Реферат: Hurricanes Essay Research Paper How a Hurricane

Hurricanes Essay, Research Paper

How a Hurricane Begins

The Equatorial trough is the area in the ocean in which the trade winds converge. It moves north and south with the seasons. This phenomena can not be explained by scientists. Hurricanes always form along or be the Equatoral trough but, never in it.

In the Tropics warm air rises, cools, sinks and then returns to the equator with the trade winds. Once this air starts sinking it warms adiabatically. This produces and layer of high level air that is warmer than the air below it. This is called a high level temperature inversion. In this rising warm air is usually trapped. This cause storms to develop closer to the surface of the Earth. If a storm is growing vigorously, and is being pushed up by unusually strong winds, then there is a possibility of it breaking through the low lying air. This causes a Tropical depression, that can be up to 40,000 feet tall.

For a tropical depression to grow further into a hurricane, there must be low pressure at the surface accompanied by high pressure at an altitude of about 56,000 feet. This forms an anticyclone Constant vertical moving air feeds the anticyclone. This causes the air to circulate around the high and low pressure areas. In the northern hemisphere the air turns clockwise around the high pressure, and counterclockwise around the low pressure areas. the directions are reversed in the Southern hemisphere. This event is called cyclone circulation.

The wind continues to strengthen until the pressure between the high and the low pressure areas can accelerate no further. It then rises until it meets the anticyclone. The anticyclone (turning clockwise) pushes the rising air outwards away from the center. By removing the rising air more air is drawn upwards. This forces the surface?s atmospheric pressure fall. This fall in pressure need not be great. The average sea-level pressure is 1,016mb. In the center of a hurricane it is 920mb-980mb. This means that the pressure only falls about 4-10%.

Now there is an area, 400m or more across, in which a storm has developed. The air in the center circulates upward towards the top and then disperses outwards. This draws more air upwards. If the air in the center becomes warmer then the air around it the sky in this area will clear, leaving a sunny area surrounded by what appears to be a wall of cloud. This is the eye, and what once was a tropical depression is now a hurricane.

Where Hurricanes Occur

Hurricanes begin as tropical depressions. These are areas where the atmospheric pressure

is just a little lower than the air around it. Hurricanes only start in the tropics, it is impossible for

them to start in places such as Minnesota. Hurricanes have different names in different places. In

the Western Hemisphere they are called hurricanes. In the Bay of Bengal they are called cyclones. Over most of the Pacific they are called typhoons. Near Indonesia they are called baguios. Also some people in Australia call them willy-nillies. Meteorologists use one common name for all of these things. They call them Tropical cyclones. Tropical cyclones are always more intense in the tropics. The opposite of tropical cyclones are anticyclones. These areas of high atmospheric pressure.

In order for a tropical cyclone to form their must be a fall of atmospheric pressure over a large area. This fall in pressure need not be great, only 20mb over a period of two days. This fall in atmospheric pressure is common in temperate latitudes, but is very unusual in the tropics where the air pressure is fairly constant over very large areas. Under certain circumstances this fall in pressure can be enough to trigger a tropical storm.

There are two main scenarios in which tropical storms turn in to hurricanes. It may be that a pocket of low pressure air becomes detached from the edge of a mid latitude weather system and spills over into the tropics as a tongue of low pressure (called a trough) extending the equator at high altitudes. On the other hand, a low pressure system on land may drift out over the sea, or wave develop along the equatoral trough. This will produce a depression that detaches itself from the equatoral trough, which reforms behind it. No matter what the causes it the depression moves westward in an easterly wave.

Minor depressions can start anywhere but will only turn into a hurricane if it crosses over an expanse of very warm sea. This confines the birth place of hurricanes to the tropics. In latitudes higher than 20 degrees the sea surface temperature is usually to low. In areas close to the equator the sea is often warm enough to start a hurricane but hurricanes never start in latitudes lower than 5 degrees. This is because of the Coriolis Effect. The Coriolis effect is needed to swing the air moving towards the low pressure area into a circular path. The effect is not strong enough to cause the swing within 5 degrees of the equator.

Vorticity will cause the air to move in a curved path and eventually cause it to rotate. After it?s momentum accelerates it forms into an even smaller radius. In order for a hurricane to form within 5 degrees of the equator air would need to converge on the low pressure region from such a vast area that there is simply not enough air available.

Taking into account the need for high sea surface temperature, and a sufficiently strong Coriolis effect, you would come to the conclusion that the area that hurricanes can form a belt over the oceans is confined to between 5 degrees and 20 degrees. This is true for both hemispheres. There is also a restriction for what time of year hurricanes can start. Hurricanes can usually only start in late summer, and fall, because this is the only time of year that the water is warm enough to start a hurricane. Sometimes hurricanes can start during other seasons but this is very rare.

Most hurricanes do not develop until the depression crosses the western side of an ocean, but some form in the North Pacific. Over the tropics air moves vertically in Hadley cell. As it moves away from the equator the Coriolis effect causes the causes the air to swing right in the Northern hemisphere and left in the Southern hemisphere (it always swings to the east). It also causes the high level air moving away from the equator to be deeper to the east of Hadley cell than on the west. The high level air then sinks (still over the tropics) and warms it self. This limits the amount of air rising from the surface. This is because the air meets with a layer of subsiding air, and since the air is warmer and less dense it can rise no further. Sometimes it is possible for this air to break through the layer of subsiding air, but since the air on the west side of Hadley cell is denser it is easier for the air to escape. This is why hurricanes start on the west side of the ocean.

Depressions can only turn into hurricanes if they pass over a warm expanse of sea. This is because the depression must collect enough water vapor to provide a layer of vapor most air deep enough to supply it with sufficient latent heat of condensation. This means if a depression goes over the land it would not collect enough water and will die out. This is another reason why most hurricanes begin on the west side of the ocean.

Hurricanes Damage

There have been many hurricanes that caused tremendous amounts of damage. In late August 1992 hurricane Andrew crossed through Southern Florida and Louisiana. It?s winds reached up to 164 MPH. It demolished 63,00 homes in Florida, and left 44,000 people homeless in Louisiana. Hurricane Andrew is the most costly hurricane in history. In these two states alone it caused damages estimated at twenty-five million dollars.

A few days later, on the other side of the world Tropical Storm Polly was forming in the China Sea. It traveled westward towards the Coast of China. There it killed 165 people and left five million without homes.

When we think of hurricane damage we think of huge buildings being crushed by the power of the wind, but in real life the things that suffer the most damage are the homes and the crops. This is especially true in the third world countries where the food is especially needed. Typhoon Cecil devastated crops in central Vietnam in May, 1989. It also demolished around 36,000 houses, but houses are a lot easier to replace than crops. In September, 1989 hurricane Hugo devested crops in the Caribbean, and the eastern United states. Like most hurricanes Hugo up rooted trees. This resulted in the destruction of orchards and forests. It Damaged the Caribbean nation forest, and the Francis Marion Nation forest in South Carolina lost more than two thirds of it?s trees and three quarters of its endangered cockade woodpeckers.

To measure hurricane damage scientist use the Saffir/Simpson scale. There are other scales that can be used but this one is the most widely used. It takes into account such factors as, the pressure in the eye, the wind speed, and the size of the storm surges.

Historic Hurricanes

Hurricanes are entirely natural event and now a days we know a lot more about them than we ever did. The information that we know now is taken from our past experiences with hurricanes. Most of this information is from recent years. Now this does not mean that there are more hurricanes now then there ever were. In older times these hurricanes were not recorded. The people would repair the damage and move on with there lives as if nothing had happen. Occasionally though some people recorded these events. In 1696 for example a party of Quakers sailing from Jamaica to Philadelphia was caught in a hurricane and shipwrecked over night, onto what is now known as Jupiter Island. this event and all the hardships suffered by the crew were recorded by Jonathan Dickinson. We know of this hurricane only because it turned a normal sea voyage into a catastrophic ship wreck.

Other storms are remembered because of the scale of their damage. In 1099, for example a hurricane moving through the English channel killed 100,000 people along the English and Dutch Coast. Besides the fact that many lives were lost there would be major economical problems. There would be shortage of people, therefore less people to employ, therefore an increase in wage rises.

Sometimes the hurricanes effected things other than economy, and population. In rare occasions hurricanes like to disrupt wars. one of the most famous of these occurred in 1281. The Mongols, who at the time ruled China and Korea, ordered the Japanese to give up all their power to them. When the Japanese refused the Mongols sent a Korean ship to the southern most Japanese island of Kyushu. Their army attacked the island and soon overcame the Japanese?s defenses. Then out of now where a hurricane came and destroyed most of the Mongol army, saving Japan. The Japanese called this hurricane kamikaze. This means ?divine wind?. They ended up turning the day of its arrival into a religious celebration.

The Kamikaze is probably the only time that a hurricane has done any good for anyone, and even in this incident many were killed. Most such storms only bring death and destruction, sometimes on a vast scale. Measured in the terms of human life, the worst hurricane ever recorded in the United States lasted from August 27 to September 15 1900. It formed in the Caribbean and found its way into Galveston, Texas, on September 8. It had winds of 77 MPH gusting up to 120 MPH. This may not sound like much of a hurricane but, like other hurricanes it brought storm surges. It was this that caused most of the damage.

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