A tropical cyclone is the generic name for all cyclone circulations originating over tropical waters. A series of environmental factors must all work in collaboration to create a tropical cyclone. A low pressure system is caused by the upwelling of warm air (Abbott, 304). If this occurs over warm ocean waters, then water evaporates creating humidity. As this humid air is then sucked into the upwelling, condensation occurs, which releases energy in the form of heat. This process heats the surrounding air which continues the cycle of updrafts. Warm air rises; drawing in more humid air which then condensates (becomes rain). At this point a positive feedback or cycle occurs which acts as a giant engine that strengthens the tropical cyclone. Tropical cyclones have different names depending upon where they occur. In the northern Atlantic and eastern Pacific, a tropical cyclone is called a hurricane. In the western Pacific Ocean it is called a typhoon. Finally in the Indian Ocean a tropical cyclone is a called a cyclone (Langley, 34).
Tropical cyclones begin as a tropical disturbance originating in the tropics or subtropics which is characterized as a low pressure system with thunderstorms, weak surface wind circulation and warm moist upwelling (Abbott, 304). If a tropical disturbance strengthens then it becomes a tropical depression which has wind speeds of no greater than 38 miles per hour. Once a Tropic Disturbance reaches wind speeds above 38 miles per hour and under 73 miles per hour it is a called a tropical storm. If a tropical Storm's sustained winds reach over 73 miles per hour then it is called a Tropical Cyclone. Each particular region which experiences Tropical Cyclones has their own way of categorizing the intensity of the storm itself.
Due to the Coriolis Effect, tropical cyclones occurring in the northern hemisphere rotate counterclockwise and clockwise in the Southern Hemisphere (Abbott, 276). This phenomenon keeps tropical cyclones from crossing the equator. As the earth rotates, it moves faster at the equator and slower towards the north and south poles. This means that when an object moves from the equator to the north, the faster speed of the rotating earth causes the object's path to be deflected to the right. Conversely, when an object moves from the northern latitudes toward the equator, the faster speed of the rotating earth causes the object's path to be deflected west. This accounts for the differing rotations of tropical cyclones. It also explains why it is rare for tropical cyclones to occur in latitudes outside of area between 5 degrees north and south of the equator.
To describe the destructive strength of hurricanes occurring in the Atlantic and eastern Pacific oceans, meteorologists use the Saffier-Simpson model which puts hurricanes into five potential categories one through five (Abbott, 305).
- Category 1 Wind Speeds 74 - 95 MPH Storm Surge 4 - 5 feet
- Category 2 Wind Speeds 96 - 110 MPH Storm Surge 6 - 8 feet
- Category 3 Wind Speeds 111 - 130 MPH Storm Surge 9 - 12 feet
- Category 4 Wind Speeds 131 - 155 MPH Storm Surge 13 - 18 feet
- Category 5 Wind Speeds 155 MPH + Storm Surge over 18 feet
It also should be noted that a storm surge can be exacerbated if it coincides with a high tide. Most deaths caused by hurricanes are actually a direct result of people who are caught in storm surges (Abbott, 313).
Plate tectonic theory is used to explain the movement of the crust which "floats" above the earth's mantle. The earth is divided into three sections, the first being the crust, the second the mantle and the third the core. The continents are the part of the crust which rise above the bodies of water that cover the earth. The crust itself is divided into different plates which are constantly moving in various directions. The outer edge of where each plate meets is called a boundary. Along these boundaries, the plates do one of three things; move parallel to each other (transform zone), crash into each other (convergent zone) or move away from each other (divergent zone). The movement of the plates can be measured in centimeters per year.
Earthquakes and Volcanoes
The majority of earthquakes occur at plate boundaries and convergent zones are the most active cause of earthquakes, volcanoes and tsunamis (Wade, 62). Convergent zones also create mountains as one plate is pushed up and crumpled while the other plate is forced underneath back into the mantle. This process occurs over millions of years. Although convergent zones generate the largest and most deadly earthquakes, transform zones are also prone to large earthquakes. Divergent zones generate small earthquakes which usually are not life threatening. The area of the world where the vast majority of earthquakes occur is the convergent or subduction zone called the Ring of Fire. The Ring of Fire is located along the boundaries of the Pacific Plate (Langley, 61).
The vast majority of volcanoes occur at plate boundaries (Abbott, 152). Subduction zones are where the most violent volcanoes occur. Divergent zones generally cause slow minor eruptions which occur at mid ocean ridges. The violence of an eruption is dependent upon two factors, the amount of gas and silica in the magma. The amount of silica within the magma affects its viscosity (Abbott, 155). If the magma contains high amounts of silica, it is said to have a high viscosity which means it flows very slowly and traps the gas in the magma. This type of magma is usually found in convergent zones and causes explosive eruptions. Lower viscosity magma has a lower silica content which allows most of the gas to escape and usually creates less violent volcanoes such as the ones which make up the Hawaiian Islands (Abbott, 156). People tend to live near volcanoes because they produce very fertile soils for growing crops.
Tsunamis are usually caused by earthquakes which occur in subduction zones. As one plate is forced downward, it sometimes catches the plate above dragging that top plate down with it. An earthquake occurs when the top plate finally breaks free, springing back up. This action causes the water sitting on top of that plate to be displaced to generate an ocean wave (Langley, 12). The resulting waves such as the December 2004 Indian Ocean earthquake can wreak death and destruction upon anyone in the path. Tsunamis can travel hundreds of miles per hour and can occur as several different waves even up to an hour apart from each other. While earthquakes are the most common cause of tsunamis, they also can be caused by landslides, volcanic eruptions and meteors (Abbott, 87).