OUR PLANET
Science of Hurricanes
BIOTRENDS
June/July
2006 -Volume 2
Issue 3
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OUR PLANET Science of Hurricanes |
BIOTRENDS June/July 2006 -Volume 2 Issue 3 |
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Melissa Victor and Michael Sugarman
What is a hurricane? A hurricane is a tropical cyclone, defined as a severe meteorological (weather) storm of swirling wind and rain that originates in certain oceanic locations, including the equatorial Atlantic region, Caribbean Sea, and the Eastern Pacific. This article will use the two terms—hurricane and tropical cyclone—interchangeably.
A hurricane's basic anatomy. A tropical cyclone is comprised of an immense rotating cloud system, wind and intense thunderstorms. Structurally, a tropical cyclone can be broken down into different meteorological components including the eye, eyewall, rainbands and a dense area of clouds with intense thunderstorm activity called the central dense overcast ( CDO ).
• The Central Dense Overcast (CDO). The CDO is a dense region of clouds and strong thunderstoms near the center of the storm. Contained within the CDO are the eye and the eyewall.
• The eye and eyewall. A strong tropical cyclone harbors an area of sinking air at the center of circulation - this is the eye. Weather in the eye is typically calm and cloud-free. 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 its diameter ranges from 2 to 200 miles. In weaker cyclones, the CDO covers the center; thus, no eye is visible. The cloud band immediately surrounding the eye is called the eyewall. The eyewall not only contains the fastest wind speeds, but is also where precipitation (rainfall) is greatest. The most severe wind damage can typically be found where a hurricane's eyewall has passed over land. The earth's orbital revolution causes the system to spin around the eye (this is known as the Coriolis effect), giving it the distinctive cyclonic characteristic.
• Rainbands. Thin clouds further out from the eye wall are termed spiral or rainbands. These bands comprise the largest area of a hurricane, and generate a large amount of precipitation. In many situations, spiral bands yield one foot of rain every day and are usually the part of the hurricane that reaches land.
This figure shows several prominent features of a tropical cyclone, including the eye, eyewall and rain bands. This figure was taken from Wikipedia.org.
What is a hurricane's driving force (source of energy)? Hurricanes are low-pressure storms fueled by heat that is released when moist air rises and condenses. Condensation, which in this case is the transformation of water from its gas form (water vapor or steam) to its liquid form, is the driving force for a hurricane. Picture a tea kettle full of water. To make water vapor, you must add heat energy to force water molecules apart and allow them to change into the gaseous state. When water molecules join back together (condense) to form liquid, the condensation releases the heat energy. This energy is converted to the motional energy of the hurricane. For a tropical cyclone to stay alive, it must remain over warm water, which supplies the large amount of atmospheric moisture (water vapor) needed. The warmer the water, the greater the rate of evaporation and thus the greater amount of energy available to drive the hurricane. The evaporation of this moisture is accelerated by the high winds and reduced atmospheric pressure in the storm. Because a hurricane's strength depends on evaporation of water, a tropical cyclone loses intensity as it travels land, and before its strength rapidly diminishes.
What drives the cyclonic rotation? Tropical cyclones rotate around a region of extreme low atmospheric pressure close to the earth's surface. Atmospheric winds occur when air flows from areas of high (denser or more air) pressure to low (less air) pressure and as they flow they are deflected in a particular direction determined by their position relative to the equator and dictated by the Coriolis Force. The Coriolis Force is a result of the rotation of the Earth on its axis. The winds push around the low–rotating right or counterclockwise in the Northern hemisphere and left or clockwise in the Southern hemisphere—creating a vortex as the area of low pressure continues to draw the air into them.
Storm surge: in the wake of a hurricane. After the eye of a hurricane passes, there is a potentially dangerous and costly effect known as the storm surge. The circular motion of the hurricane winds push "walls" of water outwards from the eye. In the Northern hemisphere, these waves are pushed in the direction of the coast on the right (relative to the forward motion) side of hurricane. This translates into the east side of hurricanes headed toward the US Gulf and Atlantic coastlines. As these waves near shore, they collect more and more water, growing above 18 feet in some instances. Storm surges can destroy beaches and may even reach towns inland. If the tide rises past a certain level, waves may actually break on houses or buildings. In some cases, the storm surge from the ocean, in combination with the immense volume of rain from the hurricane, can yield a large flood capable of drowning an entire city or town.
How are tropical cyclones classified? Tropical cyclones are categorized within three main groups based on their intensity. From weakest to strongest these categories are: tropical depressions, tropical storms and hurricanes or typhoons (depending on where in the world it forms). A tropical depression is an organized low-pressure system of swirling clouds and thunderstorms with a defined surface circulation and maximum sustained winds of no greater than 38 mph. These storms have no eye, and typically lack the spiral shape of more powerful storms. As winds increase the cyclone is classified as a tropical storm. Tropical storms are highly organized systems of thunderstorms with a defined surface circulation. They have maximum sustained winds between 39 and 73 mph. At this point, the cyclonic shape begins to develop, though most lack a distinct eye. Government weather services assign names to systems that reach this intensity (how storms are named is covered later in this article). The third category of tropical cyclones are hurricanes or typhoons, depending on the location of its formation. These storms are the most intense of tropical cyclones. They are low-pressure storm systems with sustained winds of 74 mph or greater. The eye and eyewall have completely formed and rainbands or spiral bands may organize into an outer ring of thunderstorms.
