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CA - Cloud-to-Air lightning.
Cap (or Capping Inversion) - A layer of relatively
warm air aloft (usually several thousand feet above the ground) which
suppresses or delays the development of thunderstorms. Air parcels rising into
this layer become cooler than the surrounding air, which inhibits their ability
to rise further. As such, the cap often prevents or delays thunderstorm
development even in the presence of extreme
instability. However if the cap is
removed or weakened, then explosive thunderstorm development can occur. See
CIN and
Fig. 6,
sounding.
The cap is an important ingredient in most
severe thunderstorm episodes,
as it serves to separate warm, moist air below and cooler, drier air above.
With the cap in place, air below it can continue to warm and/or moisten, thus
increasing the amount of potential instability. Or, air above it can cool,
which also increases potential instability. But without a cap, either process
(warming/moistening at low levels or cooling aloft) results in a faster release
of available instability - often before instability levels become large enough
to support severe weather development.
CAPE - Convective Available
Potential Energy. A measure of the amount of energy available for
convection. CAPE is directly related to
the maximum potential vertical speed within an updraft; thus, higher values indicate greater
potential for severe weather. Observed values in thunderstorm environments
often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may
exceed 5,000 j/kg. However, as with other indices or indicators, there are no
threshold values above which severe weather becomes imminent. CAPE is
represented on a sounding by the area
enclosed between the environmental temperature profile and the path of a rising
air parcel, over the layer within which the latter is warmer than the former.
(This area often is called positive area.) See also
CIN and Fig. 6,
sounding.
*Cb - Cumulonimbus cloud,
characterized by strong vertical development in the form of mountains or huge
towers topped at least partially by a
smooth, flat, often fibrous anvil. Also known
colloquially as a "thunderhead."
CC - Cloud-to-Cloud lightning.
Cell - Convection in the form of a single
updraft,
downdraft, or updraft/downdraft couplet,
typically seen as a vertical dome or tower
as in a cumulus or
towering cumulus cloud. A typical
thunderstorm consists of several cells (see
multi-cellular
thunderstorm).
The term "cell" also is used to describe the radar echo returned
by an individual shower or thunderstorm. Such usage, although common, is
technically incorrect.
*CG - Cloud-to-Ground lightning flash.
Chaff - Small strips of metal foil, usually
dropped in large quantities from aircraft or balloons. Chaff typically produces
a radar echo which closely resembles precipitation. Chaff drops once were
conducted by the military in order to confuse enemy radar, but now are
conducted mainly for radar testing and calibration purposes.
CIN - Convective INhibition. A
measure of the amount of energy needed in order to initiate
convection. Values of CIN typically
reflect the strength of the cap. They are
obtained on a sounding by computing the
area enclosed between the environmental temperature profile and the path of a
rising air parcel, over the layer within which the latter is cooler than the
former. (This area sometimes is called negative area.) See
CAPE and Fig. 6,
sounding.
Cirrus - High-level clouds (16,000 feet or
more), composed of ice crystals and appearing in the form of white, delicate
filaments or white or mostly white patches or narrow bands. Cirrus clouds
typically have a fibrous or hairlike appearance, and often are
semi-transparent. Thunderstorm anvils are a
form of cirrus cloud, but most cirrus clouds are not associated with
thunderstorms.
Classic Supercell - See
supercell.
Clear Slot - A local region of clearing
skies or reduced cloud cover, indicating an intrusion of drier air; often seen
as a bright area with higher cloud bases on the west or southwest side of a
wall cloud. A clear slot is believed to
be a visual indication of a rear
flank downdraft.
Closed Low - A low pressure area with a
distinct center of cyclonic
circulation which can be completely encircled by one or more
isobars or height contour lines. The term
usually is used to distinguish a low pressure area aloft from a low-pressure
trough. Closed lows aloft typically are
partially or completely detached from the main westerly current, and thus move
relatively slowly (see cutoff low).
Cloud Streets - Rows of
cumulus or cumulus-type clouds aligned
parallel to the low-level flow. Cloud streets sometimes can be seen from the
ground, but are seen best on satellite photographs.
Cloud Tags - Ragged, detached cloud
fragments; fractus or
scud.
Cold Advection - Transport of cold air
into a region by horizontal winds.
Cold-air Funnel - A
funnel cloud or (rarely) a small,
relatively weak tornado that can develop
from a small shower or thunderstorm when the air aloft is unusually cold (hence
the name). They are much less violent than other types of tornadoes.
