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Accessory Clouds
Clouds that are dependent on a larger cloud system for development
and continuance. Accessory clouds associated with the thunderstorm include roll, shelf,
mammatus, and wall clouds.
Anvil
The spreading of the upper portion of a cumulonimbus cloud into an
anvil-shaped plume usually of fibrous or smooth appearance. Strong or severe thunderstorms
often have thicker anvils with the side and bottom having a cumuliform or slowly boiling
appearance in the immediate vicinity of the parent cumulonimbus.
Collar Cloud
Frequently used as a synonym for wall cloud, although it actually is
a generally circular ring of cloud surrounding the upper portion of a wall cloud.
Cumulonimbus Cloud
The parent cloud of a thunderstorm. The cumulonimbus cloud towers
above the ordinary cumulus clouds, with stronger or severe storms often having a more
sharply outlined "hard" appearance with relatively rapid rising motions visible.
The cloud's upper portion includes the anvil. Accompanying precipitation is often heavy
and the usual occurrence of lightning and thunder with these clouds leads to the popular
names of thunderhead or thundercloud.
Cumulus Cloud
A column of rising air that has condensed into a dense, nonfibrous
cloud with distinct outlines, appearing much like a rising mound or dome. The base of the
cloud is relatively flat and dark, while the tower is usually white and sunlit. The
cumulus cloud is the first stage of a developing thunderstorm.
Downdraft
A column of generally cool air that rapidly sinks to the ground,
most often accompanied by precipitation in a shower orthunderstorm. Areas of downdraft
usually contain little cloud, and what clouds that may be present are typically
dissipating.
Flanking Line
A line of cumulus connected to and extending outward from the most
active portion of a parent cumulonimbus, usually found on the southwest side of the storm.
The cloud line has roughly a stair step appearance with the taller clouds adjacent to the
parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.
Flash Flooding
Rapidly-developing flooding of tributaries, usually as a result of
thunderstorms.
Front
A transition zone between two differing air masses. Basic types are
cold front, warm front, and stationary front. Thunderstorms can form in association with
any of these fronts, although fronts are not necessary for thunderstorm development.
Funnel Cloud
A funnel-shaped cloud extending from a towering cumulus or
cumulonimbus base. It is associated with a towering air column that is not in contact with
the ground. If a ground-based debris or dust whirl is visible below the funnel aloft, the
cloud is a tornado.
GOES-8
One of the Geostationary Operational Environmental Satellites. They
are owned and run by the National Oceanic and Atmospheric Administration (NOAA), while
NASA designs and launches them.
Gust Front
The leading edge of the thunderstorm downdraft air. Most prominent
beneath the rain-free base and on the leading edge of an approaching thunderstorm. Usually
marked by gusty, cool winds. The shelf or roll cloud sometimes accompanies the gust front,
especially when the gust front precedes a line of thunderstorms.
Hail
Precipitation in the form of balls or clumps of ice produced by
thunderstorms. Severe storms with intense updrafts are most likely
to produce large hail.
Hook Echo
A radar pattern sometimes observed in the southwest quadrant of a
tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo
is precipitation aloft around the periphery of a rotating column of air 2-10 miles in
diameter.
Lightning
Any and all of the various forms of visible electrical discharge
caused by thunderstorms. Severe thunderstorms usually have frequent -- sometimes
continuous -- lightning. However, some non-severe thunderstorms also contain frequent and
vivid electrical displays, while some severe storms are accompanied by little lightning.
Mamma Clouds
Also called mammatus, these clouds appear as hanging, rounded
protuberances or pouches on the under-surface of a cloud. With thunderstorms, mammatus are
seen on the underside of the anvil. These clouds do not produce tornadoes, funnels, hail,
or any other type of severe weather, although they often accompany severe thunderstorms.
NSSL
The National Severe Storms Laboratory.
Precipitation Shaft
A visible column of rain and/or hail falling from a cloud base. When
viewed against a light background, heavy precipitation appears very dark gray, sometimes
with a turquoise tinge. This turquoise tinge has been commonly attributed to hail, but its
actual cause is unknown.
Rain-Free Base
A horizontal, dark cumulonimbus base that has no visible
precipitation beneath it. This structure usually marks the location of the thunderstorm
updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the
rain-free base, or (2) from the rain-free base itself. This is particularly true when the
rain-free base is observed to the south or southwest of the precipitation shaft.
