This glossary contains weather-related terms that may be either heard or used by severe local storm spotters or spotter groups. Its purposes are (1) to achieve some level of standardization in the definitions of the terms that are used, and (2) provide a reference from which the meanings of any terms, especially the lesser-used ones, can be found. The idea is to allow smooth and effective communication between storm spotters and forecasters, and vice versa. This is an important necessity within the severe weather warning program. Despite advances in warning and forecasting techniques (e.g., Doppler radar), the human eye will always be a vital part of any effective warning system. Storm spotters are, and always will be, an indispensable part of the severe local storm warning program.
A complete list of terms is probably impossible to arrive at, but this list is as comprehensive as possible. Certainly it is not necessary for every spotter to know the meaning of every term contained herein. In this sense, the glossary serves as a reference. In fact, many of the terms may never be heard at all; they are included here just in case, someday, they are. (By the way, inclusion of a term in this glossary does not give license to use it freely in radio or phone communication. Use of technical terms should be kept to a minimum.) But there are some terms for which the meanings are both important and specific. The important ones are preceded by asterisks; all spotters should be familiar with the definitions of these terms before taking an active role in any spotter group.
I have written the definitions in what hopefully passes as "layman's terms." They are written to be easily understood by the storm spotter, regardless of his or her meteorological background. At times I have sacrificed technical purity for simplicity, and the result may prompt a few moans from the technical purists. So be it; this glossary wasn't written for them. Many of the terms are so closely interrelated, though, that it becomes necessary to "cross-reference" -- that is, to sue one or more terms in the definition of another. In this glossary, all terms in boldface within a definition are terms that are defined themselves elsewhere.
The gloassary is a culmination of an effort which began in the spring of 1991. Many individuals with considerable experience in severe storm research and storm spotting (or chasing) contributed to the glossary. Because of the many comments offered by these individuals, there was disagreement on the descriptions of some terms. Those terms that were identified as being somewhat more controversial are handled in the text by inclusion of a second paragraph in the description, which discusses any cautions or controversy regarding the use of the term.
One last word. Storm spotting is vital, but also can be very dangerous. No one should attempt storm spotting without first obtaining the proper training! This glossary in itself is not to be considered sufficient training material to qualify oneself as a spotter. Further training, usually provided by the National Weather Service, must be obtained through local agencies (e.g., Civil Defense) before one can be certified as a storm spotter. There is also something to be said for the so-called storm chasers, who chase storms mainly for the thrill of it (and as such are not spotters). Chasers of all levels of background and experience will no doubt find this glossary useful or at least interesting. But while I commend their enthusiasm, I must emphasize that the glossary does not condone storm chasing as a leisure activity, especially for the unprepared. Proper training and foreknowledge of the dangers are required of everyone who meets face-to-face with severe thunderstorms--regardless of the reason for the encounter.
Michael L. Branick
AC - Convective outlook issued by SELS. Abbreviation for Anticipated Convection; the term originates from the header coding (ACUS1) of the transmitted product.
ACCAS (pronounced ACK-kas) - Altocumulus Castellanus; midlevel clouds (bases generally 8 to 15 thousand feet), of which at least a fraction of their upper parts show cumulus type development. These clouds often are taller than they are wide, giving them a turret-shaped appearance. ACCAS clouds are a sign of instability aloft, and may precede the rapid development of thunderstorms.
Accessory Cloud - A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds.
Advection - Transport of an atmospheric property by the wind. See cold advection, moisture advection, warm advection.
Air mass thunderstorm - Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Air mass thunderstorms typically are associated with warm, trpoical air in the summer months; they develop during the afternoon in response to insolation, and dissipate rather quickly after sunset. They generally are less likely to be severe than other types of thunderstorms, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.
Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.
Algorithm - A computer program (or set of programs) which is designed to systematically solve a certain kind of problem. WSR-88D radars (NEXRAD ) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, VIL, accumulated rainfall, and several other parameters.
Anticyclonic rotation - Rotation in the opposite sense as the Earth's rotation, i.e. clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.
anvil - The flat, spreading top of a Cb (cumulonimbus), often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and may spread upwind (see back-sheared anvil; overshooting top.
Anvil dome - same as overshooting top or penetrating top.
