A
The process of supercooled water droplets freezing on impact with snowflakes, ice particles or other cold objects including aircraft.
Cooling of a parcel of gas by expansion, with no heat exchange between the parcel and the surrounding air.
Warming of a parcel of gas by compression, with no heat exchange between the parcel and the surrounding air.
A process where a parcel of air cools or warms due to a change in pressure and volume (expansion or compression), with no heat exchange between the parcel and the surrounding air.
Fog transported by local winds from one locality to another.
Fog resulting from the movement of moist air over a colder surface and the consequent cooling of the air to saturation.
The mass of air per unit volume.
Any deposit of ice forming on an aircraft.
The vertical distance from mean sea level to an object aloft.
A principal cloud type, forming in the middle levels of the troposphere, and appearing as a white and/or grey layer or patch with a waved aspect. In aviation forecasts and reports it is coded as AC.
A principal cloud type, forming in the middle levels of the troposphere, and appearing as a grey or bluish sheet. In aviation forecasts and reports it is coded as AS.
An uphill wind generated by the heating of a sloping surface.
The non-standard propagation of a beam of energy (radio or radar) under certain atmospheric conditions, which results in false echoes (i.e. non-precipitation) on a radar image. Usually caused by unusual rates of refraction in the atmosphere.
Aircraft equipment, such as heating elements and flexible rubber strips, used to prevent or clear structural icing.
The upper portion of a cumulonimbus cloud that spreads out under the tropopause, often in the shape of a blacksmith’s anvil, sometimes for hundreds of kilometres downstream from the parent cloud. It indicates the mature or decaying stage of a thunderstorm.
The gaseous portion of the physical environment that encircles the earth. The divisions of the atmosphere are the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.
The total weight of the atmosphere above the point of measurement.
B
An anti-clockwise shift in the wind direction.
Dust that is raised locally by the wind to a height of at least two metres above the ground. In aviation forecasts and reports it is coded as BLDU.
Snow that is raised locally by the wind to a height of at least two metres above the ground. In aviation forecasts and reports it is coded as BLSN.
The temperature at which the vapour pressure of a liquid is equal to that of the surrounding atmosphere. The boiling point of pure water at the standard sea-level pressure of 1013.25 hPa is 100°C (373°K).
Used to describe an amount of cloud covering the sky of between five and seven oktas (eighths). In aviation forecasts and reports it is coded as BKN.
Describes the relationship of the wind direction to the pressure distribution. In the southern hemisphere, if one stands with one’s back to the wind, lower pressure is to the right. Lower pressure will be to the left in the northern hemisphere.
C
Occurs when air (including warm air) with a high water vapour content is drawn into an aircraft’s carburettor (which is chilled by the fuel evaporation process coupled with the pressure reduction in the carburettor’s venturi) causing the water vapour to condense and then, if the temperature has dropped below 0°C, form ice on the surfaces of the carburettor. The ice may gradually block the venturi or cause jamming of the mechanical parts of the carburettor.
A change in the form of water, e.g. liquid to vapour, ice to liquid, ice to vapour.
A principal cloud type, forming in the high levels of the troposphere, composed of ice crystals which appear from the ground as very small elements in the form of grains or small ripples. In aviation forecasts and reports it is coded as CC.
A principal cloud type, forming in the high levels of the troposphere, composed of ice crystals which appear from the ground as a transparent sheet or veil, often creating a halo phenomenon around the sun or moon. In aviation forecasts and reports it is coded as CS.
A form of turbulence occurring in regions of marked wind shear, particularly at the boundaries of jet streams, but may also be found in strong lee waves downstream from elevated typography. It occurs in the absence of any visual clues (i.e. clouds).
A sheet of transparent ice formed by the relatively slow freezing of large supercooled water droplets, i.e. rain. Does not seriously distort aerofoil shape but does add appreciably to aircraft weight and drag.
The horizontal movement of colder air into a location of warmer air.
The height at which an adiabatically lifted air parcel will become saturated, whereupon condensation occurs. Corresponds to cloud base level.
Tiny particles upon which water vapour condenses.
Stable unsaturated air that will become unstable if saturated.
