Of Weather Terms
Air mass thunderstorm - Generally, a thunderstorm
not associated with a front or similar type of weather
feature. Air mass thunderstorms typically are associated
with warm, tropical air in the summer months; they
develop during the afternoon in response to heating,
and dissipate rather quickly after sunset. They
generally are less likely to be severe than other
types of thunderstorms.
Anvil - The flat, spreading top of a thunderstorm.
Approaching severe - A thunderstorm which
contains winds of 40 to 57 mph or hail around 1/2
inch in diameter.
Bow echo - On radar, a line of thunderstorms
that bulges outward into a bow shape. Damaging thunderstorm
winds often occur near the center of a bow echo.
Box - A severe thunderstorm watch or tornado
watch. The term derives from the fact that a watch
takes the shape of a rectangle or parallelogram
when plotted on a map.
Cap - A layer of warm air, several thousand
feet above the surface, which suppresses or delays
the development of thunderstorms. If the air is
unstable enough, explosive thunderstorm development
can occur if the cap is removed or weakened (for
example, when colder air moves in).
Cold air funnel - A funnel cloud or (rarely)
a small, relatively weak tornado that can develop
from a shower or thunderstorm when the air aloft
is unusually cold (hence the reference to "cold
Convection - In meteorology, this term is
used most often to describe the vertical transport
of heat and moisture, especially by updrafts and
downdrafts in unstable air. Showers and thunderstorms
are forms of convection.
Cumulonimbus cloud - A cloud characterized
by strong vertical development in the form of mountains
or huge towers, topped at least partially by a smooth,
flat anvil. This type of cloud is more commonly
known as a thunderstorm or thunderhead.
Cumulus - Detached clouds, generally dense
and with sharp outlines, showing vertical development
in the form of domes, mounds, or towers. Tops normally
are rounded while bases are more horizontal. Cumulus
clouds may grow into towering cumulus or cumulonimbus
Derecho - A widespread and usually fast-moving
windstorm associated with convection. Derechos include
any family of downburst clusters and can produce
damaging thunderstorm winds over areas hundreds
of miles long and more than 100 miles across.
Downburst - A strong downdraft resulting
in an outward burst of damaging winds on or near
the ground. Downburst winds can produce damage similar
to a strong tornado.
Downdraft - A small-scale column of air that
rapidly sinks toward the ground, usually accompanied
by precipitation as in a shower or thunderstorm.
Dry line - A boundary separating moist and
dry air masses. It is an important factor in severe
weather frequency in the Great Plains. It typically
lies north-south across the central and southern
High Plains during the spring and early summer,
where it separates moist air from the Gulf of Mexico
and dry desert air from the southwestern states.
The dry line typically advances eastward during
the afternoon and retreats westward at night. However,
a strong storm system can sweep the dryline eastward
into the Mississippi Valley, or even farther east,
regardless of the time of day.
Fujita Scale (or F-Scale) - A scale
of wind damage intensity in which wind speeds are
inferred from an analysis of wind damage. All tornadoes,
and most other severe local wind storms, are assigned
a single number from the scale according to the
most intense damage caused by the storm.
(weak): 40-72 mph, light damage
F1 (weak): 73-112 mph, moderate damage
F2 (strong): 113-157 mph, considerable damage
F3 (strong): 158-206 mph, severe damage
F4 (violent): 207-260 mph, devastating damage
F5 (violent): 261-318 mph, (rare) incredible damage
cloud - A funnel extending from the base of
a towering cumulus or cumulonimbus cloud, associated
with a rotating column of air that is NOT in contact
with the ground. The terms funnel cloud and tornado
are NOT interchangeable.
Gust front - The leading edge of gusty surface
winds from thunderstorm downdrafts. Passage of the
gust front is usually marked by cool, gusty winds.
The gust front often precedes the precipitation
by several minutes.
Gustnado - A small tornado, usually weak
and short-lived, that occurs along the gust front
of a thunderstorm.
