+
----
-
Tornadoes are one of weathers most deadly and fascinating
forces. Even though they are often limited by size (not being
more than one kilometer wide at most) they leave vast areas of
destruction and death behind them. They are also called twisters
or cyclones.
Tornadoes are characterized by violent winds that swirl in a
counter clockwise direction north of the equator and clockwise
south of the equator. Most people recognize them as a towering
black funnel extending downward from the base of a large
cumulonimbus cloud. It rotates at speeds up to three hundred
miles per hour (480 kpm) or in some rare cases, even faster. In
the center of the tornado, the air pressure is very low in
comparison to surrounding air pressure.
The speed of the wind is the primary cause of deaths and
destruction of property. Many people are killed by flying objects
and debris (missiles). The funnel shaped cloud travels in a skip
like movement, and usually never lasts for more than a couple of
minutes in any one given place. It is because of this skip
movement that the tornado leaves some areas wrecked while others
a few yards away almost untouched.
Certain parts of the world (ie. Australia, the Midwestern and
Southern US) are more prone to have tornadoes. They also occur
more frequently in the spring and summer months. Tornadoes
usually occur as part of a severe thunderstorm and often come in
advance of cold fronts, however, they can also occur (although
less frequently) ahead of warm fronts, and even behind cold
fronts.
The greatest killer tornado in the United States occurred
during the year 1925 in Indiana, Illinois, and Missouri. It was
the fastest and largest one ever recorded, with a destructive
path two hundred and twenty miles long and one mile wide, and
traveled at a speed of sixty miles per hour. It killed six
hundred ninety five people and injured over two thousand.
Photograph of Bezymianny,
a volcano in Kamchatka, Russia. Photograph taken by Jack Lockwood,
U.S.
Geological Survey.
A Volcano is a vent in the earth
through which hot gases and molten rock rise to the surface. A
coneshaped mountain of erupted material around such a vent is also
called a volcano. The name is taken from Vulcano, an island north of
Sicily in the Mediterranean Sea. The island was given its name by the
ancient Romans because it spewed smokelike vapor, and was thought to be
the home of Vulcan, their god of fire.
A Volcano consists of a fissure in the earth's crust, above which a cone
of volcanic material has accumulated. At the top of the cone is a
bowl-shaped vent called a crater. The cone is formed by the deposition
of molten or solid matter that flows or is ejected through the vent from
the interior of the earth. The study of volcanoes and volcanic
phenomena is called volcanology. Most volcanoes are composite landforms
built up partly of lava flows and partly of fragmental materials.
Italy's Mount
Etna, in Sicily is an example of a composite cone.
In
successive eruptions, the solid materials fall around the vent on the
slopes of the cone, while lava streams issue from the vent and from
fissures on the flanks of the cone. Thus, the cone is built up of layers
of fragmental materials and flows of lava, all inclined outward away
from the vent. Some enormous, craterlike basins, called calderas, at the
top of long-dormant or extinct volcanoes, are eventually occupied by
deep lakes, such as Crater Lake in Oregon. Some calderas are the result
of cataclysmic explosions that destroy the erupting volcano. Others form
when the subterranean magma chamber, emptied by repeated eruptions, can
no longer support the weight of the volcanic pile above it. Therefore
it collapses.
"Tsunami" is the Japanese word meaning tidal wave. A tidal wave is a
large sea wave caused by a submarine earthquake or volcanic explosion.
When the ocean floor is tilted or offset during an earthquake, a set of
waves is created. These waves are similar to the concentric waves
generated by an object dropped into the water.
Usually
tsunamis move entirely across an ocean to the shore. A tsunami can have
wavelengths of 60 to 120 mi and may reach speeds of 800 km/h. When the
wave enters shallow waters, the wave, which may have been half a meter
high out at sea, grows rapidly. When the wave reaches the shore, it may
be 50 ft. high or more. Tsunamis have incredible energy because of the
great volume of water affected. They bring waves of destruction capable
of killing thousands of residents along the coast. Towering walls of
water have struck populated coastlines with such fury that entire towns
have been destroyed. In 1896 a population of 20,000 in Sanriku, Japan
were wiped out. Tsunamis have resulted in waves as high as 135 ft. above
normal sea level.