The transfer of heat in response to a temperature gradient within an object or between objects that are in physical contact with one another. Transfer is from warmer to colder regions.
A weather chart representing conditions on a constant pressure surface, e.g. 500 hPa.
The process whereby heat is conducted away from warmer air to a colder surface.
An extensive body of air, with a more or less uniform temperature and moisture profile, which has originated over a large land mass.
A condensation trail that forms behind an aircraft when hot humid air from a jet exhaust mixes with environmental air of low water vapour and low temperature. If the increased vapour leads to saturation, the vapour will condense into water droplets and/or deposit into ice.
A cloud that owes its vertical development, and possibly its origination, to convection.
The lowest height at which condensation will occur as a result of convection due to surface heating.
D
The weight of air per unit volume.
The altitude in the International Standard Atmosphere at which a given air density is found.
A process in which a gas transforms into a solid, e.g. the process by which water vapour, in sub-freezing air, changes directly to ice without first becoming a liquid.
A nautical term for the equatorial trough, an area which typically has calm or light and variable winds.
A descent of cool air associated with convective cloud.
A wind directed down a slope, often used to describe winds produced by processes larger in scale than the slope.
Snow blown from the ground by the wind to a height of less than two metres.
Slow-falling and uniformly distributed precipitation in the form of tiny water droplets (diameters less than 0.5 millimeters), usually from stratus or stratocumulus clouds. In aviation forecasts and reports it is coded as DZ.
A line of constant potential temperature on a thermodynamic diagram. Meteorologically, the dry adiabat represents the meteorological processes that occur with the adiabatic ascent or descent of a parcel of dry air (where no liquid water is present and no condensation of water vapour occurs). As an air parcel ascends adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel; as it descends, its temperature will rise due to compression of the air parcel.
The rate of change in temperature for dry air ascending or descending adiabatically. The rate is approximately 3°C per 1000 feet.
The boundary between dry and moist air masses.
Small particles of earth or other matter suspended in the air. In aviation forecasts and reports it is coded as DU.
E
The distance between mean sea level and a point on the earth’s surface.
The geographic circle at 0° latitude on the earth’s surface.
The height at which a rising parcel of air will become equal in temperature to that of the environment, at which point it is no longer buoyant and thus will cease to rise in the atmosphere without forcing.
F
The temperature scale, developed by the German physicist Daniel Gabriel Fahrenheit in 1714, where water at the standard sea-level pressure of 1013.25 hPa has a freezing point of +32°F and a boiling point of +212°F.
The lines or bands of cumulonimbus clouds that spiral into and around the centre of a tropical cyclone.
Used to describe an amount of cloud covering the sky of one or two oktas (eighths). In aviation forecasts and reports it is coded as FEW.
A warm and dry downslope wind descending the lee side of a mountain range.
A suspension in the air, at or near the earth’s surface, of microscopic water droplets, or wet hygroscopic particles, reducing horizontal visibility to less than 1000 metres. In aviation forecasts and reports it is coded as FG.
Supercooled drizzle (i.e. drizzle existing at a temperature below 0°C) that freezes upon impact with surfaces to form glaze. In aviation forecasts and reports it is coded as FZDZ.
The temperature at which a liquid solidifies under any given set of conditions. Pure water under the standard sea-level pressure of 1013.25 hPa freezes at 0°C (32°F).
Any form of supercooled precipitation that freezes upon impact with surfaces to form glaze.
Supercooled rain that falls as liquid but freezes upon impact with surfaces to form glaze. In aviation forecasts and reports it is coded as FZRA.
The mechanical resistive force offered by one medium or body to the relative motion of another medium or body in contact with the first. In meteorology, it is the drag or resistance of the earth on the atmosphere.
The thin layer of the lower atmosphere within which wind direction and speed is subject to frictional effects caused by contact with the earth’s surface. Its depth is variable, ranging from tens of metres in a stable environment, to several kilometres in convective conditions over deserts.
The transition zone or interface between two air masses of different densities.
The passage of a front over a specific point on the earth’s surface. Changes in temperature, dew point, wind and atmospheric pressure occur with a frontal passage.
G
A weather satellite, in a west to east orbit at an altitude of 35 786 km, that maintains the same position over the equator.