Cold Pool - A region of relatively cold air,
represented on a weather map analysis as a relative minimum in temperature
surrounded by closed isotherms. Cold
pools aloft represent regions of relatively low stability, while surface-based
cold pools are regions of relatively stable air.
Collar Cloud - A generally circular ring
of cloud that may be observed on rare occasions surrounding the upper part of a
wall cloud. See
Fig. 7,
supercell.
This term sometimes is used (incorrectly) as a synonym for
wall cloud.
Comma Cloud - A
synoptic scale cloud pattern with a
characteristic comma-like shape, often seen on satellite photographs associated
with large and intense low-pressure systems.
Comma Echo - A thunderstorm radar echo which
has a comma-like shape. It often appears during latter stages in the life cycle
of a bow echo (see
Fig. 1).
Condensation Funnel - A
funnel-shaped cloud associated with rotation and consisting of condensed water
droplets (as opposed to smoke, dust, debris, etc.). Compare with
debris cloud.
Confluence - A pattern of wind flow in which
air flows inward toward an axis oriented parallel to the general direction of
flow. It is the opposite of difluence.
Confluence is not the same as convergence. Winds often accelerate as
they enter a confluent zone, resulting in speed
divergence which offsets the (apparent)
converging effect of the confluent flow.
Congestus (or
Cumulus Congestus) - same as
towering cumulus.
Convection - Generally, transport of heat
and moisture by the movement of a fluid. In meteorology, the term is used
specifically to describe vertical transport of heat and moisture, especially by
updrafts and
downdrafts in an unstable atmosphere.
The terms "convection" and "thunderstorms" often are used
interchangeably, although thunderstorms are only one form of convection.
Cbs, towering cumulus clouds, and
ACCAS clouds all are visible forms of
convection. However, convection is not always made visible by clouds.
Convection which occurs without cloud formation is called dry convection, while
the visible convection processes referred to above are forms of moist
convection.
Convective Outlook (sometimes called
AC) - A forecast containing the area(s) of
expected thunderstorm occurrence and expected severity over the contiguous
United States, issued several times daily by the SPC. The terms
approaching,
slight risk,
moderate risk, and
high risk are used to describe
severe thunderstorm potential.
Local versions sometimes are prepared by local NWS offices.
Convective Temperature - The
approximate temperature that the air near the ground must warm to in order for
surface-based convection
to develop, based on analysis of a sounding. See
Fig. 6.
Calculation of the convective temperature involves many assumptions, such
that thunderstorms sometimes develop well before or well after the convective
temperature is reached (or may not develop at all). However, in some cases the
convective temperature is a useful parameter for forecasting the onset of
convection.
Convergence - A contraction of a vector
field; the opposite of divergence.
Convergence in a horizontal wind field indicates that more air is entering a
given area than is leaving at that level. To compensate for the resulting
"excess," vertical motion may result: upward forcing if convergence
is at low levels, or downward forcing (subsidence) if convergence is at high
levels. Upward forcing from low-level convergence increases the potential for
thunderstorm development (when other factors, such as
instability, are favorable). Compare
with confluence.
Core Punch - [Slang], a penetration by a
vehicle into the heavy precipitation core of a thunderstorm.
Core punching is not a recommended procedure for storm spotting.
Cooling Degree Day (CDD)- useful in determining cooling
energy requirements. Defined as (MT - 65)= CDD,where MT is Mean Temperature
(F). If MT is less than 65 degrees F, CDD=0. MT= Mean (average)Temperature of
the day.
MT=(High Temperature+Low Temperature)/2
Cumuliform Anvil - A thunderstorm
anvil with visual characteristics resembling
cumulus-type clouds (rather than the more
typical fibrous appearance associated with cirrus). A cumuliform anvil arises from rapid
spreading of a thunderstorm updraft, and
thus implies a very strong updraft. See anvil rollover,
knuckles,
mushroom.
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. See Cb,
towering cumulus.
Cumulus Congestus (or simply
Congestus) - Same as
towering cumulus.
Cutoff Low - A
closed low which has become completely
displaced (cut off) from basic westerly current, and moves independently of
that current. Cutoff lows may remain nearly stationary for days, or on occasion
may move westward opposite to the prevailing flow aloft (i.e.,
retrogression).
"Cutoff low" and "closed low" often are used
interchangeably to describe low pressure centers aloft. However, not all closed
lows are completely removed from the influence of the basic westerlies.
Therefore, the recommended usage of the terms is to reserve the use of
"cutoff low" only to those closed lows which clearly are detached
completely from the westerlies.
