Meteorology glossary
The terms forecasters, storm spotters and chasers use, defined plainly. Each definition names its source, usually the American Meteorological Society's Glossary of Meteorology or the National Weather Service, and links to the lessons that teach the term in full.
127 terms.
A
- Absolute instability
A layer whose temperature falls with height faster than the dry adiabatic rate, called superadiabatic. Air displaced up or down keeps accelerating away from where it started.
See also Absolute stability, Conditional instability.
Source: AMS Glossary of Meteorology
- Absolute stability
A layer whose temperature falls with height more slowly than the moist adiabatic rate. Air pushed upward, saturated or not, ends up denser than its surroundings and sinks back.
See also Conditional instability, Absolute instability.
Source: AMS Glossary of Meteorology
- Adiabatic process
A process in which air exchanges no heat or mass with its surroundings. Rising air expands and cools; sinking air is compressed and warms.
See also Dry adiabatic lapse rate, Moist adiabatic lapse rate.
Source: AMS Glossary of Meteorology
- Air parcel parcel
An imaginary volume of air treated as having uniform temperature and moisture, followed as it rises or sinks. Stability is judged by comparing a lifted parcel with the air around it.
See also Lifted condensation level, CAPE.
Source: AMS Glossary of Meteorology
- Anvil incus
The flat, spreading top of a mature thunderstorm, where the updraft reaches its equilibrium level.
See also Overshooting top, Mammatus.
Source: AMS Glossary of Meteorology
B
- Bear’s cage
Storm chasers’ slang for the area of rotation in a storm wrapped in heavy rain, often at the hook echo of a high-precipitation supercell. It can only be seen at close range in poor visibility, which is what makes it dangerous.
See also Hook echo, Mesocyclone.
- Beaver tail
A smooth, flat inflow band that attaches to a supercell’s main updraft at about the height of the updraft base.
See also Inflow band, Wall cloud.
- Bounded weak echo region BWER, vault
A nearly vertical channel of weak echo surrounded on the sides and top by strong echo. It marks an updraft strong enough to carry new precipitation upward before it grows large enough to show on radar.
Source: AMS Glossary of Meteorology
- Bow echo
A bow-shaped line of storms on radar, often with swaths of damaging straight-line wind and sometimes small tornadoes. A rear-inflow jet drives the bulge, with vortices at either end.
See also Rear-inflow jet, Derecho, Quasi-linear convective system.
Source: AMS Glossary of Meteorology
- Bright band
A thin horizontal layer of high reflectivity at the height where snow melts into rain, seen mainly in steady, stratiform precipitation.
See also Reflectivity.
Source: AMS Glossary of Meteorology
- Bulk shear 0-6 km shear, bulk wind difference
The vector difference between the wind at two heights, most often the surface and 6 km. Storms become more organized and persistent as it increases; supercells are commonly associated with 35 to 40 knots and more.
See also Wind shear, Effective bulk shear.
C
- CAPE convective available potential energy
The energy a rising parcel gains from being warmer than its surroundings, in joules per kilogram. On a skew-T it is the area between the parcel path and the temperature trace from the LFC to the equilibrium level. It measures the potential strength of updrafts.
Taught in How to read a skew-T diagram.
See also CIN, Surface-based CAPE, Mixed-layer CAPE, Most-unstable CAPE.
Source: AMS Glossary of Meteorology
- Capping inversion cap, lid
A stable layer, usually an inversion, at the top of the boundary layer. It traps moisture and turbulence beneath it and can hold off thunderstorms until heating or lift breaks it.
Taught in How to read a skew-T diagram.
Source: AMS Glossary of Meteorology
- CIN convective inhibition
The energy needed to lift a parcel to its level of free convection through layers where it is cooler than its surroundings, usually a cap. Large CIN prevents storms even when CAPE is large.
Taught in How to read a skew-T diagram.
See also CAPE, Capping inversion.
Source: AMS Glossary of Meteorology
- Clear slot
A bright area of clearing, often with higher cloud bases, on the west or southwest side of a wall cloud. It marks drier air entering the storm and is taken as a visual sign of the rear flank downdraft.
See also Rear flank downdraft, Wall cloud.
