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Glossary - Meteorology

Absolute Humidity A statement of the amount of water vapour in the air at a place by the expression of it in grams per cubic metre. It is now being referred to as vapour concentration.
Accretion The growth of a precipitation element such as a snow flake, or an ice deposit by the collisions with it by supercooled water droplets.
Adiabatic Process An adiabatic process is one in which the temperature of a body of gas is changed without the transmission of heat energy into or out of the sample undergoing the change.
Advection The transfer of air over the earth horizontally. This is simply another term for horizontal wind.
Advection Fog Fog that is produced by the action of relatively warm humid air being transported over a cool surface.
Aggregation The process by which snow flakes or ice crystals are permitted to grow by collisions and resulting adherence to each other.
Air Mass A large body of air containing approximately homogeneous properties throughout. This applies especially to temperature and humidity. To qualify as an air mass, the body of air must be several thousands of kilometres across and also cover several degrees of latitude.
AIREP Aircraft weather report, provided by the pilot of an aircraft in flight.
Altocumulus White or grey, or both white and grey, patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have an apparent width of between one and five degrees.
Altostratus Greyish or bluish cloud sheet or layer of striated, fibrous or uniform appearance, totally or partly covering the sky, and having parts thin enough to reveal the sun at least vaguely, as through ground glass. Altostratus does not show halo phenomena.
Anabatic Wind A local wind which results when air adjacent to sloping ground is heated by radiation from the earth’s surface during the day.
Anemometer A device designed to permit the measurement of wind speed.
Anticyclone A pressure system consisting of a series of closed isobars surrounding an area of high pressure, with the highest pressure at the centre of the system. In most cases the resulting closed curves are in the form of a series of ovals.
Anvil Cloud The upper portion of a cumulonimbus cloud which has spread out under a stable layer of air, usually the tropopause. This shape occurs as the result of the cloud billowing from beneath.
Atmosphere The envelope of gases which surrounds the earth and is held near it by the force of gravity.
Backing A change of wind direction which occurs in an anticlockwise sense.
Beaufort Scale A scale of wind strengths adopted by Commodore (later Admiral) Francis Beaufort in 1805 determined by the state of the sea and divided into 12 stages (or forces). During the first half of the twentieth century, the scale was expanded to 17 forces.
Blocking The process which sometimes occurs in the middle latitudes when a large slow-moving anticyclone teams up with a mid-latitude depression to create a strong easterly wind flow between them. The result is to slow or even prevent their eastward migration
for a period of several days.
Blowing Dust Dust that is raised locally by the wind in such a way as to reduce the visibility (i.e. to a height of more than 2 metres above the ground), but in such a way as not to qualify as a dust storm.
Blowing Snow Fine powdery snow raised locally by the wind to a height of more than two metres above the ground.
Breeze A wind of light to moderate strength.
Buys Ballots Law The rule attributed to the Dutch meteorologist C.H. Buys Ballot, who was the first person to achieve a synoptic analysis over Europe in 1857. From it he made the observation that the wind flows at right angles to the pressure gradient in such a way that higher pressure is to the right in the Northern hemisphere. The reverse is true in the southern hemisphere.
Calm A condition in which the surface wind speed is zero.
Carbon Dioxide CO2 The fourth largest gas by volume in the atmosphere, accounting for approximately 0.03%.
Carbon Monoxide CO A noxious gas produced by the incomplete burning of substances containing carbon. It is present in the atmosphere only in minute amounts.
Castellanus A species of cloud genera which exhibits a formation of protuberances along its top and which appear in the form of cumuliform turrets. It is most commonly found with Altocumulus but may also be found with Cirrus, Cirrocumulus and Stratocumulus. It indicates the presence of convection and hence instability within the cloud.
Ceilometer An electronic device for measuring the height of the cloud base.
Celsius Scale A scale of temperature attributed to the Swedish astronomer Anders Celsius in 1742. It is based on the principle that water freezes at 0 degrees and boils at 100 degrees. Also known as the Centigrade Scale.
Cirrocumulus Thin, white patch, sheet or layer of cloud without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the elements have an apparent width of less than one degree.
Cirrostratus Transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing halo phenomena.
Cirrus From the latin, cirrus ‘lock or tuft of hair’. Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both.
Clear Air Turbulence CAT CAT is turbulence which is normally associated with cloud. It is used to describe patches of turbulence which are associated with windshears at the edges of upper troposphere jetstreams. It may be severe and highly dangerous to high-flying jet aircraft.