Cyclic Storm - A thunderstorm that
undergoes cycles of intensification and weakening (pulses) while maintaining
its individuality. Cyclic supercells are capable of producing multiple
tornadoes (i.e., a
tornado family) and/or several
bursts of severe weather.
A storm which undergoes only one cycle (pulse), and then dissipates, is
known as a pulse storm.
Cyclogenesis - Development or
intensification of a low-pressure center (cyclone).
*Cyclonic Circulation (or Cyclonic
Rotation) - Circulation (or rotation) which is in the same sense as the Earth's
rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen
from above. Nearly all mesocyclones
and strong or violent tornadoes exhibit
cyclonic rotation, but some smaller vortices, such as
gustnadoes, occasionally rotate
anticyclonically (clockwise). Compare with
anticyclonic rotation.
dBZ - Nondimensional "unit" of radar
reflectivity which represents a
logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity
factor, Z.
The value of Z is a function of the amount of radar beam energy that is
backscattered by a target and detected as a signal (or echo). Higher values of
Z (and dBZ) thus indicate more energy being backscattered by a target. The
amount of backscattered energy generally is related to precipitation
intensity, such that higher values of dBZ that are detected from precipitation
areas generally indicate higher precipitation rates. However, other factors can
affect reflectivity, such as width of the radar beam, precipitation type, drop
size, or the presence of ground
clutter or AP.
WSR-88D radars can detect reflectivities
as low as -32 dBZ near the radar site, but significant (measurable)
precipitation generally is indicated by reflectivities of around 15 dBZ or
more. Values of 50 dBZ or more normally are associated with heavy
thunderstorms, perhaps with hail, but as with most other quantities, there are
no reliable threshold values to confirm the presence of hail or severe weather
in a given situation. See VIP for threshold
dBZ values associated with each VIP level.
*Debris Cloud - A rotating
"cloud" of dust or debris, near or on the ground, often appearing
beneath a condensation funnel
and surrounding the base of a tornado.
This term is similar to dust whirl,
although the latter typically refers to a circulation which contains dust but
not necessarily any debris. A dust
plume, on the other hand, does not rotate. Note that a debris cloud
appearing beneath a thunderstorm will confirm the presence of a tornado, even
in the absence of a condensation funnel.
Delta T - A simple representation of the mean
lapse rate within a layer of the
atmosphere, obtained by calculating the difference between observed
temperatures at the bottom and top of the layer. Delta Ts often are computed
operationally over the layer between pressure levels of 700 mb and 500 mb, in
order to evaluate the amount of instability in mid-levels of the
atmosphere. Generally, values greater than about 18 indicate sufficient
instability for severe thunderstorm development.
Derecho - (Pronounced day-RAY-cho), a
widespread and usually fast-moving windstorm associated with
convection. Derechos include any family
of downburst clusters produced by an
extratropical MCS, and can produce damaging
straight-line winds over areas
hundreds of miles long and more than 100 miles across.
Dew Point (or Dew-point Temperature) - A
measure of atmospheric moisture. It is the temperature to which air must be
cooled in order to reach saturation (assuming air pressure and moisture content
are constant).
Differential Motion - Cloud motion
that appears to differ relative to other nearby cloud elements, e.g. clouds
moving from left to right relative to other clouds in the foreground or
background. Cloud rotation is one example of differential motion, but not all
differential motion indicates rotation. For example, horizontal
wind shear along a
gust front may result in differential
cloud motion without the presence of rotation.
Difluence (or Diffluence) - A pattern of wind
flow in which air moves outward (in a "fan-out" pattern) away from a
central axis that is oriented parallel to the general direction of the flow. It
is the opposite of confluence.
Difluence in an upper level wind field is considered a favorable condition
for severe thunderstorm
development (if other parameters are also favorable). But difluence is not the
same as divergence. In a difluent flow,
winds normally decelerate as they move through the region of difluence,
resulting in speed convergence which offsets the apparent diverging effect of
the difluent flow.
Directional Shear - The component of
wind shear which is due to a change in wind
direction with height, e.g., southeasterly winds at the surface and
southwesterly winds aloft. A veering
wind with height in the lower part of the atmosphere is a type of
directional shear often considered important for
tornado development.
Diurnal - Daily; related to actions which are
completed in the course of a calendar day, and which typically recur every
calendar day (e.g., diurnal temperature rises during the day, and diurnal falls
at night).