- Cold front
The leading edge of an advancing colder air mass, where cold air replaces warmer air.
See also Warm front, Dryline, Triple point.
Source: AMS Glossary of Meteorology
- Conditional instability
A layer whose lapse rate lies between the moist and dry adiabatic rates. It is stable to unsaturated air and unstable to saturated air, so instability depends on the air reaching saturation. Most thunderstorm environments are conditionally unstable.
See also Absolute stability, Absolute instability, CAPE.
Source: AMS Glossary of Meteorology
- Convective condensation level CCL
The height where the mixing ratio line for the average moisture of the lowest layers meets the temperature trace. It estimates the base of cumulus clouds formed by surface heating alone.
See also Convective temperature, Lifted condensation level.
Source: AMS Glossary of Meteorology
- Convective outlook
The Storm Prediction Center’s forecast of where thunderstorms and severe thunderstorms are expected across the contiguous United States, issued several times a day for days 1 through 8, with a map and a text discussion.
Taught in Tornado watch vs. tornado warning.
See also Outlook categories, Mesoscale discussion.
- Convective temperature
The surface temperature that must be reached for air heated at the ground to rise to the convective condensation level without being cooler than its surroundings. Forecasters use it to time the onset of convection.
See also Convective condensation level.
Source: AMS Glossary of Meteorology
- Core punch
Slang for driving into the heavy rain and hail core of a thunderstorm. The National Weather Service does not recommend it for storm spotting.
D
- Damage indicator DI
One of the structure or tree types used in Enhanced Fujita ratings, each with a table of degrees of damage and the wind speed expected to cause each.
See also Enhanced Fujita scale.
Source: AMS Glossary of Meteorology
- dBZ
Decibels of reflectivity, the logarithmic unit on radar displays: dBZ = 10 × log10(Z). Each 10 dBZ is a tenfold increase in reflectivity.
See also Reflectivity.
- Derecho
A widespread, long-lived windstorm of straight-line winds from a mesoscale convective system, with damage over a swath at least 650 km (400 miles) long and about 100 km (60 miles) wide.
See also Bow echo, Mesoscale convective system.
Source: AMS Glossary of Meteorology
- Dew point
The temperature to which air must be cooled, at constant pressure and moisture content, to become saturated. It measures the amount of water vapor in the air directly, which relative humidity does not.
See also Dew point depression, Relative humidity, Mixing ratio.
Source: AMS Glossary of Meteorology
- Dew point depression spread
The difference between the temperature and the dew point. A small depression means the air is close to saturation; on a skew-T, the two traces are close together.
See also Dew point.
Source: AMS Glossary of Meteorology
- Differential reflectivity ZDR
The ratio of the reflectivity measured with horizontally and vertically polarized pulses. Flattened raindrops give positive values; round or tumbling targets such as hail give values near zero.
See also Reflectivity.
Source: AMS Glossary of Meteorology
- Downdraft
A current of sinking air in a thunderstorm, driven largely by rain and hail and by the cooling of evaporation.
See also Updraft, Microburst, Rear flank downdraft.
Source: AMS Glossary of Meteorology
- Downdraft CAPE DCAPE
The energy available to air that sinks after being cooled by evaporating rain, from the level it starts sinking down to the ground. Larger values indicate stronger downdrafts and outflow winds.
See also CAPE, Microburst.
Source: AMS Glossary of Meteorology
- Dry adiabatic lapse rate DALR
The rate at which unsaturated air cools as it rises: about 9.8 °C per kilometer (5.4 °F per 1,000 feet). On a skew-T it is the slope of the dry adiabats.
See also Moist adiabatic lapse rate, Lapse rate.
Source: AMS Glossary of Meteorology
- Dry slot
A band of dry, mostly cloud-free air that wraps into the southern and eastern side of a low. It shows best on satellite imagery.
- Dryline
A boundary near the ground between moist and dry air, hundreds of kilometers long. In the United States it separates Gulf moisture from dry air over the southern Plains, is most common in spring, and typically moves east by day and back west at night.
See also Cold front, Triple point.
