Hurricane, tropical storm, tropical depression, tropical disturbance, extratropical cyclone
Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. The primary energy source of a tropical cyclone is the release of the heat of condensation from water vapor condensing at high altitudes. Because of this, a tropical cyclone can be thought of as a giant vertical heat engine.
The ingredients for a tropical cyclone 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 associated with this phenomenon.
Condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.
The National Center for Atmospheric Research scientists estimate the heat energy released by a hurricane equals 50 to 200 trillion watts -- about the same amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes.
The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h) depends on the region:
- hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and unofficially in the South Atlantic Ocean
- typhoon in the Northwest Pacific Ocean west of the dateline
- severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
severe cyclonic storm in the North Indian Ocean
tropical cyclone in the Southwest Indian Ocean and South Pacific Ocean east of 160°E.
Etymology
The word typhoon may come partly from the Portuguese tufao; Urdu, Persian and Arabic tufan; Greek tuphon; and written in Chinese character is a translation of typhoon.
The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracan.
The word cyclone came from the Greek word κuκλος, meaning circle.
In other areas, hurricanes have been called Bagyo in the Philippines and Taino in Haiti.
Hurricanes evolve through a life cycle of stages from birth to death. A tropical disturbance in time can grow to a more intense stage by attaining a specified sustained wind speed. The progression of tropical disturbances can be seen in the three images below.
Hurricanes can often live for a long period of time -- as much as two to three weeks. They may initiate as a cluster of thunderstorms over the tropical ocean waters. Once a disturbance has become a tropical depression, the amount of time it takes to achieve the next stage, tropical storm, can take as little as half a day to as much as a couple of days. It may not happen at all. The same may occur for the amount of time a tropical storm needs to intensify into a hurricane. Atmospheric and oceanic conditions play major roles in determining these events.
Tropical Depression
Once a group of thunderstorms has come together under the right atmospheric conditions for a long enough time, they may organize into a tropical depression. Winds near the center are constantly between 20 and 34 knots (23 - 39 mph).
A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression".
A tropical depression is designated when the first appearance of a lowered pressure and organized circulation in the center of the thunderstorm complex occurs. A surface pressure chart will reveal at least one closed isobar to reflect this lowering.
When viewed from a satellite, tropical depressions appear to have little organization. However, the slightest amount of rotation can usually be perceived when looking at a series of satellite images. Instead of a round appearance similar to hurricanes, tropical depressions look like individual thunderstorms that are grouped together.
Tropical Storms
Once a tropical depression has intensified to the point where its maximum sustained winds are between 35-64 knots (39-73 mph), it becomes a tropical storm. It is at this time that it is assigned a name. During this time, the storm itself becomes more organized and begins to become more circular in shape -- resembling a hurricane.
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34–63 knots, 39–73 mph, or 62–117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present. Government weather services assign first names to systems that reach this intensity (thus the term named storm).
The rotation of a tropical storm is more recognizable than for a tropical depression. Tropical storms can cause a lot of problems even without becoming a hurricane. However, most of the problems a tropical storm cause stem from heavy rainfall.
The near satellite image is of tropical storm Charlie (1998). Many cities in southern Texas reported heavy rainfall between 5-10 inches. Included in these was Del Rio, where more than 17 inches fell in just one day, forcing people from their homes and killing half a dozen.
Hurricanes
As surface pressures continue to drop, a tropical storm becomes a hurricane when sustained wind speeds reach 64 knots (74 mph). A pronounced rotation develops around the central core.
Hurricanes are Earth's strongest tropical cyclones. A distinctive feature seen on many hurricanes and are unique to them is the dark spot found in the middle of the hurricane. This is called the eye. Surrounding the eye is the region of most intense winds and rainfall called the eye wall (an area about 10 to 50 miles (16 to 80 kilometers) wide in which the strongest thunderstorms and winds circulate around the storm's center). Large bands of clouds and precipitation spiral from the eye wall and are thusly called spiral rain bands.
Hurricanes are easily spotted from the previous features as well as a pronounced rotation around the eye in satellite or radar animations. Hurricanes are also rated according to their wind speed on the Saffir-Simpson scale. This scale ranges from categories 1 to 5, with 5 being the most devastating. Under the right atmospheric conditions, hurricanes can sustain themselves for as long as a couple of weeks. Upon reaching cooler water or land, hurricanes rapidly lose intensity. Now, it can become a tropical storm, or a tropical depression, or an extratropical cyclone.
