Hurricane & Tropical Cyclone Formation


The term hurricane refers to a type of large storm with strong winds that emerges in Earth's tropical regions just north or south of the equator. Regardless of where they form, hurricanes always move towards the west and spin in a circular direction about the eye, or center, of the storm. These storms are only referred to as hurricanes when they occur near the Americas in the Atlantic or eastern Pacific Oceans. Storms that form in the South Pacific and Indian Oceans are usually called cyclones. Nations in East Asia like Japan and China instead use the name typhoon. Mexicans call these storms "cordonazos," they are "bagyos" in the Philippines, Haitians call them "tainos," and Australians once used the highly technical term "willy-willy." Whatever the name, these storms all share the same characteristics and are classified by the general term tropical cyclones.

Tropical cyclones are formed in four major phases. The first step occurs when a collection of small thunderstorms forms over the tropics. The hurricanes that threaten the East and Gulf coasts of the United States, for example, are the product of storms that first form over the coast of western Africa and head west across the Atlantic Ocean. The cyclones that strike East Asia and Australia, on the other hand, form over the central Pacific. Tropical cyclones emerge in these regions, between 8 and 20 latitude, because of the high humidity, light winds, and warm sea surface temperatures present in these locations. Cyclones and hurricanes can only form when the ocean water is around 80F (26.5C) or higher, and these conditions are only common during certain months of the year.

Hurricane or cyclone seasons in the Atlantic, Pacific, and Indian Oceans
Hurricane or cyclone seasons in the Atlantic, Pacific, and Indian Oceans

Weather conditions in the central Atlantic and Pacific Oceans are most favorable to hurricane formation during the summer and early fall months, which is why the period of June to October is often referred to as "Hurricane Season" in the northern hemisphere. The opposite situation occurs in the southern hemisphere since the tropical waters off Australia are warmest during the winter and spring months of January to March.

The cluster of thunderstorms that may eventually grow into a hurricane is formed during the first stage of cyclone development known as a tropical disturbance. A tropical disturbance is created when surface winds converge to create instabilities in the atmosphere that trigger the formation of storms. This situation is common near the equator where the easterly trade winds converge to create thunderstorms in a region called the Inter-Tropical Convergence Zone (ITCZ). Most Atlantic hurricanes form from another kind of tropical disturbance called the easterly wave. This disturbance emerges in the tropical easterlies and creates "waves" in the trade winds that travel toward the west. This wave causes the winds to converge together and encourages the formation of thunderstorms on the east side of the wave.

Easterly wave creating a convergence of trade winds in the Caribbean Sea
Easterly wave creating a convergence of trade winds in the Caribbean Sea

A tropical disturbance officially enters the second stage of development called a tropical depression once the sustained wind speed within the storm reaches 23 mph (37 km/h). The tropical depression gets its name from the falling surface pressures measured in the region surrounding the storm. The pressure drop occurs as water vapor within the storm condenses into water droplets and releases latent heat into the atmosphere. Latent heat is defined as the energy released when a substance changes phases, such as gas to liquid in this case.

The addition of heat causes atmospheric gases to expand, so the air inside the depression becomes less dense and rises tens of thousands of feet above the ocean surface. As its altitude increases, the air cools causing more water vapor to condense and release its heat, which draws in yet more air to rise upward. This process repeats itself again and again causing the temperature at the center of the storm to continually increase and driving the surface pressure even lower. As the pressure drops, more low-altitude air that is rich in water vapor is pulled inward to release yet more heat into the center of the storm.

This process becomes a chain reaction that pulls hot, humid air from the surface of the ocean up to high altitude where the air becomes cold and water vapor condenses into thick clouds. This growing air mass becomes increasingly dense causing the atmospheric pressure to grow. The increasing pressure pushes the growing mass of clouds outward away from the center to create the spiraling bands of clouds that hurricanes are known for. As the air mass spirals outward, its pressure decreases and the dense air plunges back towards the ocean surface where it started. It now picks up vapor again from the warm waters below and is sucked back into the center of the depression to begin its journey anew. As the cycle continues, the surface pressure at the center drops lower and lower causing the circulation of the air to strengthen and the winds to grow increasingly stronger.