Anvil rollover - [slang] a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. See cumuliform anvil, knuckles, mushroom.
AP - Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.
Approaching (severe levels) - A thunderstorm which contains winds of 35 to 49 knots (40 to 57 mph), or hail 1/2 inch or larger but less than 3/4 inch in diamater. See severe thunderstorm.
Arcus - A low, horizontal cloud formation associated with the leading edge of thunderstorm outflow (i.e., the gust front). Roll clouds and shelf clouds are both types of arcus clouds.
AVN - Aviation Model; one of the operational forecast models run at NMC. The AVN is run twice daily, with forecast output out to 72 hours.
Back-building thunderstorm - A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction.
Backing winds - Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.
In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels.
Backsheared anvil - [slang] a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential. See Figure 7; supercell.
Barber pole - [slang] a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft; drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Bear's cage - [slang] a region of storm-scale rotation, in a thunderstorm, which is wrapped in heavy precipitation. This area often coincides with a radar hook echo and/or mesocyclone, especially one associated with an HP storm.
The term reflects the danger involved in observing such an area visually, which must be done at close range in low visibility.
Beaver tail (or sometimes inflow stinger - [slang] a low, flat cloud, often shaped like a beaver tail, located in the invlow region of a thunderstorm (the east or southeast side); a particular form of feeder band. Its size and shape change as the strength of the inflow changes.
A beaver tail is not a tail cloud; the latter typically forms in the moist but cooler air on the north or northeast side of a wall cloud.
Blue watch (or blue box ) - [slang] a severe thunderstorm watch.
Bow echo - A radar acho which is linear but bent outward in a bow shape (Figure 1). Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation can also develop at either end of a bow echo, which can sometimes lead to tornado formation, especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.
Boundary layer - In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the Earth, this layer is considered to be rougly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the Earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be exactly defined.
There is a thin layer immediately above the Earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly ten meters thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.
Box (or watch box) - [slang] a severe thunderstorm or tornado watch. See blue box, red box.
Bust - [slang] an inaccurate forecast or an unsuccessful storm chase; usually a situation in which thunderstorms or severe weather are expected, but do not occur.
BWER - Bounded Weak Echo Region. Also known as a vault. Radar signature characterized by an area of weak radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft (Figure 2). This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. See WER.
CA - Cloud-to-Air lightning.
Cap (or lid) - A layer of warm air, several thousand feet above the surface, 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 Figure 6, sounding.
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. 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 the rising air parcel, over the layer within which the latter is warmer than the former. This area is often called positive area. See Figure 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.
Chaff - Small strips of metal foil, usually dropped in large quantities from aircraft or balloons in order to confuse enemy radar. Chaff typically produces a radar echo which closely resembles precipitation. Chaff drops are conducted by some military installations for testing and calibration purposes.
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 hair-like 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. See rear flank downdraft.
Closed low - A low pressure area with a distinct center of cyclonic rotation which can be completely encircled by one or more isobars or other contour lines. The term usually is used to describe a low pressure area aloft, distinguishing such an area from a low-pressure trough. Closed lows aloft typically move more slowly since they are detached from the main westerly current (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.
Collar cloud - A generally circular ring of cloud surrounding the upper part of a wall cloud. See Figure 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 Figure 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 winds converge along an axis oriented parallel to the flow along the axis; a form of convergence. Compare with difluence.
Congestus (or Cumulus Congestus ) - Same as towering cumulus.
Convection - The transport of heat (and moisture) by the movement of a fluid. In meteorology, the term is used most often to describe the vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeabley, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds, and ACCAS formations also are 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 the AC) - A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the Continental US, issued several times daily by SELS. 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 (see, e.g., OTO).
Convective temperature - The approximate temperature that the air near the ground must warm to in order for convection to develop. 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. See Figure 6, sounding.
Convergence - A contraction of a vector field, usually said of winds; the opposite of divergence. convergence in a low-level wind field results in upward forcing, thus increasing the potential for thunderstorm development (when other factors, such as instability, are favorable. See also confluence.
Core punch (or core plunge) - [slang] a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting.
