Waterspout
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 exist on a microscale, where
their environment is less than two kilometers in width. The cloud from which
they develop can be as innocuous as a moderate cumulus, or as great as a supercell. While some waterspouts are strong
and tornadic in nature, 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 they
spin 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 cloud or thunderstorm. Weak tornadoes, known
as landspouts, have been shown to develop in a
similar manner. More than one
waterspout can occur in the same vicinity at the same time. As many as nine
simultaneous waterspouts have been reported on Lake Michigan
Types
Non-tornadic
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. They usually rate no higher than EF0 on the Enhanced 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 verticalconvective 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 in connection
with severe thunderstorms, 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 rarer than their
fair-weather counterparts in that country. However, in some areas, such as
the Adriatic, Aegean and Ionian seas, tornadic waterspouts can
make up half of the total number.
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. 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. Very cold
temperatures need to be present over a body of water warm enough to
produce fog resembling steam above the water's
surface. Like the more efficient lake-effect snow events, winds
focusing down the axis of long lakes enhance wind convergence and likely
enhance their development.
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 the Great Lakes and the St. Lawrence River. 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. 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. Waterspouts are
frequently observed off the east coast of Australia, with several being
described by Joseph
Banks during
the voyage of the Endeavour in 1770.
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.
Nautical Threats
Waterspouts have long been recognized as
serious marine hazards. Stronger waterspouts can be dangerous, posing threats
to water craft, air craft and swimmers. 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.
Research and Forecasting
Szilagyi
Waterspout Index (SWI)
The Szilagyi Waterspout Index (SWI), developed by Canadian
meteorologist Wade Szilagyi, is used to predict
conditions favorable for waterspout development. The SWI ranges from −10 to
+10, where values greater than or equal to zero represent conditions favorable
for waterspout development.
International
Centre for Waterspout Research (ICWR)
The ICWR is a non governmental organization
of individuals from around the world who are interested in the field of
waterspouts from a research, operational and safety perspective. Originally a forum
for researchers and meteorologists, the ICWR has expanded interest and
contribution from storm chasers, the media, the marine and aviation communities
and from private individuals.