Cycle of circulating air releasing heat and condensation to create a tropical cyclone
Cycle of circulating air releasing heat and condensation to create a tropical cyclone

Once the sustained wind speed increases to 39 mph (63 km/h), the tropical depression enters the third stage of development called a tropical storm. Until now, the storm has not had any formal designation. Tropical depressions are usually numbered in a sequential order as they appear, but it is not until they become tropical storms that they receive an official name. The tradition of naming storms dates back to the arrival of Europeans in the Americas, but the current method of systematically naming tropical storms was not formalized until 1953. Female names were used exclusively until the late 1970s when the use of both male and female in an alternating format was adopted.

It is also during the tropical storm phase that the storm begins to take on its characteristic appearance as a rotating spiral with bands of clouds circulating around the point of lowest surface pressure at its center. The tropical storms that form in the northern hemisphere always rotate in a counter-clockwise direction due to the Coriolis force induced by Earth's rotation. Tropical storms of the southern hemisphere, however, rotate clockwise as a result of the opposing Coriolis effect.

August 1995 satellite photo of tropical cyclones in various stages of development
August 1995 satellite photo of tropical cyclones in various stages of development

As wind speed continues to increase to 74 mph (119 km/h), the tropical storm is officially classified as a tropical cyclone or hurricane. The storm now develops an eye at the center of the circulating spirals where there is very little wind, few clouds, and extremely low atmospheric pressure. The eye may extend from 5 to 40 miles (8 to 65 km) in diameter. Surrounding the eye is an area called the eyewall that circulates around the eye and contains the highest clouds. The eyewall is a region anywhere from 10 to 50 miles (16 to 80 km) across containing the strongest winds, heaviest precipitation, and most intense thunderstorms of any part of the hurricane. The most destructive region of the cyclone is the portion of the eyewall rotating in the direction of the storm's forward motion since the force of the circulating winds is combined with the motion of the storm. Radiating outward from the center are the spiraling bands of clouds that produce heavy rain and wind but are still considerably weaker than the eyewall.

Fully formed tropical cyclone with an eye, eyewall, and circulating rain bands
Fully formed tropical cyclone with an eye, eyewall, and circulating rain bands

A typical mature hurricane is about 300 miles (480 km) across, but they can be considerably larger. The largest tropical cyclone ever recorded was Typhoon Tip that developed in the northwest Pacific and struck Japan in 1979. With a diameter 1,350 miles (2,170 km) across, Tip was dubbed a "super typhoon" and would have covered an area about half the size of the continental United States. Tip's maximum sustained wind speed reached 190 mph (305 km/h) which ties with Typhoon Keith, Hurricane Camille, and Hurricane Allen as the strongest cyclone winds ever measured.

Hurricanes and cyclones typically move westward at a speed of about 10 mph (16 km/h) during the early stages of formation. The storms are steered in this direction by the trade winds that occur near the equator and blow towards the west. The Coriolis effect that determines the direction of the cyclone's circulation also influences the direction the storm takes. As a storm moves further from the equator, its course is turned towards the poles and the cyclone curves towards the north in the northern hemisphere or towards the south below the equator.

The storm may be deflected, however, due to the influence of high and low pressure systems within the atmosphere. These pressure systems may keep the storm on a westerly path or may even be powerful enough to move the cyclone towards the east. For example, some Atlantic hurricanes continue on a straight path into Mexico or Central America instead of turning north towards the US. Others are moved northeast where they skirt the US East Coast but never make landfall. In 2004, Hurricane Ivan and its remnants followed a particularly unusual path that took it through the Gulf of Mexico, north into the US, east back into the Atlantic, south towards the coast of Florida, west across the state and back into the Gulf, and making a landfall in Texas as a tropical storm before finally dying out.

The destructive power of hurricanes and cyclones comes from four primary factors. The strong winds generated inside the storm have the power to severely damage structures in their own right. The winds also pick up loose objects converting the debris into powerful projectiles that can do considerable damage. The second destructive factor is the heavy rains that the thunderstorms inside a tropical cyclone bring. Rains most often cause flooding or landslides that wash out communities and block roads. The rotating motion of cyclones can also lead to the formation of tornados with strong damaging winds of their own.

