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Funnel Cloud:
A condensation cloud, typically funnel-shaped and extending
outward from a cumuliform cloud, associated with a rotating column
of air (a vortex) that may or may not be in contact with the ground.
If the rotation is violent and in contact with the ground, the
vortex is a tornado.
Funnel clouds can occur through a variety of processes in
association with convection. For example, small funnel clouds are
infrequently seen extending from small, dissipating cumulus clouds
in environments with significant vertical wind shear in the
cloud-bearing layer.
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Tornado: A violently
rotating column of air, in contact with the ground, either pendant
from a cumuliform cloud or underneath a cumuliform cloud, and often
(but not always) visible as a funnel cloud.
When tornadoes do occur without any visible funnel cloud, debris at
the surface is usually the indication of the existence of an intense
circulation in contact with the ground. On a local scale, the
tornado is the most intense of all atmospheric circulations. Its
vortex, typically a few hundred meters in diameter, usually rotates
cyclonically (on rare occasions anticyclonically rotating tornadoes
have been observed).
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Wall Cloud: A wall cloud marks
the lower portion of a very strong usually associated with a
supercell or severe multicell storm. It typically develops near the
precipitation region of the cumulonimbus. Wall clouds that
exhibit significant rotation and vertical motions often precede
tornado formation by a few minutes to an hour.
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Shelf Cloud:
The shelf cloud is attached to the convective storm's
cloud base. Rising motion can be seen in the leading (outer)
part of the shelf cloud, while the underside appears turbulent and
tattered. |
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Scud Cloud:
They are often found in the vicinity of rainshafts. Several
mechanisms may explain their occurrence. They may represent rising
air that has greater-than-average humidity due to the
evaporation of rain or water on the ground. They may evaporate before reaching
the main cloud due to mixing with the intervening
dry air. Scud may also be attributed to the breakup of
raindrops. The breakup fragments that are able to accumulate in
regions of high humidity in rainshafts may be the source of some
scud. |
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"Gustnado":
Gustnadoes typically appear as a swirl of dust or
debris along the leading edge of the thunderstorm outflow. There is
usually no condensation funnel or other visible connection to the
cloud base above. Since they develop along the leading edge of a
storm, gustnadoes are not usually associated with a wall cloud or
rain-free cloud base, therefore, these tornadoes are typically
extremely difficult to identify visually. Some ground-based
circulations associated with thunderstorm outflow can become more
intense and more closely resemble a "traditional" tornado (that is,
they fit the Glossary definition).
The gustnado is typically associated with bow echoes or squall
lines, and should not be confused with tornadoes that may develop in
the rotating portion of the bow echo which Fujita refers to as the
comma head. |
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Landspouts:
Landspouts form when pre-existing horizontal
circulations are stretched and tilted upward by a developing
thunderstorm updraft. As with gustnadoes, landspouts do not usually
form from mesocyclones or supercells. In fact, a large number of
landspouts are observed in association with lines of cumulus
congestus or towering cumulus clouds, often before precipitation is
visible on radar. However, storm interceptors have noted the
presence of landspouts in conjunction with supercell thunderstorms,
sometimes at the same time as, but in a different part of the storm
than a supercell tornado.
Landspouts are usually visible, unlike gustnadoes, and most have
a narrow, rope-like condensation funnel extending from cloud base to
the ground. Wall clouds are not usually observed with landspouts,
and these tornadoes are typically short-lived and weak. Damage
associated with landspouts can be significant, however, with damage
in the F1 category not uncommon. |
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Cold Air Funnel: A cold
air funnel is a high based weak tornado that occurs in a cool air
mass. By high based it is meant it develops well above the earth's
surface. Since it is high based and weak they rarely impact the
earth's surface although they can look threatening. Unlike typical
tornadoes, cold air funnels develop in a shallow cool air mass and
often behind a cold frontal passage. The mixing of cool and windy
conditions in the lower troposphere with air in the middle
troposphere flowing in a different direction may spark the rotation
that spins up the funnel. If the air is moist enough and rises
enough the condensation funnel will be visible.
National Weather Service Definition: There is much debate about what causes a "cold
air funnel" and scientists are still researching this
phenomenon, however, the term "cold air funnel" is one of the
terms used by meteorologists to identify funnel clouds that are
not produced by a mesocyclone and/or are not in an environment
typically associated with severe thunderstorms. Basically all
funnel clouds are the same. The over-all weather pattern in
which they form may be different but the mechanism for producing
the funnel, namely, increasing cyclonic (or counter-clockwise)
rotation, is the same and involves the vertical stretching of
vorticity. Vorticity can basically be thought of as the amount
of "spin" inherent in the atmosphere produced by wind shear. As
cyclonic vorticity (or rotation) is stretched vertically the
area of rotation decreases so the speed of rotation must
increase to compensate. This is analogous to an ice skater
pulling in their arms and legs and increasing their rotation.
Based upon observations of cold air funnel
events during the past few years we have noticed that they seem
to generally prefer an environment when there is large scale
cyclonic (counter-clockwise) rotation associated with a large,
slow moving, closed low in the upper atmosphere. Since these
lows have been circulating in the atmosphere for a long time
(compared to the time scale of severe thunderstorms) they stir
the air and generate smaller scale rotation providing a "vorticity-rich"
environment. These lows also have a cold pool of air associated
with them resulting in unstable conditions for weak
thunderstorms to form. This is why the term cold air funnel got
its origin. However the fact that there is cold air aloft does
not govern how these funnels form, it only provides the
instability for thunderstorms (convection) to form. Based on our
observations one explanation of how cold air funnels form may be
that in the growth stage of these weak thunderstorms the updraft
stretches pre-existing smaller scale vortices within the
vorticity-rich airmass causing an increase in rotation and the
formation of a funnel. This process is similar to that shown by
research meteorologists in the formation of landspouts which
typically form on the High Plains. Cold air funnels generally
are weak, short lived, and usually do not touch down. One of the
reasons that they do not touch down and, by definition become a
tornado, may be that there is little rainfall associated with
these storms to produce a downdraft in order to generate a low
level circulation. Recent research suggests that the interaction
of descending rainfall near the surface and the thunderstorm
updraft may be very important in generating a low level
circulation which links with the upper level mesocyclone
circulation to form the tornado in typical severe thunderstorms.
These processes are not present with "cold air funnels".
NWS:
http://www.crh.noaa.gov/fsd/science/faqsummer.php
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Dust Devils: On sunny, dry days, heated air near the
ground can rise in small, spinning columns. If these columns occur
over dusty ground, a dust devil may be observable. The stronger ones
become visible when loose grass, hay or dust gathers into the whirl
and rises up the column. These dust devils can look like a weak
tornado at the bottom but rarely extend higher than 300 feet. They
are only seen in fair weather - sometimes without a cloud in the
sky. Larger dust devils can extend hundreds of feet high, toss
lawn furniture and lift objects weighing a hundred kilograms, but
are generally not a threat otherwise. Dust devils near a highway
deserve caution as vehicles passing through them can be difficult to
control. Like other weak circulations, meteorologists can tell which
days and general areas are most likely to have dust devils, but they
cannot be forecast and are not observable on Doppler radar. Severe
Weather Warnings are not usually issued for dust devils.
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