Source: AMS Glossary of Meteorology
E
- Effective bulk shear effective bulk wind difference, EBWD
Bulk shear measured from the base of the effective inflow layer to half the height of the storm’s equilibrium level, so it fits both surface-based and elevated storms. Supercells become more probable as it increases through 25 to 40 knots and more.
See also Bulk shear, Effective inflow layer.
Source: Storm Prediction Center, Effective bulk wind difference
- Effective inflow layer
The layer of air a storm can actually draw into its updraft: the levels from which a lifted parcel has at least 100 J/kg of CAPE and no more than 250 J/kg of CIN. Effective shear and helicity are measured in or from it.
See also Effective bulk shear, Storm-relative helicity.
Source: Storm Prediction Center, Effective storm-relative helicity
- Enhanced Fujita scale EF scale
The six-level scale, EF0 to EF5, that estimates tornado wind speed from damage. Surveyors match the damage to 28 damage indicators, 26 kinds of structure plus hardwood and softwood trees. The National Weather Service adopted it in 2007, replacing the Fujita scale.
See also Damage indicator, Tornado.
Source: AMS Glossary of Meteorology
- Environmental lapse rate
The rate at which temperature actually falls with height in the air around a place at a given time, as a sounding measures it. It is distinct from the rate a lifted parcel follows.
See also Lapse rate.
Source: AMS Glossary of Meteorology
- Equilibrium level EL, level of neutral buoyancy
The height at which a rising parcel becomes as dense as the air around it and stops accelerating. In a thunderstorm it is roughly where the anvil spreads out.
Taught in How to read a skew-T diagram.
See also Level of free convection, Overshooting top, Anvil.
Source: AMS Glossary of Meteorology
- Equivalent potential temperature theta-e, θe
A measure of the heat and moisture content of air together, conserved when air rises and condenses. High values near the ground mark warm, moist air that can feed strong updrafts.
See also Potential temperature, Most-unstable CAPE.
Source: AMS Glossary of Meteorology
F
- Forward flank downdraft FFD
The main downdraft in the leading part of a supercell, where most of its heavy rain and hail fall.
See also Rear flank downdraft.
- Freezing rain FZRA
Rain that falls as liquid, often supercooled, and freezes on contact with the ground and objects, coating them in ice. It needs a warm layer aloft to melt snow and a subfreezing surface.
See also Sleet.
Source: AMS Glossary of Meteorology
- Funnel cloud
A funnel-shaped cloud extending from a cloud base, marking a rotating column of air that may or may not reach the ground. When the rotation is violent and reaches the ground, it is a tornado.
See also Tornado.
Source: AMS Glossary of Meteorology
G
- Gust front
The leading edge of a storm’s cold outflow, often marked by a wind shift, a pressure jump and a drop in temperature.
See also Outflow boundary, Shelf cloud, Gustnado.
Source: AMS Glossary of Meteorology
- Gustnado
A short-lived, shallow, usually weak vortex along a gust front, made visible by swirling dust or debris. It is not considered a tornado.
See also Gust front.
Source: AMS Glossary of Meteorology
H
- Hectopascal hPa, millibar, mb
The unit of atmospheric pressure used on weather charts: 100 pascals, equal to one millibar. Sea-level pressure averages about 1013 hPa.
Source: AMS Glossary of Meteorology
- Hodograph
A plot of the wind profile as one line: the tip of each level’s wind vector, drawn from a common origin and joined from the ground upward. Its length shows the shear and its curvature the turning of the wind with height.
Taught in How tornadoes form.
See also Wind shear, Storm-relative helicity.
Source: AMS Glossary of Meteorology
- Hook echo
A hook-shaped extension of a storm’s reflectivity, usually at its right rear, formed where precipitation is drawn around the mesocyclone. It indicates conditions favorable for a tornado.
See also Mesocyclone, Rear flank downdraft, Bounded weak echo region.
Source: AMS Glossary of Meteorology
I
- Inflow band
A band of low cloud aligned with the low-level wind and moving into a storm. Inflow bands indicate the strength of the moist inflow; ones curved in a cyclonic pattern may indicate a mesocyclone.
See also Beaver tail.
