| With all the talk about "Andrew" last week, I found this interesting set of
press releases from the National Weather Service. All you ever wanted to know
about hurricanes:
** abus34 ksat 231625 ***
Public Information Statement
National Weather Service San Antonio TX
1125 am cdt sat May 23 1992
.....The Greatest Storm on Earth.....
From a vantage point in space they seem quite small, flat spirals drifting on
the sea, gentle eddies in the endless flowing of the planet's atmosphere. But
where their drift takes them across shipping lanes and islands and the coasts
of continents, their passage is commemorated by property destroyed, prospects
diminished, and death.
They are tropical children, the offspring of ocean and atmosphere, powered by
heat from the sea, driven by the easterly trades and temperate westerlies, the
high planetary winds, and their own fierce energy. In their cloudy arms and
around their tranquil core, winds blow with lethal velocity, the ocean develops
an inundating surge, and, as they move toward land, tornadoes now and then
flutter down from the advancing wall of thunderclouds.
Compared to the great cyclonic storm systems of the temperate zone they are of
moderate size, and their worst winds do not approach tornado velocities.
Still, their broad spiral base may dominate weather over thousands of square
miles, and from the earth's surface into the lower stratosphere. Their winds
may reach 200 miles per hour, and their lifespan is measured in days or weeks,
not minutes or hours. No other atmospheric disturbance combines duration,
size, and violence more destructively.
As they occur in different oceans and hemispheres, they bear names given
locally: "Baguio" in the Philippines, "Cyclone" in the Indian Ocean, "Typhoon"
in the Pacific, "Chubasco" in Baja California. In our hemisphere, the name is
"Hurricane" - The Greatest Storm on Earth.
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Hurricane - from the spanish "huracan", probably derived from the
Mayan storm god , "Hunraken"; the quiche god of thunder
and lightning, "Hurakan"; and numerous other Caribbean
indian terms for evil spirits, big wind, and the like.
(media please note: This is the first in a series of approximately 12 releases
on hurricanes. You may wish to retain these for future reference to use
individually or in a series. Between 1 and 4 will be released daily in
anticipation of the 1992 hurricane season which begins June 1st and runs
through November.)
** abus34 ksat 231841 ***
Public Information Statement
National Weather Service San Antonio TX
135 pm cdt sat May 23 1992
.....The Greatest Storm on Earth.....
There is nothing like them in the atmosphere. Even seen by sensors on
spacecraft thousands of miles above the earth, the uniqueness of these
powerful, tightly coiled storms is clear. They are not the largest storm
systems in our atmosphere, or the most violent; but they combine those
qualities as no other phenomenon does, as if they were designed to be engines
of death and destruction.
In our hemisphere, they are called hurricanes, a term which echoes colonial
spanish and caribbean indian words for evil spirits and big winds. The storms
are products of the tropical ocean and atmosphere, powered by heat from the
sea, steered by the easterly trades and temperate westerlies, and their own
fierce energy. Around their tranquil core, winds blow with lethal velocity,
the ocean develops an inundating surge, and, as they move ashore, tornadoes may
descend from the advancing bands of thunderclouds.
Hurricanes have a single benefit - they are a major source of rain for those
continental corners which fall beneath their tracks. Perhaps there are other
hidden benefits as well. But the main consequence of the hurricane is tragedy.
In asia, the price in life paid the hurricane has had biblical proportions.
Cyclone storm tides along the coast of what is now Bangladesh have killed
hundreds of thousands of persons in the past two decades.
Our hemisphere has not had such spectacular losses, but the toll has still been
tragically high. In August 1893, a great storm wave drowned more than a
thousand people in Charleston, South Carolina. In October of that same year,
nearly two thousand more perished on the gulf coast of Louisiana. The
galveston storm of 1900 took more than six thousand lives. More than 1,800
perished along the south shore of Florida's Lake Okeechobee in 1928 when
hurricane-driven waters broached an earthen levee. Cuba lost more than two
thousand to a storm in 1932. four hundred died in Florida in an intense
hurricane in September 1935 the 'labor day' hurricane that shares with 1969's
Camille the distinction of being the most severe of record. Gilbert in 1988
accounted for several hundred deaths in Monterrey, Mexico, due to flash
flooding.
Floods from 1974's hurricane fifi caused one of the western hemisphere's worst
natural disasters in history, with an estimated five thousand persons dead in
honduras, and thousands more in Nicaragua, El Salvador, Guatemala, and Belize.
In the United States, the hurricane death toll has been greatly diminished by
timely warnings of approaching storms. But damage to fixed property continues
to mount. Camille, in 1969, caused some $1.42 billion in property damage.