Clear Ice Generally, a uniform and transparent or translucent deposit of ice formed by the freezing of supercooled drizzle droplets or raindrops on objects, the surface temperature of which is below or at 0°C. Sometimes referred to as glaze ice.
Cloud A collection of minute water droplets or ice crystals which are not individually visible but as a collection are visible floating in the air above the earth’s surface, normally within the troposphere or occasionally in the lower stratosphere.
Cloud Condensation Nuclei Minute solid particles in suspension in the air, upon which water vapour condenses to form a cloud particle.
Coalescence The growth of raindrops by collisions with other water drops or the growth of ice pellets by collision with water drops or droplets.
Col A synoptic system which lies between two anticyclones (or ridges) and two depressions (or troughs) arranged at opposite sides of the system. Characterised by light variable winds.
Cold Front The boundary between a warm air mass and an advancing cold air mass.
Cold Occlusion A variety of occluded front involving air masses such that the air behind the cold front associated with a frontal depression, is colder than the air ahead of the warm front. The result is that the cold front appears to remain at the surface, while the warm sector and warm front are lifted from the surface.
Condensation The process of formation of liquid water from water vapour.
Conditional Stability Stability of a sample of air which depends upon the sample remaining unsaturated.
Conduction The process by which heat energy is transferred by direct contact from one body (or mass) to another without the transfer of matter.
Contour (Contour Line) In synoptic meteorology, a line of constant height of an isobaric surface.
Contour Analysis A method of analyzing the pressure field over a region by drawing contour lines for the height of a given pressure surface.
Contrail A trail of water droplets or ice crystals produced by an aircraft engine exhaust when products of fuel combustion add humidity to the air in suitable conditions. To be distinguished from aerodynamic trail.
Convection The process by which samples of air move vertically in the atmosphere by virtue of the fact that they are either warmer or cooler than their environment.
Convection Condensation Level CCL The level determined from an aerological diagram, at which the base of convective cloud would occur if moist air at the surface was to be warmed to the convection temperature.
Convergence The drawing together of air, usually along a line, as in a trough, or near the surface in a depression.
Coriolis Effect (Force) An apparent acceleration that air possesses by virtue of the earth’s rotation, with respect to axes fixed in the earth. It is sometimes termed geostrophic acceleration. The component of the earth’s rotation is such that wind is deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
Crepuscular Rays A photometeor consisting of clearly visible rays of light emanating from the edges of clouds which are between the observer and the luminary source (usually the sun) and centred upon it.
Cumulonimbus Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume. Under the base of this cloud which is often very dark, there are frequently low ragged clouds either merged with it or not, and precipitation.
Cumulus Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes or towers, of which the bulging upper part often resembles a cauliflower. The sunlit parts of these clouds are mostly brilliant white; their base is relatively dark and nearly horizontal. Sometimes, cumulus cloud is ragged.
Cyclone This is the collective term used to describe all systems comprising concentric closed isobars surrounding a region of lower pressure. It is thus the opposite of an anticyclone. See Tropical Cyclone.
Density The mass of air in a given volume expressed in grams per metre3.
Density Height (Altitude) The height in the ISA at which the density of air is the same as at the place being considered.
Deposition The process by which water vapour is converted directly into ice, without going through the liquid phase. It is usually referred to as “sublimation”.
Depression A depression is any synoptic system of closed isobars surrounding an area of low pressure.
Dew Hydrometeor being precipitation of water drops or droplets on surfaces on or close to the ground resulting from direct condensation of water vapour onto those surfaces when they have temperatures below the dewpoint temperature of the air in contact with them and in conditions of very light or calm wind.
Dewpoint Temperature The temperature to which a sample of moist air must be cooled at constant pressure in order that it shall become saturated.
Diurnal Variation The daily (24 hours) cyclic variation of some measurable element in meteorology such as surface wind, temperature or surface atmospheric pressure.
Divergence A condition in which air spreads out from a point or along a line. Divergence most clearly occurs near the base of an anticyclone or in a ridge of high pressure.
Doldrums The region of light variable winds associated with the equatorial trough.
Downburst The more-or-less sudden downward rush of air accompanying the onset of a shower of rain, hail or snow from a mature cumulonimbus cloud. A downburst is to be distinguished from a microburst in that it is usually more widespread and may continue for a longer time than a microburst.
Drifting Snow Fine snow lifted locally by the wind at the surface, to a height of less than two metres above the ground. See Blowing Snow.
Drizzle Hydrometeor being precipitation in the form of fine water drops (by convention, having a diameter of less than 0.5 mm), but very close together.
Droplet Liquid precipitation and cloud element having a diameter of up to about 0.2 mm.