A coating of ice, generally clear and smooth, formed on surfaces by the freezing of supercooled rain, drizzle or fog.
Was used to refer to the primary time standard, but now replaced by Coordinated Universal Time (UTC).
The leading edge of cool air rushing down and out from a thunderstorm cloud. Is usually accompanied by a drop in temperature, a wind shift and a pressure jump.
H
A deposit of interlocking ice crystals formed by direct deposition on objects such as tree branches, leaf edges, wires and poles.
I
Long-wave electromagnetic radiation which is emitted by all objects.
A set of regulations governing all aspects of civil aviation aircraft operations when a flight is conducted using instruments rather than outside visual reference.
A hypothetical vertical distribution of atmospheric temperature, pressure and density that by international agreement is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, ballistic tables, etc.
The region where southeast and northeast trade winds meet, usually located between 10 degrees north and south of the equator. It is a broad area of low pressure, located in the southern hemisphere during our summer, and in the northern hemisphere during its summer.
K
A temperature scale, designed by Lord Kelvin of Scotland in 1848, where 0°K is defined as absolute zero (where all molecular movement stops). Water freezes at 273°K, and boils at 373°K. It is used primarily for scientific purposes.
Cloud that forms in Kelvin-Helmholtz waves. The clouds, sometimes referred to as billows, are in the shape of breaking waves. These clouds are often good indicators of instability and the presence of turbulence.
Occurs when velocity shear is present within a continuous fluid, or when there is sufficient velocity difference across the interface between two fluids.
A waveform disturbance that arises from Kelvin-Helmholtz instability. Named after Lord Kelvin and Hermann von Helmholtz.
L
The rate of change of temperature with height in the atmosphere.
The energy absorbed or released during a change of state. Evaporation, melting and sublimation (a change from solid to gas) absorb heat from the surrounding air as energy is needed to weaken the individual hydrogen bonds between the water molecules. Condensation, freezing and deposition (gas to solid) release the latent heat, thus adding heat to the surrounding air.
A geographic coordinate that specifies the north-south position of a point on the earth’s surface. It is measured as the angular distance, subtended at the earth’s centre, along a meridian from a point on the earth to the equator. The equator is designated as zero degrees and the poles as 90 degrees.
The side of an obstacle that is furthest away from the wind.
A more-or-less isolated cloud, downwind of a barrier, resembling a smooth lens with sharp outlines. They mostly occur in mountain waves, and thus indicate possible severe turbulence and icing.
The height at which a parcel of saturated air becomes warmer than the surrounding air and thus begins to rise freely until it reaches its equilibrium level.
The height at which a parcel of moist air becomes saturated when it is cooled by adiabatic lifting.
A geographic coordinate that specifies the east-west position of a point on the earth’s surface. It is measured as an angle in reference to the Prime Meridian, which is designated as zero degrees longitude.
A transient strong wind concentrated in relatively narrow bands near the earth’s surface.
M
An air mass that has originated over an extensive water surface.
Disrupted air-flow resulting from wind flowing over or around terrain or manmade obstructions, whereby normal horizontal wind flow is disturbed and transformed into eddies and other irregular movements.
Atmospheric circulation in which the north and south component of motion is pronounced.
The ratio of the mass of a variable atmospheric constituent to the mass of dry air. In meteorology, the term normally refers to water vapour.
An oscillation above and downwind of a mountain caused by a disturbance in the horizontal air flow due to the high ground. They usually occur in groups, and aircraft can expect to encounter moderate to severe turbulence, uncontrolled altitude loss and possible icing if flying through them. In aviation forecasts it is coded as MTW.
A storm system composed of two or more convective cells at various stages of their life-cycle.
N
Low or middle-level thick dark cloud with more or less continuously falling rain, snow or sleet. In aviation forecasts and reports it is coded as NS.
O
P
A theoretical small and self-contained volume of air responding to meteorological processes as a single entity.
A report of in-flight weather by an aircraft pilot. Referred to as an AIREP.
An air mass that forms over a high latitude region. Continental polar air is formed over cold land surfaces and is typically very stable with low moisture.
A semi-continuous, semi-permanent boundary between polar and subtropical air masses.
