SkyStef's weather page
Advection: This is when heat or moisture is transferred horizontally. The atmosphere at all levels is usually in motion, with the low to middle troposphere being areas of significant advection, and thickness lines of 500-1000hpa are often used.
Airmass thunderstorm: Generally, a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Airmass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to insolation and dissipate rather quickly after sunset.
Anafront: A front at which the warm air is ascending the frontal surface up to high altitudes. It tends to be active with a lot of precipitation.
Anticyclogenesis: Development or intensification of a high-pressure center
Anvil: The top flatter parts of rain or storm bearing clouds.
Atmosphere: the air surrounding and bound to earth.
Backing or veering wind: When wind changes in an anti-clockwise direction i.e. from south to NE through east, this is said to have backed. When wind changes in a clockwise direction i.e. from southwest to north, through west, this is said to have veered.
Baroclinic zone: A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems.
Beaufort Wind Scale: A scale classifying wind strength in terms of observable effects both on the sea and over land.
Beaver('s) tail: A particular type of inflow band with a relatively broad, flat appearance suggestive of a beaver's tail. It is attached to a supercell's general updraft and is oriented roughly parallel to the pseudo-warm front, i.e., usually east to west or southeast to northwest.
Bow echo: A radar echo which is linear but bent outward in a bow shape. 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 sometimes can lead to tornado genesis - especially in the right (usually southern) end, where the circulation exhibits cyclonic rotation.
CA: Cloud-to-air lightning.
CAA: Cold Air Advection
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.
CC: Cloud-to-cloud lightning.
CET: Abbreviation for Central European Time (= UTC +1 hour in winter, +2 hour in summer).
CG: Cloud-to-ground lightning flash.
Cloud streets: Rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind flow. Cloud streets sometimes can be seen from the ground, but are seen best on satellite photographs.
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.
Comma cloud: A mesoscale cloud pattern (max 1000 km diameter) with a characteristic comma-like shape, often seen on satellite photographs associated with large, intense low-pressure systems.
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 that offsets the (apparent) converging effect of the confluent flow.
Contrail: Condensation trail, a Cirrus like trail of water vapor.
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. 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.
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).
Crespuscular rays: Dark blue bands which radiate from the Sun and cross the purple light, during twilight. They are shadows of clouds situated on or below the horizon. Sometimes these shadows completely cross the sky and are again seen near the anti-solar point.
Cut-Off Low: An upper level low pressure system that is no longer in the normal west to east upper air flow. Usually a cut-off low will lie to the South of the established upper air flow.
Cyclogenesis: Development or intensification of a low-pressure center.
dBZ: The colors shown on the weather radar images represent the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. The scale represents dBZ values of the energy reflected back to the radar from precipitation and other airborne material. The scale of dBZ values is also related to the intensity of rainfall.
Depression (Low): A mass of air with low atmospheric pressure.
Derecho: A squall line may be labeled a derecho if it is followed by
an extended area of damaging winds.
Dewpoint depression: The difference in degrees between the air
temperature and the dewpoint temperature.
Diurnal variation: When used in meteorology, this usually refers to the daily pattern of winds and temperatures.
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). Doppler radar is used by Belgocontrol (radar station at Melsbroek) for aviation safety purpose, and by KMI/IRM (radar station at Wideumont) for general forecast purpose.
Downburst : A strong downward rush of air which produces a blast of damaging wind on or close to the surface.
Downdraft: 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.
Dust devils: A small but rapidly rotating column of wind of short duration that is made visible by dust, sand, and debris picked up from the ground. Diameter usually ranges from a few meter to 30 meter and develop best on clear, dry, hot afternoons.
Dust Whirl: A rotating column of air rendered visible by dust.
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 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.
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.
Fog: A dense watery vapor hanging over land or sea reducing visibility less than 1 km.
Freezing level: level in atmosphere at which the temperature is 0°C.
Freezing rain: Rain that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 0 degrees Celsius or less, possibly producing a thin coating of ice. Even in small amounts, freezing rain can cause traveling problems. Large amounts can pull down power lines and tree branches. Same for Freezing Drizzle.
Front: The line that separates warm and cold fronts. Fronts are mostly accompanied by clouds usually thick enough to produce (heavy) precipitation. Behind a warm front warmer air is advected (at height), behind a cold front colder air is advected (at height). On a weather map a warm front is marked as a red line or as a black line with semicircles, a cold front as a blue line or a black line with wedges.
Frost: Crystals of frozen vapor.
Funnel cloud: A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that does not reach the ground. once a funnel cloud reaches the ground it is then called a tornado.
Gale: Wind speeds from 34 to 47 knots (61 to 85 km/h).
Geopotential: Is equivalent to the potential energy of unit mass relative to a standard level (mean sea-level by convention) and is numerically equal to the work which would be done against gravity in raising the unit mass from mean sea-level to the level at which the mass is located. When calculating the thickness of a layer, earth's gravity force plays a role. There where gravity is largest (both poles), the layer is pulled down and becomes slightly smaller.
Geopotential height: The altitude of a layer in the atmosphere. It is expressed in geopotential meters and is equal to g/9,8 times the geopotential height expressed in (geometric) meters, g being the local acceleration of gravity. It is used to define isobaric surfaces on upper level charts. Reason for using geopotential meters instead of meters is to compare heights in geographical context, without the effect of gravity, thus imitating a perfect round earth.
Gust: A sudden increase of wind speed of short duration, usually a few seconds.
Gust front: The leading edge of the downdraft from a thunderstorm. A gust front may precede the thunderstorm by several minutes and have winds that can easily exceed 70 knots (130 km/h).
Gustnado: 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.
Hail: Frozen rain or vapor that falls in showers.
Halo: Groups of optical phenomena, in the form of rings, arcs, pillars or bright spots, produced by the refraction or reflection of light by ice crystals suspended in the atmosphere.
Hectopascal (hPa): Pressure unit much used in meteorology. A hectopascal is equal to 100 pascals or 1 millibar.
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.
Hook echo: A hook echo is displayed on radar reflectivity. It is a
signature produced by precipitation held aloft that wraps around the mid-level
mesocyclone. Since the mesocyclone has counterclockwise winds, the reflectivity
signature of a hook echo will have a cyclonically shaped hook. The area free
from reflectivity inside the hook is the updraft and inflow notch region of the
supercell. A hook echo is one clue to a radar operator that a supercell has a
potential of producing a tornado. Many of the violent tornadoes associated with
classic supercells will show a distinct hook echo.
Hoar frost: Deposits of ice having a crystalline appearance, generally assuming the forms of scales, needles, feathers or fans. Formed by sublimation of water vapor from surrounding clear air.
IC: Intra-cloud lightning.
Ice crystals: A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.
Infra-Red: A method of viewing clouds via satellite to determine positions and condition.
Instability: The tendency for 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.
Inversion: A term meaning the reversal of something, in meteorology the a reversal of the normal atmospheric temperature gradient with height.
Irisation: Colors appearing on clouds, sometimes mingled, sometimes in the form of bands nearly parallel to the margins of the clouds. Green and pink predominate, often with pastel shades.
Isobar: A line on a weather chart showing places having the same atmospheric pressure at the same time.
Isobaric surfaces: A surface of equal pressure. Isobaric surfaces are used in upper level charts where geopotential heights are contoured to describe the upper level features. These charts are typically produced at standard levels such as 850hPa, 700hPa, 500hPa etc. In additional to geopotential height, these can charts are also used to display a variety of parameters such as streamlines, vorticity, moisture, temperature and so on. Also known as a constant pressure surface.
Jet streak: A local wind speed maximum within a jet stream.
Jet stream: A strong narrow current concentrated along a quasi horizontal axis in the upper troposphere or in the stratosphere, characterized by strong vertical and lateral wind shears and featuring one or more velocity maxima (jet streaks). The speed of the wind must be greater than 60 knots (31 m/s).
Katafront: A front (usually a cold front) at which the warm air descends the frontal surface. It tends to be "inactive" with few precipitation.
Knot: The unit of speed equal to 1.85 km/h (or 0.515 m/s or 1.152 mph). The knot is commonly used to report wind speed.
Left mover: 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.
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 (usually 500mb) 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.
Lightning: Luminous manifestation accompanying a sudden electrical discharge which takes place from or inside a cloud or, less often, from high structures on the ground or from mountains.
Macroburst: A large downburst affecting an area more than 4 kilometers (about 2.5 miles) across with damaging winds lasting from 5 to 20 minutes. May reach tornado intensity.
Maritime polar: A cold wet airmass from sub polar seas.
MCC - Mesoscale Convective Complex: A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:
MCS - Mesoscale Convective System: a complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.
Meridional flow: A type of atmospheric circulation pattern in which the north and south component of motion is unusually pronounced. Opposite of zonal flow.
Mesocyclone: a vertical column of (counterclockwise) rotating air within a severe thunderstorm which may be a precursor to a funnel or tornado. Typically a mesocyclone is 3-9 km in diameter. The circulation of a mesocyclone covers an area much larger than a tornado that may develop within it.
METAR: Meteorological Aviation Routine Weather Report, provided every half hour and consists of half-hourly observations at the airport, contains weather parameters like wind, visibility, weather, clouds, temperature, dewpoint and mean sea level pressure.
Microburst: A small, concentrated downburst affecting an area less than 4 kilometers (about 2.5 miles) across. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.
Mist: Suspension in the air of microscopic water droplets or wet hygroscopic particles hanging over land or sea, reducing visibility less than 5 km, but minimal 1km or more. (Criteria used in Belgium).
MSLP: Mean sea level pressure.
NOAA: National Oceanographic and Atmospheric Administration.
Nowcast: A short-term weather forecast, generally out to 6 hours or less
Occluded front-Occlusion: Fronts where the cold front has overtaken the warm front and lifted the warm air off the ground, usually meaning the end of a depression. Occluded fronts are accompanied by clouds. In the young stage they are usually thick enough to produce (heavy) precipitation. In the decaying stadium the clouds become thinner with little or no precipitation. On a weather map this front is marked as a purple line, or a black line with semicircles and wedges.
Omega blocking: A flow pattern in upper air that resembles the Greek letter Omega.
Overshooting top: A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell.
Popcorn convection: Clouds, showers, and thundershowers that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.
Precipitation (ppn): Any or all of the forms of water particles, whether liquid (e.g. rain, drizzle) or solid (e.g. hail, snow), that fall from a cloud or group of clouds and reach the ground.
Precipitation intensity: divided in (s)light, moderate and heavy (or dense). Following rules are accepted depending on the type of precipitation.
Pulse-storm: strong short lived single-cell thunderstorm, which occurs with high CAPE values.
PVA: Positive Vorticity Advection. Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves) and is a property that often enhances the potential precipitation.
Radar: Radio detection and ranging is an instrument for measuring precipitation within clouds by the signal being bounced back.
Rain: The difference between rain and drizzle is that rain has a diameter greater than 0.5mm.
Rainbow: Groups of concentric arcs with colors ranging from violet to red, produced on a "screen" of water drops (raindrops, droplets of drizzle or fog) in the atmosphere by light from the Sun or Moon. This phenomena is mainly due to the refraction and reflection of light. When rainbows are produced by the Sun, their colors are usually brilliant. When produced by the Moon, their colors are much weaker or sometimes absent.
Rain foot: A horizontal bulging near the surface in a precipitation
shaft, forming a foot-shaped prominence. It is a visual indication of a wet
Ridge: An elongated area of high pressure.
Right mover: A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather. (Supercells often are right movers.)
Rime: Deposits of ice, composed of grains more or less separated by trapped air, sometimes with crystalline appearance assuming the forms needles or scales. Formed by freezing of supercooled water droplets of surrounding moist air (fog or clouds).
Roll cloud: a low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front); roll clouds are completely detached from the thunderstorm base or other cloud features.
Runway Visual Range (RVR): An instrumentally-derived value, based on standard calibrations, that represents the horizontal distance a pilot may see down the runway from the approach end.
SAFIR: Surveillance des orages et Alerte Foudre
par Interférométrie Radioélectrique is an observation system to
detect and locate lightning via VHF waves.
Severe thunderstorm: To be classified as severe, a thunderstorm has to have wind speeds of 50 knots (90 km/h) or more, and/or hail at least 2cm or more in diameter.
Shear: Wind shear is the description for when wind changes direction, usually vertical winds but not always.
Shelf cloud: A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front), even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.
Shower: Intermittent and usually short spells of precipitation.
Single-cell: One thunderstorm cell. It has a duration of around half
Snow: Intricately shaped ice crystals that fall as precipitation.
Snowflake: White ice crystals that have combined in a complex branched hexagonal form.
Sounding: a plot of the vertical profile of temperature and dew point (and often winds) above a fixed location; used extensively in weather forecasting.
Splitting storms: A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.
Squall: Atmospheric phenomenon characterized by a very large variation of wind speed, often accompanied by a shower or thunderstorm.
Squall Line: Fictitious moving line, sometimes of considerable extent, along which squall phenomena occur.
Stable Air: Air that is colder than its surroundings and as such is resistant to upward movement.
Storm: a disturbance of the atmosphere marked by wind and usually by rain, snow, hail, sleet or thunder and lightning.
Subsidence: Sinking (downward) motion in the atmosphere, usually over a broad area.
Sublimation: The transition of a substance from the solid phase directly to the vapour phase, or vice versa, without passing through the liquid phase.
Supercell (Supercell storm): A violent thunderstorm which last for several hours, sometimes causing torrential rain and tornadoes.
TAF: Terminal Aerodrome Forecast contains a weather forecast for a specific airport for the oncoming day(s).
Tail cloud: A low tail-shaped cloud extending outward from the northern quadrant of a wall cloud. Motions in the tail cloud are toward the wall cloud with rapid updraft at the junction of tail and wall cloud. This horizontal cloud is not a funnel or tornado.
Tail-end Charlie: The thunderstorm found at the most southern end of a squall line or band of thunderstorms. Since low-level southerly inflow of warm, moist air into this storm is relatively unimpeded, such a storm often has a higher probability of strengthening to severe levels than the other storms in the line.
Temperature: The degree of hotness or coldness as measured on some definite temperature scale.
Thickness: Thickness usually refers to the depth of the 1000-500 hPa
layer in the atmosphere. However charts are also produced for thicknesses of
other layers in the atmosphere as well. The thickness gives an indication of the
mean temperature within a layer; lower thicknesses indicate colder air, higher
thicknesses warmer air.
Thunderstorm: A local storm produced by a cumulonimbus cloud, always with lightning and thunder, and usually accompanied by strong gusts of wind, heavy rain, and sometimes hail.
Tornado: A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the ground. However, the visible portion might not extend all the way to the ground.
Towering Cumulus: A large Cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil of a Cb. (Often shortened to "towering cu," and abbreviated TCU)
TREND: gives very accurate meteorological information at a specific airport and is indispensable for landing of aircraft within the next 2 hours. TREND is part of METAR.
Triple point: The intersection point between boundaries of different air masses (warm front, cold front, occlusion, etc) often a focus for severe weather development.
Tropopause: The place that marks the end of the troposphere and the start of the stratosphere marked by a change in temperature.
Trough: An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low.
Turbulence: Disturbance in the atmosphere causing gusts of varying strengths.
Unstable air: Air that is warmer than its surroundings and as such tends to rise, leading to the formation of clouds and possibly precipitation.
Updraft: A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a Cumulus cloud or an individual tower of a towering cumulus or Cb.
UTC: Abbreviation for Universal Time Coordinated and formerly known as Greenwich Mean Time (GMT).
Visibility: Greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky, or, in the case of night observations, could be seen and recognized if the general illumination were raised to the normal daylight level.
Vortex: Cyclonic flow in a relative small area.
WAA: Warm Air Advection.
Wall cloud: Lowering of a cloud base at low levels in a thunderstorm.
Waterspout: A rapidly rotating column of air extending from a cumulonimbus cloud with a circulation that reaches the surface of the water, (i.e. a tornado over water).
Wind: a natural movement of air at a velocity relative to the surface of earth.
Wind shear: the local variation of the wind speed and/or direction in a given direction. Shear usually refers to vertical wind shear (i.e., the change of wind with height) but the term is also used in Doppler radar meteorology to describe changes in radial velocity over short horizontal distances.
Wind shift: A sudden change of wind direction.
Zonal flow: Large-scale atmospheric flow in which the east-west (latitudinal) component is dominant.
Zulu time: Same as UTC, Universal Coordinated Time. It is called Zulu because "Z" is often appended to the time to distinguish it from local time.