Dry Adiabatic Lapse Rate The rate at which the temperature of an unsaturated sample of air undergoes a reduction in its temperature while being lifted in the atmosphere. This is found to be constant at 3.0°C per 1,000 ft (9.8°C per km).
Dry Season A period of between one and five months marked by an almost total absence of rain. It is a feature of some tropical areas during what would be the Winter months of the relevant hemisphere. In areas where a Mediterranean climate prevails, a dry season is characteristic of the summer months.
Dust Devil A micro-scale phenomenon consisting of a whirl of wind created by intense localised convection from heating of dry dusty ground. It is characterized by dust, sand and debris being lifted from the surface, rendering the event visible.
Duststorm A Lithometeor consisting of a condition in which visibility is reduced to less than 1000 metres by dust raised from the surface by wind.
El Niño Southern Oscillation A climatic phenomenon in which the sea surface temperature along the coasts of Peru and Equador rises. It is observed by fishermen in the region to occur about the time of Christmas. Its effect is to cause a lowering of pressure (by surface heating) in the eastern Pacific Ocean resulting in increased rainfall in South America and increased pressure and decreased rainfall in the western Pacific and Indonesia.
Environmental Lapse Rate The rate at which the temperature within the atmosphere is observed to be decreasing with height.
Equatorial Trough A trough of low pressure having a mean surface pressure of about 1011 hPa, lying between the subtropical ridge of the northern hemisphere and that of the southern hemisphere.
Equinox Those two dates each year when the sun tracks exactly along the equator, on or about the 21st March and 21st September.
Evaporation The process by which liquid water is converted into the gas water vapour.
Eye (of storm) The central region of a tropical cyclone, usually some 5 – 25 nm in diameter and fairly symmetrical. The main features are absence of rain, light winds, high and turbulent sea, and low stratus clouds, often well broken.
Fahrenheit Scale A scale of temperature developed by Gabriel Fahrenheit, a Polish physicist who invented the alcohol-in-glass and the mercury-in-glass thermometers in the early 18th century. In the Fahrenheit scale water freezes at 32° and boils at 212°.
Fog Hydrometeor consisting of water droplets in suspension in a layer of air at the surface. By international agreement, the visibility within fog is less than 1,000 metres. See vection Fog, Arctic Sea Smoke, Freezing Fog, Frontal Fog and Radiation Fog.
Föhn Wind A warm, dry wind which flows down the lee side of a mountain range exposed to a moist airflow from over the sea.
Forecast A statement of any set of meteorological conditions which is expected to occur during a nominated period of time or at a specific time in the future.
Fractus Species of cloud genera such that it appears in the form of ragged patches. It applies almost equally to cumulus and stratus clouds.
Freezing The process by which liquid water is converted into ice by the removal of heat from the water.
Freezing Fog Fog which deposits droplets onto objects at the surface; these droplets freezing almost immediately upon contact, to result in a deposit of rime.
Freezing Rain Rain which deposits drops of water onto objects; these drops freezing upon contact, to result in a deposit of clear ice or glaze.
Friction Layer The layer of air, generally accepted as extending from the earth’s surface to about 600 m (2,000 ft) above ground, in which the influence of surface friction on air motion is appreciable.
Front The boundary between two different air masses, usually marked by changes to temperature, humidity and wind direction across the boundary. Frontal types include cold front, warm front, quasistationary front and occluded front (occlusion).
Frontal Fog Fog which forms under the surface of a front when rain falling from the warm air above the frontal surface evaporates (raising the humidity), but then creates fog by condensation in the cold air beneath the front close to the earth’s surface.
Frost A condition in which the ambient surface temperature (measured at the Stevenson’s screen) is equal to or less than 0°C. It is also used to describe the deposit of ice crystals which may result from such an event. See Hoar Frost.
Funnel Cloud A funnel-shaped downward extension of cloudy air extending below the base of a cumulonimbus cloud (more rarely cumulus), but which does not reach the ground. It indicates the existence of severe to extreme thermal convection, and often indicates the formation of a tornado or spout.
Gale A condition in which the mean wind speed (measured over a period of 10 minutes), is equal to or exceeds 34 kt (force 8 on the Beaufort wind scale).
Geostationary Satellite A weather satellite in a west to east orbit around the earth above the equator, located at a height of 35,786 km above the surface. In this condition, it remains constantly over the same longitude on earth.
Geostrophic Wind The wind which results from the exact balance and equilibrium which exists between the pressure gradient force (acting down the pressure gradient) and coriolis effect (acting up the pressure gradient).