A jet stream associated with the polar front, occurring at around 23-39,000 feet above the earth’s surface. It is depicted on analysis and forecast charts when the wind is 80 knots or more.
A satellite whose orbit passes over both of the earth’s poles.
The temperature that an unsaturated parcel of dry air would have if it is brought adiabatically from its initial state to a standard pressure, typically 1000 hPa.
A line of thunderstorms that may develop ahead of an advancing cold front, and having an orientation more or less parallel to the cold front.
An elongated area of relatively low pressure that may develop ahead of an advancing cold front.
An aneroid barometer calibrated to indicate altitude by measuring atmospheric pressure and using the standard atmosphere pressure/height relations. The indicated altitude will only equal the actual altitude if the actual atmosphere is equivalent to the International Standard Atmosphere.
The altitude in the International Standard Atmosphere at which a given atmospheric pressure will be observed.
A wind that blows from one direction more frequently than any other during a given period.
A forecast weather chart. Commonly known as a prog chart.
Q
R
Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by detecting scattered or reflected radio energy.
The process by which energy is propagated through any medium by virtue of the wave motion in that medium. Electromagnetic radiation, which emits heat and light, is one form.
Fog that forms when radiational cooling at the earth’s surface lowers the temperature of the air near the ground to, or below, its dew-point. RAIN Precipitation in the form of liquid water droplets greater than 0.5 mm diameter. In aviation forecasts and reports it is coded as RA.
The cooling of the earth’s surface and the adjacent air which occurs at night when the earth’s surface suffers a net loss of heat due to outgoing radiation being greater than incoming radiation.
Precipitation in the form of liquid water droplets greater than 0.5 mm diameter. Rain precipitation is generally steady and widespread and normally falls from stratiform (layer) cloud. In aviation forecasts and reports it is coded as RA.
The ratio of the vapour pressure to the saturation vapour pressure with respect to water. Also known as the ratio of the existing amount of water vapour to that which could be held by a parcel of air. It is usually expressed as a percentage.
In relation to radar, it is the ability to read two distinct targets separately. The clearer the resolution, the nearer the two objects can be to each other and still be distinguishable.
Ice formed by the rapid freezing of supercooled water droplets when they contact an exposed object, such as an aircraft frame, forming a white and opaque granular deposit of ice.
A low-level, horizontal, tube-shaped cloud. Usually associated with a thunderstorm gust front, where the roll cloud is completely detached from the base of the cumulonimbus cloud. It will sometimes form with a cold front.
A cloud formation found in the lee of a mountain or similar barrier. The air rotates around a horizontal axis parallel to the barrier, causing a hazard to aircraft.
The maximum distance at which the runway, or lights or markers delineating it, can be seen from a position above a specified point on its centerline.
S
To add something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.
The line on a thermodynamic diagram that depicts the change in temperature of a saturated air parcel as it rises or falls and undergoes cooling or heating due to adiabatic expansion or compression.
The rate of change in temperature of a saturated air parcel as it adiabatically ascends or descends through the atmosphere. The rate varies, from approximately 1.5 to 3°C per 1,000 feet, e.g., the more water vapour present in a rising parcel of air, the greater the condensation, and thus the greater is the latent heat released, reducing the rate of cooling.
Used to describe an amount of cloud covering the sky of three or four oktas (eighths). In aviation forecasts and reports it is coded as SCT.
A type of advection fog that forms when warm moist air advects over water with a cooler temperature, and the consequent cooling of that air to below its dewpoint by the underlying cooler water.
The discontinuity in temperature and humidity that marks the leading edge of the intrusion of cool and moist maritime air associated with a sea-breeze.
The Bureau defines a severe thunderstorm as one with winds of 48 knots or greater, or hail of diameter 2 cm or larger, or tornadoes or flash floods.
Precipitation, often short-lived (but may last up to an hour) and heavy, falling from convective clouds. They usually begin and end suddenly and occur over localised areas. In aviation forecasts and reports it is coded as SH.