- Inversion temperature inversion
A layer in which temperature increases with height instead of decreasing. On a skew-T the temperature trace leans right, toward warmer values, through the layer.
See also Capping inversion.
Source: AMS Glossary of Meteorology
- Isobar
A line of constant pressure. On a surface map isobars outline highs and lows; on a skew-T they are the horizontal lines.
Source: AMS Glossary of Meteorology
- Isotherm
A line of constant temperature. On a skew-T the isotherms are straight lines sloping up to the right.
Source: AMS Glossary of Meteorology
J
- Jet streak jet max
A local maximum of wind speed within the jet stream. The air entering and leaving it rises and sinks in a set pattern, which helps trigger or suppress storms.
See also Jet stream.
Source: AMS Glossary of Meteorology
- Jet stream
A narrow band of strong wind near the top of the troposphere, in the westerlies of the middle latitudes. On upper-air charts a jet stream is marked wherever the wind reaches 50 knots or more.
See also Jet streak, Low-level jet.
Source: AMS Glossary of Meteorology
K
- K index
A thunderstorm index from the 850 to 500 hPa temperature difference and the moisture at 850 and 700 hPa. The likelihood of showers and thunderstorms rises as it increases from about 20.
See also Lifted index, Total totals index.
Source: AMS Glossary of Meteorology
L
- Landspout
An informal name for a tornado that forms under a growing cloud without a preexisting mesocyclone, with its rotation coming from near the ground. It resembles a weak waterspout over land.
Taught in How tornadoes form.
See also Tornado, Waterspout.
Source: AMS Glossary of Meteorology
- Lapse rate
The decrease of temperature with height. The environmental lapse rate is the observed profile at a place and time; a process lapse rate is the rate a lifted parcel of air follows. Comparing the two decides stability.
Taught in How to read a skew-T diagram.
See also Environmental lapse rate, Dry adiabatic lapse rate, Conditional instability.
Source: AMS Glossary of Meteorology
- Level of free convection LFC
The height at which a lifted parcel, cooling at the dry rate to its LCL and at the moist rate above it, first becomes warmer than its surroundings. Above it the parcel rises on its own.
Taught in How to read a skew-T diagram.
See also Lifted condensation level, Equilibrium level, CIN.
Source: AMS Glossary of Meteorology
- Lifted condensation level LCL
The height at which air lifted dry-adiabatically becomes saturated: the base of the cloud it forms. On a skew-T it is where the dry adiabat through the temperature meets the mixing ratio line through the dew point.
Taught in How to read a skew-T diagram.
See also Level of free convection, Convective condensation level.
Source: AMS Glossary of Meteorology
- Lifted index LI
The temperature of the air at 500 hPa minus the temperature a parcel lifted from near the surface would have there. Negative values mean the parcel is warmer than its surroundings, and so unstable.
See also K index, Total totals index.
Source: AMS Glossary of Meteorology
- Longwave trough long wave, planetary wave
One of the few large, slow-moving waves in the midlatitude westerlies, spanning thousands of kilometers. Shortwaves travel through it.
See also Shortwave trough, Ridge.
Source: AMS Glossary of Meteorology
- Low-level jet LLJ, nocturnal jet
A band of strong wind in the lowest 2 to 3 km of the atmosphere, often strengthening at night. Over the Plains it carries moisture north and increases low-level shear after dark.
See also Jet stream.
Source: AMS Glossary of Meteorology
M
- Mammatus mamma
Rounded pouches hanging from the underside of a cloud, most often a thunderstorm’s anvil.
See also Anvil.
Source: AMS Glossary of Meteorology
- Mesocyclone
The rotating updraft of a supercell: a vortex about 2 to 10 km across. Doppler radar detects it as a velocity couplet, and some mesocyclones produce tornadoes.
Taught in How tornadoes form.
See also Supercell, Velocity couplet, Tornado.
Source: AMS Glossary of Meteorology
- Mesoscale convective system MCS
A group of thunderstorms that together produce a connected area of rain about 100 km or more across in at least one direction.
See also Squall line, Derecho.
Source: AMS Glossary of Meteorology
- Mesoscale discussion MD, MCD
A short Storm Prediction Center text product describing what is happening, what is expected in the next few hours and why, often issued half an hour to several hours before a watch.
