waterspout

A waterspout near Florida. Note the two flares with smoke trails for indicating wind direction and general speed near the bottom of the photograph.

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 non-supercell tornado over water. While it is often weaker than most of its land counterparts,^[1] stronger versions spawned by mesocyclones do occur. Contrary to popular belief that waterspouts suck up water, the water seen in the main funnel cloud is actually water droplets formed by condensation.^[2] While many waterspouts form in the tropics, locations at higher latitude within temperate zones also report waterspouts, such as Europe and the Great Lakes. Although rare, waterspouts have been observed in connection with lake-effect snow precipitation bands. Waterspouts have a five-part life cycle: formation of a dark spot on the water surface, spiral pattern on the water surface, formation of a spray ring, development of the visible condensation funnel, and ultimately decay.

Formation

Waterspouts seen from the beach at Kijkduin near The Hague, the Netherlands on August 27, 2006.

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 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.^[3]

Types

Non-tornadic

A pair of waterspouts off the Bahamas

Waterspouts that are not associated with a rotating updraft of a supercell thunderstorm, are known as "non-tornadic" 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. Waterspouts of this type rapidly develop and dissipate, having life cycles shorter than 20 minutes.^[3] They usually rate no higher than EF0 on the Enhanced Fujita scale, generally exhibiting winds of less than 30 m/s (67 mph).^[4] They are most frequently seen in tropical and sub-tropical climates, with upwards of 400 per year observed in the Florida Keys.^[5] 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. ^[5][6] Fair-weather waterspouts are very similar in both appearance and mechanics to landspouts, and largely behave as such if they move ashore.^[5]

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 in connection with severe thunderstorms, but simply occurring over water.^[7] A tornado which travels from land to a body of water would also be considered a tornadic waterspout.^[8] Since the vast majority of mesocyclonic thunderstorms occur in land-locked areas of the United States, true tornadic waterspouts are correspondingly rarer than their fair-weather counterparts. However, in some areas, such as the Adriatic, Aegean and Ionian seas, tornadic waterspouts can make up half of the total number.^[9]

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.^[10][11] The term "winter waterspout" is used to differentiate between the common warm season waterspout and this rare winter season event. Very little is known about this phenomenon and only six known pictures of this event exist to date, four of which were taken in Ontario, Canada. There are a couple of critical criteria for the formation of a winter waterspout. Extremely cold temperatures need to be present over a body of warm water enough to produce fog resembling steam above the water's surface; this requires a 19°C (34°F) temperature difference between the water and the invading surface air mass. Like the more efficient lake-effect snow events, winds focusing down the axis of long lakes enhance wind convergence and likely enhance their development.^[12]

Climatology

A family of four waterspouts seen on the Great Lakes (Lake Huron) on September 9, 1999.

Four waterspouts seen in the Florida Keys on June 5, 2009.

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 the Great Lakes and the St. Lawrence River. ^[13] Waterspouts are fairly common on the Great Lakes during late summer and early fall, with a record 66+ waterspouts reported over just a seven day period in 2003. ^[14] 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. ^[15]

Life cycle

There are five stages to the waterspout life cycle. Initially, a prominent circular, light-colored disk appears on the surface of the water, surrounded by a larger dark area of indeterminate shape. After the formation of these colored disks on the water, a pattern of light and dark-colored spiral bands develop from the dark spot on the water surface. Then, a dense annulus of sea spray, called a cascade, appears around the dark spot with what appears to be an eye. Eventually, the waterspout becomes a visible funnel from the water surface to the overhead cloud. The spray vortex can rise to a height of several hundred feet or more and often creates a visible wake and an associated wave train as it moves. Eventually, the funnel and spray vortex begin to dissipate as the inflow of warm air into the vortex weakens, ending the waterspout's life cycle.^[6]

Nautical threat

Waterspouts have long been recognized as serious marine hazards. Stronger waterspouts are usually quite dangerous, posing threats to ships, planes, helicopters, and swimmers.^[16] It is recommended to keep a considerable distance from these phenomena, and to always be on alert through weather reports. The United States 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 are expected to move onshore.^[17] When close to shorelines, waterspouts can devastate nearby coral reefs and marine organisms close to towards the surface.^{[citation needed]}

The Szilagyi Waterspout Index (SWI)

The Szilagyi Waterspout Index measures waterspout potential. It ranges from -10 to +10 where SWI values greater then or equal to zero represent conditions favorable for waterspout development.

See also

Wikimedia Commons has media related to: Waterspout

• Tornadogenesis
• Funnel cloud
• Landspout
• Tornado

References

External links

General

• A series of pictures from the boat Nicorette getting impressively close to the south coast tornadic waterspout.
• A USA Today online article on waterspouts: http://www.usatoday.com/weather/wspouts.htm
• Online waterspout article
• Home video of a waterspout on Long Island Sound on 27 September 2006
• Pictures of cold-core waterspouts over Lake Michigan on 30 September 2006. Archived from the original on March 10, 2007.
• http://aoss-research.engin.umich.edu/PlanetaryEnvironmentResearchLaboratory/
• - WaterspoutVideo.com (part of existing website FloridaLightning.com) - The most comprehensive website with waterspout footage from the Florida Keys. Updated often.

Winter waterspout

• Winter Waterspout Environment Canada
• A Winter Waterspout Monthly Weather Review, February 1907.