This is SCIENCE IN THE NEWS, in VOA Special English. I’m Bob Doughty.
And I’m Barbara Klein. Our subject this week is the science of severe ocean storms.
Violent ocean storms in the northern part of the world usually develop in late summer or autumn near the equator. Scientists call them cyclones when they develop over the Indian Ocean. When they happen over the northwestern Pacific Ocean, the storms are called typhoons. And, in the eastern Pacific and the Atlantic Ocean, they are called hurricanes.
Ocean storms develop when the air temperature in one area is different from the temperature nearby. Warmer air rises, while cooler air drops. These movements create a difference in the pressure of the atmosphere.
If the pressure changes over a large area, it can cause winds to blow in a huge circle. Thick clouds form and heavy rains fall as the storm gains speed and moves over the ocean waters.
The strongest winds happen in the area known as the eyewall. It 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 as high as 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.
The National Hurricane Center in Miami, Florida, keeps watch on severe storms. It works closely with public officials and with radio and television stations to keep people informed. Experts believe this early warning system has helped reduce the number of deaths from ocean storms in recent years.
But sometimes people cannot or will not flee the path of a storm, as Hurricane Katrina showed tragically. The storm struck the coast of the Gulf of Mexico on August twenty-ninth. More than one thousand bodies have been found, most of them in Louisiana.
In the past week, coastal areas of Texas and Louisiana had to prepare for Hurricane Rita. Three million people fled to higher ground. Rita caused widespread property damage, but not as much as had been feared.
The Atlantic hurricane season continues officially until November thirtieth.
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 government planes. These planes fly into and around storms. The crews drop instruments on parachutes. The instruments report temperature, pressure, wind speed and other details.
You are listening to SCIENCE IN THE NEWS in VOA Special English.
Scientists use the Saffir-Simpson Hurricane Scale to measure the intensity of storms based on wind speed. The scale is divided into 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.
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.
Katrina was a category four when it hit land. It struck the Gulf Coast with a wind speed of about two hundred thirty kilometers an hour. But government scientists say other forces helped make Katrina the most destructive hurricane ever to hit the United States.
Scientists at the National Oceanic and Atmospheric Administration say Katrina’s air pressure was very low. The lower the air pressure, the stronger the storm. And Katrina was an unusually wide storm. The edges reached from Texas to Florida.
Katrina’s most damaging power, however, came from the water it brought. The storm surge was estimated at more than six meters, and may have been as high as nine. The storm also brought heavy rainfall.
All this water poured into Lake Pontchartrain on the north side of New Orleans. It also flooded into the Mississippi River to the south. New Orleans was built below sea level. The city is surrounded by levees made of earth and walls made of concrete. The water and wind pressure from Katrina broke through the flood dams.
The surge washed away large areas of the coastal cities of Biloxi and Gulfport, Mississippi. There was also heavy damage in Alabama.
Studies have warned for years of the risk in continued development along the Gulf Coast. Scientists have said that more hurricane barriers are needed to protect areas where people live.
Scientists at the Georgia Institute of Technology and the National Center for Atmospheric Research recently did a study of hurricanes. They say the number of the most powerful storms has increased by almost one hundred percent in the past thirty-five years. The researchers noted that ocean surface temperatures have also increased during the same period. The study appeared in Science magazine.
Peter Webster of the School of Earth and Atmospheric Sciences at Georgia Tech was one of the leaders of the research. He says the world had an average of about ten category four and five hurricanes per year in the nineteen seventies. But Professor Webster says the average has increased to eighteen per year since nineteen ninety.
The researchers say that about thirty-five percent of all hurricanes in the past ten years were category four or five. That was up from around twenty percent in the nineteen seventies. The largest increases took place in the North Pacific and Southwest Pacific, and the North and South Indian oceans.
The increase was a little smaller in the North Atlantic. But the study says North Atlantic hurricanes have increased in total number. Also, they last longer than they did before nineteen ninety-five.
Kerry Emanuel at the Massachusetts Institute of Technology recently did a study of North Atlantic and North Pacific hurricanes. His findings appeared in Nature magazine. He used a different measure of power. But he, too, found a sharp increase in the last thirty years in the intensity of hurricanes and the time they last.
Peter Webster says it is not clear if the changes are the result of global warming caused by human activity. He says researchers need a longer record of hurricane information to see if such activity is natural over time. And the professor says they also need to understand more about the part that hurricanes play in Earth's climate.
As he describes it, hurricanes help cool the oceans and control the heat balance in the atmosphere. They evaporate water and then spread the tropical heat of the oceans up into the sky.
The new studies provide more evidence of a relationship between increases in ocean surface temperatures and the intensity of hurricanes. But Professor Webster says "it is not a simple relationship," and it is difficult to understand.
He says the total number of hurricanes has decreased in the past ten years. The average time they last has decreased also. Yet sea surface temperatures reportedly have increased the most of any period back to the nineteen seventies.
SCIENCE IN THE NEWS was written by Caty Weaver. Jill Moss was our producer. Our programs are on the Web at voaspecialenglish.com. I’m Bob Doughty.
And I’m Barbara Klein. To send e-mail, write to firstname.lastname@example.org. Listen again next week for more news about science in Special English on the Voice of America.