Tropical cyclones: Formation and Decay by Steve Graham and Holli Riebeek · November 1, 2006

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TROPICAL CYCLONES: Formation and Decay


by Steve Graham and Holli Riebeek · November 1, 2006


Few things in nature can compare to the destructive force of a hurricane. Called the greatest storm on Earth, a hurricane is capable of annihilating coastal areas with sustained winds of 155 miles per hour or higher, intense areas of rainfall, and a storm surge. In fact, during its life cycle a hurricane can expend as much energy as 10,000 nuclear bombs!

The term hurricane is derived from Huracan, a god of evil recognized by the Tainos, an ancient aboriginal tribe from Central America. In other parts of the world, hurricanes are known by different names. In the western Pacific and China Sea area, hurricanes are known as typhoons, from the Cantonese tai-fung, meaning great wind. In Bangladesh, Pakistan, India, and Australia, they are known as cyclones, and finally, in the Philippines, they are known as baguios.

A Hurricane Katrina survivor holds snapshots of damage from Hurricane Andrew, which he took when he lived in Florida. (Image courtesy danakay/Flickr.)

The scientific name for a hurricane, regardless of its location, is tropical cyclone. In general, a cyclone is a large system of spinning air that rotates around a point of low pressure. Only tropical cyclones, which have warm air at their center, become the powerful storms that are called hurricanes.

Hurricane Formation and Decay

Hurricanes form over tropical waters (between 8 and 20 degrees latitude) in areas of high humidity, light winds, and warm sea surface temperatures [typically 26.5 degrees Celsius (80 Fahrenheit) or greater]. These conditions usually prevail in the summer and early fall months of the tropical North Atlantic and North Pacific Oceans, and for this reason, hurricane “season” in the northern hemisphere runs from June through November.

Tropical cyclones ususally form in each hemisphere’s summer and early fall, when ocean waters are warmest. (Graphic by Robert Simmon, NASA GSFC.)
The first sign of hurricane genesis (development) is the appearance of a cluster of thunderstorms over the tropical oceans, called a tropical disturbance. Tropical disturbances generally form in one of three ways, all of which involve the convergence of surface winds. When winds come together (converge), the force of the collision forces air to rise, initiating thunderstorms.

Thunderstorm Triggers

One trigger for convergence is the meeting of the Northern and Southern Hemisphere easterly trade winds near the equator. The meeting of these wind belts triggers numerous, daily thunderstorms in a region called the Intertropical Convergence Zone (ITCZ). Occasionally, a cluster of thunderstorms will break away from the ITCZ and organize into a more unified storm system.

Another mechanism that can lead to the formation of a hurricane is the convergence of air along the boundary between masses of warm and cold air. Along the boundary, denser cold air can help lift warm and moist air to form thunderstorms. Occasionally such boundaries, called mid-latitude frontal boundaries, drift over the Gulf of Mexico or the Atlantic Ocean off the East Coast of the United States, where developing storms can organize into hurricanes in one of two ways. Either thunderstorms organize into a large system that forms a new area of low pressure, or a pre-existing, weak, non-tropical cyclone will form along the front and will develop into a hurricane.

Cyclones that form along mid-latitude frontal boundaries are often called mid-latitude or extratropical cyclones, and they typically have cold air at upper levels over the cyclone center. In contrast, hurricanes (tropical cyclones) have warm air over their centers. To change into a tropical cyclone, the cold air over an extratropical cyclone must change to warm air. This change can happen if thunderstorms occur near the cyclone center. The thunderstorms form along the frontal boundary as warm air rises over the colder air mass. As the air rises, it cools, and water vapor condenses into clouds. The heating released by condensation then helps to warm the air, and eventually the extratropical cyclone transitions into a tropical cyclone.

Waves that occur within the dominant easterly winds over the tropical Atlantic cause areas of converging and diverging winds. The convergence forces air to rise, triggering numerous thunderstorms that can go on to become hurricanes. (Graphic by Robert Simmon, NASA GSFC.)

The last and most common mechanism that triggers the development of a cyclone is the African easterly wave, an area of disturbed weather that travels from east to west across the tropical Atlantic. Essentially, an easterly wave forms because of a “kink” in the jet of air that flows west out of Africa. The jet is created by the strong temperature difference between the Sahara Desert and the Gulf of Guinea. The warm air over the Sahara rises and, several kilometers above the surface, turns southward toward the cooler air over the Gulf. The rotation of the Earth turns this air current westward to form the African Easterly Jet, which then continues out over the Atlantic Ocean. Occasionally, a “kink” will develop in the jet and move from east to west, hence the name easterly wave. Converging winds on the east side of the easterly wave trigger the development of thunderstorms, and some of these large thunderstorm systems go on to become hurricanes. Most Atlantic hurricanes can be traced to easterly waves that form over Western Africa.