Glaze A deposit of translucent or transparent ice which results from the collision of supercooled water drops in freezing rain onto the surface of the earth or surfaces of aircraft in flight.
Gradient Wind Photometeor consisting of a mainly green coloured flash of light which appears for a second or so at the extreme upper limit of the sun at the moment of sunset. Occasionally it may be observed at sunrise. In either case, the visibility needs to be unusually clear.
Gust A feature of the wind observed at the surface, when the wind speed increases for a few moments above the mean wind speed before dying away.
Gust Front The leading edge of a localised body of cold air which has descended in the downburst from a thunderstorm, separating it from the local environment.
Hadley Cell A simple circulation of air in the atmosphere first proposed by British meteorologist George Hadley in 1735 as an explanation for the Trade winds. This theory has since been refined by the introduction of a cell in the middle latitudes in which air rises in the mid-latitude depressions, flows westwards and equatorwards in the upper troposphere and descends into the sub-tropical anticyclones.
Hail Hydrometeor. Precipitation in the form of small balls of ice, generally having diameters between 5 - 50 mm
falling from cumulonimbus cloud, either individually or collected together into larger irregular lumps.
Haze Lithometeor consisting of a suspension of extremely small solid particles (such as bacteria, pollen etc) in
concentrations sufficient to reduce the visibility to some extent and often rendering the colours of objects seen through it as yellowish, reddish or bluish. The reduction of visibility is seldom to less than 10 km.
Heat A form of energy, normally expressed in Joules. It takes two forms namely sensible heat which when
added to a substance raises its temperature, and latent heat which when added to or removed from a substance changes its state.
Heat Low See Thermal Depression.
High The term used in synoptic meteorology to indicate a high pressure system. See Anticyclone.
Hoar Frost Hydrometeor consisting of a deposit of ice crystals or frozen dew drops, in the form of needles, feathers,
fans etc, adhering to the upper and vertical surfaces of any object exposed at night. The cooling results from radiation from the affected surface sufficient to render its temperature less than 0°C.
Humidity The quality of air such that it contains the gas water vapour. Humidity may be expressed as dewpoint temperature, mixing ratio, relative humidity, specific humidity or vapour pressure.
Hurricane The term used in the Caribbean Sea, the Gulf of Mexico and adjacent areas, to describe tropical revolving storms which are the equivalent of tropical cyclones. See Cyclone.
Hygrometer One of a number of types of devices used to measure the humidity of air. See Psychrometer.
Ice Water in its solid form. It occurs both in the air (as ice crystals, snow, hail etc.) and at the surface of the earth (as rime, hoar frost, glaze, icebergs etc).
Ice Nuclei Particles in suspension on the atmosphere, which act as nuclei for deposition to form ice cloud particles. They consist mainly of quartz sand, volcanic ash and glacial clay, having dimensions of 5 to 50μ and, in clean air, are found in concentrations of 0.1-10 per litre. See Cloud Condensation Nuclei.
Insolation The action of exposure to solar radiations. The term is often used to describe the intensity of such exposure. It is actually short for incoming solar radiation.
Inter-Tropical Convergence Zone ITCZ A feature of the equatorial trough which exhibits as a zone of more or less concentrated convergence near the surface, of the air masses emanating from the sub-tropical high pressure belts of the northern and southern hemisphere. The ITCZ is characterized by more or less widespread build-up of convective cloud and thunderstorms in bands lying along the axis of the equatorial trough.
International Standard Atmosphere ISA A set of atmospheric conditions adopted by the International Civil Aviation Organisation (ICAO) and used as the basis of comparison for determining the performance of aircraft. Also known as international standard atmosphere or ISA.
Inversion A layer of air through which the normal temperature lapse rate is inverted, i.e. the temperature increases with height. It is a layer which is characterized by strong static stability.
Isobar A line drawn on a chart or graph, representing constant pressure.
Isobaric Analysis An analysis of a region shown on a map, consisting of a series of isobars drawn to represent the horizontal pressure field and depicting areas of high pressure (anticyclones), low pressure (depressions) and associated features including the locations of fronts.
Isotach A line drawn on a map joining places having equal wind speed.
Isotherm A line drawn on a map joining places having equal temperature.
Isothermal Layer A layer of air through which the temperature lapse rate is zero; i.e. the temperature remains constant with height. It is a layer through which the air has strong static stability.
Jetstream A fast-moving, relatively narrow current of air travelling on an almost horizontal track, characterized by sharp lateral and vertical windshears and having a speed at its centre of at least 60 kt.
Most jetstreams are located near the tropopause and associated with areas of relatively steep horizontal temperature gradients. See Low-level jet.