The formation of tropical cyclones is the topic of extensive ongoing research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:
1. Sea surface temperatures above 26.5 degrees Celsius (79.7 degrees Fahrenheit) to at least a depth of 50 meters (164 feet). Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.
2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through the world's tropical oceans.
4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan, the strongest storm to be so close to the equator, started its formation at 9.7 degrees north.)
5. Lack of vertical wind shear (change in wind velocity or direction over height). High levels of wind shear can break apart the vertical structure of a tropical cyclone.
Extratropical cyclone
Another storm system is the extratropical cyclone.
An extratropical cyclone (also called Wave Cyclone) is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses. From space, extratropical storms have a characteristic comma-shaped cloud pattern. Extratropical cyclones can also be dangerous because their low-pressure centers cause powerful winds.
These images from the Multi-angle Imaging SpectroRadiometer (MISR) portray an occluded extratropical cyclone situated in the Southern Ocean, about 650 kilometers south of the Eyre Peninsula, South Australia.
The left-hand image, a true-color view from MISR’s nadir (vertical-viewing) camera, shows clouds just south of the Yorke Peninsula and the Murray-Darling river basin in Australia. Retrieved cloud-tracked wind velocities are indicated by the superimposed arrows. The image on the right displays cloud-top heights. Areas where cloud heights could not be retrieved are shown in black. Both the wind vectors and the cloud heights were derived using data from multiple MISR cameras within automated computer processing algorithms. The stereoscopic algorithms used to generate these results are still being refined, and future versions of these products may show modest changes.
Extratropical cyclones are the dominant weather system at midlatitudes, and the term is used generically for regional low-pressure systems in the mid- to high-latitudes. In the southern hemisphere, cyclonic rotation is clockwise. These storms obtain their energy from temperature differences between air masses on either side of warm and cold fronts, and their characteristic pattern is of warm and cold fronts radiating out from a migrating low pressure center which forms, deepens, and dissipates as the fronts fold and collapse on each other. The center of this cyclone has started to decay, with the band of cloud to the south most likely representing the main front that was originally connected with the cyclonic circulation.
These views were acquired on October 11, 2001, and the large view represents an area of about 380 kilometers x 1900 kilometers.
A strong tropical cyclone consists of the following components.
Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of rain bands and thunderstorm activity surrounding the central low. Tropical cyclones with symmetrical CDO tend to be strong and well developed.
Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
Eyewall: The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eyewall replacement cycles, in which outer eye walls form to replace inner ones. The mechanisms that make this occur are still not fully understood.
Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.
Large-scale winds
Although tropical cyclones are large systems generating enormous energy, their movements over the earth's surface are often compared to that of leaves carried along by a stream. That is, large-scale winds—the streams in the earth's atmosphere—are responsible for moving and steering tropical cyclones. The path of motion is referred to as a tropical cyclone's track.
The major force affecting the track of tropical systems in all areas are winds circulating around high-pressure areas. Over the North Atlantic Ocean, tropical systems are steered generally westward by the east-to-west winds on the south side of the Bermuda High, a persistent high-pressure area over the North Atlantic. Also, in the area of the North Atlantic where hurricanes form, trade winds, which are prevailing westward-moving wind currents, steer tropical waves (precursors to tropical depressions and cyclones) westward from off the African coast toward the Caribbean and North America.
Coriolis effect
he earth's rotation also imparts an acceleration (termed the Coriolis Acceleration or Coriolis Effect). This acceleration causes cyclonic systems to move toward the earth's poles in the absence of strong steering currents. Thus, tropical cyclones in the Northern Hemisphere are deflected toward the north pole and cyclones in the Southern Hemisphere are deflected toward the South Pole, if no strong pressure systems are counteracting the Coriolis Acceleration.
Interaction with high and low pressure systems
Finally, when a tropical cyclone moves into higher latitude, its general track around a high-pressure area can be deflected significantly by winds moving toward a low-pressure area. Such a track direction change is termed recurve. A hurricane moving from the Atlantic toward the Gulf of Mexico, for example, will recurve to the north and then northeast if it encounters winds blowing northeastward toward a low-pressure system passing over North America. Many tropical cyclones along the U.S. East Coast and in the Gulf of Mexico are eventually forced toward the northeast by low-pressure areas which move from west to east over North America.