Floods from Agnes in 1972 cost an estimated $2.1 billion. Damage estimates
exceeded $3 billion along the texas coast from 1983's Alicia. Hurricane
Gilbert in 1988 added to the property damage. The final total on 1989's
hurricane Hugo is still being tallied - by far the most destructive storm,
dollar-wise, in our country's history.
That is why such first-order technical accomplishments as early detection and
timely warning of approaching hurricanes are not enough for some scientists.
For them, there is the persistent goal of somehow disarming The Greatest Storm
on Earth.
In the United States, hurricane warning and research are focused in NOAA, the
National Oceanic and Atmospheric Administration of the U. S. Department of
Commerce. And there, as the planet orbits toward the summer solstice,
scientists, technicians, pilots, and others brace for another season of great
storms.
** abus34 ksat 231859 ***
Public Information Statement
National Weather Service San Antonio TX
200 pm cdt sat May 23 1992
.....The Greatest Storm on Earth.....
It is the northern summer. The illusion of a moving sun caused by our planet's
year-long orbit brings that star's direct rays northward to the equator, then
toward the Tropic of Cancer. Behind this illusory solar track the sea and air
grow warmer, and the polar air of winter beats its seasonal retreat.
This northward shift of the sun brings the season of tropical cyclones to the
northern hemisphere. Along our coasts and those of Asia it is time to look
seaward.
Over the western Pacific, the tropical cyclone season is never quite over, but
varies greatly in intensity. Every year, conditions east of the Philippines
send a score of violent storms howling toward asia, but it is worst from June
through October.
Southwest of Mexico, eastern Pacific hurricanes develop during spring and
summer. Most of these will die at sea as they move over colder ocean waters or
perish over the desert or strike the lower California coast as squalls. But
there are destructive exceptions when storms curve back toward Mexico.
Along our Atlantic and Gulf coasts, the nominal hurricane season lasts from
June through November. Early in this season, the western Caribbean and Gulf of
Mexico are the principal areas of origin. In July and August, this center
begins an eastward shift; by early September a few storms are being born as far
east as the Cape Verde islands off Africa's west coast. Again after
mid-september, most storms begin in the western Caribbean and Gulf of Mexico.
In an average year, more than one hundred disturbances with hurricane potential
are observed in the Atlantic, Gulf, and Caribbean; but fewer than ten of these
reach the tropical storm stage, and only about six mature into hurricanes. On
average, three of these hurricanes strike the United States, where they may
kill between 50 and 100 people somewhere between Texas and Maine and cause
hundreds of millions of dollars property damage. In a worse-than-average year,
the storms may account for several hundred deaths, and property damage totaling
billions of dollars.
For NOAA, the National Oceanic and Atmospheric Administration, the hurricane
season means another hazard from the atmosphere, at a time when tornadoes,
floods, and severe storms are playing havoc elsewhere on the continent.
Meteorologists with NOAA's National Weather Service monitor the massive flow of
data that might contain the early indications of a developing storm somewhere
over the warm sea - cloud images from satellites, meteorological data from
hundreds of surface stations and ships, balloon probes of the atmosphere and
information from hurricane hunting aircraft.
In NOAA's environmental research laboratories, scientists wait for nature to
furnish additional specimens of the great storms to probe, analyze - eventually
to modify, if they can be modified with beneficial results.
** abus34 ksat 241613 ***
Public Information Statement
National Weather Service San Antonio TX
1115 am cdt sun May 24 1992
.....The Greatest Storm on Earth.....
The national hurricane center flexes the warning service when a tropical
disturbance is detected and confirmed by aerial reconnaissance. These initial
messages are in the form of bulletins, which alert the system that a suspicious
area is under surveillance. bulletins are also picked up by news media, where
the word goes out that NOAA, the National Oceanic and Atmospheric
Administration, is watching a disturbance for future development over the
Atlantic, Gulf, or Caribbean. If the disturbance intensifies into a tropical
cyclone, a time-honored tradition is applied - it is given a name.
The early caribbean practice of naming hurricanes for the saint on whose day
they occurred was never used in this country, but our substitute was a
cumbersome latitude-longitude identification. The advent of high-speed
communications, together with the confusion which arose when more than one
tropical cyclone was in progress in the same area, forced a change. For a
time, tropical cyclones were designated by letters of the alphabet (e.g.,
a-1943), and by the World War II phonetic alphabet (Able, Baker, Charlie); and
it has been suggested that the storms be named from the international civil
aviation organization's phonetic system (Alpha, Bravo, Cocoa), the letters of
the greek alphabet (Alpha, Beta, Gamma), the names of animals (Antelope, Bear,
Coyote), and descriptive adjectives (annoying, blustery, churning). It has
also been suggested that the storms carry the names of well-known
personalities, places, and things, and the names of mythological figures.
