cyclones

Other-Science
The terms hurricane and typhoon are regionally specific names for a strong tropical cyclone. A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organised convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.
 
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called tropical depressions. Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a tropical storm and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
 
 Hurricane (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160° E) Typhoon (the Northwest Pacific Ocean west of the dateline) Severe tropical cyclone (the Southwest Pacific Ocean west of 160° E or Southeast Indian Ocean east of 90° E) Severe cyclonic storm (the North Indian Ocean) Tropical cyclone (the Southwest Indian Ocean)
 
In meteorology, a tropical cyclone (or tropical storm, typhoon, or hurricane, depending on strength and location) is a type of low-pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.
 
The center of a cyclone is a low-pressure region. Pressure gradient force, from high- to low-pressure regions, causes high winds around these regions. Wind flow around a large cyclone is almost invariantly anticlockwise, in the northern hemisphere, and clockwise, in the southern hemisphere, due to the Coriolis effect. Anticyclonic large storms are extremely rare on Earth, though Jupiter’s Great Red Spot storm is anticyclonic.
 
Types of Cyclones
 
Tropical cyclones (also known as tropical storms, hurricanes and typhoons) are cyclones which form over warm ocean waters and draw their energy from the evaporation and condensation of that water. They are characterised by a strong area of low pressure at the surface and an area of higher pressure aloft. Tropical cyclones are associated with strong thunderstorms, high winds, and flooding. They form over the tropic oceans especially in the Carribean region and South East Asia which also affecting the northly parts of Australia.
 
Extratropical cyclones lie somewhere in between tropical cyclones and mid-latitude cyclones, drawing a portion of their energy through the evaporation and condensation of ocean water, and some if it through horizontal temperature gradients in the atmosphere. They move out of the extratropical regions towards the polar regions, bringing precipitation in the form of rain or drizzle. Nearly always called by the term low-pressure cells in North America, they often form quickly along cold fronts that have stagnated after moving into an area where warm, moist air exists. The warm, moist air is less dense, therefore it overruns the more dense cold air at and behind the cold front. A cyclonic motion is imparted to the ascending air, naturally, forming a shallow cyclone. Usually, a part of the cold front will develop into a warm front, giving the frontal zone a shape (when drawn on a weather chart) that is called a “wave”. An old name for such a system is “warm wave” (in the United States).
 
In the beginning, the sea-level pressure is not very low; typically, it is about 1009 millibars (1009 hectopascals) (hPa) (SI). Intensification or “deepening” of the extratropical cyclone progresses slowly. Often, the sea-level pressure in the low-pressure cell fails to drop below 1000 millibars (1000 hectopascals) (hPa) (SI). Rapidly falling air pressure is very rare or non-existent, even after several days have passed and the storm has moved to higher latitudes. Extratropical cyclones are generally mild storms with surface winds of 7-15 knots. The band of precipitation that is associated with the warm front is often extensive. They tend to move along a predictable path at a moderate rate of progress. They are much more common in the United States than in Europe which is located above the arid Sahara desert. The dry, arid air of the Sahara desert is not conducive to the formation of extratropical cyclones.
 
Transpiration occurs when plants take in water through the roots and release it through the leaves, a process that can clean water by removing contaminants and pollution.
 
Mid-latitude cyclones
 
In the North Pacific ocean, the Kuroshio or Japan current moves warm water into the northern North Pacific ocean. In the North Atlantic ocean, the Gulf Stream moves warm water into the northern North Atlantic ocean. The warm water at high latitudes provides the energy that brings on the generation of subpolar cyclones or low-pressure cells that exhibit characteristics which differentiate them from extratropical cyclones.
 
They are tall and often exceed 30,000 feet (c. 10km) in height. Above the surface of the earth, the air temperature near the center of the storm is colder than the air to the north, east, south, and west of it. For that reason, the storms are called cold-core lows. A suitable chart to examine is the 700 millibars (hectopascals) (hPa) (SI) chart promulgated by Canada. The information is at about 10,000 feet or 3,000 meters. At first, the storms move from high latitudes to lower latitudes, i.e., they travel towards the southeast over North America and Europe, then eastwards. As the storms move across land, the air pressure may fall rapidly to below 980 millibars (980 hectopascals) (hPa) (SI).
 
Similar storms may appear, at times, at very high latitudes. The very cold storms are called subarctic cyclones or low-pressure cells in the Northern Hemisphere and subantarctic cyclones or low-pressure cells in the Southern Hemisphere.
 
Tornadoes
 
In North America, tornadoes are sometimes described as cyclones because they involve powerful winds around a low-pressure vortex. However, they differ from other cyclones by their very local nature; most cyclones are massive storms, tornados are comparatively small but extremely powerful. Tornados occur on too local a scale for the Coriolis effect to determine the direction of rotation; for this reason, tornado winds sometimes flow anticyclonically, or opposite the direction dictated by the Coriolis effect.
 
Hurricane
 
A tropical cyclone in which winds attain speeds greater than 74 mi (119 km) per hr. Wind speeds reach over 190 mi (289 km) per hr in some hurricanes. The term is often restricted to those storms occurring over the N Atlantic Ocean; the identical phenomenon occurring over the W Pacific Ocean is called a typhoon; a tropical cyclone around Australia is called a willy-willy; and over the Indian Ocean, a tropical cyclone. Hurricanes have a life span of 1 to 30 days. They weaken and are transformed into extratropical cyclones after prolonged contact with the colder ocean waters of the middle latitudes, and they rapidly decay after moving over land areas.
 
