A hurricane is a powerful, spiraling storm that begins over a warm sea, near the equator. It is characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. In the northern hemisphere, cyclonic storms are called hurricanes or typhoons and their winds blow in an anti-clockwise circle. In the southern hemisphere, these are known as cyclones, whose winds blow in a clockwise circle. Depending on its location and strength, a tropical cyclone is referred to by names such as  hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. The word hurricane, used in the North Atlantic and Northeast Pacific, is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracán. Huracan became the Spanish term for hurricanes.

Classification

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. For example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane.

A hurricane or typhoon is a system with sustained winds of at least 74 miles per hour . A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The central calm region of the storm is called the "Eye". The diameter of the eye varies between 30 and 50 km and is a region free of clouds and has light winds. Around this calm and clear eye, there is the "Wall Cloud Region" of the storm about 5O km in extent, where the gale winds, thick clouds with torrential rain, thunder and lightning prevail. Away from the "Wall Cloud Region", the wind speed gradually decreases. The eye is often visible in satellite images as a small, circular, cloud-free spot. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 195 miles per hour (314 km/h).

Formation

Cyclones are formed from simple thunderstorms. However, these thunderstorms can only grow to cyclone strength with cooperation from both the ocean and the atmosphere. First of all, the ocean water itself must be warmer than 26.5 degrees Celsius (81°F). The heat and moisture from this warm water is ultimately the source of energy for cyclones. Cyclones will weaken rapidly when they travel over land or colder ocean waters - locations where their heat and/or moisture sources do not exist.

Related to having warm ocean water, high relative humidity in the lower and middle troposphere are also required for cyclone development. These high humidity reduce the amount of evaporation in clouds and maximizes the latent heat released because there is more precipitation. The vertical wind shear in a tropical cyclone's environment is also important. Wind shear is defined as the amount of change in the wind's direction or speed with increasing altitude.

When the wind shear is weak, the storms that are part of the cyclone grow vertically, and the latent heat from condensation is released into the air directly above the storm, aiding in development. When there is stronger wind shear, this means that the storms become more slanted and the latent heat release is dispersed over a much larger area.

Types

Hurricanes are measured in 5 categories on the Saffir Simpson scale.

Category 1 Minimal Wind speeds of 74-95 mph
Category 2 Moderate 96-110 mph
Category 3 Extensive III-130 mph
Category 4 Extreme 131-155 mph
Category 5 Catastrophic Over 155 mph

Many atlantic hurricanes named - Allen, Andrew, Isabel, Ivan, Dean, Felix, Emily, etc. have reached to category 5. Hurricane names are set by an international committee of the World Meteorological Organisation.

Size

One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than 222 kilometres , then the cyclone is "very small" . A radius between 333 kilometres to 666 kilometres are considered "average-sized". "Very large" tropical cyclones have a radius of greater than 888 kilometres. Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10-5 s-1 from its

center.

Time

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active whilst November is the only month with all the tropical cyclone basins active.

In the Northern Atlantic Ocean, a distinct hurricane season occurs from June to November, sharply peaking from late August through September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, the tropical cyclone year begins on July and runs all year round and encompasses the tropical cyclone seasons which run from November until the end of April with peaks in mid-February to early March.

Hurricane Accounts & Destruction

Tropical cyclones that cause extreme destruction are rare, although when they occur, they can cause great amounts of damage or thousands of fatalities. The 1970 Bhola cyclone is the deadliest tropical cyclone on record, killing more than 300,000 people and potentially as many as 1 million after striking the densely populated Ganges Delta region of Bangladesh on 13 November 1970. The North Indian cyclone basin has historically been the deadliest basin. Elsewhere, Typhoon Nina killed nearly 100,000 in China in1975 . The Great Hurricane of 1780 is the deadliest Atlantic hurricane on record, killing about

22,000 people in the Lesser Antilles. In 1982, the unnamed tropical depression that eventually became Hurricane Paul killed around 1,000 people in Central America.

Hurricane Katrina is estimated as the costliest tropical cyclone worldwide, causing $81.2 billion in property damage with overall damage estimates exceeding $100 billion. Katrina killed at least 1,836 people after striking Louisiana and Mississippi as a major hurricane in August 2005. Hurricane Andrew is the second most destructive tropical cyclone in U.S history, with damages totaling $40.7 billion and with damage costs at $31.5 billion. Hurricane Ike is the third most destructive tropical cyclone in U.S history. The Galveston Hurricane of 1900 is the deadliest natural disaster in the United States, killing an estimated 6,000 to 12,000 people in Galveston, Texas. Hurricane Mitch caused more than 10,000 fatalities in Latin America. Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage. In March 2004, Cyclone Gafilo struck northeastern Madagascar as a powerful cyclone, killing 74, affecting more than 200,000, and becoming the worst cyclone to affect the nation for more than 20 years.

Forecasting

Tropical cyclones far from land are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler radar. Radar plays a crucial role around landfall by showing a storm's location and intensity every several minutes.

High-speed computers and sophisticated simulation software allow forecasters to produce computer models that predict tropical cyclone tracks based on the future position and strength of high - and low - pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. However, scientists are less skillful at predicting the intensity of tropical cyclones. The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development.

Preparedness

One thinking is fighting the storm and to subdue its violence; the other thinking is to learn to live with it. Hurricane preparedness encompasses the actions and planning taken before a tropical cyclone strikes to mitigate damage and injury from the storm. Preparedness may involved preparations made by individuals as well as centralized efforts by governments or other organizations.

Organised Preparedness

Hurricane mitigation uses policies to make buildings and other infrastructure more resistant to the effects of tropical cyclones.

The World Meteorological Organization (WMO) has established in 1972, a Tropical Cyclone Project (TCP) with the objective of assisting the member countries to increase their capabilities to detect and forecast the approach and landfall of the tropical cyclones and to develop schemes to organize and execute disaster prevention and preparedness measures. One such plan that is in operation for assisting the countries adjoining the Bay of Bengal and the Arabian Sea is the panel on the tropical cyclones of World Meteorological Organization (WMO) and the Economic and Social Council for Asia and Pacific (ESCAP). RMC New Delhi is issuing advisory bulletin to the member countries, whenever, there is a cyclonic storm in North Indian Ocean.

Individual Preparedness

The most important decision in individual preparedness is determining if and when to evacuate an area that will be affected by a tropical cyclone If evacuation is not possible or necessary, other preparedness actions include storing supplies, securing a home against extreme winds and rain, and making plans with others prior to the storm's landfall.

Hurricane preparedness kits usually include drinkable water, sealed pre-prepared meals first-aid kits, prescription medications in sealed containers, waterproof battery-powered or hand-crank-powered flashlights and radios, a whistle or other sound-signaling device, a multi-tool with a knife, identification and medical cards, any necessary medical records, waterproof bags or portable waterproof containers, and other supplies helpful to a survival situation.

Safety Tips

Some of the safety tips that we can follow are:

• Listen to TV or radio for weather bulletins
• Board up garage and porch doors and stay away from windows
• Leave home if you might be affected by storms tide or flooding
• Turn off electricity and water
• Fill containers including drinking water
• Use phone only for emergency
• Follow evacuation routes to nearest shelter

Conclusion

Apart from the loss of human lives, natural disasters inflict severe damage to ecology and economy of the region. Space technology has made significant contribution in preparedness, prevention and relief of disaster management. With the help of satellites India has developed an operational mechanism for disaster warning to avoid damages.