Joule A unit of work defined as the force of one Newton having moved a body through a distance of one metre in the direction of the force.
Katabatic Wind A local wind which results when air close to the ground is cooled by conduction at night after the ground has cooled by radiation. If this occurs over sloping ground, the result is a layer of cold air under relatively warm air above it, and the cooled air subsequently drains to the bottom of the slope.
Land Breeze A local wind which operates towards the sea (or large body of water), usually reaching its peak at or just after dawn. It is caused by air over the water being warmer than that over the land following cooling of the latter by radiation during the night. The warmer air over the water is less dense and so rises to create a weak pressure gradient along the coastline.
Lapse Rate The rate at which temperature decreases with height in the atmosphere. Since this is the normal condition within the troposphere, lapse rate is considered positive when temperature decreases with height.
Latent Heat The heat energy absorbed or emitted by a substance during a change of state of that substance, without a change of its temperature. In meteorology, this is almost always applied to water. Heat is added to water during melting, evaporation and sublimation but is removed during deposition, condensation and freezing.
Lee Waves Vertical wave-like motion of air which on occasions, occurs to the lee side of a mountain range which is set across a wind flow. These are more commonly known as mountain waves or standing waves.
Leeward The direction from a point or feature which is downwind of that point i.e. sheltered from the wind (opposite of windward).
Lenticularis A species of cloud that exhibits the shape of lenses or almonds. These clouds usually have sharp outlines and are non-fibrous. Clouds of this type often appear to remain stationary for some time, although their shape may appear to change during that time. The term is normally applied to altocumulus but occasionally applies to cirrocumulus and stratocumulus.
Lifting Condensation Level LCL The level determined from an aerological diagram, at which the base of cloud would occur if stable air was lifted by any mechanism from the surface.
Lightning An electrometeor consisting of a luminous manifestation associated with a sudden discharge of electricity which occurs within a cloud, between clouds or between a cloud and the ground. It is a feature of thunderstorms and is associated exclusively with cumulonimbus clouds.
Line Squall A band of thunderstorms, often associated with an active cold front. It contains strong gusty winds and shears, heavy showers, possibly with hail and a marked drop in temperature.
Lithometeor A meteor consisting of a collection of particles which do not contain water, more or less suspended in the air or lifted from the surface by wind. Their importance lies in the fact that they reduce the visibility. Examples of lithometeors include duststorm, haze, sandstorm, sea salt, smoke etc.
Local Wind Any wind of a more or less localised nature which has been produced by some mechanism other than the distribution of pressure of a synoptic nature in the region affected. Local winds include Land breeze, sea breeze, anabatic wind, katabatic wind and Föhn wind.
Low A term in meteorology used to describe a region of low pressure. See Depression.
Mamma Supplementary feature associated with certain cloud types, consisting of udder-like protuberances hanging from the base of the cloud. It is most commonly observed under cumulonimbus and altostratus clouds but also seen under cirrus, cirrocumulus, altocumulus and stratocumulus.
Melting The process by which ice is converted into liquid water. This occurs by the supply of heat to the ice.
Mesopause The upper boundary of the mesosphere which separates it from the thermosphere, and which exists at a height of about 80 km above the earth’s surface.
Mesosphere That layer of the atmosphere which lies above the stratosphere, marked by the fact that temperature decreases with height through it. At its base (the stratopause) the temperature is in the order of minus 5°C and it decreases to about minus 95°C at its top (the mesopause).
METAR See Routine Observation
Microburst A highly localized very strong shaft of descending air occurring under a cumuliform cloud to the surface where it spreads to create strong radiating winds. The rate of descent in the shaft may exceed 6,000 fpm and the winds near the surface may exceed 50 kt, The whole phenomenon is usually confined within a radius of about two nm and its life span is usually not more than about 20 minutes.
Mist A hydrometeor consisting of a suspension of water droplets (sometimes accompanied by smoke particles) near the surface, in such a way as to reduce the visibility to between 1,000 and 3,000 metres. It is similar to fog, but less concentrated.
Mixing Ratio The ratio of the mass of water vapour in a sample of air to the mass of the dry air with which it is associated. It is expressed in grams per kilogram (of dry air).
Monsoon A seasonal wind which generally flows from an ocean towards and over a land mass, caused by heating of the land during the summer months.
The associated weather normally consists of widespread rain so that the monsoon has come to be used to describe the rainy season in those tropical regions where this is a feature.
Monsoon Low A Thermal Depression which forms over a large land mass during the summer months and become a more or less permanent feature during that season. In Australia, and other regions affected by the Monsoon, it results in the Wet Season.
