Hurricanes, also called tropical cyclones, are fierce storms that start over warm, tropical waters and grow into giant, wind-driven cyclones. They can do terrible damage to anything in their path.
Hurricanes start as tropical storms in the warmer areas (tropical and subtropical latitudes) of the northern Atlantic. This area is often just referred to as the tropics. Hurricanes are called by many names. The hurricanes that form in the western Pacific Ocean are called typhoons, while hurricanes in India are often called cyclones.
Tropical storms form because the tropics have lots of warm sunshine that heats the ocean and evaporates ocean water into rising cumulus clouds. In the tropics these clouds often become thunderstorms by late afternoon. If many thunderstorms form in a row in this region it is called a squall line. As a squall line passes, it brings gusty wind and heavy rain.
If a line of tropical thunderstorms form over warm ocean water with humid air and winds, this can come together to form a tropical wave (or tropical disturbance). The tropical wave can grow into a tropical depression as the winds pick up. Soon it can grow into a tropical storm. At this point, meteorologists give it a name. Sometimes a tropical storm then grows into a hurricane. It has officially become a hurricane if the winds reach 75mph (64 knots).
The National Weather Service started naming hurricanes with women’s names in 1953. In 1978, storm names began alternating between male and female names and including Spanish, French and Dutch names. For any given year, storm names are decided (ahead of time) by the World Meteorological Organization. If a storm takes many human lives, that name is never used again.
An average hurricane is about 300 miles (about 500 km) wide. They are circular storms that spin counter clockwise in the Northern Hemisphere and wrap around a central eye. Interestingly, hurricanes in the Southern Hemisphere, hurricanes spin clockwise! Hurricanes form between 5° and 20° latitudes and not on the equator. This is because there is no spinning wind force at the equator.
Hurricanes build when surface winds over the ocean meet to form a “wave” – an area of low pressure. Low pressure leads to convection – this is when warm air rises and cold air sinks. The conditions for forming a hurricane are best in June through November when the ocean temperature is about 80° (26°C).
It is thought that all the hurricanes that form in the Atlantic begin with a tropical wave over Africa – but only if Africa is having a wet year. In drought years in Africa, there are fewer hurricanes over the Atlantic. Most hurricanes last less than a week. They begin to lose power when they hit colder water or land.
The eye of a hurricane has few clouds and only light wind. When the eye of a hurricane passes over an area, sometimes people think the storm is over. As a matter of fact, this is a very dangerous time to be outside during a hurricane. The eye of a hurricane is surrounded by a ring of strong thunderstorms called the eye wall. The hurricane eye wall has some of the storm’s strongest and most damaging wind. As the eye passes over an area, serious damage can result from the wind, rain and lightening in the following eye wall. The high winds of a hurricane come with heavy rain and big waves. This can cause dangerous hurricane flooding.
The deadliest hurricane on record in the United States happened in 1900. It made land fall in Galveston, Texas. Though it was not the most powerful hurricane on record, it caused the most deaths – more than 6,000 people. This happened for several reasons. First, in 1900, weather officials did not have the modern technology we have today for tracking storms. They could not judge where the hurricane would travel in enough time to warn people. The second reason was Weather Bureau policy at the time – that a hurricane warning might cause people to panic. This policy changed after that storm.
In more recent history, hurricane Katrina hit U.S. shores in 2005. The storm surge destroyed the levee protecting New Orleans, Louisiana and flooding the city. More than 1,300 people died in the storm and following flood.
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