It appears that the feminization of tropical cyclones began during WORLD War
II, when weathermen plotting the movement of storms across vast theaters of
operations identified them alphabetically, using the names of girls. George R.
Stewart's novel, "Storm" (Random House, 1941), may have been the first
published account of this practice. Whatever the origin, the use of ladies
names for tropical cyclones had been persistent. Even though some alternative
recommendations have had merit, the practice continued, and had been official
weather service policy from 1953 until 1979.
In 1960, a semi-permanent list of four sets of names in alphabetical order was
introduced. In 1971, the list was expanded to ten sets of names. A separate
set of names was used each year, beginning with the first name in each set.
The letters q, u, x, y and z were not included because of the scarcity of names
beginning with those letters. after ten years, when the ten sets of names had
been used, the sets were to be used over again in the same manner. The names
of noteworthy hurricanes were permanently retired from the list and replaced by
another. Hurricane Carla, which struck the Texas coast in September of 1961,
was one such hurricane to have her name retired.
Typhoons and Pacific hurricanes had also been feminized. In the eastern north
Pacific, the alphabetical listing of names was prepared in sets of four, and
designations were cycled from year to year. In the central and western north
pacific, the practice differed because of the high incidence of tropical
cyclones. The four sets prepared for typhoons originating there were not
cycled annually. Instead, all names were used consecutively, regardless of the
year. For example, if the 1991 typhoon season had ended with typhoon Virginia,
then 1992 would have begun with typhoon Winifred, both from the same set of
names.
In 1979, the national weather service practice of using female names only for
the naming of hurricanes was modified to include men's names to identify every
other tropical cyclone. The 1991 hurricane season included Bob, Claudette and
Danny, among others.
The 1992 hurricane season will begin with Andrew, and will be followed by
Bonnie...Charley...Danielle...Earl...Frances...Georges...Hermine...
Ivan...Jeanne...Karl...Lisa...Mitch...Nicole...Otto...Paula...Richard...
Shary...Tomas...Virginie and Walter.
** abus34 ksat 241643 ***
Public Information Statement
National Weather Service San Antonio TX
1145 am cdt sun May 24 1992
.....The Greatest Storm on Earth.....
The day is past when a hurricane could develop to maturity far out to sea and
go unreported until its thrust toward land. Earth-orbiting satellites operated
by noaa, the national oceanic and atmospheric administration, keep the earth's
atmosphere under virtually continuous surveillance, night and day. Long before
a storm has evolved even to the point of ruffling the easterly wave, scientists
at NOAA's National Hurricane Center (a National Weather Service forecast
office) in Miami, Florida, have begun to watch the disturbance. In the
satellite data coming in from polar-orbiting and geostationary spacecraft, and
in reports from ships and aircraft, they look for subtle clues that mark the
development of hurricanes - cumulus clouds covered by the cirrostratus deck of
a highly organized convective system; showers that become steady rains;
dropping atmospheric pressure; intensification of the tradewinds, or a westerly
wind component there.
Then, if this hint of a disturbance blooms into a tropical storm, a
time-honored convention is applied: it receives a name. This practice began
in the 1940's, when wartime weathermen plotted the movement of storms -
informally named for wives and sweethearts - across vast theaters of
operations. It has been weather service policy since 1953. since 1979,
tropical storms have alternated between men and women names. the system uses an
alphabetical series of names - Andrew, Bonnie, Charley, Danielle - that changes
each year, and is different for the north Atlantic and the eastern north
Pacific. Typhoons also receive names in alphabetical order; but because they
occur throughout the year, these storms are named consecutively without regard
to the year - that is, a calendar year of typhoon activity could begin with
typhoon Veronica.
The first hurricane warning in the united states was flashed in 1873, when the
signal corps warned against a storm approaching the coast between Cape May, New
Jersey, and New London, Connecticut. Today, naming a storm is a signal which
brings a considerably more elaborate warning system to readiness.
Long-distance communication lines and preparedness plans are flexed.
No one had deliberately flown an airplane into a hurricane before the afternoon
of July 27, 1943, when army air corps major Joseph P. Duckworth flew a
single-engined AT-6 into a hurricane off Galveston. he made the trip twice that
day, the first time taking navigator Lt. Ralph O'hair, the second, taking
weather officer Lt. William Jones- Burdick. There have been "hurricane
hunters" ever since.
As an Atlantic hurricane drifts closer to land, it comes under surveillance by
weather reconnaissance aircraft of the U. S. Air Force, the famous "hurricane
hunters," who bump through the turbulent interiors of the storms to obtain
precise fixes on the position of the eye, and measure winds and pressure
fields. Despite the advent of satellites, the aircraft probes are the most
detailed information hurricane forecasters receive. The hurricanes are also
probed by "flying laboratories" from NOAA's research facilities center in
Miami. finally, the approaching storm comes within range of a radar network
stretching from Brownsville to Boston, and from Miami to the Lesser Antilles.