Track forecasting
Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system.
With their understanding of the forces that act on tropical cyclones, and a wealth of data from earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. High-speed computers and sophisticated simlulation software allow forecasters to produce computer models that forecast tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. But while track forecasts have become more accurate than 20 years ago, scientists say they are less skillful at predicting the intensity of tropical cyclones. They attribute the lack of improvement in intensity forecasting to the complexity of tropical systems and an incomplete understanding of factors that affect their development.
Landfall
In the case of a storm, landfall is when the center of the storm (the eye) reaches land. Therefore, hurricane conditions are experienced on land well in advance of landfall... in fact, for a storm which moves steadily inland, the storm as experienced at the point of landfall is half over at the time of landfall. To coordinate a coastal evacuation, policymakers would NOT calculate backwards from the anticipated time of landfall, but instead might calculate from the anticipated time of some chosen wind speed radius reaching land.
Dissipation
A tropical cyclone can cease to have tropical characteristics in several ways:
- It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms become disorganized areas of low pressure within a day or two of landfall. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose strength. This is, however, the cause of many storm fatalities, as the dying storm unleashes torrential rainfall, and in mountainous areas, this can lead to deadly mudslides.
- It remains in the same area of ocean for too long, sucking up all the warm water. Without warm surface water, the storm cannot survive.
- It experiences wind shear, causing the convection to lose direction and the heat engine breaks down.
- It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms.
- It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extratropical cyclones.
Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane force) winds and drop several inches of rainfall. When a tropical cyclone reaches higher latitudes or passes over land, it may merge with weather fronts or develop into a frontal cyclone, also called extratropical cyclone. In the Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach Europe as a European windstorm.
Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds, a Category 5 hurricane has the highest. The rankings are not absolute in terms of effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. In fact, tropical systems of less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides.
The National Hurricane Center classifies hurricanes of Category 3 and above as Major Hurricanes. The Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h, equivalent to a strong Category 4 storm) as Super Typhoons.
The definition of sustained winds recommended by the World Meteorological Organization (WMO) and used by most weather agencies is that of a 10-minute average. The U.S. weather service defines sustained winds based on 1-minute average speed measured about 10 meters (33 ft) above the surface.
The Saffir-Simpson Hurricane Scale is a scale classifying hurricanes by the intensity of their sustained winds, developed in 1969 by civil engineer Herbert Saffir and National Hurricane Center director Bob Simpson. Classifications are used to gauge the likely damage and flooding a hurricane will cause upon landfall. The Saffir-Simpson Hurricane Scale is used only to describe hurricanes forming in the Atlantic Ocean and northern Pacific Ocean east of the International Date Line. Other areas use their own classification schemes, such as cyclones and typhoons.
The Australian Bureau of Meteorology uses a 1-5 scale called tropical cyclone severity categories. Unlike the Saffir-Simpson Scale, severity categories are based on strongest wind gusts and not sustained winds. Severity categories are scaled somewhat lower than the Saffir-Simpson Scale, with a severity category 2 tropical cyclone being roughly equivalent to a Saffir-Simpson category 1 hurricane.
The initial scale was developed by Saffir while on commission from the United Nations to study low-cost housing in hurricane-prone areas. While performing the study, Saffir realized there was no simple scale for describing the likely effects of a hurricane. Knowing the utility of the Richter magnitude scale in describing earthquakes, he devised a 1–5 scale based on wind speed that showed expected damage to structures. Saffir gave the scale to the NHC, and Simpson added in the effects of storm surge and flooding. It does not take into account rainfall or location, which means a Category 3 hurricane that hits a major city will likely do far more damage than a Category 5 hurricane that hits a remote area.
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, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline, in the landfall region. Note that all winds are using the U.S. 1-minute average.
Category One Hurricane:
Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft above normal. No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricane Lili of 2002 made landfall on the Louisiana coast as a Category One hurricane. Hurricane Gaston of 2004 was a Category One hurricane that made landfall along the central South Carolina coast.
