Meteorologists have divided the development of a tropical cyclone into four stages: Tropical disturbance, tropical depression, tropical storm, and full-fledged tropical cyclone. When the water vapor from the warm ocean condenses to form clouds, it releases its heat to the air. The warmed air rises and is pulled into the column of clouds.
Evaporation and condensation continue, building the cloud columns higher and larger. A pattern develops, with the wind circulating around a center like water going down a drain. As the moving column of air encounters more clouds, it becomes a cluster of thunderstorm clouds, called a tropical disturbance. As the thunderstorm grows higher and larger, the air at the top of the cloud column is cooling and becoming unstable. As the heat energy is released from the cooling water vapor, the air at the top of the clouds becomes warmer, making the air pressure higher and causing winds to move outward away from the high pressure area.
This movement and warming causes pressures at the surface to drop. Then air at the surface moves toward the lower pressure area, rises, and creates more thunderstorms.
Winds in the storm cloud column spin faster and faster, whipping around in a circular motion. When the winds reach between 25 and 38 mph, the storm is called a tropical depression. When the wind speeds reach 39 mph, the tropical depression becomes a tropical storm.
Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are called "hurricanes. Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface.
Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below. A cumulonimbus cloud. A tropical cyclone has so many of these, they form huge, circular bands. Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then that "new" air becomes warm and moist and rises, too.
As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface. Storms that form north of the equator spin counterclockwise. This air rises in discrete parcels, causing thundery showers to form.
These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface. There are various trigger mechanisms required to transform these cloud clusters into a tropical cyclone. These trigger mechanisms depend on several conditions being 'right' at the same time.
The most influential factors are:. The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air. The development of the surface depression causes an increase in the strength of the trade winds. The spiralling winds accelerate inwards and upwards, releasing heat and moisture as they do so. As the depression strengthens it becomes a tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds.
There is a relatively small area of intense horizontal winds at the surface, often well over m. Further aloft at about six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiralling winds leaving the tropical cyclone core. At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface.
This is called the eye of the storm. Great amounts of energy are transferred when warm water is evaporated from tropical seas. This energy is stored within the water vapour contained in moist air.
The release of heat energy warms the air locally, causing a further decrease in pressure aloft. Consequently, air rises faster to fill this area of low pressure, and more warm, moist air is drawn off the sea, feeding further energy to the system. Thus, a self-sustaining heat engine is created. This relatively small amount of mechanical energy equates to a power supply of 1.
Tropical cyclones are named to provide ease of communication between forecasters and the general public, regarding forecasts, watches and warnings. Since the storms can often last a week or even longer, and more than one can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described. Names were first used widely in World War II and were subsequently adopted by all regions. In most regions pre-determined alphabetic lists of alternating male and female names are used.
However, in the western North Pacific and North Indian Oceans the majority of names used are not personal names.
While there are a few male and female names, most are names of flowers, animals, birds, trees, foods or descriptive adjectives. The names are also not allocated in alphabetical order, but are arranged by the name of the Asian country which contributed the name. Many tropical cyclones eventually drift far enough from the equator to move into areas dominated by westerly winds found in the middle latitudes. These winds tend to reverse the direction of the tropical cyclone to an eastward path. As the tropical cyclone moves polewards it picks up forward speed and may reach 30 m.
An average tropical cyclone can travel about to miles a day, or about 3, miles before it dies out. Tropical cyclones also occur in various parts of the Pacific Ocean, and can affect coastal regions of Mexico, south-east Asia, north-east Australia and the south Pacific islands. In the northern hemisphere most tropical cyclones occur between June and November with a peak in September.
However, in the western North Pacific it is not unusual to have the occasional tropical cyclone outside of this period. In the southern hemisphere the season usually lasts from November to April. Tropical cyclones are occasionally observed in the South Atlantic, but this is a very rare occurrence. The map above shows the areas of the world affected by tropical cyclones.
Past tropical cyclones. Other phenomena which can be just as damaging than the wind frequently accompany tropical cyclones:.
These phenomena can cause major destruction, especially when the tropical cyclone's path takes it over land. However, a path over land also causes the destruction of the tropical cyclone itself. As it moves over land, its energy source is depleted and friction across the land surface distorts the air flow.
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