Through the lifetime of the hurricane, advisories from the National Hurricane
Center and other warning offices give the storm's position and what the
forecasters in miami expect the storm to do. As the hurricane drifts to within
a day or two of its predicted landfall, these advisories begin to carry
hurricane watch and warning messages, telling people when and where the
hurricane is expected to strike, and what its effects are likely to be. Not
until the storm has decayed over land and its cloudy elements and great cargo
of moisture have blended with other brands of weather, does the hurricane
emergency end.
This system works well. The death toll in the united states from hurricanes
has dropped steadily as noaa's hurricane tracking and warning apparatus has
matured. Although the accuracy of hurricane forecasts has improved over the
years, any significant improvements ust come from quantum jumps in scientific
understanding.
The forecasters also know that science will never provide a full solution to
the problems of hurricane safety. The rapid development of america's coastal
areas has placed millions of people with little or no hurricane experience in
the path of these lethal storms. For this vulnerable coastal population, the
answer must be community preparedness and public education in the hope that
education and planning before the fact will save lives and lessen the impact of
the hurricane.
** abus34 ksat 241723 ***
Public Information Statement
National Weather Service San Antonio TX
1220 pm cdt sun May 24 1992
.....The Greatest Storm on Earth.....
Once generated, a hurricane tends to survive while it is over warm water, for
it is the temperature difference between air and water that drives and sustains
the storm system. But the forces which control its movement are destructive;
they drive the storm ashore or over the colder water beyond the tropics where
it will fill and die, or be resurrected as a storm of another type. This
thrust away from the tropics is the clockwise curve which takes typhoons of the
tropical Pacific across the coastlines of Japan and northern Asia, and the
hurricanes of the tropical north Atlantic, Caribbean, and Gulf of Mexico across
the eastern United States.
Even before a hurricane forms, the embryonic storm has forward motion, driven
by the easterly flow in which it is embedded. While this easterly drift is
small - less than 20 miles per hour - inten- sification is favored; greater
movement generally inhibits intensification during the early stages. When the
hurricane matures, greater forward motion is frequently accompanied by
intensification. The intensification which often follows acceleration is
usually shortlived. at temperate latitudes, a few hurricanes reach forward
speeds of 60 miles per hour.
Forecasting the direction this steering current will take the hurricane is
complicated by several factors. The hurricane winds mask the basic current
over a large area, both horizontally and vertically. Also, the steering
mechanism is not completely understood. It is not known, for example, what
fraction of the storm's forward motion comes from its own internal energy and
what portion is applied by the basic current, which is ordinarily the dominant
force.
The tracks of hurricanes are as individual as the storms themselves. no two
tracks are precisely superimposed, and only the most general trends can be
established. A hurricane drifting westward past cuba may seem poised to
recurve north and east across Florida, only to dither, then spin off through
the Gulf of Mexico - to Yucatan or New Orleans or Brownsville. Or a hurricane
may follow a course from birth to death whose only consistency is an erratic,
aimless looping across the tropics. Although most hurricanes ultimately
recurve, there have been numerous exceptions. Hurricane Inez of September
1966, which doubled back instead of recurving and finally died in central
Mexico, is a case in point.
From generation, a hurricane is acted upon by those forces which finally
destroy it. At middle altitudes, air flows through the cyclonic vortex,
cooling the warm core and acting as a thermal brake on storm intensity.
Friction between the storm and ocean is only slightly inhibiting. It is
possible that, without frictional effects, there would not be enough low-level
inflow to keep the storm fueled with moist, warm air. Over land, frictional
effects are greater and contribute to filling and dissipating the storm -
although it is the loss of energy from the sea, not friction, which finally
kills a hurricane.
Hurricane intensity is unquestionably linked to the warmth of ocean waters in
its path. The storms do not form over water much below 80 degrees fahrenheit,
and decreases in water temperature have a direct influence on the rate at which
the hurricane decays. Off California, small hurricanes deteriorate rapidly
once they reach the cold eastern Pacific waters; larger hurricanes in the
Atlantic may travel for longer periods over cold north Atlantic water, the rate
of decay also being a function of storm size and intensity.
Over land, a hurricane decays rapidly. Without its heat source, and with the
added effects of frictional drag, the circulation is rapidly destroyed.
Hurricane rains, however, may continue even after the winds are much depleted.
It has been estimated that hurricane rainfall - with or without destructive
winds - accounts for nearly a fourth of the southeastern United States' annual
precipitation.
Many hurricanes are transformed into extratropical cyclones at higher
latitudes, or combine with existing temperate-zone distrbances. In these
cases, the storm circulation expands over a large area and becomes a major
atmospheric feature. Storms moving up the Atlantic coast of the United States
are often in the throes of this transformation as they strike into New
England. In such instances, external forces sometimes intensify the hurricanes
enough to overcome the dissipating effects of friction and reduced supply of
heat from the sea.