Mountain Waves A system of vertical waves in the wind flow which lies over and to the leeward of a prominent mountain range set across a wind flow of at least moderate strength.
Nimbostratus Cloud genus defined as a grey cloud layer, often dark, the appearance of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches the ground. It is thick enough throughout to blot out the sun. Low, ragged clouds frequently occur below the layer, with which they may or may not merge.
Nitrogen N A chemically inactive gas which forms the majority of the mixture of gases making up air. By volume, it amounts to 70.09 percent. Meteorologically, it is of no importance on its own apart from the fact that it contributes significantly to the density and pressure of the air.
Observation A statement of an individual or a collection of meteorological data or phenomena, observed to have occurred at a nominated time.
Occlusion (Occluded Front) A condition which exists when the cold front travelling around the centre of a frontal depression, catches up with the warm front and lifts the warm sector of the depression from the surface. See Warm Occlusion and Cold Occlusion.
Okta A unit used to describe the amount of the celestial dome that is obscured by cloud. One okta is the equivalent of one eighth of the sky covered.
Orographic Lifting The process by which cloud is formed when air is lifted by topography and cooled adiabatically to a temperature below its dewpoint. The cloud base is then found to be at the lifting condensation level (LCL) for the air.
Oxygen O A gas which forms part of the total mix of gases which constitute air and amounting to about 20.95% by volume. It does have a great value in the formation of the ionosphere.
Ozone O3 A gas which is most highly concentrated around the level of the stratopause and which is important for the fact that it absorbs ultra-violet radiations from the sun.
Polar Aurora An electrometeor consisting of luminous phenomenon which appears in the upper atmosphere, in the form of arcs, bands or curtains.
They are caused in magnetic polar regions by the eruption of ionised particles from the sun during a solar flare, and channelled to those regions by the earth’s magnetism. Most polar aurora occur in the band between 100 and 400 km above the surface.
Polar Front The boundary that lies along the middle latitudes of each hemisphere, separating the warm air mass associated with the tropics and sub-tropics from the cold air mass associated with polar regions.
Polar Front Jet A jetstream associated with the Polar Front, located at about 30,000 ft above the surface, observed mostly in winter over southern Australia.
Precipitable Water Is the rainfall that would be recorded in a rain gauge if all of the water vapour in an equivalent column of air extending above a weather station had been converted into rain in the conditions existing at the time of the observation. It is established from a radio sonde observation.
Precipitation That group of hydrometeors which results in the deposit of water through the air, usually onto the surface. It includes water in liquid or solid form falling from the sky, whether visible or not, and any deposit of liquid or solid water by condensation or deposition.
Pressure Gradient The horizontal difference in pressure as depicted by a synoptic analysis using isobars or contour lines.
Pressure Height (Altitude) The height indicated by a pressure altimeter when its sub-scale is set to 1013.5 hPa.
Prevailing Wind The direction of the wind which applies for the majority of the time at a place, either over a given period (such as a season), or at all times.
Prognosis An alternative term for a Forecast.
Psychrometer A device comprising two thermometers, one of which measures the air temperature of the environment, and the other has its bulb surrounded by a piece of muslin which is kept wet with pure water. The purpose of the device is to measure the humidity of the environment.
Quasi-Stationary Front A portion of the Polar front which appears to travel along its length rather than across it and thus appears to be almost stationary over a period of some hours. It is common that the genesis of a new frontal depression is likely to occur on this feature if a wave develops on it.
Radiation The process by which heat is transferred by way of electromagnetic waves. As such, this is the only way heat can be transferred through a vacuum, and thus through the ether between the sun and the earth. It is also a major means of transferring heat within the atmosphere. The visibility within the layer is reduced to below 1,000 metres.
Radiation Fog Fog that results when air close to the surface is cooled by conduction to cold ground at night and subsequently mixed by a light wind to create a layer containing condensed water droplets at a temperature equivalent to its dewpoint.
Radiation Inversion An inversion which has resulted from cooling of ground by terrestrial radiation at night.
Radiosonde A device consisting of a small radio transmitter and sensors for pressure, temperature and humidity.
Radiosondes are attached to hydrogen-filled balloons and released from a network of observing stations at regular intervals (usually twice each day), to measure those variables above the station to a height of about 100,000 ft (30 km or 10 hPa).
Rain A Hydrometeor, consisting of a precipitation of water drops, falling from a cloud. Generally the diameters of these drops varies from 0.5 mm to about 6 mm.