When a tsunami strikes the shore, it creates a number of waves with
troughs that are lower than normal sea level. Each following wave is
higher than the one before it. The period between waves is 10 to 30
minutes. This usually gives people ample time to escape to high ground
after the first wave.
Most tsunamis originate along the Ring of Fire. The Ring of Fire is a
area of volcanoes and seismic activity 24,000 mi long. It encircles the
Pacific Ocean. Since the year 1819, more than 40 tsunamis have struck
the Hawaiian Islands. A tsunami warning system has been developed in
areas such as Hawaii, where many devastating tidal waves occur. Hawaii,
the highest risk area, averages one tsunami every year with a damaging
occurrence every 7 years. Alaska, also at high risk, averages a tsunami
every 1.75 years and a damaging event every 7 years. The warnings are
provided by seismograph records. Seismographs help determine the
location of where a submarine earthquake occured. These earthquakes
usually originate in one of the deep trenches in the Pacific Ocean
floor.
One of the largest and most destructive tsunamis ever recorded traveled
at least half way around the world in 1883 after the collapse of
Krakatoa, a volcano in Indonesia. Waves up to 100 ft. high caused great
damage along the coast of Sumatra.
In 1964, an Alaskan earthquake generated a tsunami with waves between 10
and 20 feet high along parts of the California, Oregon, and Washington
coasts. This tsunami caused more than $84 million in damage in Alaska
and 123 fatalities in Alaska, Oregon, and California.
Although tsunamis are rare along the Atlantic coastline, a severe
earthquake on November 18, 1929, in the Grand Banks of Newfoundland
generated a tsunami that caused considerable damage and loss of lives at
Placentia Bay, Newfoundland.
In 1946, a tsunami with waves of 20 to 32 feet crashed into Hilo,
Hawaii, flooding the downtown area and killing 159 people.
The Tsunami Warning Centers in Honolulu, Hawaii, and Palmer, Alaska,
monitor disturbances that trigger tsunamis. When a tsunamis is recorded,
it is tracked and a tsunami warning is issued to the threatened area.
Most deaths during a tsunami are a result of drowning. Associated risks
include flooding, polluted water supplies, and damaged gas lines.
Since 1945, more people have been killed as a result than as a direct
result of an earthquake's groundshaking.
Even today the phenomenon of lightning is still
not fully understood. It is awesome and frightening to many and
because of the mystery that surrounds it, several cultures have
developed different beliefs about it. Some African cultures
believe that when someone is hit by lightning that he/she was
cursed even before the event. The Romans and Greeks believed that
the gods were fighting and this was one of their weapons.
Lightning kills and injures many people, starts thousands of
forest fires, and causes millions of dollars in property damage
every year.
Lightning is characterized by the discharge of
electricity between rain clouds or between a rain cloud and the
earth. It is usually seen as an arc of extremely bright light
which can be many kilometers in length, however, there are other
forms as well. Accompanying the lightning is the giant roar of
thunder. The thunder is caused by the expansion of air that has
been heated by the lightning which then collides with cooler air,
creating the sound of an explosion.
Thunderstorms are the most common types of
storms, and thunder itself, although frightening, is not
dangerous. It is the lightning that causes the problems.
Lightning and thunder occur together, however we hear the thunder
after we see the lightning. Sound waves travel about one mile in
five seconds, while light travels at more than 186,000 miles per
second. Therefore, lightning is seen immediately when it occurs,
and thunder is heard a little later.
The different sounds of thunder (deep roar/loud
crash) is caused by the different types of lightning, for
example, the thunder that has the sharp crackle like sound is
caused when the large trunk of lightning forks into many
branches. (See types of lightning)
An earthquake is a shaking of the ground caused by the sudden
dislocation of material within the earth's outer layer, or crust. When
forces pushing on a mass of rock overcome the friction holding the rock
in place and blocks of rock slip against each other a earthquake may
occur. Some earthquakes are so slight, and some occur in such remote
areas, that they are barely felt. Others are so violent that they cause
extensive damage.