Formation of Hurricanes
 
A cyclone that eventually reaches hurricane intensity first passes through two intermediate stages known as tropical depression and tropical storm. Hurricanes start over the oceans as a collection of storms in the tropics. The deepening low-pressure center takes in moist air and thermal energy from the ocean surface, convection lifts the air, and high pressure higher in the atmosphere pushes it outward. Rotation of the wind currents tends to spin the clouds into a tight curl; as the winds reach gale force, the depression becomes a tropical storm. The mature hurricane is nearly circularly symmetrical, and its influence often extends over an area 500 mi (805 km) in diameter.
 
As a result of the extremely low central pressure (often around 28.35 in./960 millibars but sometimes considerably lower, with a record 25.69 in./870 millibars registered in a 1979 NW Pacific typhoon) surface air spirals inward cyclonically (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere), converging on a circle of about 20 mi (30 km) diameter that surrounds the hurricane’s “eye.” The circumference of this circle defines the so-called eye wall, where the inward-spiraling, moisture-laden air is forced aloft, causing condensation and the concomitant release of latent heat; after reaching altitudes of tens of thousands of feet above the surface, this air is finally expelled toward the storm’s periphery and eventually creates the spiral bands of clouds easily identifiable in satellite photographs.
 
The upward velocity of the air and subsequent condensation make the eye wall the region of heaviest precipitation and highest clouds. Because the outward increase in pressure is greatest there, the eye wall is also the region of maximum wind speed. By contrast, the hurricane eye is almost calm, experiences little or no precipitation, and is often exposed to a clear sky. Temperatures in the eye are 10°F to 15°F (5°C–8°C) warmer than those of the surrounding air as a result of sinking currents at the hurricane’s core.
 
Movement and Occurrence of Hurricanes
 
Hurricanes and typhoons usually move westward at about 10 mph (16 kph) during their early stages and then curve poleward as they approach the western boundaries of the oceans at 20° to 30° lat., although more complex tracks are common. In the Northern Hemisphere, incipient hurricanes usually form over the tropical Atlantic Ocean and mature as they drift westward; hurricanes also form off the west coast of Mexico and move northeastward from that area. Between June and November, an average of six tropical storms per year mature into hurricanes along the east coast of North America, often over the Caribbean Sea or the Gulf of Mexico. Two of these storms will typically become major hurricanes (categories 3 to 5 on the Saffir-Simpson scale). One to three hurricanes typically approach the U.S. coast annually, some changing their direction from west to northeast as they develop; as many as six hurricanes have struck the United States in one year. Hurricanes and typhoons of the N Pacific usually develop sometime between May and December; typhoons and tropical cyclones of the Southern Hemisphere favor the period from December through April; Bay of Bengal and Arabian Sea tropical cyclones occur either between April and June or September and December, the times of the onset and retreat of the monsoon winds.
 
Damage Caused by Hurricanes
 
High winds are a primary cause of hurricane-inflicted loss of life and property damage. Another cause is the flooding resulting from the coastal storm surge of the ocean and the torrential rains, both of which accompany the storm. The Saffir-Simpson scale is the standard scale for rating the severity of a hurricane as measured by the damage it causes. It classifies hurricanes on a hierarchy from category 1 (minimal), through category 2 (moderate), category 3 (extensive), and category 4 (extreme), to category 5 (catastrophic). A supertyphoon is equivalent to a category 4 or 5 hurricane.
 
Only three category-5 storms have hit the United States since record-keeping began—the 1935 Labor Day hurricane, which devastated the Florida Keys, killing 600; Hurricane Camille in 1969, which ravaged the Mississippi coast, killing 256; and Andrew in 1992, which leveled much of Homestead, Fla. Hurricane Katrina in 2005 was a category-5 storm at peak intensity over the central Caribbean, Mitch in 1998 was a category-5 storm at its peak over the W Caribbean, and Gilbert in 1988 was a category-5 storm at its peak. Gilbert was the strongest Atlantic tropical cyclone of record until Wilma in 2005, which was at its peak while category-5 storm over the W Caribbean. The 1970 Bay of Bengal tropical cyclone killed about 300,000 persons, mainly by drowning. The deadliest U.S. hurricane was the 1900 Galveston storm, which killed 8,000–12,000 people and destroyed the city. Hurricane Katrina (2005), one of the worst natural disasters in U.S. history, was economically the most destructive U.S. storm, devastating the SW Mississippi and SE Lousiana coasts, flooding New Orleans, killing some 1,200 people, and leaving hundreds of thousands homeless. Hugo (1989) in South Carolina and Opal (1995) and Charley, Ivan, and two others (2004) in Florida, and Rita (2005) in Louisiana and Texas also caused billions of dollars worth of damage. Weak hurricanes can still cause major flooding and damage, even when downgraded to a tropical storm, as did Hurricane Agnes (1972).
 
To decrease such damage several unsuccessful programs have studied ways to “defuse” hurricanes in their developing stages; more recent hurricane damage-mitigation steps have included better warning systems involving real-time satellite imagery. A hurricane watch is issued when there is a threat of hurricane conditions within 24–36 hours. A hurricane warning is issued when hurricane conditions (winds greater than 74 mph/119 kph or dangerously high water and rough seas) are expected in 24 hours or less.

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