** abus34 ksat 251520 ***
Public Information Statement
National Weather Service San Antonio TX
1015 am cdt mon May 25 1992
.....The Greatest Storm on Earth.....
Given that the hurricane, as an engine, is inefficient and hard to start and
sustain, once set in motion, once mature, it is an awesome natural event
indeed.
The young storm stands upon the sea as a whirlwind of awful violence. its
hurricane-force winds cover thousands of square miles, and gale force winds -
winds of 33 to 55 knots - cover areas ten times larger. along the contours of
its spiral run bands of dense clouds from which torrential rains fall. These
spiral rainbands ascend in decks of cumulus and cumulonimbus clouds to the high
upper atmosphere, where condensing water vapor is swept off as ice-crystal
wisps of cirrus clouds by high-altitude winds. Lightning glows in the
rainbands, and this cloudy terrain is whipped by turbulence. in the lower few
thousand feet, air flows in toward the center of the cyclone, and is whirled
upward through ascending columns of air near the center. Above 40,000 feet,
this cyclonic pattern is replaced by an anticyclonic circulation - the
high-level pump which is the exhaust system of the hurricane engine.
At lower levels, where the hurricane is most intense, winds on the rim of the
storm follow a wide pattern, like the slower currents on the rim of a
whirlpool; like those currents, these winds accelerate as they approach the
central vortex. This inner band is the eyewall, where the storm's worst winds
are felt, and where moist air entering at the surface is chimneyed upward,
releasing heat to drive the storm. In most hurricanes, these winds exceed 90
knots - nearly twice that in extreme cases. Maximum winds run still higher in
typhoons.
Hurricane winds are produced, as all winds are, by differences in atmospheric
pressure, or density. The pressure gradient - the rate of pressure change with
distance - produced in hurricanes is the sharpest n the atmosphere, excepting
only the pressure change believed to exist across the narrow funnel of a
tornado.
Atmospheric pressure is popularly expressed as the height of a column of
mercury that can be supported by the weight of the overlying air at a given
time. In North America, barometric measurements at sea level seldom go below
29 inches of mercury (982 millibars), and in the tropics it is generally close
to 30 inches (1,016 millibars) under normal conditions. Hurricanes drop the
bottom out of those normal categories. The labor day hurricane that struck the
Florida keys in 1935 had a central pressure of only 26.35 inches (892
millibars). and the change is swift: pressure may drop an inch (33 millibars)
an hour, with a pressure gradient change of a tenth of an inch (3 millibars)
per mile. Gilbert in september 1988 had the lowest pressure ever observed in
an Atlantic storm - a corrected 888 millibars.
At the center of the storm is a unique atmospheric entity, and a persistent
metaphor for order in the midst of chaos - the "eye" of the hurricane. It is
encountered suddenly. From the heated tower of maximum winds and
thunderclouds, one bursts into the eye, where winds diminish to something less
than 15 knots. Penetrating the opposite wall, one is abruptly in the worst of
winds again.
A mature hurricane orchestrates more than a million cubic miles of atmosphere.
Over the deep ocean, waves generated by hurricane winds can reach heights of 50
feet or more. Under the storm center the ocean surface is drawn upward like
water in a giant straw, forming a mound a foot or so higher than the
surrounding ocean surface. This mound translates into coastal surges of 20
feet or more. Besides this surge, massive swells pulse out through the upper
layers of the sea - Pacific surfers often ride the oceanic memory of distant
typhoons. hurricane Eloise, which struck the Florida panhandle in September
1975, taught scientists something new about the influence of passing hurricanes
on the marine environment. Expendable bathythermographs dropped from NOAA
(National Oceanic and Atmospheric Administration) research aircraft ahead of,
in, and in the wake of the storm showed that the ocean was disturbed to depths
of hundreds of feet by a passing hurricane, and 'remembered' hurricane passage
with internal waves that persisted for weeks after the storm had gone. The
same storm also demonstrated that a passing hurricane can be felt deep in the
seafloor sediments. Sea-surface temperatures are significantly cooled by the
turbulent mixing and upwelling of cooler subsurface water caused by passing
hurricanes. Following Gilbert's odyssey across the Bay of Campeche, ssts
cooled 10 degrees fahrenheit or more.
While a hurricane lives, the transaction of energy within its circulation is
immense. The condensation heat energy released by a hurricane in one day can
be the equivalent of energy released by fusion of four hundred 20-megaton
hydrogen bombs. One day's released energy, converted to electricity, could
supply the united states' electrical needs for about six months.