Taught in Tornado watch vs. tornado warning.
See also Convective outlook, Tornado watch.
- Microburst
A concentrated downdraft affecting an area less than 4 km across, with peak winds lasting 2 to 5 minutes. Microbursts can be wet or dry and are a serious hazard to aircraft.
See also Downdraft, Downdraft CAPE, Virga.
Source: AMS Glossary of Meteorology
- Mixed-layer CAPE MLCAPE, mean-layer CAPE
CAPE computed for a parcel with the average temperature and moisture of the lowest 100 hPa, about the lowest kilometer. It represents daytime updraft air better than a surface parcel does.
See also CAPE, Surface-based CAPE.
- Mixing ratio
The mass of water vapor per mass of dry air, usually in grams per kilogram. It stays constant as unsaturated air rises or sinks, which is why the LCL is found along a mixing ratio line.
See also Dew point, Lifted condensation level.
Source: AMS Glossary of Meteorology
- Moist adiabatic lapse rate saturated adiabatic lapse rate
The rate at which saturated air cools as it rises. Condensation releases heat, so the rate is always less than the dry rate, and it varies with temperature and pressure.
See also Dry adiabatic lapse rate.
Source: AMS Glossary of Meteorology
- Most-unstable CAPE MUCAPE
CAPE computed for the most buoyant parcel in the lowest 300 hPa, the one with the highest equivalent potential temperature. When that parcel starts above the ground, the instability is elevated.
See also CAPE, Equivalent potential temperature.
- Multicell thunderstorm multicell
A cluster of short-lived storm cells whose combined outflow forms one gust front, along which new cells form every 5 to 15 minutes.
See also Supercell, Gust front.
O
- Occluded front occlusion
A front that forms as a low deepens into colder air and its cold front overtakes the warm front, separating the air behind the cold front from the air ahead of the warm front.
See also Cold front, Warm front.
Source: AMS Glossary of Meteorology
- Occluded mesocyclone
A mesocyclone that the rear flank downdraft has completely wrapped around at low levels, cutting off its supply of warm, moist air.
See also Mesocyclone, Rear flank downdraft.
- Outflow boundary
The edge of air cooled by thunderstorms as it spreads along the ground. It can last from minutes to more than a day and often sets off new storms where it meets other boundaries.
See also Gust front.
Source: AMS Glossary of Meteorology
- Outlook categories marginal, slight, enhanced, moderate, high
The five levels of severe weather risk on the Storm Prediction Center’s outlooks: marginal, slight, enhanced, moderate and high. A high risk means a severe weather outbreak is expected, from numerous intense, long-tracked tornadoes or a long-lived derecho.
Taught in Tornado watch vs. tornado warning.
See also Convective outlook.
- Overshooting top penetrating top
A dome rising above a thunderstorm’s anvil where the updraft carries past its equilibrium level. A large, persistent one indicates a strong updraft.
See also Anvil, Equilibrium level.
Source: AMS Glossary of Meteorology
P
- Particularly dangerous situation PDS
Wording added to a watch in rare situations when long-lived, strong and violent tornadoes are possible, or to a severe thunderstorm watch for an intense windstorm.
Taught in Tornado watch vs. tornado warning.
See also Tornado watch.
- Potential temperature theta, θ
The temperature air would have if brought dry-adiabatically to a pressure of 1000 hPa. It stays constant for unsaturated air moving up or down, so each dry adiabat on a skew-T is a line of constant potential temperature.
See also Equivalent potential temperature, Dry adiabatic lapse rate.
Source: AMS Glossary of Meteorology
- Precipitable water PW, PWAT
The total water vapor in a column of air, expressed as the depth of water it would make if all of it condensed. It is a measure of how much moisture is available to a storm, not of how much rain will fall.
Source: AMS Glossary of Meteorology
Q
- Quasi-linear convective system QLCS
A line of storms, such as a squall line or bow echo, containing small vortices that can behave like low-level mesocyclones and produce tornadoes.
Taught in How tornadoes form.
See also Squall line, Bow echo.
