Hurricanes, in part because of the large area they cover, are one of the most dangerous and damaging of all natural disasters. According to National Center for Atmospheric Research numbers, each year hundreds hurricanes cause an average of more than $5 billion in property damage and more than 20 deaths in the United States alone, arguably one of the best prepared countries in the world. If not for the advance warnings provided by modern forecasting techniques, the death tolls would be dramatically higher.
While our understanding of hurricanes is far from perfect, scientists do understand many of the factors that cause hurricanes to form, strengthen and ultimately collapse as they run out of energy.
What defines a hurricane?
A hurricane is a tropical storm with winds of 74 mph or more, according to Louisiana Homeland Security and Emergency Preparedness. The winds circulate around the storm's center or eye, traveling counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Hurricanes can include dangerous winds, extremely heavy rains, storm surges, and may even spawn tornadoes.
What conditions are necessary for the formation and growth of a hurricane?
Hurricanes draw their energy from the warm waters of the tropical oceans, requiring surface water temperatures of 81 degrees or higher, says the University of Illinois. They form over oceans because moisture in the air is also required. Moist air holds more energy in the form of latent heat. Tropical winds also play a role. Low wind shear allows the storms to build vertically, concentrating the energy of latent heat from the atmosphere over a smaller area, increasing the intensity of the storm and likelihood of it developing into a hurricane. Hurricanes form in the tropics along either side of the equator but not right on it, because they also require help from the Earth's Coriolis force to produce their initial spin or rotation.
What is the difference between a hurricane, a typhoon and a tropical cyclone?
These three terms all refer to the same type of storm. The only real difference is the location, says the University of North Carolina. In North America, we use the term hurricane, but the same kinds of storms are called typhoons in the northern-west Pacific Ocean and tropical cyclones in the Indian Ocean and southern-west Pacific Ocean. Tropical cyclone is also a generic term for tropical depressions with circulating winds whether or not they reach hurricane strength.
What are the growth stages of a hurricane?
The conditions required for a hurricane may start out as a series of thunderstorms called a tropical depression that have a central region of low pressure and begin to develop some cohesive rotation. As the associated winds increase in speed to 39 mph to 74 mph, the rotation of the storm becomes more organized and more circular. At this point, it will be called a tropical storm and receive an official name. As a tropical storm strengthens further a distinctive eye forms in the center of the swirling winds. Minimum sustained wind speeds of 74 miles per hour or greater make the storm a hurricane.
How does a hurricane get stronger?
Hurricanes are a self-reinforcing system. They are fueled by warm, moist air flowing toward a central region of low pressure. The incoming air is pushed vertically near the storm's center. As it rises, the air cools and can no longer hold as much moisture. The moisture in the air condenses out in the form of precipitation. As it does so, heat held by the water vapor is released into the surrounding air. As the air is heated, it expands, pushing air away from the storm center at the upper level. This reduces the amount of air above the center of the storm (measuring in the mass or weight of the air), which lowers the air pressure at the lower levels of the hurricane's center. Reduced air pressure here draws even more air toward the center at the lower level and the hurricane's strengthening cycle repeats continuously as long as the air being drawn into the system has enough "latent heat" trapped in water vapor to keep the cycle going.
Why do hurricanes seem to aim for the east coast of the United States?
Hurricanes that form along the east or west sides of North America are generally pushed in a northeasterly direction due to the prevailing direction of the tropical trade winds in that location. As they move up the east coast, they get pushed along by the Gulf Stream which tends to push them back out over the North Atlantic.
Why do hurricanes weaken or dissipate after they make landfall?
Hurricanes require a constant influx of warm, very moist air to sustain themselves. As they move north into cooler waters or the eye moves over land, there simply isn't enough moisture or latent heat to allow the hurricane to strengthen. As its latent heat is released, it runs out of energy and the system weakens.
Do hurricanes cause tornadoes?
Yes. The strong winds circling in the hurricane's eye wall can produce very intense tornadoes either within the eye wall itself or just outside it. Hurricane-spawned tornadoes can occur with little warning and can be very dangerous even in areas that are not accustomed to tornado activity.
What is a storm surge?
A storm surge occurs when the winds of a hurricane are blowing toward the shoreline from the ocean. They literally push the surface water toward they land where it piles up, getting deeper and deeper as long as the winds continue in the same direction. The storm surge can be compounded by incoming tides resulting in flooding far inland.
Brad Sylvester spent more than 18 years working in the consumer electronics industry before becoming a full-time freelance writer. He has a keen interest in science and the environment, and possesses a nearly insatiable curiosity about almost everything. You can follow him on Twitter @back2n8ure.