** abus34 ksat 251620 ***
Public Information Statement
National Weather Service San Antonio TX
1115 am cdt mon May 25 1992
.....The Greatest Storm on Earth.....
Hurricane formation was once believed to be something like the genera tion of
thunderstorms, where surface heating causes air to rise, water vapor to
condense, and thunderclouds to grow. But, in hurricanes, there was an
additional element - the inexplicable drop in atmospheric pressure that
organized these elements and spun them into a cyclonic spiral.
This theory left much to be desired. Few hurricanes mature in the doldrums,
where this kind of activity is usually found, and there are too many seemingly
ideal convective situations there and too few hurricanes. And what of the drop
in atmospheric pressure?
Even storms as neatly constructed and carefully observed as hurricanes present
scientists with an infinitude of unknowns. There is no full understanding of
what triggers a hurricane, or how a disturbance in the easterly trades or a
disturbance along the ITCZ is transformed into a mature storm.
This is what seems to happen. Some starter mechanism - the intruding polar
trough, easterly wave, an eddy from an active ITCZ - stimulates an area of
vertical air motion. The initial disturbance creates a region into which
low-level air from the surrounding area begins to flow, accelerating the
convection already occurring inside the disturbance. As water vapor in the
ascending moist columns condenses (releasing large amounts of heat energy to
drive the wind system), the vertical circulation acquires greater organization;
and the horizontal form of the disturbance becomes the familiar cyclonic
spiral, in which (in the northern hemisphere) the movement of low- and
mid-level air is counter-clockwise. Now the storm is an embryo hurricane.
High-altitude winds help the developing hurricane's massive vertical transport
exhaust air into the upper reaches of the troposphere, the lower level of the
atmosphere where most vertical mixing and weather occur. These winds pump
ascending air out of the cyclonic system into the clockwise circulation of a
high-altitude anticyclone, carrying air well away from the disturbance before
it can sink to the surface again.
Thus, a large-scale vertical circulation is set up in which low-level air
spirals up the cyclonic core of the disturbance, and is exhausted at high
altitudes. This pumping action - and the heat released by the ascending,
water-bearing air - explains the sudden drop in atmospheric pressure at the
surface. The drop produces the hurricane's uniquely steep pressure gradient,
along whose contours winds are accelerated to hurricane speeds.
It is generally believed that the interaction of low-level and high-altitude
wind systems at scales larger than the hurricane's determines the intensity the
storm will attain. Scientists also believe planetary wind systems, displaced
northward, set up an essential large-scale flow which supports the budding
storm, and that the development of hurricanes is often preceded by high-level
warming and low-level inflow, in some balance that is not fully understood.
There is still much to be learned. For example, researchers have begun to
ponder the connection between thunderclouds over the hotplate of central Africa
and disturbances in the easterly tradewind belt, and how these are affected by
the massive natural cloud seeding of saharan dust. Recent investigation has
shown a correlation between el nino and Atlantic hurricane formation.
** abus34 ksat 251653 ***
Public Information Statement
National Weather Service San Antonio TX
1145 am cdt mon May 25 1992
.....The Greatest Storm on Earth.....
Hurricanes are the unstable, unreliable creatures of a moment in our planet's
natural history. But their brief life ashore can leave scars that never quite
heal. In the late-1970's, the hand of 1969's Camille could still be seen along
the Mississippi gulf coast. Most of a hurricane's destructive work is done by
the general rise in the height of the sea called storm surge, wind, and
flood-producing rains.
Hurricane winds can be the least destructive of these, although there are
important exceptions like 1971's celia, whose high winds did most of the
storm's destructive work. These winds are a force to be reckoned with by
coastal communities deciding how strong their structures should be. For
example, normal atmospheric pressure at sea level is about two thousand pounds
per square foot. As winds increase, pressure against objects is added at a
disproportionate rate. Pressure mounts with the square of wind velocity, so
that a tenfold increase in wind speed increases pressure one-hundredfold.
Thus, 20-knot wind increases atmospheric pressure by about two pounds per
square foot; a wind of 200 knots increases atmospheric pressure by more than
225 pounds per square foot. For some structures, this added force is enough to
cause failure. Tall structures like radio towers can be worried to destruction
by gusting hurricane-force winds. Winds also carry a barrage of debris that
can be quite dangerous.
All the wind damage does not necessarily come from the hurricane. As the storm
moves shoreward, interactions with other weather systems can produce tornadoes,
which work around the fringes of the hurricane. Although hurricane-spawned
tornadoes are not the most violent form of these whirlwinds, they have added to
the toll we pay the hurricane.