Hook echo - A radar pattern characterized
by a hook-shaped (or figure 6-shaped) extension
of a thunderstorm echo, usually in the southwest
part of the storm. A hook is often associated with
a mesocyclone, and indicates favorable conditions
for tornado development.
Instability - The tendency for air parcels
to accelerate when they are displaced from their
original position; the greater the instability,
the greater the potential for severe thunderstorms.
Inversion - Usually used in reference to
temperature; an increase in temperature with height
(which is the reverse of what usually occurs in
Mammatus clouds - Rounded, sack-like protrusions
hanging from the underside of a cloud (usually a
thunderstorm anvil). These clouds do not produce
severe weather. They often accompany severe thunderstorms,
but may accompany non-severe thunderstorms as well.
Mesoscale Convective Complex (MCC) - A large
complex of thunderstorms, generally round or oval-shaped,
which normally reaches peak intensity at night.
An MCC must meet certain criteria for size, duration,
and shape. MCCs typically form during the afternoon
and evening in the form of several isolated thunderstorms,
during which the potential for severe weather is
greatest. During peak intensity, the primary threat
shifts toward heavy rain and flooding.
Mesoscale Convective System (MCS) - A term
often used to describe a cluster of thunderstorms
that does not meet the size, duration, or shape
criteria of an MCC. It is a complex of thunderstorms,
which may be round-shaped or in a line, and normally
persists for several hours or more.
Mesocyclone - A region of rotation, typically
2 to 6 miles in diameter, often found on the southwest
part of a supercell. The circulation of a mesocyclone
covers an area much larger than the tornado which
MAY develop within it. This is technically a radar
term defining a signature of rotation on Doppler
radar that meets specific criteria for magnitude,
vertical depth, and duration.
Microburst - A small, concentrated downburst
affecting an area less than about 2.5 miles across.
Most microbursts are rather short-lived (5 minutes
or so ), but on rare occasions have been known to
last up to 30 minutes.
Outflow boundary - A boundary separating
thunderstorm-cooled air (outflow) from the surrounding
air; similar in effect to a cold front, with the
passage marked by a wind shift and usually a drop
in temperature. Outflow boundaries may persist for
24 hours or more after the thunderstorms that generated
them dissipate, and may travel hundreds of miles
from their area of origin. New thunderstorms often
develop along outflow boundaries, especially near
the point of intersection with another boundary
(cold front, dry line, another outflow boundary).
Overrunning - Relatively warm moist air moving
above another air mass of greater density (colder
air). Embedded thunderstorms sometimes develop in
such a pattern; severe thunderstorms (mainly with
large hail) can occur, but tornadoes are unlikely.
Pulse storm - A thunderstorm within which
a brief period (pulse) of strong updraft occurs,
during and immediately after which the storm produces
a short episode of severe weather. These storms
generally are not tornado producers, but often produce
large hail and/or damaging winds.
Risks (Severe Thunderstorm) - The Storm Prediction
Center (SPC) assesses risks of severe thunderstorms
in its convective outlooks. The risks are as follows:
SLGT risk implies that well-organized severe
thunderstorms are expected but in small numbers
and/or low coverage. Within a slight risk area,
5-29 reports of 1 inch hail or larger, and/or
3-5 tornadoes, and/or 5-29 wind events (i.e winds
of 58 mph or greater) are forecast.
MDT risk implies a greater concentration
of severe thunderstorms, and in most situations,
a greater magnitude of severe weather. Within
a moderate risk area, at least 30 reports of 1
inch hail or larger, and/or 6-19 tornadoes, or
numerous wind events (30 that might be associated
with a squall line, bow echo or derecho) are forecast.
HIGH risk almost always means a major severe
weather outbreak is expected, with great coverage
of severe weather and enhanced likelihood of extreme
weather (i.e. violent tornadoes or extreme convective
wind events over a large area). Within a high
risk area, expect at least 20 tornadoes with at
least 2 of them rated F3+, or an extreme derecho
causing 50+ widespread wind events with numerous
higher end wind (80+ mph) and structural damage
Roll cloud - A low, horizontal, tube-shaped
cloud associated with a thunderstorm gust front.