Divergence - The expansion or spreading out
of a vector field; usually said of horizontal winds. It is the opposite of
convergence. Divergence at upper
levels of the atmosphere enhances upward motion, and hence the potential for
thunderstorm development (if other factors also are favorable).
Doppler Radar - Radar that can measure
radial velocity, the instantaneous
component of motion parallel to the radar beam (i.e., toward or away from the
radar antenna).
*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. Although
usually associated with thunderstorms, downbursts can occur with showers too
weak to produce thunder. See dry and
wet microburst.
Downdraft - A small-scale column of air that
rapidly sinks toward the ground, usually accompanied by precipitation as in a
shower or thunderstorm. A downburst is
the result of a strong downdraft.
Downstream - In the same direction as a
stream or other flow, or toward the direction in which the flow is moving.
Dry Adiabat - A line of constant
potential temperature on a
thermodynamic chart. See
Fig. 6,
sounding.
Dry Line - A boundary separating moist and dry
air masses, and an important factor in severe weather frequency in the Great
Plains. It typically lies north-south across the central and southern high
Plains states during the spring and early summer, where it separates moist air
from the Gulf of Mexico (to the east) and dry desert air from the southwestern
states (to the west). The dry line typically advances eastward during the
afternoon and retreats westward at night. However, a strong storm system can
sweep the dry line eastward into the Mississippi Valley, or even further east,
regardless of the time of day. A typical dry line passage results in a sharp
drop in humidity (hence the name),
clearing skies, and a wind shift from south or southeasterly to west or
southwesterly. (Blowing dust and rising temperatures also may follow,
especially if the dry line passes during the daytime; see
dry punch). These changes occur in
reverse order when the dry line retreats westward. Severe and sometimes
tornadic thunderstorms often develop along a dry line or in the moist air just
to the east of it, especially when it begins moving eastward. See
LP storm.
Dry-line Bulge - A bulge in the
dry line, representing the area where dry
air is advancing most strongly at lower levels (i.e., a surface
dry punch). Severe weather potential is
increased near and ahead of a dry line bulge.
Dry-line Storm - Generally, any
thunderstorm that develops on or near a dry
line. The term often is used synonymously with
LP storm, since the latter almost always
occurs near the dry line.
Dry Microburst - A
microburst with little or no
precipitation reaching the ground; most common in semi-arid regions. They may
or may not produce lightning. Dry microbursts may develop in an otherwise
fair-weather pattern; visible signs may include a
cumulus cloud or small
Cb with a high base and high-level
virga, or perhaps only an
orphan anvil from a dying rain
shower. At the ground, the only visible sign might be a
dust plume or a ring of blowing dust
beneath a local area of virga. Compare with
wet microburst.
Dry Punch - [Slang], a surge of drier air;
normally a synoptic-scale or
mesoscale process. A dry punch at the
surface results in a dry line
bulge. A dry punch aloft above an
area of moist air at low levels often increases the potential for severe
weather.
Dry Slot - A zone of dry (and relatively
cloud-free) air which wraps east- or northeastward into the southern and
eastern parts of a synoptic scale
or mesoscale low pressure system. A dry
slot generally is seen best on satellite photographs.
Dry slot should not be confused with clear slot, which is a
storm-scale phenomenon.
Dust Devil - A small atmospheric vortex not
associated with a thunderstorm, which is made visible by a rotating cloud of
dust or debris (dust whirl). Dust
devils form in response to surface heating during fair, hot weather; they are
most frequent in arid or semi-arid regions.
*Dust Plume - A non-rotating
"cloud" of dust raised by
straight-line winds. Often
seen in a microburst or behind a
gust front.
If rotation is observed, then the term dust whirl or
debris cloud should be used.
*Dust Whirl - A rotating column of air
rendered visible by dust. Similar to debris cloud; see also
dust devil,
gustnado,
tornado.
Dynamics - Generally, any forces that produce
motion or affect change. In operational meteorology, dynamics usually refer
specifically to those forces that produce vertical motion in the atmosphere.
ECMWF - European Center for
Medium-Range Weather Forecasting. Operational references
in forecast discussions typically refer to the ECMWF's medium-range forecast
model. See MRF, UKMET.
Elevated Convection -
Convection occurring within an elevated
layer, i.e., a layer in which the lowest portion is based above the earth's
surface. Elevated convection often occurs when air near the ground is
relatively cool and stable, e.g., during periods of
isentropic lift, when an unstable
layer of air is present aloft. In cases of elevated convection, stability
indices based on near-surface measurements (such as the
lifted index) typically will
underestimate the amount of instability present. Severe weather is
possible from elevated convection, but is less likely than it is with
surface-based convection.