Floods from hurricane rainfall are quite destructive. A typical hurricane
brings 6 to 12 inches of rainfall to the area it crosses, and some have brought
much more. The resulting floods have caused great damage and loss of life,
especially in mountainous areas, where heavy rains mean flash floods. One of
the most widespread floods in United States history was caused by the remnants
of hurricane Agnes in 1972. Rains from the dying hurricane brought disastrous
floods to the entire Atlantic tier of states, causing 118 deaths and some $2.1
billion in property damage. Flash floods from torrential rains in dissipating
hurricane Gilbert in September 1988 killed over 200 people in Monterrey,
Mexico.
The hurricane's worst killer comes from the sea, in the form of storm surge,
which claims nine of every ten victims in a hurricane.
As the storm crosses the continental shelf and moves close to the coast, mean
water level may increase 15 feet or more. The advancing storm surge combines
with the normal astronomical tide to create the hurricane storm tide. In
addition, wind waves 10 feet high or more are superimposed on the storm tide.
This buildup of water level can cause severe flooding in coastal areas,
particularly when the storm surge coincides with normal high tides. Because
much of the United States' densely populated coastline along the Atlantic and
Gulf coasts lies less than 10 feet above mean sea level, the danger from storm
surges is great.
Wave and current action associated with the surge also causes extensive damage.
Water weighs some 1,700 pounds per cubic yard; extended pounding by frequent
waves can demolish any structures not specifically designed to withstand such
forces.
Currents set up along the coast by the gradient in storm surge heights and wind
combine with the waves to severely erode beaches and coastal highways. Many
buildings withstand hurricane winds until, their foundations undermined by
erosion, they are weakened and fail. Storm tides, waves, and currents in
confined harbors severely damage ships, marinas, and pleasure boats. In
estuarine and bayou areas, intrusions of salt water endanger the public health
- and create bizarre effects like the salt-crazed snakes fleeing Louisiana's
flooded bayous.
** abus34 ksat 261739 ***
Public Information Statement
National Weather Service San Antonio TX
1240 pm cdt tue May 26 1992
.....The Greatest Storm on Earth.....
When the bermuda high is weak and south of its normal position, and the
easterly trades are shallow (below about 15,000 feet), an elongated area of low
pressure - a trough - embedded in the temperate westerlies may push southward
into the tropics. When the southern end of the intruding trough slows its
eastward movement, or is trapped in the easterly trades, a hurricane may
develop; or the trough may separate to become another weather-maker - the
easterly wave.
Once embedded in the deep easterly current, this westward-drifting region of
low-pressure tends to organize low-level circulation into alternate areas of
converging and diverging airflow. Where air is diverging (high pressure),
weather is fine. But where convergence occurs (low pressure), the depth of the
moist layer increases, and convection produces heavy cumulus and cumulonimbus
clouds which build to heights above 30,000 feet.
Many of the waves in the easterlies have been traced back to western Africa,
where they are most evident in the middle and low levels of the atmosphere as
they cross Africa's atlantic coast. These systems may travel thousands of
miles with little change in form. But where the waves are destabilized by
intense convection or by some external force - for example, high-level winds
that promote greater organization of the circulation in the wave - they may
curl inward. The vertical circulation accelerates, and a vortex develops that
sometimes reaches hurricane intensity.
Easterly waves are present over some part of the Caribbean almost daily from
June through September, the months of highest incidence of hurricanes. The
waves are also present to a lesser extent in May, October, and November.
The tropical easterly tradewind belt of the northern hemisphere has a mirror
image in a similar belt of winds south of the equator. A highly simplified
view of the atmosphere would show these tradewind belts separated by an
equatorial region of low pressure, popularly called the doldrums - what
meteorologists call the intertropical convergence zone, or ITCZ.
The ITCZ varies considerably more than this simple view indicates. sometimes it
is so weak as to be nearly undetectable, and almost completely free of
significant weather. At other times it can be an intensely active zone a
hundred miles or more wide, with cloud tops rising to the stratosphere and
weather as violent as that in the continental squall lines which sweep our own
great plains.
The ITCZ also follows the sun. In February the ITCZ moves from a position near
the equator to its extreme limit near about twelve degrees north latitude in
August; however, its day-to-day surface position varies greatly.
This interhemispheric borderland has a flow consisting mainly of eddies
(individual currents in a moving fluid) that drift westward, and its intense
activity is thought to be associated with the movement of these eddies into the
tropics.
While the ITCZ is in the southern range of its annual migration (it is never
south of the equator) the effect of the earth's rotation is small; but as it
shifts northward, the influence of the rotating globe - the coriolis force - is
great enough to permit a circulation to develop that can evolve into the tight,
violent eddy of a tropical cyclone.
What is true for hurricanes in the north Atlantic is generally true for the
hurricanes of the eastern north Pacific, and similar storms like the cyclones
of the Indian Ocean, and the western Pacific's typhoon. the storms tend to be
born over warm water, spun from disturbances in the equatorial tradewind belts.