Storm surge created by a tropical cyclone
Storm surge created by a tropical cyclone

The final and usually most destructive force wrought by a hurricane is called the storm surge. The storm surge is a rise in the level of the ocean that can reach as much as 35 ft (10.5 m) above normal. The surge is created by strong winds that push up a wall of water ahead of the cyclone and washes inland as the storm moves ashore. The storm surge is created on the right-hand side of the cyclone (left-hand side in the southern hemisphere) ahead of that portion of the storm rotating towards the coast. The destructive power of the surge comes from its ability to wash over shorelines, flood areas far inland, and then rush back out to sea as the waters subside.

The strength of these destructive forces increases as the hurricane becomes more powerful. Two researchers named Herbert Saffir and Robert Simpson developed a system called the Saffir-Simpson Scale to predict the level of flooding and damage expected during a hurricane. The scale consists of five categories based primarily on the hurricane's maximum sustained winds but also taking into account its central pressure and storm surge. The weakest hurricanes are Category 1 or 2 that do relatively little damage. Categories 3, 4, and 5 are much more powerful, can do intense damage to property, and pose great risk to life.

Categories of hurricanes and their relative destructive power
Categories of hurricanes and their relative destructive power

The factors that cause a tropical cyclone to decay are directly related to the process by which they form. Even if conditions at the ocean surface are favorable to hurricane formation, strong wind shears or a temperature inversion at high altitudes can rob the storm of the convection air currents required to form a tropical depression. The depression could also be absorbed by another, stronger region of low pressure that disrupts the cyclone formation process.

If the storm does grow into a cyclone, several factors can cause it to lose strength and dissipate. Moving over land deprives the storm of the warm water on which it depends, and most hurricanes decay to a low-pressure system within a couple days of landfall. Similarly, the storm may remain in the same waters for too long and consume all the available heat or move into colder waters that cannot supply the heat needed to keep the cyclone active.

As a hurricane decays, it again reverts through phases similar to those that first formed the tropical cyclone. Hurricane Katrina, for example, was first named on 24 August 2005 when it reached tropical storm strength off the Atlantic coast of Florida. Tropical Storm Katrina grew to hurricane strength the following day just before moving ashore through southern Florida as a Category 1 hurricane. Katrina soon weakened to a tropical storm again after making landfall, but regained its strength once it reached the warm waters of the Gulf of Mexico on 26 August. Hurricane Katrina spent three days over the Gulf and rapidly increased in strength as it turned north towards the Gulf Coast. Katrina strengthened from Category 2 to Category 3 on the 27th, Category 4 on the 28th, and a massive Category 5 with sustained winds of 175 mph (280 km/h) later that day.

Hurricane Katrina and an approximation of its path
Hurricane Katrina and an approximation of its path

Katrina weakened slightly to a Category 4 before making landfall on 29 August in Louisiana and weakened again to a Category 3 by the time it made its third landfall in Mississippi. The distinction made little difference, however, since Katrina was still the fourth strongest hurricane ever to hit the United States and devastated large areas of Louisiana, Mississippi, and Alabama. As Katrina raced north, it finally lost hurricane strength and was downgraded to a tropical storm about 100 miles (160 km) inland. The storm was again downgraded to a tropical depression the next day while over Tennessee and continued north towards the Great Lakes. The remnants of Katrina moved further north into eastern Canada before finally dying out four days after striking the Gulf Coast.

Although Atlantic hurricanes tend to be the best known in the US, the most active cyclone areas are in the Pacific and Indian Oceans. The greatest number of tropical storms forms in the northwestern Pacific and threaten East Asia. On average, about 27 tropical storms appear in this region of the world each year and about 17 of these become typhoons. The second most active region is the eastern Pacific where storms form off the coast of Mexico and move northwest towards Hawaii before eventually dissipating in the cold waters south of Alaska. This part of the world experiences an average of 16 tropical storms and nine hurricanes per year. The southwest Indian Ocean is also a very active area where a yearly average of 13 tropical storms and seven cyclones pose a menace to Madagascar and the east coast of Africa. The fourth most active region is the northern Atlantic where about eleven tropical storms and six hurricanes form in a typical year.