Energy Helicity Index (or EHI) -
An index that incorporates vertical shear
and instability, designed for the
purpose of forecasting supercell
thunderstorms. It is related directly to storm-relative
helicity in the lowest 2 km (SRH, in
m2/s2) and CAPE (in
j/kg) as follows:
EHI = (CAPE x SRH)/160,000.
Thus, higher values indicate unstable conditions and/or strong vertical
shear. Since both parameters are important for severe weather development,
higher values generally indicate a greater potential for severe weather. Values
of 1 or more are said to indicate a heightened threat of
tornadoes; values of 5 or more are rarely
observed, and are said to indicate potential for violent tornadoes. However,
there are no magic numbers or critical threshold values to confirm or predict
the occurrence of tornadoes of a particular intensity.
Enhanced V - A pattern seen on satellite
infrared photographs of thunderstorms, in which a thunderstorm
anvil exhibits a V-shaped region of colder
cloud tops extending downwind from the thunderstorm core. The enhanced V
indicates a very strong updraft, and
therefore a higher potential for severe weather.
Enhanced V should not be confused with V
notch, which is a radar signature.
Enhanced Wording - An option used by
the SPC in tornado and severe thunderstorm
watches when the potential for
strong/violent tornadoes, or unusually
widespread damaging straight-line winds, is high. The statement "THIS IS A
PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING
TORNADOES" appears in tornado watches with enhanced wording. Severe
thunderstorm watches may include the statement "THIS IS A PARTICULARLY
DANGEROUS SITUATION WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS,"
usually when a derecho event is occurring
or forecast to occur. See PDS watch.
Entrance Region - The region
upstream from a wind speed maximum in a
jet stream (jet max), in which air is approaching
(entering) the region of maximum winds, and therefore is accelerating. This
acceleration results in a vertical circulation that creates
divergence in the upper-level winds in
the right half of the entrance region (as would be viewed looking along the
direction of flow). This divergence results in upward motion of air in the
right rear quadrant (or
right entrance region) of
the jet max. Severe weather potential sometimes increases in this area as a
result. See also exit region,
left exit region.
Equilibrium Level (or EL) - On a
sounding, the level above the level of
free convection (LFC) at which the temperature of a rising air parcel again
equals the temperature of the environment. (See
Fig. 6,
sounding.)
The height of the EL is the height at which thunderstorm
updrafts no longer accelerate upward.
Thus, to a close approximation, it represents the height of expected (or
ongoing) thunderstorm tops. However, strong updrafts will continue to rise past
the EL before stopping, resulting in storm tops that are higher than the EL.
This process sometimes can be seen visually as an
overshooting top or
anvil dome.
The EL typically is higher than the tropopause, and is a more accurate
reference for storm tops.
Eta Model - One of the operational numerical
forecast models run at NCEP. The Eta is run
twice daily, with forecast output out to 48 hours.
Exit Region - The region
downstream from a wind speed maximum in
a jet stream (jet max), in which air is moving away from
the region of maximum winds, and therefore is decelerating. This deceleration
results in divergence in the
upper-level winds in the left half of the exit region (as would be viewed looking
along the direction of flow). This divergence results in upward motion of air
in the left front quadrant (or
left exit region) of the jet max. Severe weather potential sometimes increases
in this area as a result. See also entrance region,
right entrance region.
F scale - See
Fujita Scale.
Feeder Bands - Lines or bands of low-level
clouds that move (feed) into the updraft
region of a thunderstorm, usually from the east through south (i.e., parallel
to the inflow). Same as inflow bands.
This term also is used in tropical meteorology to describe spiral-shaped
bands of convection surrounding, and
moving toward, the center of a tropical cyclone.
*Flanking Line - A line of
cumulus or
towering cumulus clouds connected
to and extending outward from the most active part of a
supercell, normally on the southwest
side. The line normally has a stair-step appearance, with the tallest clouds
closest to the main storm, and generally coincides with the
pseudo-cold front. See
Fig. 3, HP storm, and
Fig. 7,
supercell.
Forward Flank Downdraft - The
main region of downdraft in the forward,
or leading, part of a supercell, where
most of the heavy precipitation is. Compare with
rear flank downdraft. See
pseudo-warm front, and
Fig. 7, supercell.