Animations: West African Coast (4.2 MB Quicktime)

Equatorial Overview (5.4 MB Quicktime)

Hurricanes are commonly formed by easterly waves. The waves are “kinks” in the African Easterly Jet, a strong wind that blows over the Atlantic from the West African coast. The easterly waves trigger strong thunderstorms that move eastward. Over the warm waters of the Atlantic, the thunderstorms embedded in the easterly wave can grow into a hurricane under the right conditions.

These images and animations show an easterly wave crossing the African coast during late summer 2005. The system later developed into Hurricane Irene. Cold cloud tops appear white in these thermal-infrared images, acquired by the Meteosat-8 satellite. As the easterly wave, visible as a circular area of clouds, approached the African coast it began to rotate. On August 4, the system became a tropical depression over the Atlantic. (Images copyright 2005 EUMETSAT, provided by the British Atmospheric Data Center.)

Increasing Organization

Given favorable conditions, the tropical disturbance can become better organized into a more unified storm system. As the storm organizes, surface air pressures fall in the area around the storm and winds begin to spin in a cyclonic circulation (counter-clockwise in the Northern Hemisphere). Surface pressures fall when water vapor condenses in rising air and releases energy, or latent heat, into areas within the tropical disturbance. The heat boosts the air (increases the buoyancy), so it continues rising. To compensate for the rising air, surrounding air sinks. As this air sinks towards the surface, it is compressed by the weight of the air above it and warms. The pressure rises at the top of the layer of warming air, pushing air at the top of the layer outward. Because there is now less air in the layer, the weight of the entire layer is less, and the pressure at the ocean surface drops. The drop in pressure draws in more air at the surface, and this air converges near the center of the storm to form more clouds.

Hurricanes intensify when condensation of water vapor in rising air releases heat energy into storm, setting off a chain reaction. The heat makes the surrounding air more buoyant, causing it to rise further. To compensate for the rising air, surrounding air sinks. The sinking air is compressed by the weight of the air above it, and it warms. The pressure rises at the top of the layer of warmed air, pushing air outward. As the air spreads outward, the total air pressure at the surface drops. The more the pressure drops, the more the winds intensify, drawing more heat and moisture from the ocean surface. (Graphic by Robert Simmon, NASA GSFC.)

Like an ice skater whose body spins faster as his arms are drawn inward, air near the surface speeds up as it spirals in towards the center of the low pressure area. The increasing winds that spin around the center of the storm draw heat and moisture from the warm ocean surface, providing more fuel for the rising motions that produce the clouds and increase the temperatures.

A chain reaction (or feedback mechanism) is now in progress, as the rising temperatures in the center of the storm cause surface pressures to drop even more. The lower the surface pressure, the more rapidly air flows into the storm at the surface, increasing the winds and causing more thunderstorms. More thunderstorms release more heat, forcing air at higher altitudes outward. The air pressure at the surface drops even more, triggering stronger winds, and so on.

The storm takes the distinctive, spiraling hurricane shape because of the Coriolis Effect, generated by the rotation of the Earth. This is the same force that causes the south-blowing African jet to bend westward over the Atlantic, spawning easterly waves. In the Northern Hemisphere, the Earth’s rotation causes moving air to veer to the right. As air rushes towards the low-pressure center of the storm at the Earth’s surface, it curves right. If the storm is far enough from the equator (generally at least 8 degrees of latitude), the deflection or curvature is great enough that the air starts spinning counterclockwise around the center of the storm.
Once sustained wind speeds reach 37 kilometers (23 miles) per hour, the tropical disturbance is called a tropical depression. As winds increase to 63 kilometers (39 miles) per hour, the cyclone is called a tropical storm and receives a name, a tradition started with the use of World War II vintage code names such as Able, Baker, Charlie, etc. For a number of years beginning in 1953, female names were used exclusively until the late 1970s, when storm names began to be alternated between male and female names. Finally, when wind speeds reach 119 kilometers (74 miles) per hour, the storm is classified as a hurricane.

The Saffir-Simpson Scale

In the early 1970s, a classification system was designed to quantify the level of damage and flooding expected from a hurricane. This system was conceived by Herbert Saffir, a consulting engineer, and Robert Simpson, then the director of the National Hurricane Center. Using a mix of structural engineering and meteorology, they constructed the Saffir-Simpson Hurricane Intensity Scale, or simply, the Saffir-Simpson Scale. Consisting of 5 categories (1 being the weakest and 5 being the strongest), the scale corresponds to a hurricane’s central pressure, maximum sustained winds, and storm surge. Sustained wind speeds are the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf in the landfall region. Categories 3, 4, and 5 are considered major (intense) hurricanes, capable of inflicting great damage and loss of life.

