The deadliest natural disaster to affect the United States was
when a category 4 hurricane hit Galveston Island, Texas, on
September 8, 1900. Galveston is a low-lying barrier island located
south of Houston and in 1900 served as a wealthy port city. Residents
of coastal Texas received early warning of an approaching
hurricane from a Cuban meteorologist, but most chose to ignore
this advice. Later, perhaps too late, U.S. forecasters warned of an
approaching hurricane, and many people then evacuated the
island to move to relative safety inland. However, many others
remained on the island. In the late afternoon the hurricane moved
in to Galveston, and the storm surge hit at high tide covering the
entire island with water. Even the highest point on the island was
covered with one foot of water. Winds of 120 miles per hour (190
km/hr) destroyed wooden buildings, as well as many of the
stronger brick buildings. Debris from destroyed buildings crashed
into other structures, demolishing them and creating a moving
mangled mess for residents trapped on the island. The storm continued
through the night, battering the island and city with 30-foot
(9-m) high waves. In the morning, residents who found shelter
emerged to see half of the city totally destroyed, and the other half
severely damaged. But worst of all, thousands of bodies were
strewn everywhere, 6,000 on Galveston Island, and another 1,500
on the mainland. There was no way off the island as all boats and
bridges were destroyed, so survivors were in additional danger of
disease from the decaying bodies. When help arrived from the
mainland, the survivors needed to dispose of the bodies before
cholera set in, so they put the decaying corpses on barges, and
dumped them at sea. However, the tides and waves soon brought
the bodies back, and they eventually had to be burned in giant
funeral pyres built from wood from the destroyed city. Galveston
was rebuilt, and a seawall built from stones was supposed to protect
the city; however, in 1915, another hurricane struck Galveston,
claiming 275 additional lives.
The Galveston seawall has since been reconstructed and is
higher and stronger, although some forecasters believe that even
this seawall will not be able to protect the city from a category 5
hurricane. The possibility of a surprise storm hitting Galveston
again is not so remote, as demonstrated by the surprise tropical
storm of early June 2001. Weather forecasters were not successful
in predicting the rapid strengthening and movement of this storm,
which dumped 23–48 inches (58–122 cm) of rain on different parts
of the Galveston-Houston area and attacked the seawall and
coastal structures with huge waves and 30-mile-per-hour (48-
km/hr) winds. Twenty-two people died in the area from the surprise
storm, showing that even modern weather forecasting cannot
always adequately predict tropical storms. It is best to heed early
warnings and prepare for rapidly changing conditions when hurricanes
and tropical storms are approaching vulnerable areas.
in the Northern Hemisphere (clockwise in the Southern
Hemisphere) in toward the eye wall, moving faster and generating
huge waves as they approach the center. Wind speeds
increase toward the center of the storm and the atmospheric
pressure decreases to a low in the eye, uplifting the sea surface
in the storm center. Surface air flows in toward the eye
of the hurricane, then moves upward, often above nine miles
(15 km), along the eye wall. From there it moves outward in
a large outflow, until it descends outside the spiral rain
bands. Air in the rain bands is ascending, whereas between
the rain bands, belts of descending air counter this flow. Air
in the very center of the eye descends to the surface. Hurricanes
drop enormous amounts of precipitation, typically
spawn numerous tornadoes, and cause intense coastal damage
from winds, waves, and storm surges, where the sea surface
may be elevated many meters above its normal level.
Most hurricanes form in the summer and early fall over
warm tropical waters when the winds are light and the
humidity is high. In the North Atlantic, hurricane season generally
runs from June through November, when the tropical
surface waters are warmer than 26.5°C (80°F). They typically
begin when a group of unorganized thunderstorms are acted
on by some trigger that causes the air to begin converging and
spinning. These triggers are found in the intertropical convergence
zone that separates the northeast trade winds in the
Northern Hemisphere from the southeast trade winds in the
Southern Hemisphere. Most hurricanes form within this zone,
between 5° and 20° latitude. When a low-pressure system
develops in this zone in hurricane season, the isolated thunderstorms
can develop into an organized convective system
that strengthens to form a hurricane. Many Atlantic hurricanes
form in a zone of weak convergence on the eastern side
of tropical waves that form over North Africa, then move
westward where they intensify over warm tropical waters.
In order for hurricanes to develop, high level winds
must be mild, otherwise they would disperse the tops of the
growing thunderclouds. In addition, high level winds must
not be descending, since this would also inhibit the upward
growth of the thunderstorms. Once the mass of thunderstorms
is organized, hurricanes gain energy by evaporating
water from the warm tropical oceans. When the water vapor
condenses inside the thunderclouds, this heat energy is then
converted to wind energy. The upper level clouds then move
outward, causing the storm to grow stronger, and decreasing
the pressure in the center of the storm. The low pressure in
the storm’s center draws the outlying thunderstorms in
toward the surface low, and these rain bands then spiral
inward because of the Coriolis force. The clouds spin progressively
faster as they move inward, because of the conservation
of angular momentum.
