waterspout

[waw-ter-spout, wot-er-]

waterspout, tornado occurring at sea or over inland waters. The characteristic funnel-shaped cloud is formed at the base of a cumulus-type cloud and extends downward to the water surface, where it picks up spray. Waterspouts are most frequent in tropical regions, but are not uncommon in higher latitudes.

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Licensed from Columbia University Press

Waterspout

A waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud) that occurs over a body of water and is connected to a cumuliform cloud. In the common form, it is a nonsupercell tornado over water, and brings the water upward. It is weaker than most of its land counterparts.

Formation

Waterspouts exist on a microscale, where their environment is less than two kilometers in width. Their bigger cloud that develops them can be as innocuous as a moderate cumulus, or as great as a supercell. While some waterspouts are strong (tornadic) like their land-based counterparts, most are much weaker and caused by different atmospheric dynamics. They normally develop in moisture-laden environments with little vertical wind shear along lines of convergence, such as land breezes, lake effect bands, lines of frictional convergence from nearby landmasses, or surface troughs. Waterspouts normally develop as their parent clouds are in the process of development, and it is theorized that they spin up as they move up the surface boundary from the horizontal shear near the surface, and then stretch upwards to the cloud once the low level shear vortex aligns with a developing cumulus or thunderstorm. Weak tornadoes, known as landspouts, have been shown to develop in a similar manner.

Types

Non-tornadic

Waterspouts that are not associated with a rotating updraft of a supercell thunderstorm, are known as "nontornadic" or "fair-weather waterspouts", and are by far the most common type.

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

"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.

Climatology

Though the majority occur in the tropics, they can seasonally appear in temperate areas throughout the world, and are common across the western coast of Europe as well as the British Isles and several areas of the Mediterranean and Baltic Sea. They are not restricted to saltwater; many have been reported on lakes and rivers including all five Great Lakes and the Saint-Lawrence river. They are more frequent within 100 kilometers (60 mi) from the coast than farther out at sea. Waterspouts are common along the southeast U.S. coast, especially off southern Florida and the Keys and can happen over seas, bays, and lakes worldwide. Approximately 160 waterspouts are currently reported per year across Europe, with the Netherlands reporting the most at 60, followed by Spain and Italy at 25, and the United Kingdom at 15. They are most common in late summer. In the Northern Hemisphere, September has been pinpointed as the prime month of formation.

Nautical threat

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.

Winter waterspout or Snowspout

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:

Extremely cold temperatures present over a body of warm water enough to produce fog resembling steam above the water's surface; this usually requires temperatures of or colder if the water temperature is no warmer than .
Lake-effect snows in a clustered or banded formation must be present and going on.
The synoptic or environmental winds must be extremely light; usually less than .