Seen from above the Earth, a hurricane has a soft, serene beauty. But experienced from within, hurricanes display a relentless and terrifying violence. Named after Huracan, a Caribbean god of evil, these are the largest storms on Earth. Churning across oceans and sweeping over continents, hurricanes combine the energies of wind and water into an irresistible force.
Hurricanes are severe storms defined by high velocity winds that rotate around a central, low-pressure core. Similar tropical storms are called cyclones or typhoons in other parts of the world. They form over tropical oceans between about 5 and 20 degrees of latitude, where seawater is hot enough to give them strength, and the rotation of the Earth makes them spin.
These storms are spawned by the interaction of the sea and the air. Their creation requires just a few simple conditions – a weather disturbance, such as a thunderstorm, that pulls in surface air from all directions and about 200’ of water 80° F, or warmer, at the top of the ocean.
Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere. This humid air is then dragged aloft when converging winds collide and turn upwards. At higher altitudes, water vapor starts to condense into clouds and rain, releasing heat that warms the surrounding air, causing it to rise as well. As the air far above the sea rushes upward, even more warm moist air spirals in from along the surface to replace it.
As long as the base of this weather system remains over warm water, and its top is not sheared apart by high altitude winds, it will strengthen and grow. More and more heat and water will be pumped into the air. The pressure at its core will drop further and further, sucking in wind at ever increasing speeds. Over several hours to days, the storm will intensify, finally reaching hurricane status when the winds that swirl around it reach sustained speeds of 74 miles per hour or more.
A fully developed hurricane is a highly organized and complex system of wind, clouds, and rain. Although meteorologists can break a hurricane down into just a few major elements, they are far from understanding the formation and behavior of these features. At the center of a hurricane is the eye, a nearly circular area of eerily fair weather, 5 to 120 miles across. Surrounding the eye is the eyewall, a towering ring of clouds that contains the hurricane’s most powerful winds and heaviest rainfall. Spiraling into the eyewall are the rain bands, wide zones of intense thunderstorms that sweep outward for 50 to 300 miles. In between these bands, rainfall and wind speed diminish.
Even as the familiar rotating spiral structure of a hurricane begins to form, the storm is pushed into motion by the Earth’s prevailing winds. In the tropics, the trade winds blow to the west. Thus, storms that form in the prolific hurricane breeding grounds of the eastern Atlantic are driven slowly but inexorably towards the Caribbean, Gulf of Mexico, and Central and North America.
Eventually, hurricanes turn away from the tropics and into mid-latitudes. Once they move over cold water or over land, and lose touch with the hot water that powers them, these storms weaken and break apart. The lifespan of a hurricane may be as brief as day or as long as several weeks. Most of it is spent in the empty reaches of the open ocean where the wind and waves it generates menace the occasional sailor or delight distant surfers.
But when hurricanes come onshore, they change from weather curiosities to natural disasters. In a single day, the average hurricane releases 200 times the energy generated by all the world’s electrical power plants combined. This energy can devastate both the human and natural landscape. Hurricane force winds often extend across an area 150 miles wide. Gale force winds may occur over a 300-400 mile wide stretch. The force of these winds and the debris they hurl can demolish buildings, power lines, and trees.
But although hurricanes and high winds are synonymous, water causes the most death and destruction. The strong winds and low air pressure in a hurricane create a dome of water 15 - 20 feet high and 50 -100 miles wide, called a storm surge. When the storm makes landfall, so does the surge. Topped by crashing waves and driven with enormous force, a storm surge can devastate low-lying coastal areas. Dangerous flooding comes not just from the sea, but also from the skies. Torrential rains often fall over a wide area for several days, leading to both flash and long-term flooding.
Hurricanes hold the dubious distinction of causing both the deadliest and the most damaging natural events in United States history. In 1900, the Galveston Hurricane killed more than 8,000 people. Just over 100 years later, Hurricane Katrina racked up damages of 100 billion dollars or more. In that same interval, cyclones and typhoons cost hundreds of thousands of lives and many billions of dollars worldwide.
The human misery and economic impact inflicted by hurricanes has spurred intense research into predicting storm development and formation. Although scientists are still unable to forecast the track of individual storms with precision, they have made strides in understanding hurricane dynamics and seasonal storm patterns.
Recent studies have shown a clear link between ocean surface temperatures and tropical storm intensity – warmer waters fuel more energetic storms. As ocean temperatures have warmed over the last several decades, the frequency of Category 4 and 5 hurricanes has increased.
Most studies agree that over the next one or two decades, hurricane frequency will increase substantially as a result of natural climate cycles. Some research also suggests that storm intensity will increase as well, due to warmer sea surface temperatures induced by global warming. The implications of these predictions are unsettling, especially for the United States. Well over half of the population lives in coastal and near coastal areas that are vulnerable to hurricane damage.