Causes of Earthquakes
Earthquakes are caused by stresses below the earth's outer surface.
These stresses usually build up until the rocks fracture along a "fault
plane." This causes vibrations, also known as seismic waves. Seismic
waves will then travel in all directions from the area of fracture. In
large earthquakes seismic waves may be detected over the entire earth.
Earthquakes can be caused by volcanoes in certain cases. Nuclear
explosions under the ground can create waves that are very similar to
natural seismic waves. The seismic energy created in a atomic bomb is
one hundred-thousandth that of the largest earthquake.
Effects of Earthquakes
Earthquakes produce various damaging effects to the areas they act upon.
This includes damage to buildings and in worst cases the loss of human
life. The effects of the rumbling produced by earthquakes usually
leads to the destruction of structures such as buildings, bridges, and
dams. They can also trigger landslides. An example of how an earthquake
can lead to even more destruction is the 1959 earthquake near Hebgen,
Montana. It caused a land slide that killed several people and blocked
the Madison River. Due to the fact that the Madison River was blocked, a
lake was created which later flooded the nearby town of Ennis.
Besides producing floods and destroying buildings, earthquakes that take
place under the ocean can sometimes cause tsunamis, or tidal waves.
Tsunamis are high and long walls of water which travel at a very rapid
rate. They are notorious for destroying entire populations and cities
near coastlines. In 1896 Sanriku, Japan, with a population of 20,000,
suffered such a fate.
How to protect yourself during an earthquake
1.) If an earthquake is occuring the most important thing to do is to
DROP and COVER. Drop
and cover means to fall on to the floor and get under something for
protection. During an earthquake, if you are indoors, it is very
important to stay calm and take cover under a heavy object.
2.) If you are outdoors, stay as far away from buildings as possible.
3.) Stay away from glass or anything that could fall.
4.) If you are in a crowded area, do not even consider running for the
nearest exit. Everyone will be doing that, and crowding will lead to
even more injuries.
Take cover under something heavy and stay away from things that could
fall on you.
It is also very important to remain as calm as possible.
5.) Be prepared for aftershocks after the initial earthquake has ended.
Aftershocks are follow-up earthquakes. They are smaller than the first
one, but still are very dangerous.
Hurricanes are one of natures many destructive
forces. The word comes from West Indian; huracan ("big
wind"). Hurricanes are seasonal storms and are most
prevalent in August and September. They develop from easterly
waves, which can later develop into a tropical depression with
winds up to 31 miles per hour. Later, it might develop into a
tropical storm with winds up to 73 miles per hour, and eventually
a hurricane. Thunderstorms often form with hurricanes and then
produce waterspouts.
Hurricanes are huge tropical cyclones that originate over
oceans near the equator, such as the Caribbean Sea and the Gulf
of Mexico. Usually they follow a parabola shaped path and
sometimes do not reach any land at all. However, if they do reach
land, destruction of property is imminent. Winds whirl in a
counter clockwise direction in the northern hemisphere and
clockwise in the southern hemisphere. Wind speeds of a hurricane
range from seventy to one hundred fifty miles per hour. These
winds blow circularly around a low-pressure center known as the
eye of the storm. Hurricanes cause numerous deaths in addition to
the millions of dollars of property damage each year due to the
intense winds and huge tidal surges.
One of the most destructive elements of a hurricane is the
huge waves that are formed because of the strong winds and heavy
rains. These waves called a storm surge can rise several feet
above water levels, especially during high tide. The worst
hurricane in the United States was the one during the year of
1900 in Galveston Texas, where a hurricane swept a fifteen foot
wall of water out of the Gulf of Mexico and into the city,
washing away and killing six thousand people. About 90 percent of
deaths during a hurricane are caused by drowning. Another
devastating outcome of a hurricane is the mudslides that can
follow. These are caused by the heavy rains and extensive
deforestation in some areas. A good example of this was during
the month of July 1998, when a huge mudslide that followed a
storm in a little Himalayan village in India, killed hundreds of
people.
الأحد 11 يناير 2015 - 16:04 من طرف 
موضوع: بحث الكوارث الطبيعية بالانجليزية 