The strength of hurricanes is measured using the Saffir-
Simpson scale, which measures the damage potential of a
storm, considering such factors as the central barometric pressure,
maximum sustained wind speeds, and the potential
height of the storm surge. Category 1 hurricanes have central
pressures of greater than 980 millibars, sustained winds of
74–95 miles per hour (119–153 km/hr), and a likely 4–5 foot
(1–1.5 m) storm surge. Damage potential is minimal, with likely
effects including downed power lines, ruined crops, and
minor damage to weak parts of buildings. Category 2 hurricanes
have central barometric pressures at 979–965 millibars,
maximum sustained winds of 96–110 miles per hour (155–177
km/hr), and 6–8 foot (1.8–2.4 m) storm surges. Damage is typically
moderate, including roof and chimney damage, beached
and splintered boats, and destroyed crops, road signs, and traffic
lights. Category 3 hurricanes have central barometric pressures
falling of 964 and 945 millibars, sustained winds of
111–130 miles per hour (179–209 km/hr), and storm surges of
9–12 feet (2.7–3.6 m). Category 3 hurricanes are major storms
capable of extensive property damage, including uprooting
large trees, and the destruction of mobile homes and poorly
constructed coastal houses. Category 4 storms can be devastating,
with central barometric pressures falling to 940–920 millibars,
sustained winds of 131–155 miles per hour (211–249
km/hr), and storm surges of 13–18 feet (4–5.5 m). These
storms typically rip the roofs off homes and businesses, destroy
sea piers, and throw boats well inland. Waves may breach seawalls
causing large-scale coastal flooding. Category 5 storms
are truly massive with central barometric pressures dropping
below 920 millibars, maximum sustained winds above 155
miles per hour (249 km/hr), and storm surges over 18 feet (5.5
m). Storms with this power rarely hit land, but when they do
they are capable of leveling entire towns, moving large
amounts of coastal sediments, and causing large death tolls.
Hurricanes inflict some of the most rapid and severe damage
and destruction to coastal regions and sometimes cause
large numbers of deaths. The number of deaths from hurricanes
has been reduced dramatically in recent years with our
increased ability to forecast the strength and landfall of hurricanes,
and our ability to monitor their progress with satellites.
However, the costs of hurricanes in terms of property damage
have greatly increased, as more and more people build expensive
homes along the coast. The greatest number of deaths
from hurricanes has been from effects of the storm surge.
Storm surges typically come ashore as a wall of water that
rushes onto land at the forward velocity of the hurricane, as
the storm waves on top of the surge are pounding the coastal
area with additional energy. For instance, when Hurricane
Camille hit Mississippi in 1969 with 200-mile-per-hour winds
(322 km/hr), a 24-foot (7.3-m) high storm surge moved into
coastal areas, killing most of the 256 people that perished in
this storm. Winds and tornadoes account for more deaths.
Heavy rains from hurricanes also cause considerable damage.
Flooding and severe erosion is often accompanied by massive
mudflows and debris avalanches, such as those caused by Hurricane
Mitch in Central America in 1998. In a period of several
days, Mitch dropped 25–75 inches (63.5–190.5 cm) of rain on
Nicaragua and Honduras, initiating many mudslides that were
the main cause of the more than 11,000 deaths from this single
storm. One of the worst events was the filling and collapse of a
caldera on Casitas volcano—when the caldera could hold no
more water, it gave way sending mudflows (lahars) cascading
down on several villages, killing 2,000 people.
Many cyclones are spawned in the Indian Ocean.
Bangladesh is a densely populated low-lying country, sitting
mostly at or near sea level, between India and Myanmar. It is
a delta environment, built where the Ganges and Brahmaputra
rivers drop their sediment eroded from the Himalaya
Mountains. It sits directly in the path of many Bay of Bengal
tropical cyclones and has been hit by seven of the nine most
deadly hurricane disasters in the history of the world. On
November 12 and 13 of 1970, a category 5 typhoon hit
Bangladesh with 155-mile-per-hour (249.5 km/hr) winds, and
a 23-foot (7-m) high storm surge that struck at the astronomically
high tides of a full moon. The result was devastating,
with 400,000 human deaths and half a million farm animals
perishing. Again in 1990, another cyclone hit the same area,
this time with a 20-foot (6-m) storm surge and 145-mile-perhour
(233 km/hr) winds, killing another 140,000 people and
another half-million farm animals.
Hurricane Andrew was the most destructive hurricane in
U.S. history, causing more than $30 billion in damage in
August of 1992. Andrew began to form over North Africa
and grew in strength as it was driven across the Atlantic by
the trade winds. On August 22 Andrew had grown to hurricane
strength and moved across the Bahamas with 150-mileper-
hour (241 km/hr) winds, killing four people. On August
24 Andrew smashed into southern Florida with a nearly 17-
foot (5.2-m) high storm surge, steady winds of 145 miles per
hour (233 km/hr), and gusts to 200 miles per hour (322
km/hr). Andrew’s path took it across a part of south Florida
that had hundreds of thousands of poorly constructed homes
and trailer parks, and hurricane winds caused intense and
widespread destruction. Andrew destroyed 80,000 buildings,
severely damaged another 55,000, and demolished thousands
of cars, signs, and trees. In southern Florida, 33 people died.
By August 26 Andrew had traveled across Florida, losing
much of its strength, but had moved back into the warm
waters of the Gulf of Mexico and regained much of that
strength. On August 26 Andrew made landfall again, this time
in Louisiana with 120-mile-per-hour (193 km/hr) winds,
where it killed another 15 people. Andrew’s winds stirred up
the fish-rich marshes of southern Louisiana, where the muddied
waters were agitated so much that the decaying organic
material overwhelmed the oxygen-rich surface layers, suffocating
millions of fish. Andrew then continued to lose strength
but dumped flooding rains over much of Mississippi.
See also AIR PRESSURE; BEACH; CLOUDS; EXTRATROPICAL
CYCLONES; POLAR LOW; STORM SURGES; THUNDERSTORMS;
TRADE WINDS.
Tidak ada komentar:
Posting Komentar
Catatan: Hanya anggota dari blog ini yang dapat mengirim komentar.