Below is a listing of the wind speed ranges for each class. The likelihood of tornadoes occurring in a region is first determined by analyzing the general atmospheric conditions. When the conditions appear to be favorable for development of tornadoes, then the National Weather Service or NWS may issue a tornado watch . This simply means that conditions are favorable and concern is warranted. In order to attempt to monitor the formation or existence of a tornado, weather radar has been used for many years.
Conventional radar measures the reflected signal off precipitation in clouds.
Because of the structure of typical severe thunderstorm that produces a tornado, the shape of the precipitation region may resemble a hook. The return from conventional radar from such a thunderstorm is called a hook echo. A precipitation-free region near the end of the hook is where tornadoes are likely to exist, and is called the vault. Recently, Doppler Radars are becoming more common. These measure the frequency shifts of the returning signals caused by the Doppler effect.