Assigning hurricane intensity based on wind speed: The Saffir-Simpson hurricane scale. Based on a hurricane's sustained maximum wind intensity, a number 1 thru 5 is assigned, with storms given a 1 having the lowest winds, and those given a 5 having the highest. The Saffir-Simpson Scale is only used for hurricanes that form in the Atlantic and Northern Pacific Oceans, east of the International Date Line. Other regions label their tropical cyclones as "cyclones" and "typhoons", and use their own classification schemes.
Category 1: A category one hurricane has winds of 74 to 95 mph with a storm surge generally 4-5 ft above normal. Although most building structures do not sustain significant damage, there may be some coastal flooding and minor damage to ships in port.
Category 2: A category two hurricane has wind speeds between 96 and 110 mph and a storm surge 6-8 feet above normal. Buildings may sustain roof, door and window damage, and a few trees may be blown down. These storms may also cause damage to mobile homes, poorly constructed signs and piers.
Category 3: Category three hurricanes have winds between 111 and 130 mph with a storm surge 9-12 ft above normal. These storms cause some serious structural damage to small residences and utility buildings, with some mobile homes easily being destroyed. There would be extensive shrubbery and tree damage with stripped foliage and large trees blown over. In addition, terrain under 5 ft above sea level may be flooded up to 8 miles inland.
Category 4: Category four hurricanes have winds of 131 to 155 mph with storm surges 13-18 ft above normal sea level. These effects can cause extensive wall failures with some roofs blowing completely off some. Shrubs, trees, and most signs are blown down. There is complete destruction of mobile homes, and extensive damage to doors and windows. Terrain lower than 10 ft above sea level may be flooded, requiring massive evacuations of residential areas as far inland as 6 miles.
Category 5: Category five hurricanes have winds greater than 155 mph, with a storm surge generally greater than 18 ft above normal. This can result in complete roof failure on many residences and industrial buildings, and small utility buildings blowing over or away entirely. There will also be major damage to the lower floors of structures that are 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 of the shoreline is recommended when a storm of this magnitude is predicted.
It is important to note that an increase in a hurricane's ranking may not necessarily reflect its capacity to cause damage relative to a weaker one. Many other factors, such as population density and effectiveness of hurricane warning systems and evacuations play a role in the overall effect a storm has. For instance, a Category 2 hurricane striking a major urban area will likely do more damage and cause more casualties than a Category 5 striking a rural region. In fact, tropical systems of less than hurricane strength can produce significant damage and casualties, especially from flooding and landslides.
How hurricanes are named. When naming hurricanes began, only female names were used. Often times, the names of prominent women were used; such was the case for hurricane Bess in 1949, named after the First Lady, Bess Truman. In 1979, a woman's organization began protesting that men's names should also be used. Since then, at the beginning of each season, an alphabetical list of names, both male and female, is drawn up. They start with names that begin with "A" and slowly go down the list. The lists are decided upon, depending on the regions, either by committees of the World Meteorological Organization, or by national weather offices involved in the forecasting of the storms. Each year, the names of particularly destructive storms (if there were any) are "retired" and new names are chosen to take their place. The "gender" of the season's first storm also alternates year to year. The first storm of an odd-numbered year gets a feminine name, while the first storm of an even-numbered year gets a masculine name. Six lists of names are prepared in advance, and each list is used once every six years. The five letters, Q, U, X, Y and Z, are omitted in the Atlantic , while only Q and U are omitted in the Eastern Pacific. If there are more than 21 named storms in an Atlantic season or 24 in an Eastern Pacific season, the rest are named as letters from the Greek alphabet. Thus, the twenty-second storm of the season would be called Alpha, while the twenty-third would be Beta and so on. This was necessary during the 2005 season, when the list using the traditional alphabet was exhausted.
Why are storms named? Storms reaching tropical storm level or greater are named to assist in recording insurance claims, to assist in warning people of the coming storm and to emphasize that these are potentially destructive storms that should not be ignored.
When is hurricane season? The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from June 1st to November 30th , with the season peaking between mid-August and late October. However, deadly hurricanes can occur at anytime during the season. The zones where hurricanes form and the tracks they take are generally related to the time of year. Consequently, different areas of the country have varying levels of risk during different months. Patterns can vary considerably from year to year, although researchers have not found any relation between storm activity early in the hurricane season and activity in the rest of the period. We do know that over many years hurricanes have cycles of greater and lesser activity.
Hurricanes and the future. Recent scientific findings state that hurricanes are likely to intensify as global warming becomes increasingly worse (refer to the article on global warming in this issue). Specifically, when global warming affects the oceans' temperature, it will lead to more moisture in the air. The two factors of water temperature and moisture in the air can combine to create a massive hurricane. The increase in water temperatures will provide fuel for more intense hurricanes. It is conceivable that hurricane seasons in the future may last year round.
National Oceanic & Atmospheric Administration.
HowStuffWorks.
Centers for Disease Control and Prevention.
Wikipedia: The free online encyclopedia.
http://en.wikipedia.org/wiki/Tropical_cyclone
The Why Files. Science behind the news.
Tropical Cyclone. Atlantic Basin hurricanes and tropical storms.
Hurricanes: Answers and more from Answers.com
http://www.answers.com/topic/hurricane
