The best way to describe how hurricanes form and where hurricanes occur begins with a look at the relationship between air temperature, atmospheric pressure, water vapor and wind. Hurricane researchers believe that two important factors are atmospheric pressure and high altitude air temperatures.
According to the Hurricane Research Center, a sudden drop in atmospheric pressure at the surface of the sea and a one to three degree increase in air temperature at 20,000 to 40,000 feet occurring around 24 hours before the storm develops are the primary catalysts in early stages of development of this powerful phenomenon. A third important factor contributing to be storm’s development is the interaction of low and high altitude winds that circulate in a counter-clockwise direction around the center of a low-pressure area.
After decades of research, hurricanes are now being computer-modeled as gigantic heat engines, powered by temperature changes at the sea surface and driven by powerful high-altitude winds. In this model, rising air currents form a definable column of wind and rain that vents the warm surface heat energy upwards into the upper atmosphere.
The heat engine model exhibits the properties of a chimney with hot air convection currents that pick up moisture from the ocean’s surface, propelling the vapor to high altitudes where the intense heat from the summer sun further heats them. At the same time, the moisture condenses, releasing more heat into the atmosphere, causing further increases in temperature within a defined area. This, in its own turn, enhances the rising air currents and upward motion of the water vapor. If there is sufficient upward motion and localized wind shears at both low and high altitudes, a low-pressure center develops at the surface level. Surface winds blowing into the low-pressure center generate a vortex about the center of the disturbance and results in the creation of a column of rising air, the so-called chimney effect. In the center of the chimney is the calm eye of the storm, which average around 14 miles in diameter. Because of the chimney effect, rotating winds rising from the surface to the altitudes between 20,000 and 60,000 feet high will steadily increase in velocity.
Upon reaching a surface wind velocity of 75 mph, these powerful summer storms become classified as hurricanes by the National Hurricane Center. Hurricanes are classified according to the Saffir-Simpson Hurricane Scale, which assesses the velocity of a storm’s intensity. Hurricane categories range from Category 1 for storms with relative low-velocity winds, of 75 to 95 mph and up to Category 5 for mega-storms like Camille in 1969, Andrew in 1992, and Hurricane Katrina in 2005. Category 5 hurricanes have winds greater than 155 mph and a storm surge generally greater than 18 ft above normal.
Hurricanes release an unbelievable amount of energy into the atmosphere. In one day, the energy released is equivalent to 400 – 20-megaton hydrogen bombs. The most significant damage, however, results from the surge of seawater that floods coastal areas as the hurricane mightily pushes its way onto land. As a result of the strength of hurricane winds and storm surge, recent hurricanes, like Hurricanes Katrina and Ike, are regarded among the most serious natural disasters in our nation’s history.
Predicting the path a hurricane for disaster preparedness is very important to low-lying coastal areas, especially those with high-density populations, such as New Orleans. Hurricanes arrive on land in the company of a massive surge of water pushed ahead of the storm by the high winds. The storm surge preceding a hurricane’s landfall can reach over twenty feet high and travel miles inland up rivers, canals and channels before receding into the ocean. The massive weight and velocity of any storm surge have devastated entire coastal areas resulting in tremendous losses of human life and property. The best hurricane preparedness plans for coastal cities call for massive evacuation ahead of the storm’s arrival.