Smaller drops are spherical in shape while larger drops tend to be flattened on their lower surface by the action of drag. Drops of larger than 6mm in diameter tend to break up into smaller drops.
Rainbow A group of concentric arcs with colours ranging from violet to red, produced by the refraction and reflection of light from the sun or moon, on a screen of water drops in the atmosphere, seen when the luminary is behind the observer.
Rainy Season See Wet Season.
Relative Humidity The mass of water vapour observed in a sample of air expressed as a percentage of the mass of water vapour that the air could sustain (before saturation), at that temperature.
Ridge (of High Pressure) An elongated extension of high pressure away from an anticyclone (high pressure system). The term is also used to describe a series of individual anticyclones to form a chain of such systems (as the sub-tropical ridge) which exists in each hemisphere between the equatorial trough and the mid-latitude depressions.
Rime A hydrometeor consisting of a deposit of ice resulting from the almost instantaneous freezing of fog droplets, cloud droplets or fine drizzle droplets when they are caused to collide with an object whose temperature is at or below freezing temperature.
Roaring Forties A term used to describe the strong, frequently gale force, westerly winds which prevail in the latitude band between 40° and 50° in each hemisphere.
These winds are associated with the travelling midlatitude depressions that form along the polar front.
Rotor A closed eddy lying parallel with the ground, located in the crest of a Mountain wave under suitable conditions.
Rotor Cloud A cloud which may form in the upper half of a rotor given sufficient humidity in the air associated with mountain waves.
Route Forecast ROFOR A forecast provided for a flight in circumstances when routine forecasts are not available or cannot be provided.
A route forecast will be provided on request subject to sufficient notice being provided by the pilot.
Routine Observation METAR An observation of weather conditions made at a predetermined time at an aerodrome equipped with an automatic weather station (AWS) or manned by official observers of the Bureau of Meteorology. By convention, routine observations are made at time on the hour and 30 minutes past the hour.
Runway Visual Range The maximum distance that a pilot would be expected to be able to see along a runway intended for take-off or landing. It is estimated from a position at a height 5 metres above the runway threshold.
Saint Elmo’s Fire An electrometeor consisting of a more or less continuous luminous electrical discharge usually of weak intensity emanating from prominent objects on the earth’s surface or from the tips of aerofoils on aircraft in flight.
Sandstorm A lithometeor consisting of a collection of sand particles lifted from the surface by a strong (usually gale-force) wind, in sufficient concentrations to reduce the visibility at ground level to below 1,000 metres. In Australia, sandstorms are usually confined to those areas where sandy deserts exist.
Saturated Adiabatic Lapse Rate SALR The rate at which a saturated parcel of air undergoes a reduction of its temperature whilst it is being lifted in the atmosphere.
Saturation A condition of air such that it contains all of the mass of water vapour that it can sustain at its current temperature and pressure, in other words, that its vapour pressure is at its maximum for that temperature.
Sea Breeze An important local wind which operates from over a large water body towards a land area under the influence of heating by solar radiation of the air over the land during the day. As the air over the land is warmer and less dense than that over the water, a localised pressure gradient is set up which favours the flow of air towards the land.
Secondary Depression A depression which has formed on a wave of an existing front (usually the cold front of a depression), and contained within the circulation of a larger depression (the primary).
Sensible Heat Sensible heat is literally the heat that can be felt.
It is the energy moving from one system to another that changes the temperature rather than changing its phase.
For example, it warms water rather than melting ice.
Severe Thunderstorm A thunderstorm containing conditions which include any of, wind gusts of 48 kt or more, hailstones of 2 cm or more, or tornadoes. Severe thunderstorms can also be expected to produce torrential rain and frequent lightning.
Shear See Windshear.
Shower Precipitation in the form of rain, hail or snow, falling from cumuliform cloud (usually cumulonimbus). Showers are characterized by more or less sudden commencement and cessation with at least a partial clearing of the sky between showers.
SIGMET Information concerning en route weather phenomena which may affect the safety of aircraft operations. In practice, it is a warning issued by MET for transmission to pilots, concerning conditions forecast to occur in an area and which constitute a potential hazard.
SIGWX Prognosis An area forecast, issued to pilots for operations above 10,000 ft altitude and which takes the form of a chart. Two types of charts are produced; namely SIGWX high (for operations between FL185 and FL445, and SIGWX medium for operations between FL100 and FL250. Each type shows the predicted locations of features such as jetstreams, moderate or severe turbulence, the tropopause height and areas of Cumulonimbus clouds as well as significant surface features, if appropriate, using approved symbols.
