Hurricanes, Tropical Cyclones, and Typhoons

Hurricanes, tropical cyclones and typhoons are high-speed, spiraling storms with winds that reach a constant speed of 119 km/h (74 mph). The storms which originate in the Atlantic Ocean are called hurricanes. The ones that reach the United States and Europe usually start off the coast of Africa. The air currents there move westward and they take the hurricanes with them. In the South Pacific and the Indian Ocean, these storms are called tropical cyclones. Typhoon is the name given to this phenomenon in the North Pacific.

Technical Name of Storm	Region Storm Occurs	Storm Season
Hurricane	western North Atlantic, central and eastern North Pacific, Caribbean, Gulf of Mexico	June to November
Tropical Cyclone	southwest Indian Ocean	November to April
Severe Cyclonic Storm	western South Pacific, southeast Indian Ocean	November to April
Very Severe Cyclonic Storm	Bay of Bengal, Arabian Sea	April to June
Typhoon	western North Pacific	May to November

Science behind the storms

With seasonal changes come changes in temperature. These changes mostly come from the shape, tilt, and movement of the Earth. Because the Earth is round, parts of the Earth closer to the Equator face the Sun more directly, while parts closer to the North and South Poles are angled away from the Sun. Sunlight and heat are concentrated on the parts of the Earth angled toward the sun, because the light and heat are spread over a smaller surface area. The strip of the Earth's surface that most directly faces the Sun isn't always exactly the Equator; it changes as the Earth orbits the Sun because the Earth's axis is tilted.

As summer approaches, (May in the Northern Hemisphere and November in the Southern Hemisphere), the tilt of the Earth shifts the concentration of the Sun’s rays away from the Equator. In the Northern Hemisphere, the concentration of sunlight moves northward toward the Tropic of Cancer, warming the ocean waters on the way. Ocean water must be at least 60 m (200 ft) deep for a tropical storm to occur. Then in July and August, the concentration of the Sun’s rays moves southward back toward the Equator, continuing to warm the already warmed water. Very warm water, at least 27°C (80°F), is a necessary ingredient for the creation of a tropical storm. The concentration of sunlight continues to move southward to the Tropic of Capricorn as summer nears in the Southern Hemisphere. The same process of warming the ocean water then occurs there, explaining the difference in the storm seasons for the two hemispheres.

Due to evaporation and convection, the air above the ocean water becomes warm and moist. This air is pushed up by the surrounding colder denser air as it sinks. This causes wind. The Earth’s rotation causes the wind to begin to swirl as it rises. As the warm moist air rises, it becomes denser as it cools and begins to sink into the center of the storm. This center will become the eye of the hurricane if the storm progresses to that stage. The air continues to rise and sink as it spirals. In the Atlantic Ocean, above the Equator, there are typically one hundred storms during hurricane season that begin this way. Roughly 10% of these storms become tropical storms and only 6% produce winds that are strong enough to be classified as hurricanes. The strongest winds are in the eye wall, the area surrounding the eye. The winds spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Hurricanes/tropical cyclones cause torrential rain. As a tropical storm approaches land, it also pushes a wall of water called a storm surge onto the shore. Flooding results, which can go inland for several miles. Once the storm reaches land, or even colder water, the storm dies out. It needs to be over warm water to survive.

Classification of hurricanes

In 1972, American scientists Herbert Saffir and Robert Simpson developed the Saffir-Simpson Scale to classify the severity of hurricanes. This scale ranks hurricanes by the intensity of wind speeds. Hurricanes are divided into five categories.

Category	Sustained Winds	Types of Damage Due to Hurricane Winds
1	74-95 mph 64-82 knots 119-153 km/h	Very dangerous winds will produce some damage: Damage to unanchored mobile homes; shrubs/small trees uprooted. Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.
2	96-110 mph 83-95 knots 154-177 km/h	Extremely dangerous winds will cause extensive damage: Some coastal flooding; damage to vegetation. Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks.
3 (major)	111-129 mph 96-112 knots 178-208 km/h	Devastating damage will occur: Inland flooding; trucks/cars swept away; trees uprooted; damage to small boats/structures; damage to all structures by floating debris. Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.
4 (major)	130-156 mph 113-136 knots 209-251 km/h	Extreme damage will occur: Roads/bridges washed away; major erosion of beaches. Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.
5 (major)	157 mph or higher 137 knots or higher 252 km/h or higher	Catastrophic damage will occur: Extensive flooding. A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Storm tracking and forecasting

Storms are classified by their wind speed. Once the speed reaches 63 km/h (39 mph), the weather condition is called a tropical storm and is given a name. This process begins the warning system. If the wind speed reaches 119 km/h (74 mph), the storm is reclassified as a hurricane. As part of this hurricane/tropical storm tracking system, the World Meteorological Organization (located in Geneva, Switzerland) maintains the lists of these names for all tropical storms. In each region of the world, names for those lists are chosen based on regional preference and are rotated each year. Tropical cyclones can be given men’s or women’s names, as well as names of animals and flowers. Lists of names for Atlantic hurricanes are alternating men and women’s names in alphabetical order. The names of the some of the worst tropical storms have been retired and will never be used again. If a name is retired in one of the lists of Atlantic hurricane names, it is replaced with a name that begins with the same letter.

As technology and research techniques have advanced over the years, meteorologists have been able to be more accurate in spotting potential tropical storms. In the early days of forecasting, scientists relied on “storm spotters,” people in a position to report weather conditions that could result in tropical storms. These would include sailors and residents of coastal regions. Weather conditions “spotted” would include the type of clouds and the amount of steady rain, as well as the force of the wind. Later, airplane pilots were included as spotters. During the 1940’s, specially equipped aircraft began flying into storms and reporting the conditions. In 1960, meteorologists started using weather satellites. This was a major advancement, giving scientists the ability to forecast tropical storms and hurricanes before they touched land. Also during the 1960’s, the advances in radar capabilities increased the accuracy of hurricanes forecasts. Doppler weather radar was added in the late 1980’s. Scientists also use a host of surface stations to measure wind speed as the hurricane touches land. If these stations are damaged by the winds, the structural damage of buildings and boats and the distribution of debris aid in the estimation of wind speed.

Meteorologists continue to reanalyze data from past hurricanes as newer techniques and technologies come into play. This process increases the scientists’ ability to develop reliable hurricane models and to accurately forecast hurricanes.