Hurricanes

How Does a Hurricane Start? The hurricane takes its name from the West Indian word huracan which means "big wind." Storms that occur over the Atlantic or the eastern Pacific Oceans are called hurricanes. The same kind of storm that forms over the western Pacific or Indian Oceans is called a typhoon. This name comes from the Chinese word taifun or “great wind.”

Hurricanes and typhoons are not just violent winds. They are giant, whirling storms that develop in a special way. Hurricanes form only in the tropics where extremely moist air and heat are concentrated over the ocean, near the equator. The water temperature must be at least 80o Fahrenheit both day and night. Evaporation of the warm water into the atmosphere over the ocean makes the air very moist. Winds blowing across the ocean in different directions begin to push masses of warm, moist air toward each other. This event is called convergence. When the air masses collide, the air in the center starts to rise, forming an updraft. At high altitudes, the moist air of the updraft begins to cool and water droplets form. These water droplets form clouds. Large cumulonimbus clouds begin to grow and thunderstorms develop. More thunderstorms form as more convergence and updrafts occur. If the thunderstorms do not dissipate, they may start to gather together. This formation is called a tropical disturbance. Many more thunderstorms join the disturbance. This weather event becomes large enough to be influenced by forces created from the Earth's rotation.

The tropical disturbance begins to swirl and becomes a vortex of thunderstorms. Updrafts are continuously pulling more air into the disturbance. When the winds begin to blow continuously at 23 miles per hour, the storm becomes a tropical depression. The tropical depression continues to gain power and becomes a tropical storm when the wind speed becomes 40 miles per hour. At any time, the disturbance, depression, or storm can run out of hot, moist air and weaken or die out. If it continues to gain strength and reaches 74 miles per hour we call it a hurricane.

Hurricanes have top wind speeds of at least 74 miles per hour, but wind speed can reach 180 miles per hour. The closer you are to the storm's center, the faster the wind will be. The top wind speed will be reached within 60 miles from the center of the hurricane. As you move away from the center, wind speed is slower. At 300 miles from the center, the wind speed may be only 18 miles per hour. The energy of a hurricane comes from the heat released when water vapor condenses to liquid water. The atmosphere above a tropical ocean is the only place enough warm, moist air is available to produce the energy necessary to create a hurricane.

The movement of a hurricane is somewhat predictable. It is so large that it moves with the Earth's wind currents that surround it. These wind currents are very large and steady and don't change course abruptly. Therefore, hurricanes usually travel in one of these wind currents until they meet another wind current, then they may change direction. Sometimes one of these storms stalls over an area for days.

A hurricane covers a very large area. Sometimes a tropical storm can have a cloud system that is 2,000 miles in diameter. Typically, a hurricane is about 300 miles across. That is about the distance from Chicago, Illinois to Columbus, Ohio. An average hurricane is about 800 to 5,000 times as wide as an average tornado. Hurricanes usually travel across the sea and land at 10 to 32 miles per hour. Some may travel at speeds up to 50 miles per hour. The path of a hurricane usually covers thousands of miles, most of it over the ocean.

It is very important to track these huge storms and to make accurate predications about their movements. Many people live in areas affected by hurricanes. If the National Hurricane Center scientists believe a hurricane is threatening to reach a populated area within 24 hours, they will issue a hurricane warning. People prepare by gathering and sheltering property and boarding up homes and businesses. Sometimes people will even be evacuated from an area if the forecast calls for an extremely strong storm. Many lives have been saved by these preparations.

To study conditions inside hurricanes, teams of pilots and weather scientists fly regular missions into these storms. They get measurements of wind speed, temperature, air pressure, and other weather conditions at different altitudes. These investigations help scientists make predictions about hurricane formation and movement. The National Weather Service names hurricanes to quickly identify them. The names are assigned in alphabetical order alternating between female and male names. There are separate lists of names for hurricanes in the Atlantic and Pacific oceans.

Hurricanes and their potential for destruction are rated using a scale from 1 to 5 called the Saffir-Simpson scale. A Category 1 hurricane is the least destructive and a Category 5 hurricane is the most destructive. There are three types of damage caused by hurricanes:

▪Wind Damage
Hurricane-force winds, 74 mph or more, can destroy poorly constructed buildings and mobile homes. Debris, such as signs, roofing material, siding, and small items left outside, become flying missiles in hurricanes. Winds often stay above hurricane strength well inland.

▪Storm Surge Damage
Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. The surge of high water topped by waves is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property.

▪Flood Damage
Widespread torrential rains often in excess of 6 inches can produce deadly and destructive floods. This is the major threat to areas well inland.

Even Category 1 hurricanes can cause death, property damage and flooding and should be taken very seriously. Coastal areas are often evacuated by the police when a hurricane is approaching.

Saffir/Simpson Hurricane Scale