Slant Visibility The greatest distance at which an object may be seen and identified along the line of sight inclined to the ground (i.e. from an aircraft in flight).
Smoke A lithometeor consisting of particles of combustion.
Near the source, this may include ash particles and burned-out embers. Further from the source, the larger particles can be expected to have precipitated out, leaving only minute particles, resulting in smoke haze.
Snow A hydrometeor being a precipitation of ice crystals, most of which are branched or star-shaped. At temperatures above -5°C, most of these are collected together in the form of snow flakes.
Solar Radiation The transfer of heat energy from the sun towards the earth in the form of electro magnetic waves of relatively short length concentrated between 0.15 and 4.0μ. This includes the spectrum containing Ultra violet, visible light and the shorter wave infra red. See Terrestrial Radiation.
Solstice The days of the year when the sun is located furthest from overhead the equator. On and around the 21st June, the overhead sun is near the latitude 23° 17′North. This represents the winter solstice for the southern hemisphere and the summer solstice of the northern hemisphere. On and around the 21st December, the overhead sun is near the latitude 23° 17′South. This represents the summer solstice of the southern hemisphere and the winter solstice of the northern hemisphere.
Southern Oscillation Index An index developed by G.T. Walker, based on measurements of atmospheric pressure, surface temperature and rainfall and used to assist in the prediction of El Niño events. See El Niño Southern Oscillation and Walker Circulation.
Special Observation SPECI An observation of surface meteorological conditions occurring at an aerodrome. A SPECI will be issued at the commencement of, change in intensity or cessation of specified conditions considered significant to aircraft operations at that aerodrome.
A SPECI is similar to a METAR, but is not issued in accordance with a fixed schedule, unless it coincides with the timing of a METAR. In such a case, it would replace the METAR for that time.
Specific Heat Specific heat is the amount of heat energy required to raise the temperature of a body per unit of mass.
In SI units, specific heat (symbol: c) is the amount of heat in joules required to raise 1 gram of a substance 1 Kelvin.
Specific Humidity The mass of water vapour in a sample of air expressed as the ratio of the mass of the water vapour to the mass of the moist air with which it is associated. It is expressed in grams per kilogram (of moist air containing the water vapour). It is not much used since Mixing Ratio is preferred. See Mixing Ratio.
Spout See Waterspout.
Squall A more or less sudden increase in the mean wind speed at a place by at least 16 knots to at least 22 knots and lasting for at least one minute. A squall is often but not always, associated with a shower or thunderstorm, but it also frequently accompanies the passage of a front, with or without showers.
Squall Line A narrow band of active thunderstorms, either continuous or with breaks. It is not associated with a cold front.
Stability The condition of the air over a place when a sample (or parcel) of the air, having been lifted by any means, tends to return to its original level as the result of a change of its temperature by the adiabatic effect. See Adiabatic Process and Parcel Method.
Standard Atmosphere See International Standard Atmosphere (ISA).
Standing Waves See Mountain Waves.
Steam Fog Hydrometeor consisting of fog which results from condensation into cold air of water vapour which has been released from a relatively warm underlying water surface. It is most often observed over ponds, pools and lakes early on cold winter mornings. See Arctic Sea Smoke.
Stevenson Screen The standard housing for thermometers and other instruments used to measure temperature and humidity near the earth’s surface, designed by the engineer Thomas Stevenson, father of Robert Louis Stevenson.
Stratocumulus Cloud genus. Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud, which almost always has dark parts, composed of tessellations, rounded masses, rolls etc., that are non-fibrous and which may or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.
Stratopause The upper boundary of the stratosphere, which separates it from the mesosphere. It is generally located at about 150,000 ft (about 46 km) above the surface.
Stratosphere The second layer of the atmosphere that lies between the troposphere and the mesosphere. Its lower boundary is the tropopause and its upper boundary is the stratopause. It is characterized by the fact that temperature is generally increasing with height through it and that most of the atmospheric ozone is contained within it. See Stratopause.
Stratus Cloud genus. Generally grey cloud layer with a fairly uniform base, which may give drizzle, ice prisms or snow grains. When the sun is visible through the cloud, its outline is clearly discernable.
Stratus does not show halo phenomena except, possibly, at very low temperatures. Sometimes stratus appears in the form of low ragged patches.
Streamlines Lines drawn on a synoptic analysis to depict the direction of flow of wind. Streamline Analysis may replace Isobaric Analysis in tropical regions where the geostrophic assumption breaks down due to lack of Coriolis Effect. See Synoptic Analysis.
Sublimation The process by which water ice (solid water) is converted directly into vapour, without passing through the liquid phase. See Deposition.