Storm Warnings: Trying to Understand the Causes of Hurricanes
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This is SCIENCE IN THE NEWS in VOA Special English. I'm Bob Doughty. And I'm Faith Lapidus. Powerful storms are called hurricanes when they form over the Atlantic Ocean and the eastern Pacific. They are called typhoons in the northwestern Pacific, and cyclones when they develop over the Indian Ocean. Whatever we call them, these storms are the subject of our program this week.
Severe ocean storms in the northern half of the world generally develop in late summer or early autumn near the equator.
Storms can result when the air temperature in one area is different from that of another. Warmer air rises and cooler air falls. These movements create a difference in the pressure of the atmosphere.
If the pressure changes over a large area, winds start to blow in a huge circle. High-pressure air is pulled into a low-pressure center.
Severe ocean storms happen less often in the southern hemisphere. There the season of greatest activity is between December and March.
South of the equator, the winds flow in the same direction as the hands on a clock. North of the equator, they flow counter-clockwise.
This is because as Earth turns, air is pulled to the right in the northern hemisphere. In the southern hemisphere, air is pulled to the left.
Storms can get stronger and stronger as they move over warm ocean waters. The strongest, fastest winds of a hurricane are found in the eyewall. This is the area that surrounds the center, or eye, of the storm. The eye itself is calm by comparison, with light winds and clear skies.
Winds in severe ocean storms can reach speeds of more than two hundred fifty kilometers an hour. Up to fifty centimeters of rain can fall. Some storms have produced more than one hundred fifty centimeters of rain.
These storms also cause high waves and ocean surges. A surge is a continuous movement of water that may reach six meters or more. The water smashes across low coastal areas. Surges are commonly responsible for about ninety percent of all deaths from ocean storms.
Weather scientists use computers to create models that show where a storm might go. Models combine information such as temperatures, wind speed, atmospheric pressure and the amount of water in the atmosphere.
Scientists collect the information with satellites, weather balloons and devices floating in the world's oceans. They also collect information from ships and passenger flights and from planes that fly into and around storms. The crews drop instruments on parachutes to record temperature, pressure, wind speed and other conditions.
The Saffir-Simpson Hurricane Scale is a way to rate storms based on wind speed. It provides an idea of the amount of coastal flooding and property damage that might be expected.
The scale is divided into five categories. A Category One storm has winds of about one hundred twenty to one hundred fifty kilometers an hour. It can damage trees and lightweight structures. It can also cause some flooding.
Wind speeds in a Category Two hurricane can reach close to one hundred eighty kilometers an hour. These storms are often powerful enough to break windows or blow the roof off a house.
Winds between about one hundred eighty and two hundred fifty kilometers an hour represent categories three and four. Anything even more powerful is a Category Five hurricane.
An Australian weather scientist began to call storms by women's names before the end of the nineteenth century. During World War Two, weather scientists called storms by the names of their wives or girlfriends. The weather service in the United States officially started to use women's names for storms in nineteen fifty-three. In nineteen seventy-nine, it began to use men's names, too.
Scientists decide on lists of names years in advance. They agree on them at meetings of the World Meteorological Organization.
Naming storms is part of the job of the National Hurricane Center near Miami, Florida. Storms get a name when they reach a wind speed of sixty-two kilometers an hour, even if they never develop into hurricanes.
The first name used in a storm season begins with the letter A, the second with B and so on. The same list of names is not used again for at least six years. And different lists are used for different parts of the world.
For example, names for storms in the Atlantic this year include Alberto, Debby, Florence, Joyce and Oscar. Storm names for the central North Pacific include Akoni, Hana, Lala and Oka.
Letters of the Greek alphabet had to be used for the first time last year to name storms in the Atlantic. That was the plan -- to call storms Alpha, Beta, Gamma and so on -- if there were ever more than twenty-one named storms in a season. In fact, there were twenty-eight.
The two thousand five Atlantic hurricane season was the first on record with fifteen hurricanes. Four reached Category Five strength, also a first. And the National Oceanic and Atmospheric Administration says it was the first season with four major hurricanes to hit the United States.
The most destructive was Katrina. More than one thousand eight hundred people were killed along the Gulf of Mexico coast, most of them in Louisiana.
There is debate about the influence of global warming on hurricanes. Scientists have found no simple answers.
A new study has just been published online in the Proceedings of the National Academy of Sciences, in the United States. Scientists examined rising ocean temperatures in areas of the Atlantic and Pacific where hurricanes form. They found an eighty-four percent chance that humans have caused most of the observed rise over the last one hundred years. They say warming sea surface temperatures are mainly the result of an increase in greenhouse gases released into the atmosphere.
Earlier research examined temperature changes over very large ocean areas, such as all of the Atlantic or Pacific. The new study involved much smaller hurricane formation areas. The researchers say they used most of the world's computer climate models to study the causes of the temperature changes.
The study involved scientists from ten research centers. These included Lawrence Livermore National Laboratory in California and the National Center for Atmospheric Research in Colorado. Tom Wigley from the Colorado team says: "The best explanation for these changes has to include a large human influence."
Hurricanes are highly complex. The researchers say increasing ocean temperatures are not the only cause of hurricane intensity, but are likely to be one of the major influences.
A year ago, Kerry Emanuel of the Massachusetts Institute of Technology reported an increase in the intensity of hurricanes since the nineteen seventies. He linked it to tropical sea-surface temperatures rising as a result of normal long-term changes and global warming. He said his results suggested that future warming may lead to storms of increasingly destructive power.
But other experts say not all storms get stronger as waters get warmer. Some do, others do not. They say major storms may develop when bodies of water reach about twenty-eight degrees Celsius. Other conditions, however, must also be present. These include a large amount of water in the air and low wind speed in the upper atmosphere.
Studies have also looked at changes in the number of the most powerful hurricanes each year. Peter Webster and others at the Georgia Institute of Technology reported that the number of intense storms almost doubled in the past thirty-five years.
But some other scientists found different results when they looked at different periods.
One of them was Patrick Michaels of the University of Virginia and the Cato Institute, and currently a visiting professor at Virginia Tech. He says the rate of category four and five hurricanes in the Atlantic is the same now as it was in the nineteen forties and fifties. He says this shows that natural forces are at work, not global warming caused by humans.
Frank Lepore is the public affairs officer at the National Hurricane Center. He says disagreements like these show the great difficulty involved in trying to understand what causes hurricanes.
In July, ten scientists released a statement about what they called the main hurricane problem facing the United States. They warned that the problem is development in coastal areas at risk from hurricanes.
They called on government leaders not to support policies that have increased the density of population and wealth in these areas. Their message was that building in risky coastal areas only increases the amount of destruction when a hurricane strikes.
SCIENCE IN THE NEWS was written by Nancy Steinbach and Caty Weaver and produced by Brianna Blake. I'm Bob Doughty. And I'm Faith Lapidus. A link to the National Hurricane Center, along with transcripts and audio files of our programs, can be found at voaspecialenglish.com. Listen again next week for more news about science in Special English on the Voice of America.