A thermodynamic diagram with a skewed temperature scale on the horizontal axis and a logarithmic scale of pressure on the vertical axis. It is used to plot radiosonde soundings which give a vertical profile of air temperature and dew point temperature through the atmosphere. Forecasters use it to evaluate and forecast air parcel behaviour. Some values that can be determined are the Convective Condensation Level, the Lifting Condensation Level, and the Level of Free Convection.
Small particles produced by combustion that are suspended in the air (a transition to haze may occur when the smoke particles have travelled great distance and when the larger particles have settled out, the remaining haze particles become widely scattered through the atmosphere). In aviation observations and forecasts it is coded as FU.
A luminous and sometimes audible electric discharge that occurs from objects, especially pointed ones, when the local electrical field (voltage) attains a strength of near 1000 volts per centimetre. It often occurs during the latter phases of a violent thunderstorm. It typically appears on the extremities of aircraft, lightning rods and steeples.
A state of the atmosphere in which a lifted air parcel will sink to its equilibrium level once the lifting mechanism ceases, due to the air parcel being denser (cooler) than the surrounding air.
A mathematical model of the atmosphere which is standardised so that predictable calculations can be made.
An atmospheric wave that is stationary with respect to the earth’s surface.
Fog that forms when when cool air, passing over warm water, reaches its saturation point due to water evaporating from the warm water into the cooler air. Fog rising in the convection currents above the water give rise to a steaming appearance.
The boundary zone between the stratosphere and the mesosphere. In the stratosphere the temperature increases with height, with the stratopause being the point of maximum temperature, prior to a decrease in temperature in the mesosphere.
The layer of the atmosphere located between the troposphere and the mesosphere. It is characterized by an increase in temperature with height and an absence of convective clouds and associated turbulence.
The process of ice changing directly into water vapour.
A descending motion of air in the atmosphere, usually with the implication that it extends over a broad area such as occurs with a high pressure system.
A band of relatively strong winds found between 20 and 40° latitude in the middle and upper troposphere. Jet streams form near boundaries of adjacent air masses with significant differences in temperature.
T
The layer of the atmosphere located between the mesosphere and outer space. It is a region of increasing temperature with height, and includes all of the exosphere and most of the ionosphere.
The sound emitted by rapidly expanding gases along the channel of a lightning discharge.
Thunderstorms are associated with convective clouds (cumulonimbus) and are more often accompanied by precipitation. They are often short-lived and usually occur over localised areas. Aviation hazards associated with thunderstorms include wind shear, loss of visibility, low cloud, lightning strikes and possible large hail. In aviation forecasts and reports it is coded as TS.
A vertically developed cumulus cloud, often a precursor to cumulonimbus. In aviation forecasts and reports it is coded as TCU.
Located at 23.5 degrees north, it is the most northern point on the earth where the sun is directly overhead (on 21 June).
Located at 23.5 degrees south, it is the most southern point on the earth where the sun is directly overhead (on 22 December).
An air mass that forms in the tropics or subtropics. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid land and is very hot and dry.
An area of organized convection, originating in the tropics or occasionally the subtropics, that maintains its identity for 24 hours or more, but has no closed wind circulation. It is often the first developmental stage of a tropical cyclone.
The region of the earth located between the Tropic of Cancer, at 23.5 degrees north and the Tropic of Capricorn, at 23.5 degrees south.
The boundary zone or transition layer between the troposphere and the stratosphere.
The lowest layer of the atmosphere. Is characterised by clouds, weather and a decrease in temperature with increasing altitude.
Irregular fluctuations occurring in fluid motions.
V
An anabatic wind which forms during the day by heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up mountain slopes.
A clockwise shift in the wind direction.
A measure of the opacity of the atmosphere. It is the greatest distance one can see prominent objects with unaided normal eyesight.
Rules that govern civil aircraft flight under conditions that allow navigation by visual reference to the earth’s surface.
W
A disruption of the airflow behind a moving aircraft that produces turbulence.
The horizontal movement of warmer air into a location.
Water in its gaseous form.
A small-scale, rapidly rotating column of wind, formed thermally and thus most likely to develop on clear, dry and hot afternoons. Often called a dust devil when made visible by the dust, dirt or debris it picks up. In aviation forecasts and reports it is coded as PO.
Occurs when clouds and the earth’s surface seem to blend, resulting in the horizon becoming erased.