BARBARA KLEIN: This is SCIENCE IN THE NEWS, in VOA Special English. I'm Barbara Klein.
BOB DOUGHTY: And I'm Bob Doughty. Today we remember Hurricane Katrina and tell about the science of severe ocean storms.
BARBARA KLEIN: Many Americans are observing the fifth anniversary of one of the nation's worst natural disasters. Hurricane Katrina reached the state of Louisiana on the morning of August twenty-ninth, two thousand five. It was the costliest hurricane in American history, and one of the deadliest.
Radio and television programs, concerts and films are recalling the storm and its effects on the nation. Literary readings and religious observances also are marking the event.
Hurricane Katrina struck hardest in Louisiana, Mississippi and Alabama. Death and destruction from the hurricane and its effects extended along the Gulf Coast. More than one thousand eight hundred people were killed.
BOB DOUGHTY: The storm formed over the Bahamas on August twenty-third, two thousand five. The next day, it grew strong enough for scientists to call it a tropical storm. Then it moved toward the United States. It first reached land in south Florida on August twenty-fifth.
At that time, the National Hurricane Center said the winds were at a top continuing speed of more than one hundred thirty kilometers per hour. Experts identified the storm as a hurricane. They named it Katrina, and rated it as the least severe type of hurricane. Still, it caused flooding and killed people in Florida.
BARBARA KLEIN: Hurricane Katrina weakened again after striking Florida. Later it moved to the Gulf of Mexico. The Gulf's warm waters helped it gain strength. At one point, the storm's winds were blowing at more than two hundred sixty-eight kilometers per hour. Experts increased its rating to the most severe hurricane.
Time passed, and the winds again weakened. Then Hurricane Katrina reached land in Louisiana. Its speed had fallen to about two hundred kilometers per hour when it struck near New Orleans.
But the wind was strong enough to pick up trees, vehicles and buildings. It threw them into the air like toys. Walls of water flooded over the land. Intense rain fell. Then Hurricane Katrina struck land again, this time at the border of Mississippi and Louisiana. Again, there was loss of life and terrible destruction.
BOB DOUGHTY: Severe ocean storms in the northern part of the world usually develop in late summer or early 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 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 falls. These movements create a difference in the pressure of the atmosphere.
BARBARA KLEIN: If the pressure changes over a large area, winds start to blow in a huge circle. High pressure air is pulled toward a low pressure center. Thick clouds form and heavy rains fall as the storm gains speed and moves over the ocean waters. Storms can get stronger as they move over warm ocean waters.
The strongest, fastest winds of a hurricane blow in the area known as the eyewall. It surrounds the center, or eye, of the storm. The eye itself is calm by comparison.
Wind speeds in severe ocean storms can reach 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 strikes low coastal areas. Surges are commonly responsible for about ninety percent of all deaths from ocean storms.
BOB DOUGHTY: 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 deaths from ocean storms in recent years.
But sometimes people cannot or will not flee the path of a storm. That is what happened in many places in New Orleans.
BARBARA KLEIN: Weather scientists use computer programs to create models that show where a storm might go. The programs 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 attached to parachutes. The instruments report temperature, pressure, wind speed and other conditions.
BOB DOUGHTY: Scientists use the Saffir-Simpson Hurricane Scale to measure the intensity of 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 groups or categories.
The mildest hurricane is called category one. It has winds of about one hundred twenty to one hundred fifty kilometers an hour. This storm can damage trees and lightweight structures. It can also cause 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 roofs off houses.
Winds between about one hundred eighty and two hundred fifty kilometers an hour represent categories three and four. An even more powerful storm is a category five hurricane.
BARBARA KLEIN: The National Oceanic and Atmospheric Administration said Hurricane Katrina was a strong category three hurricane when it hit land in Louisiana. But researchers say other forces than its wind speed helped cause Katrina's extensive destruction. NOAA scientists say Katrina's air pressure was very low. The lower the air pressure, the stronger the storm. And Katrina was also an unusually wide storm.
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 meters.
BOB DOUGHTY: 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 and destroyed many areas of New Orleans. The surge washed away large areas of the coastal cities of Biloxi and Gulfport, Mississippi. There was also heavy damage in Alabama.
BARBARA KLEIN: Some scientists believe climate change affects major storms. Some say the warming of Earth's atmosphere is already making the storms worse. Other scientists have published studies that disagree.
Earlier this year, a special World Meteorological Organization committee reported on severe storms. The committee's work appeared in the journal "Nature Geoscience." Ten scientists took part. The experts represented both sides of the debate about global warming. They reached no clear answer about whether global warming had already intensified storms. Still, the committee made some predictions.
BOB DOUGHTY: They said global warming might cause more powerful ocean storms in the future. They said the overall strength of storms measured by wind speed might increase two to eleven percent by the year twenty-one hundred. And there might be an increase in the number of the most severe storms. But there might be fewer weak and moderate storms.
The current Atlantic Ocean hurricane season began in June. Weather experts say fewer severe storms than usual have struck since then. Experts had predicted above-normal numbers of storms during the season, which continues through November.
BARBARA KLEIN: This SCIENCE IN THE NEWS was written by Jerilyn Watson. It was produced by June Simms. You can read scripts and download audio at 51voa.com. You can also find us on Facebook, Twitter, YouTube and iTunes at VOA Learning English. I'm Barbara Klein.
BOB DOUGHTY: And I'm Bob Doughty. Listen again next week for more news about science in Special English on the Voice of America.