Weather experts 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 oceans. They also receive information from ships and passenger airplanes and other flights. Government scientists use specially-equipped planes to fly into and around storms. The crews drop instruments tied to parachutes. The instruments collect information about temperature, pressure and wind speed.

Scientists use the Saffir-Simpson Hurricane Scale to measure the intensity of storms based on wind speed. This 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 a category one. A hurricane called “Cat One” has winds of about 120 to 150 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 180 kilometers an hour. These storms are often powerful enough to break windows or blow the tops off houses.

Winds between about 180 and 250 kilometers an hour represent categories three and four. A more powerful storm is a category five hurricane. At its fiercest point, Hurricane Sandy was called a “Cat Three.”