temperature, possibly due to increasing anthropogenic greenhouse gases, increase the energy available to tropical cyclones. It is possible and likely that global warming could increase the intensity of hurricanes in the time to come. How global warming affects hurricane frequency is still uncertain.

(Emanuel  2006)