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.