Meteorologists monitor weather systems in tropic areas to study the formation of tropical cyclones as well as to predict when tropical cyclones will occur and where they will go. As a clouds and wind build meteorologists use different terms to describe the weather system, including tropical disturbance, tropical depression, tropical storm and tropical cyclone. The name of the weather system depends on wind speed.
A tropical disturbance refers to a group of thunderclouds that have formed over an area of low pressure and are rotating slightly. Viewed from a satellite, a disturbance will appear disorganized without a distinctive shape--more like a group of thunderstorms than a tropical cyclone. This weather system will only be called a disturbance if it has maintained rotation for over 24 hours.
A tropical disturbance can evolve into a tropical depression if the maximum sustained wind speed increases to 38 miles per hour (61 kmph). These storms are usually about the same size (diameter) of a disturbance, but the rotation of the storm is more defined. A depression can technically be called a tropical cyclone, but won't receive the official designation of hurricane, typhoon or cyclone unless it becomes bigger.
Once a depression reaches the maximum speed of 73 mph (117 kmph) it becomes a tropical storm. At this stage, the system will look much more organized, beginning to take on the distinctive shape of a tropical cyclone as the circulation of wind intensifies.
These systems only receive the designation of hurricane, typhoon or cyclone when wind speed exceed 73 mph. As a hurricane continues to intensify, their strength is categorized by something known as the Saffir-Simpson scale. This scale includes the designations one through five. Hurricanes designated between three and five are known as Major Hurricanes or Intense Hurricanes.
