A waterspout is a cyclonic wind event resembling a tornado that occurs over open bodies of water, sometimes in small groups called a waterspout cluster or family. Unlike tornadoes over land that often become visible due to the dust and debris present within them, a waterspout is made up of swirling droplets of water. Many people assume that a waterspout is nothing more than a tornado over water. This is not entirely true, however, as some waterspouts can develop in relatively fair weather conditions unlike the stormy conditions that normally produce tornadoes. For this reason, waterspouts are categorized in one of two ways — tornadic or fair-weather.
A tornadic waterspout forms from the same type of conditions that produce tornadoes — usually very strong thunderstorms that can also produce very heavy rain, rough seas, high winds, and hail. Such a waterspout can form over water or over land. A tornado that forms over land and moves over water becomes a waterspout. It is no different than a normal tornado, except that it exists over water. It is even possible for a tornado to move over water, becoming a waterspout, then move on shore again as a tornado.
Sometimes a waterspout forms under relatively calm conditions and is called a fair weather waterspout. This term can be somewhat misleading, however, as this type of waterspout usually forms under a bank of low lying cumulus clouds. Calling these types of waterspouts fair weather waterspouts is a reference to the fact that they can form when no storms are present, and even though they usually occur under a cloud bank, it is possible for the weather to be very calm and even fairly sunny. It is rare for such a waterspout to make landfall and if it does, it generally dissipates very quickly.
The two types of waterspouts differ in several ways besides the conditions under which they form. Tornadic waterspouts can be dangerous and destructive and can have winds as strong as a tornado. Fair weather waterspouts, while potentially dangerous if they directly contact small watercraft, are generally much weaker than tornadic spouts, and tend to be more short-lived. Tornadic waterspouts start in the cloud layer and extend downwards but fair weather waterspouts start at water level and extend upwards. Fair weather waterspouts, which generally only form over fairly large bodies of water, occur more often than the tornadic variety, which can form anywhere a tornado is possible, although neither type of waterspout is particularly common.