Front - A boundary or transition zone between two
air masses of different density, and thus (usually) of different temperature. A
moving front is named according to the advancing air mass, e.g., cold front if
colder air is advancing.
Fractus - Ragged, detached cloud fragments;
same as scud.
Fujita Scale (or F Scale) - A scale of
wind damage intensity in which wind speeds are inferred from an analysis
of wind damage:
F0 (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.
All tornadoes, and most other severe
local windstorms, are assigned a single number from this scale according to the
most intense damage caused by the storm.
*Funnel Cloud - A
condensation funnel extending
from the base of a towering
cumulus or Cb, associated with a rotating
column of air that is not in contact with the ground (and hence
different from a tornado). A condensation
funnel is a tornado, not a funnel cloud, if either a) it is in contact
with the ground or b) a debris cloud
or dust whirl is visible beneath it.
Ground Clutter - A pattern of radar
echoes from fixed ground targets (buildings, hills, etc.) near the radar.
Ground clutter may hide or confuse precipitation echoes near the radar antenna.
Gunge - [Slang], anything in the atmosphere that
restricts visibility for storm spotting, such as fog, haze, precipitation
(steady rain or drizzle), widespread low clouds (stratus), etc.
Gust Front - The leading edge of gusty
surface winds from thunderstorm downdrafts; sometimes associated with a
shelf cloud or
roll cloud. See also
downburst,
gustnado,
outflow boundary.
Gustnado (or Gustinado) - [Slang], gust front
tornado. A small tornado, usually weak and
short-lived, that occurs along the gust
front of a thunderstorm. Often it is visible only as a
debris cloud or
dust whirl near the ground. Gustnadoes
are not associated with storm-scale
rotation (i.e. mesocyclones); they are
more likely to be associated visually with a
shelf cloud than with a
wall cloud.
Heating Degree Day (HDD)- useful in determining heating
energy requirements. Defined as (65 - MT)= HDD,where MT is Mean Temperature
(F). If MT is greater than 65 degrees F, HDD=0. MT= Mean (average)Temperature
of the day.
MT=(High Temperature+Low Temperature)/2
Helicity - A property of a moving fluid which
represents the potential for helical flow (i.e. flow which follows the pattern
of a corkscrew) to evolve. Helicity is proportional to the strength of the
flow, the amount of vertical wind shear, and
the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is
computed from the vertical wind profile in the lower part of the atmosphere
(usually from the surface up to 3 km), and is measured relative to storm
motion. Higher values of helicity (generally, around 150
m2/s2 or more) favor the development of mid-level
rotation (i.e. mesocyclones). Extreme
values can exceed 600 m2/s2.
High Risk (of
severe thunderstorms) - Severe
weather is expected to affect more than 10 percent of the area. A high risk is
rare, and implies an unusually dangerous situation and usually the possibility
of a major severe weather outbreak. (See slight risk,
moderate risk,
convective outlook.)
Hodograph - A plot representing the vertical
distribution of horizontal winds, using polar coordinates. A hodograph is
obtained by plotting the end points of the wind vectors at various altitudes,
and connecting these points in order of increasing height. Interpretation of a
hodograph can help in forecasting the subsequent evolution of thunderstorms
(e.g., squall line vs.
supercells,
splitting vs. non-splitting
storms, tornadic vs. nontornadic storms, etc.).
Hook (or Hook Echo) - A radar
reflectivity pattern characterized by
a hook-shaped extension of a thunderstorm echo, usually in the right-rear part
of the storm (relative to its direction of motion). A hook often is associated
with a mesocyclone, and indicates
favorable conditions for tornado
development. See
Fig. 2, BWER,
and Fig. 7,
supercell.
HP Storm or HP Supercell - High-Precipitation
storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy
precipitation (often including hail) falls on the trailing side of the
mesocyclone (Fig. 3).
Precipitation often totally envelops the region of rotation, making visual
identification of any embedded tornadoes
difficult and very dangerous. Unlike most classic supercells, the region of
rotation in many HP storms develops in the front-flank region of the storm
(i.e., usually in the eastern portion). HP storms often produce extreme and
prolonged downburst events, serious
flash flooding, and very large damaging hail events.
Mobile storm spotters are strongly advised to maintain a safe distance from
any storm that has been identified as an HP storm; close observations (e.g.,
core punching) can be extremely
dangerous. See bear's cage.
Humidity - Generally, a measure of the water
vapor content of the air. Popularly, it is used synonymously with
relative humidity.
Glossary (A-B)
Glossary (I-R)
Glossary (S-Z)
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