Category Two Hurricane:
Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8 feet above normal. 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. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Frances of 2004 made landfall over the southern end of Hutchinson Island, Florida as a Category Two hurricane. Hurricane Isabel of 2003 made landfall near Drum Inlet on the Outer Banks of North Carolina as a Category 2 hurricane.
Category Three Hurricane:
Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 9-12 ft above normal. 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 trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from 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 with several blocks of the shoreline may be required. Hurricanes Jeanne and Ivan of 2004 were Category Three hurricanes when they made landfall in Florida and in Alabama, respectively.
Category Four Hurricane:
Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally 13-18 ft above normal. 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. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. 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). Hurricane Charley of 2004 was a Category Four hurricane made landfall in Charlotte County, Florida with winds of 150 mph. Hurricane Dennis of 2005 struck the island of Cuba as a Category Four hurricane.
Category Five Hurricane:
Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18 ft above normal. 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. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. 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. Only 3 Category Five Hurricanes have made landfall in the United States since records began: The Labor Day Hurricane of 1935, Hurricane Camille (1969), and Hurricane Andrew in August, 1992. The 1935 Labor Day Hurricane struck the Florida Keys with a minimum pressure of 892 mb--the lowest pressure ever observed in the United States. Hurricane Camille struck the Mississippi Gulf Coast causing a 25-foot storm surge, which inundated Pass Christian. Hurricane Andrew of 1992 made landfall over southern Miami-Dade County, Florida causing 26.5 billion dollars in losses--the costliest hurricane on record. In addition, Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone on record with a minimum pressure of 888 mb.
The Saffir-Simpson Hurricane scale (pdf).
Pictures storm are also available on the NASA Goddard Space Flight Center web site
(http://rsd.gsfc.nasa.gov/rsd/images/preview.html)
Alerts, suggestions, pictures, and information are available on the below web sites:
Federal Emergency Management Agency's (www.fema.gov)
Atlantic Oceanographic and Meteorological Laboratory (AOML) (www.aoml.noaa.gov)
Operational Significant Event Imagery (OSEI) (www.osei.noaa.gov)
The ingredients for a tropical cyclone 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 associated with this phenomenon.
Condensation as a driving force is the primary difference which distinguishes tropical cyclones from other meteorological phenomena. Mid-latitude cyclones, for example, draw their energy mostly from pre-existing temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The evaporation of this moisture is driven by the high winds and reduced atmospheric pressure present in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength will diminish rapidly.
The National Center for Atmospheric Research scientists estimate the heat energy released by a hurricane equals 50 to 200 trillion watts -- about the same amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes.
Terms for tropical cyclones and their etymology
The term used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h) depends on the region:
- hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline, and unofficially in the South Atlantic Ocean
- typhoon in the Northwest Pacific Ocean west of the dateline
- severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
severe cyclonic storm in the North Indian Ocean
tropical cyclone in the Southwest Indian Ocean and South Pacific Ocean east of 160°E.
Etymology
The word typhoon may come partly from the Portuguese tufao; Urdu, Persian and Arabic tufan; Greek tuphon; and written in Chinese character is a translation of typhoon.
The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracan.
The word cyclone came from the Greek word κuκλος, meaning circle.
In other areas, hurricanes have been called Bagyo in the Philippines and Taino in Haiti.
Stages of Development
Hurricanes evolve through a life cycle of stages from birth to death. A tropical disturbance in time can grow to a more intense stage by attaining a specified sustained wind speed. The progression of tropical disturbances can be seen in the three images below.
Hurricanes can often live for a long period of time -- as much as two to three weeks. They may initiate as a cluster of thunderstorms over the tropical ocean waters. Once a disturbance has become a tropical depression, the amount of time it takes to achieve the next stage, tropical storm, can take as little as half a day to as much as a couple of days. It may not happen at all. The same may occur for the amount of time a tropical storm needs to intensify into a hurricane. Atmospheric and oceanic conditions play major roles in determining these events.
Tropical Depression
Once a group of thunderstorms has come together under the right atmospheric conditions for a long enough time, they may organize into a tropical depression. Winds near the center are constantly between 20 and 34 knots (23 - 39 mph).A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression".A tropical depression is designated when the first appearance of a lowered pressure and organized circulation in the center of the thunderstorm complex occurs. A surface pressure chart will reveal at least one closed isobar to reflect this lowering.
