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Glossary of Weather Terms

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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.
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