Rotating storm systems that originate over tropical and subtropical oceans are called tropical cyclones. As a tropical cyclone gains intensity, it becomes a hurricane. Inside a hurricane, the barometric pressure at the ocean's surface drops to extremely low levels. This central low pressure draws in warm, moist ocean air, and thunderstorms swirl around the center of these massive storms.
When a cyclone reaches hurricane strength, its low-pressure center is called the “eye” of the storm. The stronger the pressure difference at the ocean surface, the higher the wind speed. Acting like fuel that feeds more energy into the storm, moisture from the warm water is converted to heat in the thunderstorms that spiral into bands of rain that form around the eye. As air is forced into the eye, it rises rapidly and condenses, releasing large amounts of heat into the atmosphere. This refuels the hurricane, and the lower the barometric pressure at the center of the storm, the stronger the hurricane.
The destruction caused by few natural disasters can compare to the destructive force of a hurricane. During its life cycle, these storms can expend as much energy as 10,000 nuclear bombs. With sustained wind speeds of 249 kilometers per hour (155 mph) or more, intense rains and storm surges, hurricanes are capable of destruction of coastal areas. Hurricanes reaching Category 3 and higher are considered major hurricanes.
The Saffir-Simpson scale of hurricane intensity is based on measurements of wind speed, height of storm surges and central barometric pressure in millibars. The Saffir-Simpson scale ranges from Category 1 hurricanes with a barometric pressure of greater than 980 millibars that cause minimal damage, to Category 5 hurricanes with a central pressure of less than 920 millibars. Category 5 hurricanes are capable of causing catastrophic damage.
With only 892 millibars of central barometric pressure, the Labor Day Hurricane struck the Florida Keys in 1935 and was classified as a Category 5. Another Category 5 storm with a central pressure of 909 millibars, Hurricane Camille made landfall in Mississippi in 1969. Hurricane Andrew with a central pressure of 922 millibars was also a Category 5 and struck southeastern Florida in 1992. Category 5 Hurricane Charley made landfall in Punta Gorda, Florida, in 2004 with a central pressure of 941 millibars. Although it was classified as a strong Category 3 storm, Hurricane Katrina at 920 millibars caused widespread devastation along many areas of the central Gulf Coast and had the third lowest central pressure ever recorded.