When viewed from a satellite, tropical depressions appear to have little organization. However, the slightest amount of rotation can usually be perceived when looking at a series of satellite images. Instead of a round appearance similar to hurricanes, tropical depressions look like individual thunderstorms that are grouped together.
Tropical Storms
Once a tropical depression has intensified to the point where its maximum sustained winds are between 35-64 knots (39-73 mph), it becomes a tropical storm. It is at this time that it is assigned a name. During this time, the storm itself becomes more organized and begins to become more circular in shape -- resembling a hurricane.A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34–63 knots, 39–73 mph, or 62–117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present. Government weather services assign first names to systems that reach this intensity (thus the term named storm).The rotation of a tropical storm is more recognizable than for a tropical depression. Tropical storms can cause a lot of problems even without becoming a hurricane. However, most of the problems a tropical storm cause stem from heavy rainfall.
The near satellite image is of tropical storm Charlie (1998). Many cities in southern Texas reported heavy rainfall between 5-10 inches. Included in these was Del Rio, where more than 17 inches fell in just one day, forcing people from their homes and killing half a dozen.
Hurricanes
As surface pressures continue to drop, a tropical storm becomes a hurricane when sustained wind speeds reach 64 knots (74 mph). A pronounced rotation develops around the central core.Hurricanes are Earth's strongest tropical cyclones. A distinctive feature seen on many hurricanes and are unique to them is the dark spot found in the middle of the hurricane. This is called the eye. Surrounding the eye is the region of most intense winds and rainfall called the eye wall (an area about 10 to 50 miles (16 to 80 kilometers) wide in which the strongest thunderstorms and winds circulate around the storm's center). Large bands of clouds and precipitation spiral from the eye wall and are thusly called spiral rain bands.Hurricanes are easily spotted from the previous features as well as a pronounced rotation around the eye in satellite or radar animations. Hurricanes are also rated according to their wind speed on the Saffir-Simpson scale. This scale ranges from categories 1 to 5, with 5 being the most devastating. Under the right atmospheric conditions, hurricanes can sustain themselves for as long as a couple of weeks. Upon reaching cooler water or land, hurricanes rapidly lose intensity. Now, it can become a tropical storm, or a tropical depression, or an extratropical cyclone.
The formation of tropical cyclones is the topic of extensive ongoing research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible:
1. Sea surface temperatures above 26.5 degrees Celsius (79.7 degrees Fahrenheit) to at least a depth of 50 meters (164 feet). Warm waters are the energy source for tropical cyclones. When these storms move over land or cooler areas of water they weaken rapidly.
2. Upper level conditions must be conducive to thunderstorm formation. Temperatures in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist.
3. A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through the world's tropical oceans.
4. A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan, the strongest storm to be so close to the equator, started its formation at 9.7 degrees north.)
5. Lack of vertical wind shear (change in wind velocity or direction over height). High levels of wind shear can break apart the vertical structure of a tropical cyclone.
Extratropical cyclone
Another storm system is the extratropical cyclone.
An extratropical cyclone (also called Wave Cyclone) is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses. From space, extratropical storms have a characteristic comma-shaped cloud pattern. Extratropical cyclones can also be dangerous because their low-pressure centers cause powerful winds.
These images from the Multi-angle Imaging SpectroRadiometer (MISR) portray an occluded extratropical cyclone situated in the Southern Ocean, about 650 kilometers south of the Eyre Peninsula, South Australia.The left-hand image, a true-color view from MISR’s nadir (vertical-viewing) camera, shows clouds just south of the Yorke Peninsula and the Murray-Darling river basin in Australia. Retrieved cloud-tracked wind velocities are indicated by the superimposed arrows. The image on the right displays cloud-top heights. Areas where cloud heights could not be retrieved are shown in black. Both the wind vectors and the cloud heights were derived using data from multiple MISR cameras within automated computer processing algorithms. The stereoscopic algorithms used to generate these results are still being refined, and future versions of these products may show modest changes.
Extratropical cyclones are the dominant weather system at midlatitudes, and the term is used generically for regional low-pressure systems in the mid- to high-latitudes. In the southern hemisphere, cyclonic rotation is clockwise. These storms obtain their energy from temperature differences between air masses on either side of warm and cold fronts, and their characteristic pattern is of warm and cold fronts radiating out from a migrating low pressure center which forms, deepens, and dissipates as the fronts fold and collapse on each other. The center of this cyclone has started to decay, with the band of cloud to the south most likely representing the main front that was originally connected with the cyclonic circulation.
