Fair-weather waterspouts occur in coastal waters and are associated with dark, flat-bottomed, developing convective cumulus towers. They usually rate no higher than EF0 on the Fujita scale, generally exhibiting winds of less than 30 m/s (67 mph). They are most frequently seen in tropical and sub-tropical climates, with upwards of 400 per year observed in the Florida Keys. They typically move slowly, if at all, since the cloud they are attached to is horizontally static, being formed by vertical convective action instead of the subduction/adduction interaction between colliding fronts. Fair-weather waterspouts are very similar in both appearance and mechanics to landspouts, and largely behave as such if they move ashore.
"Tornadic waterspouts", also accurately referred to as "tornadoes over water", are formed from mesocyclonic action in a manner essentially identical to traditional land-based tornadoes, but simply occurring over water. A tornado which travels from land to a body of water would also be considered a tornadic waterspout.
Since the vast majority of mesocyclonic thunderstorms occur in land-locked areas of the United States, true tornadic waterspouts are correspondingly more rare than their fair-weather counterparts. Like all tornadoes, they possess an intensity commensurate to the system which spawned them, but are generally limited in both power and lifespan by the disruptive thermo- and hydrodynamic effects bodies of water tend to have on the complex mesocyclonic action needed to sustain a powerful tornado. Water is also a great deal heavier than the dirt, dust, and debris commonly ingested by a tornado.
Waterspouts have long been recognized as serious marine hazards. Lucretius wrote about whirling columns that descended from the sky into the ocean and put sailors "into great peril"; history is filled with examples of ships being destroyed or damaged by them.
Stronger waterspouts are usually quite dangerous, posing threats to ships, planes, and swimmers. It is recommended to keep a considerable distance from these phenomena, and to always be on alert through weather reports. The U.S. National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.
A winter waterspout, also known as a snow devil, an icespout, an ice devil, a snonado, or a snowspout, is an extremely rare instance of a waterspout forming under the base of a snow squall. The term "winter waterspout" is used to differentiate between the common warm season waterspout and this rare winter season event which will form over breaks in ice covered body of waters at temperatures of −18 °C (−0.4 F) or colder. Very little is known about this rare phenomenon and only six known pictures of this event exist to date, four of which were taken in Ontario, CanadaEnvironmental Conditions There are three critical criteria for the formation of a winter waterspout: