Glossary of Weather Terms

Home , Air mass, Altitude, Anabatic wind, Barometer, Block (meteorology), Cloud

REFERENCE MATERIAL Glossary of Weather Terms

Bureau of Meteorology › Weather Services › Aviation

A ALTITUDE The vertical distance from indicates the mature or decaying stage mean sea level to an object aloft. of a thunderstorm. ACCRETION The process of supercooled water droplets freezing on ALTOCUMULUS A principal cloud AIRCRAFT ICING Any deposit of ice impact with snow-flakes, ice particles type, forming in the middle levels of forming on an aircraft. or other cold objects including aircraft. the troposphere, and appearing as a white and/or grey layer or patch with a ATMOSPHERE The gaseous portion ADIABATIC COOLING Cooling of a waved aspect. In aviation forecasts and of the physical environment that parcel of gas by expansion, with no reports it is coded as AC. encircles the earth. The divisions of heat exchange between the parcel and the atmosphere are the troposphere, the surrounding air. ALTOSTRATUS A principal cloud type, the stratosphere, the mesosphere, the forming in the middle levels of the ionosphere, and the exosphere. ADIABATIC HEATING Warming of a troposphere, and appearing as a grey parcel of gas by compression, with no or bluish sheet. In aviation forecasts ATMOSPHERIC PRESSURE The total heat exchange between the parcel and and reports it is coded as AS. weight of the atmosphere above the the surrounding air. point of measurement. ANABATIC WIND An uphill wind ADIABATIC PROCESS A process generated by the heating of a sloping B where a parcel of air cools or warms surface. due to a change in pressure and BACKING A counter clockwise shift in volume (expansion or compression), ANEMOMETER An instrument used the wind direction. with no heat exchange between the to measure wind speed and direction. BAROMETER An instrument for parcel and the surrounding air. ANEROID BAROMETER A non- measuring atmospheric pressure. Two ADVECTION The horizontal transport liquid instrument used to measure types of barometer are the aneroid of any property in the atmosphere by atmospheric pressure. barometer and the mercury barometer. the movement of air. ANOMALOUS PROPAGATION BLOWING DUST Dust that is raised ADVECTION FOG Fog resulting from The non-standard propagation of a locally by the wind to a height of at the movement of moist air over a beam of energy (radio or radar) under least two metres above the ground. colder surface and the consequent certain atmospheric conditions, which In aviation forecasts and reports it is cooling of the air to saturation. results in false echoes (i.e. non- coded as BLDU. precipitation) on a radar image. Usually BLOWING SNOW Snow that is raised ADVECTED FOG Fog transported caused by unusual rates of refraction in locally by the wind to a height of at by local winds from one locality to the atmosphere. another. least two metres above the ground. ANTI-ICING EQUIPMENT Aircraft In aviation forecasts and reports it is AIR DENSITY The mass of air per unit equipment, such as heating elements coded as BLSN. volume. and flexible rubber strips, used to BOILING POINT The temperature at prevent or clear structural icing. AIR MASS An extensive body of which the vapour pressure of a liquid air with approximately uniform ANTICYCLONE An extensive is equal to that of the surrounding temperature and moisture horizontal spiral movement of the atmosphere. The boiling point of characteristics. atmosphere around and away from a pure water at the standard sea-level central region of high pressure. The pressure of 1013.25 hPa is 100°C ALTIMETER An instrument used to (373°K). determine altitude using atmospheric spiral motion is anticlockwise in the southern hemisphere and clockwise in pressure (QNH). BROKEN CLOUD Used to describe the northern hemisphere. an amount of cloud covering the sky ANVIL The upper portion of a of between five and seven oktas cumulonimbus cloud that spreads (eighths). In aviation forecasts and out under the tropopause, often in reports it is coded as BKN the shape of a blacksmith’s anvil, BUOYANCY In meteorology, it is sometimes for hundreds of kilometres the vertical force acting upon an air downstream from the parent cloud. It parcel as a result of a difference in moon. In aviation forecasts and reports CONTACT COOLING The process density between the air parcel and its it is coded as CS. whereby heat is conducted away from surrounding environment. warmer air to a colder surface. CIRRUS A principal cloud type, BUYS BALLOT’S LAW Describes the forming in the high levels of the CONTINENTAL AIR MASS An relationship of the wind direction to the troposphere, composed of ice crystals extensive body of air, with a more-or- pressure distribution. In the southern less uniform temperature and moisture which appear from the ground as white hemisphere, if one stands with one’s profile, which has originated over a tufts or filaments. In aviation forecasts back to the wind, lower pressure is to large land mass. and reports it is coded as CI. the right. Lower pressure will be to the left in the northern hemisphere. CONTOUR A line joining points of CLEAR AIR TURBULENCE (CAT) A equal value on a surface. form of turbulence occuring in regions C of marked wind shear, particularly at CONTRAIL A condensation trail may the boundaries of jet streams, but may form behind an aircraft when hot CARBURETTOR ICING Occurs when humid air from the a jet exhaust mixes also be found in strong lee waves. air (including warm air) with a high with environmental air of low water It occurs in the absence of any visual water vapour content is drawn into vapour and low temperature. If the clues (i.e. clouds). an aircraft’s carburettor (which is increased vapour leads to saturation, the vapour will condense into water chilled by the fuel evaporation process CLEAR ICE A sheet of transparent droplets and/or deposit into ice. coupled with the pressure reduction in ice formed by the relatively slow the carburettor’s venturi) causing the freezing of large supercooled water CONVECTION In meteorology, it is the water vapour to condense and then, droplets, i.e. rain. Does not seriously vertical transport of heat and moisture, if the temperature has dropped below distort aerofoil shape but does add especially by updrafts and downdrafts 0°C, form ice on the surfaces of the appreciably to aircraft weight and drag. in an unstable atmosphere. carburettor. The ice may gradually block COLD AIR ADVECTION The horizontal CONVECTIVE CLOUD A cloud that the venturi or cause jamming of the movement of colder air into a location owes its vertical development, and mechanical parts of the carburettor. of warmer air. possibly its origination, to convection.

CEILOMETER An instrument for COLD FRONT The leading edge of CONVECTIVE CONDENSATION estimating cloud amount and height. an advancing cold air mass that is LEVEL The lowest height at which replacing warmer air. condensation will occur as a result of CELSIUS TEMPERATURE SCALE (°C) convection due to surface heating. A temperature scale, named after the CONDENSATION Change of state CONVERGENCE The condition that Swedish astronomer Anders Celsius, from vapour to liquid. exists as a result of a net horizontal where water at the standard sea-level CONDENSATION NUCLEI Tiny inflow of air into a region. Convergent pressure of 1013.25 hPa has a freezing particles upon which water vapour winds at lower levels are associated point of 0°C and a boiling point of condenses. with upward motion. 100°C. CONDENSATION LEVEL The height at COORDINATED UNIVERSAL TIME CHANGE OF STATE A change in the which an adiabatically lifted air parcel (UTC) The primary time standard by form of water, e.g. liquid to vapour, ice will become saturated, whereupon which the world regulates clocks and to liquid, ice to vapour. condensation occurs. Corresponds to time. In aviation forecasts and reports cloud base level. it is signified by the letter Z. CIRROCUMULUS A principal cloud type, forming in the high levels of the CONDITIONAL INSTABILITY Stable CORIOLIS FORCE (EFFECT) An troposphere, composed of ice crystals unsaturated air that will become apparent force on a moving particle unstable if saturated. that arises solely from the earth’s which appear from the ground as very rotation acting as a deflecting force. small elements in the form of grains or CONDUCTION The transfer of heat It acts to the left in the southern small ripples. In aviation forecasts and in response to a temperature gradient hemisphere and to the right in the reports it is coded as CC. within an object or between objects northern hemisphere. It is greatest that are in physical contact with one at the poles and nonexistent at the CIRROSTRATUS A principal cloud another. Transfer is from warmer to equator. type, forming in the high levels of the colder regions. troposphere, composed of ice crystals CUMULONIMBUS A principal cloud CONSTANT PRESSURE CHART A which appear from the ground as a type, with bases forming in the low weather chart representing conditions transparent sheet or veil, often creating levels of the troposphere, characterised on a constant pressure surface, e.g. by a large vertical extent, and often a halo phenomenon around the sun or 500 hPa. capped by an anvil-shaped cirrus cumulonimbus or towering cumulus semi-arid areas and is most likely to cloud. It is often accompanied by rain cloud. The outward burst of air creates develop on clear, dry, hot afternoons in showers, turbulence, icing and gusty damaging winds at or near the earth’s response to surface heating. In aviation surface winds; and sometimes also surface. The term microburst is forecasts and reports it is coded as PO. by lightning, thunder, hail, microbursts used to describe a downburst which and/or tornadoes. In aviation forecasts causes damage over an area with DUSTSTORM A phenomenon and reports it is coded as CB horizontal dimensions of less than four characterized by strong winds and kilometres. dust-filled air over a large area, and CUMULUS A principal cloud type, reducing visibility to below 1000 forming in the low levels of the DOWNDRAFT A descent of cool air metres. In aviation forecasts and troposphere, characterised by flat associated with convective cloud. reports it is coded as DS. bases and dome or cauliflower-shaped upper surfaces. Small, separate DOWNSLOPE WIND A wind directed . E cumulus are associated with fair down a slope, often used to describe ECHO In meteorology, it is used to weather, but may grow into towering winds produced by processes larger in refer to the appearance on a radar cumulus or cumulonimbus. In aviation scale than the slope. display of the radio signal reflected forecasts and reports it is coded as CU. DRIFTING SNOW Snow blown from from a target (e.g. cloud). the ground by the wind to a height of D less than two metres. EDDY A small disturbance in the wind DENSITY The weight of air per unit that can produce turbulence. volume. DRIZZLE Slow-falling and uniformly distributed precipitation in the form ELEVATION The distance between DENSITY ALTITUDE The altitude in the of tiny water droplets (diameters less mean sea level and a point on the International Standard Atmosphere at than 0.5 millimeters), usually from earth’s surface. which a given air density is found. stratus or stratocumulus clouds. In EQUATOR The geographic circle at 0° aviation forecasts and reports it is latitude on the earth’s surface. DEPOSITION A process in which a coded as DZ. gas transforms into a solid, e.g. the EQUILIBRIUM LEVEL The height at process by which water vapour, in DRY ADIABAT A line of which a rising parcel of air will become sub-freezing air, changes directly to ice constant potential temperature equal in temperature to that of the without first becoming a liquid. on a thermodynamic diagram. environment, at which point it is no Meteorologically, the dry adiabat longer buoyant and thus will cease to DEW Water in the form of small liquid represents the meteorological rise in the atmosphere without forcing. drops that form on grass and other processes that occur with the adiabatic objects near the ground when the air ascent or descent of a parcel of dry EQUATORIAL TROUGH The quasi- temperature falls below its dew point, air (where no liquid water is present continuous area of low pressure usually overnight. and no condensation of water vapour located between the subtropical high occurs). As an air parcel ascends DEW POINT The temperature to pressure belts of the northern and adiabatically, its pressure decreases which air must be cooled, at constant southern hemispheres, and moving and its temperature falls due to the pressure and water vapour content, in north and south with the seasons. expansion of the air parcel; as it order for saturation to occur. If the air descends, its temperature will rise due EVAPORATION The physical process is cooled further, some of the water to compression of the air parcel. by which a liquid, such as water, is vapour will condense to liquid. transformed into its gaseous state. DRY ADIABATIC LAPSE RATE The DIURNAL Pertaining to actions that rate of change in temperature for EYE An area of clear skies that are completed within twenty four dry air ascending or descending develops in the centre of a tropical hours, and recur every twenty four adiabatically. The rate is approximately cyclone. It is characterised by light hours. 3°C per 1000 feet. winds and no rainfall. DIVERGENCE Horizontal outflow of DRY LINE The boundary between dry EYE WALL An organized band of air from a particular region. Divergence and moist air masses. cumulonimbus clouds that surrounds at lower levels is associated with a the eye of a tropical cyclone. downward movement of air. DUST Small particles of earth or other matter suspended in the air. In aviation DOLDRUMS A nautical term for F forecasts and reports it is coded as DU. the equatorial trough, an area which FAHRENHEIT TEMPERATURE SCALE typically has calm or light and variable DUST DEVIL A small and rapidly (°F) The temperature scale, developed winds. rotating column of wind made visible by the German physicist Daniel Gabriel by the dust, dirt or debris picked up by Fahrenheit in 1714, where water at the DOWNBURST A severe localised the wind. It usually occurs in arid or standard sea-level pressure of 1013.25 downdraft of wind from a hPa has a freezing point of +32°F and a depth is variable, ranging from tens GUST A sudden and brief increase in boiling point of +212°F. of metres in a stable environment, wind speed. to several kilometres in convective FEEDER BANDS The lines or bands conditions over deserts. GUST FRONT The leading edge of of cumulonimbus clouds that spiral cool air rushing down and out from into and around the centre of a tropical FRONT The transition zone or interface a thunderstorm cloud. Is usually cyclone. between two air masses of different accompanied by a drop in temperature, densities. a wind shift and a pressure jump FEW Used to describe an amount of cloud covering the sky of one or two FRONTAL PASSAGE The passage HAIL Precipitation that originates oktas (eighths). In aviation forecasts of a front over a specific point on in convective clouds, such as and reports it is given as FEW. the earth’s surface. Changes in cumulonimbus, in the form of balls temperature, dew point, wind and or irregular pieces of ice. In aviation FOEHN A warm and dry downslope atmospheric pressure occur with a forecasts and reports it is coded as wind descending the lee side of a frontal passage. GR, with small hail coded as GS. mountain range. FUNNEL CLOUD A violent, rotating H FOG A suspension in the air, at or near column of air visibly extending toward the earth’s surface, of microscopic the earth’s surface from the base of a HAZE Dust and/or smoke particles water droplets, or wet hygrscopic towering cumulus or cumulonimbus suspended in the air, and reducing the particles, reducing horizontal visibility cloud. A funnel cloud reaching the visibility to 5000 metres or less. In to less than 1000 metres. In aviation ground is called a tornado if over aviation forecasts and reports it is forecasts and reports it is coded as FG. land, and a waterspout if over water. coded as HZ. In aviation forecasts and reports it is FREEZING DRIZZLE Supercooled HIGH PRESSURE SYSTEM An area of coded as FC. drizzle (i.e. drizzle existing at a pressure maximum with diverging and temperature below 0°C) that freezes anti-clockwise winds in the southern upon impact with surfaces to form G hemisphere and clockwise in the glaze. In aviation forecasts and reports GEOSTATIONARY SATELLITE A northern hemisphere. it is coded as FZDZ. weather satellite, in a west to east orbit at an altitude of 35 786 km, that HOARFROST A deposit of interlocking FREEZING POINT The temperature maintains the same position over the ice crystals formed by direct deposition at which a liquid solidifies under any equator. on objects such as tree branches, leaf given set of conditions. Pure water edges, wires and poles. under the standard sea-level pressure GEOSTROPHIC WIND A wind that of 1013.25 hPa freezes at 0°C (32°F). blows parallel to straight isobars above I the friction layer, wherein the coriolis ICING The formation of ice, rime or FREEZING PRECIPITATION Any force exactly balances the horizontal hoarfrost on an aircraft. form of supercooled precipitation that pressure gradient force. freezes upon impact with surfaces to INFRARED Long-wave GLAZE A coating of ice, generally clear form glaze. electromagnetic radiation which is and smooth, formed on surfaces by emitted by all objects. FREEZING RAIN Supercooled rain the freezing of supercooled rain, drizzle that falls as liquid but freezes upon or fog. INSOLATION Solar radiation or heating impact with surfaces to form glaze. received at the earth’s surface. The GRADIENT WIND A wind that blows In aviation forecasts and reports it is name is derived from INcoming SOLar parallel to curved isobars above coded as FZRA radiATION. the friction layer, wherein the wind FRICTION The mechanical resistive direction is determined by the balance INSTABILITY A state of the force offered by one medium or body of three forces, i.e. the coriolis force, atmosphere where an air parcel lifted to the relative motion of another the centrifugal force and the horizontal vertically will freely accelerate upward medium or body in contact with the pressure gradient force. once the lifting mechanism ceases. first. In meteorology, it is the drag The air parcel will form cumulus-type GREENWICH MEAN TIME (GMT). or resistance of the earth on the clouds if sufficient moisture is present. atmosphere. Was used to refer to the primary time standard, but now replaced by INSTRUMENT FLIGHT RULES A set FRICTION LAYER (BOUNDARY Coordinated Universal Time (UTC). of regulations governing all aspects of LAYER, SURFACE LAYER) The thin civil aviation aircraft operations when a GROUND CLUTTER A pattern of radar layer of the lower atmosphere within flight is conducted using instruments echoes from fixed ground objects such which wind direction and speed is rather than outside visual reference. subject to frictional effects caused by as buildings, hills or other objects on or contact with the earth’s surface. It’s close to the ground. INTERNATIONAL STANDARD KELVIN-HELMHOLTZ CLOUD Cloud resembling a smooth lense with ATMOSPHERE (ISA) A hypothetical that forms in Kelvin-Helmholtz waves. sharp outlines. They mostly occur in vertical distribution of atmospheric The couds, sometimes referred to as mountain waves, and thus indicate temperature, pressure and density that billows, are in the shape of breaking possible severe turbulence and icing. by international agreement is taken to waves. These clouds are often good be representative of the atmosphere indicators of instability and the LEVEL OF FREE CONVECTION The for purposes of pressure altimeter presence of turbulence. height at which a parcel of saturated air calibrations, aircraft performance becomes warmer than the surrounding calculations, ballistic tables, etc. KELVIN TEMPERATURE SCALE air and thus begins to rise freely until it (°K) A temperature scale, designed reaches its equilibrium level. INTERTROPICAL CONVERGENCE by Lord Kelvin of Scotland in 1848, ZONE The region where southeast where 0°K is defined as absolute zero LIFTING CONDENSATION LEVEL The and northeast trade winds meet, (where all molecular movement stops). height at which a parcel of moist air usually located between 10 degrees Water freezes at 273°K, and boils at becomes saturated when it is cooled north and south of the equator. It is 373°K. It is used primarily for scientific by adiabatic lifting. a broad area of low pressure, located purposes. LIGHTNING A visible electrical in the southern hemisphere during discharge produced by a our summer, and in the northern KNOT A unit of speed equivalent to cumulonimbus cloud. It can occur hemisphere during its summer. 1.852 kilometers per hour. In aviation forecasts and reports it is signified by between cloud and ground, between INVERSION An increase in KT. clouds, within a single cloud, or temperature with increasing between a cloud and surrounding air. altitude, which is opposite to the L LONGITUDE A geographic coordinate usual decrease of temperature with LAND BREEZE A diurnal coastal or lake that specifies the east-west position increasing altitude. breeze that blows offshore. It is caused of a point on the earth’s surface. It is ISOBAR A line on a chart connecting by the temperature differences measured as an angle in reference points of equal pressure. between a water surface and adjacent to the Prime Meridian, which is land. designated as zero degrees longitude. ISOTACH A line on a chart connecting points of equal wind speed LAPSE RATE The rate of change LOW LATITUDES The latitude belt of temperature with height in the between the equator and 30 degrees ISOTHERM A line on a chart connecting atmosphere. north and south of the equator. points of equal temperature. LATENT HEAT The energy absorbed LOW LEVEL JET A transient strong J or released during a change of state. wind concentrated in relatively narrow Evaporation, melting and sublimation (a bands near the earth’s surface. JET STREAM An area of strong winds change from solid to gas) absorb heat concentrated in a relatively narrow from the surrounding air as energy LOW PRESSURE SYSTEM An area band. Is most commonly used to refer is needed to weaken the individual of pressure minimum with converging to a stream of maximum winds hydrogen bonds between the water winds rotating clockwise in the embedded in the mid-latitude molecules. Condensation, freezing and southern hemisphere and anticlockwise westerlies concentrated in the upper deposition (gas to solid) release the in the northern hemisphere. troposphere. latent heat, thus adding heat to the surrounding air. M K MARITIME AIR MASS An air mass LATITUDE A geographic coordinate KATABATIC WIND A drainage wind that has originated over an extensive that specifies the north-south position generated by air being cooled by water surface. of a point on the earth’s surface. It is conduction along a slope. The cooled measured as the angular distance, air flows downhill as a katabatic wind. MEAN SEA LEVEL The height of the subtended at the earth’s center, along sea surface, measured with respect KELVIN-HELMHOLTZ WAVE A wave- a meridian from a point on the earth to to land-based benchmarks, after form disturbance that arises from the equator. The equator is designated averaging out variations due to tides Kelvin-Helmholtz instability. Named as zero degrees and the poles as 90 and waves. In aviation it is used as a after Lord Kelvin and Hermann von degrees. measurement reference for altitude at Helmholtz. flight levels. LEE (LEESIDE/LEEWARD) The side of KELVIN-HELMHOLTZ INSTABILITY an obstacle that is furthest away from MEAN SEA LEVEL PRESSURE Occurs when velocity shear is present the wind. (MSLP) The atmospheric pressure at within a continuous fluid, or when there mean sea level. LENTICULAR CLOUD A more-or-less is sufficient velocity difference across isolated cloud, downwind of a barrier, the interface between two fluids. MECHANICAL TURBULENCE MULTICELL STORM A storm system PRE-FRONTAL SQUALL LINE A line Disrupted air-flow resulting from wind composed of two or more convective of thunderstorms that may develop flowing over or around terrain or man- cells at various stages of their life- cycle. ahead of an advancing cold front, and made obsrtuctions, whereby normal having an orientation more or less horizontal wind flow is disturbed and N parallel to the cold front. transformed into eddies and other NIMBOSTRATUS Low or middle-level PRE-FRONTAL TROUGH An elongated irregular movements. thick dark cloud with with more or less area of relatively low pressure that continuously falling rain, snow or sleet. MERIDIONAL FLOW Atmospheric may develop ahead of an advancing In aviation forecasts and reports it is circulation in which the north and south cold front. coded as NS. component of motion is pronounced. PRESSURE ALTIMETER An aneroid METAR A French acronym for a O barometer calibrated to indicate meteorological report from an OCCLUDED FRONT A front formed altitude by measuring atmospheric aerodrome at a routine time (1/2 when a cold front overtakes a warm pressure and using the standard hourly) when conditions are better than front. atmosphere pressure/height relations. specified thresholds. It is the primary The indicated altitude will only format in aeronautical meteorology OROGRAPHIC LIFTING Occurs where equal the actual altitude if the actual for reports of surface meteorological the flow of air is forced up and over atmosphere is equivalent to the information at an aerodrome. physical barriers such as mountains. International Standard Atmosphere.

MICROBURST Used to refer to a P PRESSURE ALTITUDE The altitude in severe downburst of wind, usually the International Standard Atmosphere PARCEL A theoretical small and from a thunderstorm, over an area of at which a given atmospheric pressure self-contained volume of air less than four kilometers in diameter. will be observed. responding to meteorological The term downburst is used to refer to processes as a single entity. larger diameters. PRESSURE GRADIENT The pressure change that occurs over a fixed PILOT REPORT A report of in-flight MIST A suspension in the air, at or distance. weather by an aircraft pilot. Referred to near the earth’s surface, of microscopic as an AIREP. water droplets or wet hygrscopic PREVAILING WIND A wind that blows from one direction more frequently particles which reduce the horizontal POLAR AIR MASS An air mass that than any other during a given period. visibility to less than 5000 metres but forms over a high latitude region. not less than 1000 metres. In aviation Continental polar air is formed over PROGNOSTIC CHART A forecast forecasts and reports it is coded as BR. cold land surfaces and is typically very weather chart. Commonly known as a stable with low moisture. MIXING RATIO The ratio of the mass prog chart. of a variable atmospheric constituent to POLAR FRONT A semi-continuous, Q the mass of dry air. In meteorology, the semi-permanent boundary between term normally refers to water vapour. polar and subtropical air masses. QUASI-STATIONARY FRONT A front which is nearly stationary or moving MONSOON The seasonal shift of POLAR-FRONT JET A jet stream very slowly. winds caused by the much greater associated with the polar front, annual variation of temperature over occurring at around 23-39,000 feet R large land areas compared with above the earth’s surface. It is depicted RADAR Acronym for RAdio Detection neighbouring ocean surfaces, which on analysis and forecast charts when And Ranging. An electronic instrument results in an excess of air pressure the wind is 80 knots or more. over the land areas in winter and a used to detect distant objects and deficit in summer. The monsoon is POLAR-ORBITING SATELLITE A measure their range by detecting strongest on the southern and eastern satellite whose orbit passes over both scattered or reflected radio energy. sides of Asia. of the earth’s poles. RADIATION The process by which MOUNTAIN WAVE An oscillation POTENTIAL TEMPERATURE The energy is propagated through any above and downwind of a mountain temperature that an unsaturated parcel medium by virtue of the wave motion caused by a disturbance in the of dry air would have if it is brought in that medium. Electromagnetic horizontal air flow due to the high adiabatically from its initial state to a radiation, which emits heat and light, is ground. They usually occur in groups, standard pressure, typically 1000 hPa. one form. and aircraft can expect to encounter RADIATIONAL COOLING The cooling severe turbulence and icing if flying PRECIPITATION In meteorology it is of the earth’s surface and the adjacent through them. In aviation forecasts it is any product of the condensation of air which occurs at night when the coded as MTW. atmospheric water vapour that falls under gravity. earth’s surface suffers a net loss of heat due to outgoing radiation being field (voltage) attains a strength of near SEVERE THUNDERSTORM The greater than incoming radiation. 1000 volts per centimeter. It often Bureau defines a severe thunderstorm occurs during the latter phases of a as one with winds of 48 knots or RADIATION FOG Fog that forms violent thunderstorm. It typically greater, or hail of diameter 2 cm or when radiational cooling at the earth’s appears on the extremities of aircraft, larger, or tornadoes or flash floods. surface lowers the temperature of the lightning rods and steeples. air near the ground to, or below, its SHOWER Precipitation from a dew-point. SANDSTORM A strong wind carrying convective cloud that is characterised sand through the air and reducing by its sudden beginning and ending, RAIN Precipitation in the form of liquid visibility to less than 1000 metres. In changes in intensity, and rapid changes water droplets greater than 0.5 mm aviation observations and forecasts it is in the appearance of the sky. It occurs diameter. In aviation forecasts and coded as SS. in the form of rain (SHRA), snow reports it is coded as RA. (SHSN), or hail (SHGR). SATURATE To add something to the RELATIVE HUMIDITY The ratio of point where no more can be absorbed, SKEW T-LOG P DIAGRAM A the vapour pressure to the saturation dissolved, or retained. In meteorology, thermodynamic diagram with a vapour pressure with respect to water. it is used when discussing the amount skewed temperature scale on the Also known as the ratio of the existing of water vapor in a volume of air. horizontal axis and a logarithmic amount of water vapour to that which scale of pressure on the vertical could be held by a parcel of air. It is SATURATED ADIABAT The line axis. It is used to plot radiosonde usually expressed as a percentage. on a thermodynamic diagram that soundings which give a vertical profile depicts the change in temperature of of air temperature and dew point RESOLUTION In relation to radar, it is a saturated air parcel as it rises or falls temperature through the atmosphere. the ability to read two distinct targets and undergoes cooling or heating due Forecasters use it to evaluate and separately. The clearer the resolution, to adiabatic expansion or compression. forecast air parcel behaviour. Some the nearer the two objects can be to values that can be determined are the each other and still be distinguishable. SATURATED ADIABATIC LAPSE Convective Condensation Level, the RATE The rate of change in Lifting Condensation Level, and the RIDGE An elongated area of high temperature of a saturated air parcel Level of Free Convection. pressure. as it adiabatically ascends or descends through the atmosphere. The rate RIME Ice formed by the rapid freezing SMOKE Small particles produced varies, from approximately 1.5 to 3°C of supercooled water droplets when by combustion that are suspended per 1,000 feet, e.g., the more water they contact an exposed object, such in the air (a transition to haze may vapour present in a rising parcel of air, as an aircraft frame, forming a white occur when the smoke particles the greater the condensation, and thus and opaque granular deposit of ice. have traveled great distance and the greater is the latent heat released, when the larger particles have settled ROLL CLOUD A low-level, horizontal, reducing the rate of cooling. out, the remaining haze particles tube-shaped cloud. Usually associated become widely scattered through the SCATTERED Used to describe an with a thunderstorm gust front, where atmosphere). In aviation observations amount of cloud covering the sky of the roll cloud is completely detached and forecasts it is coded as FU . three or four oktas (eighths). In aviation from the base of the cumulonimbus forecasts and reports it is coded as cloud. It will sometimes form with a SNOW Frozen precipitation in the SCT. cold front. form of ice crystals that combine in a complex branched hexagonal form as SEA-BREEZE A diurnal coastal breeze ROTOR CLOUD A cloud formation they fall towards the earth’s surface. It that blows onshore due a temperature found in the lee of a mountain or most often falls from stratiform clouds, differential between the land and the similar barrier. The air rotates around but can fall as snow showers from water. a horizontal axis parallel to the barrier, cumuliform cloud. In aviation forecasts causing a hazard to aircraft. SEA-BREEZE FRONT The discontinuity and reports it is coded as SN, or SHSN in temperature and humidity that for snow showers. RUNWAY VISUAL RANGE (RVR) marks the leading edge of the intrusion The maximum distance at which the SPECI A special report of surface of cool and moist marine air associated runway, or lights or markers delineating meteorological information at an with a sea-breeze. it, can be seen from a position above a aerodrome. They are only issued when specified point on its centerline. SEA FOG A type of advection fog that specific criteria are met. Has the same forms when warm moist air advects format as a METAR, except for the S over water with a cooler temperature, name. ST. ELMO’S FIRE A luminous and and the consequent cooling of that SQUALL A sudden onset of strong sometimes audible electric discharge air to below its dewpoint by the winds with speeds increasing by at that occurs from objects, especially underlying cooler water. least 16 knots and sustained at 22 or pointed ones, when the local electrical more knots for at least one minute. The STRATOSPHERE The layer of the THUNDERSTORM A cumulonimbus intensity and duration is longer than atmosphere located between the cloud characterized by thunder and that of a gust. In aviation forecasts and troposphere and the mesosphere. lightning and associated gusty surface reports it is coded as SQ. It is characterized by an increase winds, hail, rain, turbulence, icing and, in temperature with height and an under the most severe conditions, SQUALL LINE THUNDERSTORMS absence of convective clouds and microbursts and/or tornadoes. In A continuous line of thunderstorms associated turbulence. aviation forecasts and reports it is accompanied by a surface gust front coded as TS. at the line’s leading edge. In aviation STRATUS A principal cloud type, forecasts it is coded as SQL TS. forming in the low levels of the TORNADO A rotating column of air troposphere and normally existing extending between a cumulonimbus STABLE ATMOSPHERE A state of as a flat layer that does not exhibit cloud and the ground. It is the the atmosphere in which a lifted air individual elements. In aviation most destructive of all storm-scale parcel will sink to its equilibrium level forecasts and reports it is coded as ST. atmospheric phenomena (microbursts once the lifting mecahism ceases, due can be just as severe but they occur to the air parcel being denser (cooler) SUBLIMATION The process of a ice over a shorter time period). than the surrounding air. changing directly into water vapour. TOWERING CUMULUS A vertically STANDARD ATMOSPHERE SUBSIDENCE A descending motion developed cumulus cloud, often a A mathematical model of the of air in the atmosphere, ususally with precursor to cumulonimbus. In aviation atmosphere which is standardised so the implication that it extends over a forecasts and reports it is coded as TCU. that predictable calculations can be broad area such as occurs with a high- made. pressure system. TRADE WINDS Two belts of prevailing wind that blow easterly from the STANDING WAVE An atmospheric SUBTROPICAL JET A band of subtropical high pressure centres wave that is stationary with respect to relatively strong winds found between in each hemisphere towards the the earth’s surface. 20 and 40° latitude in the middle equatorial trough. Primarily lower-level and upper troposphere. Jet streams winds, they are characterised by their STEAM FOG Fog that forms when form near boundaries of adjacent air great consistency of direction. To the when cool air, passing over warm masses with significant differences in north of the trough they blow from water, reaches its saturation point temperature. the northeast, and to the south of the due to water evaporating from the trough they blow from the southeast. warm water into the cooler air. Fog SUPERCELL A cumulonimbus cloud rising in the convection currents above characterised by a rotating and TROPICS The region of the earth the water give rise to a steaming long-lived, intense updraft. They can located between the Tropic of Cancer, appearance. potentially result in the most severe at 23.5 degrees north and the Tropic of thunderstorms, capable of producing Capricorn, at 23.5 degrees south. STRATIFORM Clouds that exhibit extremely large hail, damaging winds extensive horizontal development (in and violent tornadoes. TROPICAL AIR MASS An air mass contrast to the vertical development of that forms in the tropics or subtropics. cumuliform clouds). SUPERCOOLING The reduction of Maritime tropical air is produced over the temperature of a liquid below its oceans and is warm and humid, while STRATOCUMULUS A principal freezing point without it becoming a continental tropical air is formed over cloud type, forming in the low levels solid. arid land and is very hot and dry. of the troposphere and existing in a relatively flat layer but having individual SYNOPTIC CHART Any chart that TROPICAL CYCLONE A non-frontal elements, from which drizzle can depicts meteorological or atmospheric low pressure system that develops fall. It can form from cumulus clouds conditions over a large area at a given over tropical waters with winds of 34 becoming more stratified when they time. knots or more. In Australia, the term push up into a stable atmospheric severe tropical cyclone is used when layer. In aviation forecasts and reports T winds reach or exceed 64 knots. it is coded as SC. THERMOSPHERE The layer of the TROPICAL DISTURBANCE An area atmosphere located between the STRATOPAUSE The boundary zone of organized convection, originating mesosphere and outer space. It is a between the stratosphere and the in the tropics or occasionally the region of increasing temperature with mesosphere. In the stratosphere the subtropics, that maintains its identity height, and includes all of the temperature increases with height, for 24 hours or more, but has no exosphere and most of the ionosphere. with the stratopause being the point closed wind circulation. It is often the of maximum temperature, prior to THUNDER The sound emitted by first developmental stage of a tropical a decrease in temperature in the rapidly expanding gases along the cyclone. mesosphere. channel of a lightning discharge. TROPIC OF CANCER Located at surrounding atmosphere, the lower WARM AIR ADVECTION The 23.5 degrees north, it is the most levels of air heat and rise, flowing up horizontal movement of warmer air northern point on the earth where mountain slopes. into a location. the sun is directly overhead (on 21 June). VAPOUR PRESSURE The pressure WARM FRONT The leading edge of exerted by gaseous molecules. In an advancing warm air mass that is TROPIC OF CAPRICORN Located meteorology, it is that part of total replacing a relatively colder air mass. at 23.5 degrees south, it is the most atmospheric pressure due to water southern point on the earth where vapour content. WATER VAPOUR Water in its the sun is directly overhead (on 22 gaseous form. December). VEERING A clockwise shift in the wind direction. WAVE LENGTH The distance TROPOPAUSE The boundary zone between two successive wave or transition layer between the VIRGA Precipitation that falls crests separated by a trough. troposphere and the stratosphere. from clouds but evaporates before reaching the ground. WHIRLY WIND A small-scale, rapidly TROPOSPHERE The lowest layer of rotating column of wind, formed the atmosphere. Is characterised by VISIBILITY A measure of the thermally and thus most likely clouds, weather and a decrease in opacity of the atmosphere. It is to develop on clear, dry and hot temperature with increasing altitude. the greatest distance one can see afternoons. Often called a dust devil prominent objects with unaided when made visible by the dust, dirt TROUGH An elongated area of low normal eyesight. or debris it picks up. atmospheric pressure. VISUAL FLIGHT RULES (VFR) WHITEOUT Occurs when clouds TURBULENCE Irregular fluctuations Rules that govern civil aircraft and the earth’s surface seem to occurring in fluid motions. flight under conditions that allow blend, resulting in the horizon navigation by visual reference to the becoming erased. U earth’s surface. WIND DIRECTION The direction UNSTABLE ATMOSPHERE An from which the wind is blowing. atmosphere in which air parcels rise W buoyantly due to the rising air parcel WAKE TURBULENCE A disruption WIND SHEAR A wind direction and/ being less dense (warmer) than the of the airflow behind a moving or speed change over a vertical or surrounding air. aircraft that produces turbulence. horizontal distance.

UPDRAFT A small-scale current of WALKER CIRCULATION A deep air with marked vertical motion. east-west circulation induced by the contrast between the warm waters V of the western Pacific and the cooler waters of the eastern Pacific. VALLEY BREEZE An anabatic wind which forms during the day by heating of the valley floor. As the ground becomes warmer than the

Airservices Australia is the official distributor of aviation forecasts, warnings and observations issued by the Bureau of Meteorology. Flight briefing services are available at: www.airservicesaustralia.com. Telephone contact details for elaborative briefings are contained in Airservices’ Aeronautical Information Publication Australia (AIP), which is available online through their website.

The Australian Bureau of Meteorology provides a wide range of observations, synoptic charts, satellite and radar images, forecast and warnings via the internet at www.bom.gov.au.