These views were acquired on October 11, 2001, and the large view represents an area of about 380 kilometers x 1900 kilometers.
Cyclone structure
A strong tropical cyclone consists of the following components.
Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.
Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of rain bands and thunderstorm activity surrounding the central low. Tropical cyclones with symmetrical CDO tend to be strong and well developed.Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
Eyewall: The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eyewall replacement cycles, in which outer eye walls form to replace inner ones. The mechanisms that make this occur are still not fully understood.
Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.
Movement, track and dissipation
Large-scale winds
Although tropical cyclones are large systems generating enormous energy, their movements over the earth's surface are often compared to that of leaves carried along by a stream. That is, large-scale winds—the streams in the earth's atmosphere—are responsible for moving and steering tropical cyclones. The path of motion is referred to as a tropical cyclone's track.
The major force affecting the track of tropical systems in all areas are winds circulating around high-pressure areas. Over the North Atlantic Ocean, tropical systems are steered generally westward by the east-to-west winds on the south side of the Bermuda High, a persistent high-pressure area over the North Atlantic. Also, in the area of the North Atlantic where hurricanes form, trade winds, which are prevailing westward-moving wind currents, steer tropical waves (precursors to tropical depressions and cyclones) westward from off the African coast toward the Caribbean and North America.
Coriolis effect
he earth's rotation also imparts an acceleration (termed the Coriolis Acceleration or Coriolis Effect). This acceleration causes cyclonic systems to move toward the earth's poles in the absence of strong steering currents. Thus, tropical cyclones in the Northern Hemisphere are deflected toward the north pole and cyclones in the Southern Hemisphere are deflected toward the South Pole, if no strong pressure systems are counteracting the Coriolis Acceleration.
Interaction with high and low pressure systems
Finally, when a tropical cyclone moves into higher latitude, its general track around a high-pressure area can be deflected significantly by winds moving toward a low-pressure area. Such a track direction change is termed recurve. A hurricane moving from the Atlantic toward the Gulf of Mexico, for example, will recurve to the north and then northeast if it encounters winds blowing northeastward toward a low-pressure system passing over North America. Many tropical cyclones along the U.S. East Coast and in the Gulf of Mexico are eventually forced toward the northeast by low-pressure areas which move from west to east over North America.
Track forecasting
Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system.
With their understanding of the forces that act on tropical cyclones, and a wealth of data from earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. High-speed computers and sophisticated simlulation software allow forecasters to produce computer models that forecast tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. But while track forecasts have become more accurate than 20 years ago, scientists say they are less skillful at predicting the intensity of tropical cyclones. They attribute the lack of improvement in intensity forecasting to the complexity of tropical systems and an incomplete understanding of factors that affect their development.
Landfall
In the case of a storm, landfall is when the center of the storm (the eye) reaches land. Therefore, hurricane conditions are experienced on land well in advance of landfall... in fact, for a storm which moves steadily inland, the storm as experienced at the point of landfall is half over at the time of landfall. To coordinate a coastal evacuation, policymakers would NOT calculate backwards from the anticipated time of landfall, but instead might calculate from the anticipated time of some chosen wind speed radius reaching land.
Dissipation
A tropical cyclone can cease to have tropical characteristics in several ways:
- It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms become disorganized areas of low pressure within a day or two of landfall. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose strength. This is, however, the cause of many storm fatalities, as the dying storm unleashes torrential rainfall, and in mountainous areas, this can lead to deadly mudslides.
- It remains in the same area of ocean for too long, sucking up all the warm water. Without warm surface water, the storm cannot survive.
- It experiences wind shear, causing the convection to lose direction and the heat engine breaks down.
- It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms.
- It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extratropical cyclones.
Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane force) winds and drop several inches of rainfall. When a tropical cyclone reaches higher latitudes or passes over land, it may merge with weather fronts or develop into a frontal cyclone, also called extratropical cyclone. In the Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach Europe as a European windstorm.
Categories and ranking
Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds, a Category 5 hurricane has the highest. The rankings are not absolute in terms of effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. In fact, tropical systems of less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides.