River Flood
Usually occurs on rivers, after flash flooding has occurred on
streams and tributaries. River floods develop and reach their peak more slowly than flash
floods. In many cases, the river flood peak occurs after the rain has ended.
Roll Cloud
A relatively rare, low-level horizontal, tube-shaped accessory cloud
completely detached from the cumulonimbus base. When present, it is located along the gust
front and most frequently observed on the leading edge of a line of thunderstorms. The
roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll
clouds are not and do not produce tornadoes
Scud Clouds
Low cloud fragments often seen in association with and behind
thunderstorm gust fronts. these clouds are ragged and wind-torn and are not usually
attached to the thunderstorm base. Scud clouds do not produce severe weather. In some
cases, when clouds are attached to the thunderstorm base, they can be mistaken for wall
clouds or tornadoes.
Severe Thunderstorm
A thunderstorm that goes from the mature stage to the severe stage
before dissipating. Severe thunderstorms are most efficient "machines" because
the updraft remains strong for a long time. They also occasionally contain rotation on a
broad scale. Because of its structure, the severe storm may last for hours beyond the
lifetime of a normal thunderstorm while producing large hail, high winds, torrential rain,
and possible tornadoes. Officially, a thunderstorm is classified as severe if 50-knot (58
mph) winds are measured, 3/4-inch or larger hail occurs, or funnel clouds or tornadoes
develop.
Shelf Cloud
A low-level horizontal accessory cloud that appears to be
wedge-shaped as it approaches. It is usually attached to the thunderstorm base and forms
along the gust front. The leading edge of the shelf is often smooth and at times layered
or terraced. It is most often seen along the leading edge of an approaching line of
thunderstorms, accompanied by gusty straight winds as it passes overhead and followed by
precipitation. The underside is concave upward, turbulent, boiling, or wind-torn.
Tornadoes rarely occur with the shelf cloud.
Squall Line
Any line or narrow band of active thunderstorms. The term is usually
used to describe solid or broken lines of strong or severe thunderstorms.
Straight Winds
Winds associated with a thunderstorm, most frequently found with the
gust front. These winds originate as downdraft air reaches the ground and rapidly spreads
out, becoming a strong and horizontal flow. Damaging straight winds, although relatively
rare themselves, are much more common than are tornadoes.
Tail Cloud
A low tail-shaped cloud extending outward from the northern quadrant
of a wall cloud. Motions in the tail cloud are toward the wall cloud with rapid updraft at
the junction of tail and wall cloud. This horizontal cloud is not a funnel or tornado.
Thunderstorm
A local storm (accompanied by lightning and thunder) produced by a
cumulonimbus cloud, usually with gusty winds, heavy rain, and sometimes hail. Non-severe
thunderstorms rarely have lifetimes over two hours. A typical non-severe thunderstorm life
cycle consists of three stages: cumulus stage, mature stage, and dissipation stage.
Tornado
A violently rotating narrow column of air in contact with the ground
and extending from a thunderstorm base. The tornado is most often found in the southwest
quadrant of the storm, near the trailing edge of the cumulonimus cloud. Tornadoes and
funnel clouds are usually pendant from wall clouds or directly from the thunderstorm base,
within a few miles to the southwest of the precipitation shaft. The spinning motion of a
tornado is most often left to right on the backside (counterclockwise). Tornadoes have
been called twisters and cyclones, but these words are all synonyms for the most violent
storm on earth, with estimated wind speeds up to 300 mph.
Updraft
Warm moist air which rises and condenses into a visible cumulus or
cumulonimbus cloud. Once the cloud forms, it depends on the updraft for continuance and
further development.
Virga
Wisps or streaks of rain falling out of a cloud but not reaching the
earth's surface. When seen from a distance, these streaks can be mistaken for funnels or
tornadoes.
Wall Clouds
A local and often abrupt lowering of a rain-free cumulonimbus base
into a low-hanging accessory cloud, from 1 to 4 miles in diameter. The wall cloud is
usually situated in the southwest portion of the storm below an intense updraft marked by
the main cumulonimbus cloud and associated with a very strong or severe thunderstorm. When
seen from several miles away, many wall clouds exhibit rapid upward motion and rotation in
the same sense as a tornado, except with considerably lower speed. A rotating wall cloud
usually develops before tornadoes or funnel clouds by a time which can range from a few
minutes up to possibly an hour. Sometimes other long-hanging accessory clouds are
mistakenly identified as wall clouds.
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