R
- Radial velocity Doppler velocity
The part of the wind blowing directly toward or away from the radar, which is all a Doppler radar measures. Wind blowing across the beam reads as zero.
See also Velocity couplet, Velocity aliasing.
Source: AMS Glossary of Meteorology
- Radiosonde weather balloon, sonde
An instrument package carried aloft by a balloon that measures temperature, humidity and pressure and radios them to the ground. Its tracked position gives the wind.
Taught in How to read a skew-T diagram.
See also Sounding.
Source: AMS Glossary of Meteorology
- Rear flank downdraft RFD
Sinking air on the back side of a supercell that wraps around the mesocyclone. It is often visible as a clear slot beside the wall cloud, and the rain and hail it carries can appear on radar as a hook echo.
Taught in How tornadoes form.
See also Forward flank downdraft, Clear slot, Hook echo.
- Rear-inflow jet
A current of air entering a mesoscale convective system from behind and flowing through its trailing rain area toward the leading line.
See also Bow echo.
Source: AMS Glossary of Meteorology
- Reflectivity Z
The power returned to the radar by precipitation, expressed on a logarithmic scale in dBZ. Higher values indicate heavier precipitation or larger particles.
See also dBZ.
Source: AMS Glossary of Meteorology
- Relative humidity RH
The ratio of the water vapor in the air to the most it could hold at that temperature, as a percentage. Because it depends on temperature, it changes through the day even when the moisture does not.
See also Dew point, Mixing ratio.
Source: AMS Glossary of Meteorology
- Ridge
An elongated area of relatively high pressure, where the flow curves anticyclonically. The opposite of a trough; ridges usually bring sinking air and fair weather.
See also Shortwave trough.
Source: AMS Glossary of Meteorology
S
- Scud
Ragged, low cloud fragments below a storm’s main cloud base, often near rain shafts. Scud is commonly mistaken for a funnel or wall cloud.
See also Wall cloud.
Source: AMS Glossary of Meteorology
- Severe thunderstorm
A thunderstorm producing a tornado, wind of 58 mph (50 knots) or more, or hail 1 inch or larger. A significant severe storm produces wind of 75 mph (65 knots) or more or hail 2 inches or larger.
See also Severe thunderstorm watch, Severe thunderstorm warning.
- Severe thunderstorm warning
A local National Weather Service product meaning a severe thunderstorm is indicated by radar or reported. Severe thunderstorms can produce tornadoes with little or no warning.
See also Severe thunderstorm, Tornado warning.
- Severe thunderstorm watch
A Storm Prediction Center product meaning conditions are favorable for severe thunderstorms in and near the watch area, usually for 4 to 8 hours.
See also Severe thunderstorm, Tornado watch.
- Shelf cloud
A low, wedge-shaped cloud attached to the base of a storm along its gust front. Air rises along its leading edge and its underside looks turbulent and ragged. Unlike a wall cloud, it marks outflow, not the updraft.
See also Gust front, Wall cloud.
Source: AMS Glossary of Meteorology
- Shortwave trough shortwave, short wave
A small, fast-moving wave in the upper-level westerlies, riding within the larger long waves. Rising air ahead of a shortwave trough often sets off storms.
See also Longwave trough, Ridge.
Source: AMS Glossary of Meteorology
- Significant tornado parameter STP
A composite index multiplying mixed-layer CAPE, cloud base height, effective helicity, effective shear and inhibition, each scaled to a typical value. Most significant tornadoes (EF2 and stronger) have occurred with values above 1, and most nontornadic supercells with values below 1.
Taught in How tornadoes form.
See also Supercell composite parameter, Storm-relative helicity, Mixed-layer CAPE.
Source: Storm Prediction Center, Significant tornado parameter
- Skew-T log-P diagram skew-T
The chart used to plot soundings: temperature against the logarithm of pressure, with the isotherms rotated 45 degrees to separate them from the dry adiabats. Areas on it are proportional to energy, so CAPE and CIN can be read as areas.
Taught in How to read a skew-T diagram.
Source: AMS Glossary of Meteorology
- Sleet ice pellets
In the United States, ice pellets: raindrops or melted snowflakes that refreeze while falling through a cold layer near the ground. In British usage, and colloquially in parts of the United States, sleet means a mix of rain and snow.