The National Hurricane Center classifies hurricanes of Category 3 and above as Major Hurricanes. The Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h, equivalent to a strong Category 4 storm) as Super Typhoons.
The definition of sustained winds recommended by the World Meteorological Organization (WMO) and used by most weather agencies is that of a 10-minute average. The U.S. weather service defines sustained winds based on 1-minute average speed measured about 10 meters (33 ft) above the surface.
The Saffir-Simpson Hurricane Scale is a scale classifying hurricanes by the intensity of their sustained winds, developed in 1969 by civil engineer Herbert Saffir and National Hurricane Center director Bob Simpson. Classifications are used to gauge the likely damage and flooding a hurricane will cause upon landfall. The Saffir-Simpson Hurricane Scale is used only to describe hurricanes forming in the Atlantic Ocean and northern Pacific Ocean east of the International Date Line. Other areas use their own classification schemes, such as cyclones and typhoons.
The Australian Bureau of Meteorology uses a 1-5 scale called tropical cyclone severity categories. Unlike the Saffir-Simpson Scale, severity categories are based on strongest wind gusts and not sustained winds. Severity categories are scaled somewhat lower than the Saffir-Simpson Scale, with a severity category 2 tropical cyclone being roughly equivalent to a Saffir-Simpson category 1 hurricane.
The initial scale was developed by Saffir while on commission from the United Nations to study low-cost housing in hurricane-prone areas. While performing the study, Saffir realized there was no simple scale for describing the likely effects of a hurricane. Knowing the utility of the Richter magnitude scale in describing earthquakes, he devised a 1–5 scale based on wind speed that showed expected damage to structures. Saffir gave the scale to the NHC, and Simpson added in the effects of storm surge and flooding. It does not take into account rainfall or location, which means a Category 3 hurricane that hits a major city will likely do far more damage than a Category 5 hurricane that hits a remote area.
The 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, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline, in the landfall region. Note that all winds are using the U.S. 1-minute average.
Category One Hurricane:
Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft above normal. No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricane Lili of 2002 made landfall on the Louisiana coast as a Category One hurricane. Hurricane Gaston of 2004 was a Category One hurricane that made landfall along the central South Carolina coast.
Category Two Hurricane:
Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8 feet above normal. 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. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Frances of 2004 made landfall over the southern end of Hutchinson Island, Florida as a Category Two hurricane. Hurricane Isabel of 2003 made landfall near Drum Inlet on the Outer Banks of North Carolina as a Category 2 hurricane.
Category Three Hurricane:
Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 9-12 ft above normal. 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 trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from 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 with several blocks of the shoreline may be required. Hurricanes Jeanne and Ivan of 2004 were Category Three hurricanes when they made landfall in Florida and in Alabama, respectively.
Category Four Hurricane:
Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally 13-18 ft above normal. 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. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. 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). Hurricane Charley of 2004 was a Category Four hurricane made landfall in Charlotte County, Florida with winds of 150 mph. Hurricane Dennis of 2005 struck the island of Cuba as a Category Four hurricane.
Category Five Hurricane:
Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18 ft above normal. 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. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. 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. Only 3 Category Five Hurricanes have made landfall in the United States since records began: The Labor Day Hurricane of 1935, Hurricane Camille (1969), and Hurricane Andrew in August, 1992. The 1935 Labor Day Hurricane struck the Florida Keys with a minimum pressure of 892 mb--the lowest pressure ever observed in the United States. Hurricane Camille struck the Mississippi Gulf Coast causing a 25-foot storm surge, which inundated Pass Christian. Hurricane Andrew of 1992 made landfall over southern Miami-Dade County, Florida causing 26.5 billion dollars in losses--the costliest hurricane on record. In addition, Hurricane Gilbert of 1988 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone on record with a minimum pressure of 888 mb.
The Saffir-Simpson Hurricane scale (pdf).
Pictures storm are also available on the NASA Goddard Space Flight Center web site
(http://rsd.gsfc.nasa.gov/rsd/images/preview.html)
Alerts, suggestions, pictures, and information are available on the below web sites:
Federal Emergency Management Agency's (www.fema.gov)
Atlantic Oceanographic and Meteorological Laboratory (AOML) (www.aoml.noaa.gov)
Operational Significant Event Imagery (OSEI) (www.osei.noaa.gov)
24/09/2005
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