Regions of tropical cyclone activity around the world
Regions of tropical cyclone activity around the world

Other regions known for tropical cyclone activity include the southwestern Pacific where storms threaten Oceania and eastern Australia as well as the southeastern Indian Ocean that generates cyclones striking western Australia. The northern Indian Ocean generally produces few storms but these have been some of the most destructive in history due to the large death tolls wrought in nations like India, Bangladesh, Thailand, and Sri Lanka. The southern Atlantic Ocean is also an area of tropical activity but tends to produce very few storms.

Cyclonic storms are not unique to Earth either. The gas giants of the outer solar system also exhibit similar storm activity. These storms feed off internal heating and convective currents through the dense gases that make up the planet's atmosphere. Although giant storms have been seen on Saturn and Neptune, they pale in comparison to the massive storms on Jupiter. The most famous by far is the Great Red Spot that circles the planet's southern hemisphere. The storm was discovered by British astronomer Robert Hooke in 1664 and has been raging for over 340 years.

Jupiter's Great Red Spot and numerous smaller storms
Jupiter's Great Red Spot and numerous smaller storms

The Great Red Spot is essentially the opposite of a hurricane on Earth. While Earth's cyclones are low-pressure systems, the Great Red Spot is a region of high pressure called an anti-cyclonic storm. Like Earth's cyclones, the Coriolis effect also drives the circulation of this storm, but its anti-cyclonic nature causes the Great Red Spot to rotate counterclockwise in the opposite direction a cyclone would spin. The Great Red Spot has persisted for so long since Jupiter has no land to disrupt the convective currents that sustain the storm, and it shows no signs of weakening.
- answer by Molly Swanson
- answer by Joe Yoon, 11 September 2005

A Note About Willy-Willies:

We have received a few messages from Australians who point out that the use of the term "willy-willy" as another name for a cyclone is only a myth. They say the term instead refers to small whirlwinds or dust devils that bear little resemblance to an enormous hurricane. Nevertheless, countless textbooks, encyclopedias, weather sources, and official websites from prestigious institutions like NASA and NOAA continue to include "willy-willy" as a name used for cyclones in Australia. Why is this?

Upon conducting research with the assistance of a meteorology professor in Australia, we've learned that the truth lies somewhere in between. Meteorological records from the 1800s and early 1900s make it clear that "willy-willy" was indeed used extensively as a synonym for cyclone or hurricane. A good example can be found in the report Australian Hurricanes and Related Storms by H. A. Hunt published by the Commonwealth Meteorologist in 1925. The report includes a map with a caption reading "Track of a Typical 'Willy-Willy'" that clearly illustrates the path of a tropical cyclone. Still more examples can be found in a document from the Australian weather bureau titled Tropical Cyclonic Storms on the North-West Coast of Western Australia from the 1910s. Quotes from this report include:

"When the barometer shows signs of falling with an easterly wind, conditions are conducive to the development or approach of willy-willies, and precautions should be taken accordingly."

"As a precautionary measure during the willy-willy season, hourly readings of the barometer and the direction of the wind should be made by mariners and entered in the ships log."

"12-18 Jan 1899: S.S. Tangier encountered willy-willy on 12th off N.W. coast"

Nevertheless, the meaning of the term willy-willy is not always clear in these documents. Though there are many cases where willy-willy is used interchangeably with the words hurricane and cyclone, there are others where it appears to be meant as a whirlwind or tornado spawned by a hurricane, similar to the modern definition of the term. It would seem that the meaning of the phrase willy-willy gradually evolved during the twentieth century and it was no longer used in conjunction with cyclones after the 1930s. Today, "willy-willy" is used exclusively for small whirlwinds, dust clouds, or dirt devils while large tropical storms like hurricanes are known as cyclones.
- answer by Greg Alexander, 14 September 2005


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