See also Freezing rain.
Source: AMS Glossary of Meteorology
- Sounding upper-air observation
A vertical profile of the atmosphere, most often one complete radiosonde flight: temperature, dew point and wind from the ground to the stratosphere. A model can produce one for any place and hour.
Taught in How to read a skew-T diagram.
See also Radiosonde, Skew-T log-P diagram, Hodograph.
Source: AMS Glossary of Meteorology
- Spectrum width SW
The spread of radial velocities within one radar sample volume. It is used to find turbulence, boundaries and mesocyclones and to judge how reliable the velocity data are.
See also Radial velocity.
- Squall line
A line of thunderstorms, continuous or broken, with a connected area of rain. A type of mesoscale convective system that is much longer than it is wide.
See also Quasi-linear convective system, Bow echo, Mesoscale convective system.
Source: AMS Glossary of Meteorology
- Stationary front
A front that is not moving, or is moving slower than about 5 knots.
See also Cold front, Warm front.
Source: AMS Glossary of Meteorology
- Storm-relative helicity SRH
A measure of how much the air flowing into a storm is spinning in the direction it moves, in m²/s², usually for the lowest 1 or 3 km. It gauges the potential for a rotating updraft. Values over 100 in the lowest kilometer and 250 in the lowest 3 km suggest an increased tornado threat with supercells.
Taught in How tornadoes form.
See also Hodograph, Streamwise vorticity, Significant tornado parameter.
- Streamwise vorticity
Spin about an axis that points along the direction the air is moving, like a thrown football. Air with streamwise vorticity flowing into an updraft gives the updraft rotation.
See also Vorticity, Storm-relative helicity.
Source: AMS Glossary of Meteorology
- Supercell
A thunderstorm built around a single, long-lived rotating updraft that persists far longer than it takes air to rise through it, often much longer than 10 to 20 minutes. Supercells form in strong vertical wind shear and produce most violent tornadoes and very large hail.
Taught in How tornadoes form.
See also Mesocyclone, Multicell thunderstorm, Rear flank downdraft.
Source: AMS Glossary of Meteorology
- Supercell composite parameter SCP
A composite index of most-unstable CAPE, effective helicity, effective shear and inhibition, each scaled to a threshold for supercells. Larger values mean more overlap of the ingredients for right-moving supercells.
See also Significant tornado parameter.
Source: Storm Prediction Center, Supercell composite parameter
- Surface-based CAPE SBCAPE
CAPE computed for a parcel lifted from the surface.
See also CAPE, Mixed-layer CAPE, Most-unstable CAPE.
T
- Three-body scatter spike TBSS, hail spike
A false streak of weak reflectivity extending away from the radar behind a core of large hail, caused by energy scattering from the hail to the ground and back. It often has near-zero velocity and high spectrum width.
See also Reflectivity.
- Tornadic vortex signature TVS
A Doppler velocity signature of a tornado or a developing tornado aloft: extreme inbound and outbound velocities about one beamwidth apart. It raises the probability of a tornado but does not confirm one.
See also Velocity couplet, Mesocyclone.
Source: AMS Glossary of Meteorology
- Tornado
A rapidly rotating column of air extending from the ground to the base of a cumuliform cloud, often with circulating debris or dust at the ground. A visible funnel is not required. Most are 2 km or less across and last about 100 to 1,000 seconds.
Taught in How tornadoes form.
See also Funnel cloud, Mesocyclone, Enhanced Fujita scale.
Source: AMS Glossary of Meteorology
- Tornado emergency
The highest tier of a tornado warning, for a severe threat to human life and catastrophic damage from a tornado confirmed by a reliable source or by radar evidence such as debris.
Taught in Tornado watch vs. tornado warning.
See also Tornado warning.
- Tornado warning
A product from the local National Weather Service office meaning a tornado is occurring or imminent, detected by radar or reported, for a small polygon, valid 15 to 45 minutes. Take shelter immediately.
Taught in Tornado watch vs. tornado warning.
See also Tornado watch, Tornado emergency.