Category 1: Winds 119-153 km/hr (74-95 mph). Storm surge generally 4-5 feet 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. Winds from Hurricane Katrina damaged this sign in Dequincy, Louisiana. (Image Copyright © sidehike/Flickr.)

Category 2: Winds 154-177 km/hr (96-110 mph). 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. Winds from Hurricane Wilma shattered windows in Fort Lauderdale, Florida, in 2005. (Image Copyright © Gary Musik/Flickr.)

Category 3: Winds 178-209 km/hr (111-130 mph). Storm surge generally 9-12 ft above normal. Some structural damage to small residences and utility buildings, with a minor amount of curtain wall (non-load-bearing exterior wall) 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. This photo shows extensive damage to private homes and boats in Christian Pass, Mississippi, in the aftermath of Hurricane Katrina. (Image Copyright © Andrea Booher/Federal Emergency Management Agency.)

Category 4: Winds 210-249 km/hr (131-155 mph). Storm surge generally 13-18 feet above normal. More extensive curtain wall 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 feet above sea level may be flooded. Hurricane winds completely toppled this house in North Carolina. (Image Copyright © brighterworlds’ photos/Flickr.)

Category 5: Winds greater than 249 km/hr (155 mph). Storm surge generally greater than 18 feet 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 feet above sea level and within 500 yards of the shoreline. This photo shows the aftermath of Hurricane Andrew in 1992. (Image Copyright © Greenpeace.)

Hurricane Anatomy

During hurricane development, certain characteristics become more prominent as the storm strengthens. At the center of the hurricane is the eye, a cloud-free area of sinking air and light winds that is usually from 10 to 65 kilometers in diameter. As air rises in the thunderstorms surrounding the eye, some of it is forced towards the center, where it converges and sinks. As this air sinks, it compresses and warms to create an environment (mostly) free of clouds and precipitation. The eye is the calmest part of the storm because the strong surface winds converging towards the center never actually reach the exact center of the storm, but instead form a cylinder of relatively calm air.

(Top) Surrounding the eye of the hurricane is a ring of thunderstorms, called the eyewall. Rainbands surround the eye of the storm in concentric circles. In the eyewall and in the rainbands, warm, moist air rises, while in the eye and around the rainbands, air from higher in the atmosphere sinks back toward the surface. The rising air cools, and water vapor in the air condenses into rain. Sinking air warms and dries, creating a calm, cloud-free area in the eye. (Middle) Low pressure at the ocean surface in the heart of the hurricane draws in surrounding air. These spiraling winds pick up speed as they approach the eye, pulling more heat and moisture from the ocean surface. (Bottom). The stronger the convection in the thunderstorms becomes, the more rain they produce. The more rain they produce, the more heat they release into the surrounding atmosphere, further fueling the storm. (Graphics Copyright © National Center for Atmospheric Research/The COMET Program.)

Bordering the eye of a mature hurricane is the eye wall, a ring of tall thunderstorms that produce heavy rains and very strong winds. The most destructive section of the storm is in the eye wall on the side where the wind blows in the same direction as the storm’s forward motion. For example, in a hurricane that is moving due west, the most intense winds would be found on the northern side of the storm, since the hurricane’s winds are added to the storm’s forward motion.

Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion, suitably called spiraling rain bands. The rain bands are capable of producing heavy bursts of rain and wind, perhaps one-half or two-thirds the strength of those associated with the eye wall.

Weakening Factors

Even when the conditions are ripe for hurricane formation at the surface, the storm may not form if the atmospheric conditions five to ten kilometers above the surface are not favorable. For example, around the area of 20 degrees latitude, the air aloft is often sinking, due to the presence of the sub-tropical high—a semi-permanent high pressure system in the subtropical regions. The high pressure pushes air towards the surface. The sinking air warms and creates a temperature inversion, an extremely stable air layer in which temperature increases with altitude, the opposite of the usual temperature profile in the lower atmosphere. Called the trade wind inversion, this warm layer is very stable, which makes it difficult for air currents to rise and form thunderstorms and (eventually) hurricanes. In addition, strong upper-level winds tend to rip apart developing thunderstorms by dispersing the latent heat and preventing the warming temperatures that lead to lower air pressure at the surface.

At the surface, hurricanes can diminish rather quickly given the right conditions. These conditions include the storm moving over cooler water that can’t supply warm, moist tropical air; the storm moving over land, again cutting off the source of warm, moist air; and finally, the storm moving into an area where strong winds high in the atmosphere disperse latent heat, reducing the warm temperatures aloft and raising the surface pressure.


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