Waterspout

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Waterspouts on the beach of Kijkduin near The Hague , the Netherlands on 2006 August 27.

A waterspout near Florida.

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 is weaker than most of its land counterparts.^[1]

1 Formation
2 Types
3 Deterioration
4 Climatology
5 Nautical threat
6 References
7 See also
8 External links

[edit] Formation

Waterspouts exist on the microscale, meaning their environment is less than two kilometers in width. 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 nearly 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 horizonal 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.^[2] An outbreak of over 66 waterspouts occurred in the Great Lakes in late September and early October 2003 along a lake effect band. Their parent cloud can be as innocuous as a moderate cumulus, or as significant as a supercell.

[edit] Types

Waterspouts, not associated with a rotating updraft of a supercell thunderstorm, are "nontornadic" or "fair-weather waterspouts,"^[3] while those associated with mesocyclones are called "tornadic waterspouts". Tornadic waterspouts are generally more intense than nonsupercell waterspouts, and really just resemble tornadoes on the water.

Fair weather waterspouts occur in coastal waters and are associated with usually weak, developing convective towers. They usually are F0, comprised of winds of less than 30 m/s. These are arguably the most common waterspouts in tropical climates, with upwards of 400 per year observed in the Florida Keys.^[4] Fair-weather waterspouts are similar to landspouts.^[4]

A pair of waterspouts off the Bahamas

[edit] Deterioration

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[edit] 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.^[3] They are more frequent within 100 kilometers (62 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, a couple climatologies have both pinpointed September as the prime month of formation.^[5]

[edit] 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 either of 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.