Roll clouds are completely detached from the base
of the thunderstorm and appear to be "rolling" about
a horizontal axis. They are NOT horizontal funnel
Rope - A narrow, often contorted funnel associated
with the decaying stage of a tornado.
Scud - The common name for stratus fractus
clouds. They are small, ragged, low cloud fragments
that are usually not attached to a larger cloud
base (such as the base of a thunderstorm). They
are often seen with and behind cold fronts and thunderstorm
gust fronts. These clouds do not produce severe
weather. When they are near, or attached to the
base of the thunderstorm, they can be mistaken for
Severe thunderstorm - A thunderstorm with
winds of 58 mph (50 knots) or more, or hail 3/4
inch in diameter or larger. Structural damage may
imply the occurrence of a severe thunderstorm. Hail
and wind are the two elements for classifying a
thunderstorm as severe; lightning and flooding rains
are elements of the thunderstorm itself.
SPC - The Storm Prediction Center in Norman,
OK; formerly known as the National Severe Storms
Forecast Center or SELS. SPC issues convective outlooks
and all severe thunderstorm and tornado watches
for the lower 48 states.
Squall line - A solid or nearly solid line
or band of thunderstorms.
Straight-line winds - Generally, any wind
that is not associated with rotation; used mainly
to differentiate thunderstorm winds from tornadic
winds. Straight-line winds originate as a downdraft
of rain-cooled air, which reaches the ground and
spreads out rapidly, producing a potentially damaging
gust of wind up to 100 mph. In recent years, there
have been several occasions on which winds greater
than 100 mph have been measured.
Suction vortex - A small but very intense
vortex within a tornado circulation. Several suction
vortices typically are present in a multiple-vortex
tornado. Much of the extreme damage associated with
violent tornadoes is attributed to suction vortices.
Supercell - A relatively long-lived thunderstorm
with a persistent rotating updraft. Supercells are
rare, but are responsible for a remarkably high
percentage of severe weather events - especially
tornadoes, extremely large hail, and damaging wind
Tornado - A violently rotating column of
air in contact with the ground.
Towering cumulus - A large cumulus cloud
with great vertical development, usually with a
cauliflower-like appearance, but lacking the characteristic
anvil of a cumulonimbus cloud.
Updraft - A small-scale current of rising
air. If the air is sufficiently moist, then the
moisture condenses to become a cumulus cloud or
an individual tower of a towering cumulus or a cumulonimbus.
Upslope flow - Air that flows toward higher
terrain, and hence is forced to rise.
Virga - Streaks or wisps of precipitation
falling from a cloud but evaporating before reaching
Wall cloud - A local, often abrupt lowering
from the rain-free base of a thunderstorm. Wall
clouds can range from a fraction of a mile up to
nearly 5 miles in diameter, and normally are found
on the south or southwest side of the thunderstorm.
When seen from within several miles, many wall clouds
exhibit rapid upward motion and counterclockwise
rotation. Rotating wall clouds usually develop before
strong or violent tornadoes, by anywhere from a
few minutes up to nearly an hour. Wall clouds must
be monitored visually for signs of persistent, sustained
Waterspout - In general, a tornado occurring
over water. Specifically, it normally refers to
a small, relatively weak rotating column of air
over water beneath a cumulonimbus cloud or a towering
cumulus cloud. (Waterspouts are most frequently
observed in shallow waters off the coasts of Texas
and Florida. However, they have occurred in Arkansas
on some of the larger lakes, and on a few occasions,
in the Arkansas River.)
The terms in this glossary were taken from several
National Weather Service publications. This is not
an all-inclusive list of all terms associated with
severe weather. The terms here were selected because
they are the most frequently used in Missouri --
in weather summaries; and in interviews with the