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 Cbs, 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" are often 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 useage 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 storms are capable of producing multiple tornadoes and/or several bursts of severe weather. A storm which undergoes only one cycle (pulse), and then dissipates, is known as a pulse storm.
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.
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.
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.
Dewpoint - 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 held constant.
Difluence (or Diffluence ) - A pattern of wind flow in which the air spreads laterally in a fan-out pattern from a central axis oriented parallel to the flow along the axis. Difluence in the upper level winds is considered a favorable condition for severe thunderstorm development. See confluence, divergence.
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.
Divergence - The expansion or spreading out of a vector field; usually said of 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).
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.
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 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 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 may follow 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 or just east of the dry line, especially when it begins moving eastward. See LP storm.
Dry line bulge - A bulge in the dry line, represengint 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, and 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 a 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. See 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 but intense vortex made visible by dust or debris (dust whirl), which forms in response to surface heating during fair, hot weather. Dust devils 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 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.
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.
Enhanced wording - An option used by in some tornado and severe thunderstorm watches when the potential for strong to violent tornadoes, or unusually widespread damaging straight-line winds, is considered high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH THE POSSIBILITY OF VERY DAMAGING TORNADOES" is included 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.
F scale - See Fujita scale.
Feeder band - A line or band 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 band.
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. See Figure 5, HP storm, and Figure 7, supercell.
Forward flank downdraft - The main region of downdraft in the downwind part of a supercell; where most of the heavy precipitation is. Compare with rear flank downdraft; see Figure 7, supercell.
Front - An interface or transition zone between two air masses of different density, and therefore (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 (also F scale ) - A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:
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 nearby precipitation echoes.
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 clouds. See also downburst, gustnado, outflow boundary.
Gustnado - [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.
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, and is measured relative to storm motion. Higher values of helicity favor the formation of rotation at low levels (i.e. mesocyclones).
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 with 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 southwest part of the storm. A hook often is associated with a mesocyclone), and indicates favorable conditions for tornado development. See Figure 2, BWER), and Figure 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 (Figure 3). Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes very 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).
Inflow bands (or feeder bands ) - Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and hence its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.
Inflow jets - Local jets of air near the ground flowing inward toward the base of a tornado.
Inflow notch - A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with a V notch. Supercell thunderstorms often exhibit inflow notches, usually in the southwest quadrant of a classic supercell but sometimes in the eastern part of an HP storm.
Inflow stinger - A beaver tail cloud with a stinger-like shape.
Insolation - Incoming solar radiation. Solar heating; sunshine.
Instability - The tendency of air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather--the greater the instability, the greater the potential for severe thunderstorms. See lifted index and Figure 6, sounding.
Inversion - Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e. an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap. See Figure 6, sounding.
Isobar - A line connecting points of equal pressure.
Jet streak - A local wind speed maximum within a jet stream.
Jet stream - Relatively strong winds concentrated in a narrow stream in the atmosphere; normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day and week to week. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength, and orientation of the jet stream or jet streams.
Knuckles - [slang] lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds. See also cumuliform anvil, anvil rollover.
Laminar - Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air travelling in parallel layers or sheets.
Landspout - [slang] a small, weak tornado; generally, one that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalent of waterspouts.
Lapse rate - The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height, a sign of instability. See Figure 6, sounding.
Large scale - See synoptic scale.
Left mover - A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm. See also right mover.
LEWP - Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line (Figure 4). The potential for strong outflow and damaging straight-line winds is increased near the bulge, which often resembles a bow echo. Severe weather potential is also increased with storms near the crest of an LEWP.
Lifted index (or LI) - A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability--the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However, there are no "magic numbers" or threshold LI values below which severe weather becomes imminent. See Figure 6, sounding.
Loaded gun (sounding) - [slang] a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it. See Figure 6, sounding.
Longwave trough - A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the northern hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot, cold, dry, stormy) over periods ranging from days to months. Smaller disturbances (e.g. shortwave troughs typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).
Low-level jet - [abbrev LLJ] A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).
The term may also be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.
LP storm (or LP supercell ) - Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core (Figure 5). LP storms often exhibit a striking visual appearance; the main tower is often bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification is often difficult, so visual reports are very important. LP storms almost always occur on or near the dry line, and therefore are sometimes referred to as dry line storms.