There is little tropical cyclone activity south of the equator, except for the
ocean area west of Australia during the southern summer.
** abus34 ksat 261935 ***
Public Information Statement
National Weather Service San Antonio TX
230 pm cdt tue May 26 1992
.....The Greatest Storm on Earth.....
Over the Atlantic, the budding summer brings other changes as well. a
semipermanent zone of high pressure - the familiar 'high' of weather maps -
returns to a surface position centered near Bermuda and the Azores. This
clockwise spiral of descending air, or anticyclone, lies between the temperate
and tropical bands of prevailing winds, and, during summer and early autumn,
dominates the north Atlantic atmosphere.
North of the high-pressure system, prevailing westerlies flow eastward in a
deep layer extending from the surface to altitudes of 40,000 feet or more.
Near the center of the Bermuda high, winds are variable. But to the south, the
surface flow of air is predominantly to the west - the easterly tradewinds. In
the northern summer, these easterlies may deepen until they reach from the
surface to the stratosphere, and cover vast areas of the tropics; or they may
break up into small vorticies (a whirlpool is a vortex) which drift westward
into the caribbean or Gulf of Mexico. It is in this tradewind current that
most Atlantic hurricanes are born.
The characteristic sinking of air within the anticyclone produces layers in the
deep current of the easterly trades. As air sinks to levels of greater
atmospheric pressure, it is heated by compression, producing at lower altitudes
what is called a temperature inversion - a condition in which air, instead of
cooling with altitude, cools only to the inversion altitude, then grows warmer,
before it begins to cool again at greater heights. Beneath the inversion
layer, air sweeps for hundreds of miles over the surface of the sea, receiving
a charge of moisture to altitudes of several thousand feet from evaporated
ocean water.
Convection, or vertical motion, in this lower, heated layer, and intermittent
intrusions of moist air, weaken the inversion, which gradually dissipates as
the low-level air continues its trajectory above the warm sea. This vertical
penetration of the inversion permits water vapor to be lifted to cooler
altitudes, where it condenses into clouds and raindrops. As water vapor
condenses, it releases heat energy into the atmosphere. This latent heat
energy is gradually transported to higher levels, changing the vertical
character to the tradewind belt. Sometimes the easterly flow is sufficiently
disturbed by such processes that rain areas become concentrated, and the
concentration intensifies into a storm.
** abus34 ksat 262113 ***
Public Information Statement
National Weather Service San Antonio TX
415 pm cdt tue May 26 1992
.....The Greatest Storm on Earth.....
From a vantage point in space they seem quite small, flat spirals drifting on
the sea, gentle eddies in the endless flowing of the planet's atmosphere. But
where their drift takes them across shipping lanes and islands and the coasts
of continents, their passage is commemorated by property destroyed, prospects
diminished, and death.
They are tropical children, the offspring of ocean and atmosphere, powered by
heat from the sea, driven by the easterly trades and temperate westerlies, the
high planetary winds, and their own fierce energy. In their cloudy arms and
around their tranquil core, winds blow with lethal velocity, the ocean develops
an inundating surge, and, as they move toward land, tornadoes now and then
flutter down from the advancing wall of thunderclouds.
Compared to the great cyclonic storm systems of the temperate zone they are of
moderate size, and their worst winds do not approach tornado velocities.
Still, their broad spiral base may dominate weather over thousands of square
miles, and from the earth's surface into the lower stratosphere. Their winds
may reach 200 miles per hour, and their lifespan is measured in days or weeks,
not minutes or hours. No other atmospheric disturbance combines duration,
size, and violence more destructively.
As they occur in different oceans and hemispheres, they bear names given
locally: "baguio" in the Philippines, "Cyclone" in the Indian Ocean, "Typhoon"
in the Pacific, "Chubasco" in Baja California. In our hemisphere, the name is
"Hurricane" - The Greatest Storm on Earth.
The 1992 list of names for the Atlantic Ocean, Caribbean Sea and Gulf of Mexico
are: Andrew...Bonnie...Charley...Danielle...Earl...Frances...
Georges...Hermine...Ivan...Jeanne...Karl...Lisa...Mitch...Nicole...
Otto...Paula...Richard...Shary...Tomas...Virginie and Walter.
The 1992 list of names for eastern Pacific tropical storms and hurricanes are:
Agatha...Blas...Celia...Darby...Estelle...Frank...
georgette...Howard...Isis...Javier...Kay...Lester...Madeline...Newton...
Orlene...Paine...Roslyn...Seymour...Tina...Virgil...Winifred...Xavier...
Yolanda and Zeke.
(This is the last in a series of public releases on Hurricanes - The Greatest
Storm on Earth, prior to the beginning of the 1992 hurricane season which
begins June 1st and continues through November.
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