Source: NWS Instruction 10-511, WFO Severe Weather Products Specification
- Tornado watch
A Storm Prediction Center product meaning conditions are favorable for tornadoes in and near the watch area, usually for 4 to 8 hours.
Taught in Tornado watch vs. tornado warning.
See also Tornado warning, Particularly dangerous situation.
- Total totals index TT
A stability index from the 850 hPa temperature and dew point and the 500 hPa temperature. Thunderstorms become increasingly likely from about 30, and severe thunderstorms are considered likely at 50 or more.
See also K index, Lifted index.
Source: AMS Glossary of Meteorology
- Triple point
The meeting point of two or more boundaries, such as a cold front, warm front and dryline, separating three air masses. It is usually near the center of low pressure and a common place for storms to start.
See also Dryline, Cold front, Warm front.
Source: AMS Glossary of Meteorology
U
- Updraft
A current of rising air in a storm. A tilted updraft is a sign of vertical wind shear, which favors severe storms.
See also Downdraft.
- UTC Z, Zulu time, Coordinated Universal Time
The time at the prime meridian, used for all weather observations and forecasts so they share one clock. Called Greenwich Mean Time before 1972. 00Z is 7 pm CDT the previous evening; 12Z is 7 am CDT.
V
- Velocity aliasing velocity folding
An error in which a radial velocity faster than the radar’s maximum unambiguous (Nyquist) velocity is displayed as a wrong value, often of the opposite sign.
See also Radial velocity.
- Velocity couplet rotational couplet
Adjacent areas of strong inbound and outbound radial velocity, indicating rotation or divergence. A tight, strong couplet is how radar shows a mesocyclone or tornado.
See also Radial velocity, Mesocyclone, Tornadic vortex signature.
- Virga
Streaks of rain or snow falling from a cloud that evaporate before reaching the ground. The evaporation cools the air and can drive dry microbursts.
See also Microburst.
Source: AMS Glossary of Meteorology
- Virtual temperature
The temperature dry air would need to have the same density as a given sample of moist air. Moist air is lighter than dry air at the same temperature, so CAPE and CIN are computed with it.
See also CAPE.
Source: AMS Glossary of Meteorology
- Volume coverage pattern VCP
The scanning strategy a WSR-88D runs: its set of elevation angles and rotation rates, chosen for the weather at hand.
See also WSR-88D.
- Vorticity
A measure of the spin of the air at a point. In weather analysis it usually means spin about a vertical axis; counterclockwise spin, positive in the Northern Hemisphere, is cyclonic.
See also Streamwise vorticity, Shortwave trough.
Source: AMS Glossary of Meteorology
W
- Wall cloud
An abrupt lowering of a thunderstorm’s cloud base, usually a kilometer or more across, beneath a strong updraft. Wall clouds with strong rotation and rising motion often precede tornadoes by a few minutes to an hour.
See also Shelf cloud, Scud, Funnel cloud.
Source: AMS Glossary of Meteorology
- Warm air advection warm advection, WAA
The transport of warmer air into a region by the wind. It is associated with rising air on the large scale.
Source: AMS Glossary of Meteorology
- Warm front
A front that moves so that warmer air replaces colder air.
See also Cold front, Occluded front.
Source: AMS Glossary of Meteorology
- Waterspout
Any tornado over water; most often a nonsupercell tornado. They are most frequent in the subtropics in the warm season, and more are reported in the lower Florida Keys than anywhere else.
Source: AMS Glossary of Meteorology
- Wet-bulb temperature
The temperature air reaches when cooled to saturation by evaporating water into it. It lies between the temperature and the dew point, and sets how cold falling rain can make the air beneath a cloud.
See also Dew point.
Source: AMS Glossary of Meteorology
- Wind shear shear
The change of the wind’s speed or direction over a distance. In severe weather, vertical shear, the change with height, is what organizes storms and gives them rotation.
See also Bulk shear, Hodograph.
Source: AMS Glossary of Meteorology
- WSR-88D NEXRAD
Weather Surveillance Radar, 1988 Doppler: the National Weather Service’s radar network, which added Doppler velocity and spectrum width to reflectivity.
See also Volume coverage pattern, Reflectivity, Radial velocity.