Meteorological Glossary

ABSOLUTE HUMIDITYIn a system of moist air, the ratio of the mass of water vapor to the total volume of the system; usually expressed as grams per cubic meter (g/m³ ).

ACCURACYThe degree with which an instrument measures a variable in terms of an accepted standard value or true value; usually measured in terms of inaccuracy but expressed as accuracy; often expressed as a percentage of full-scale range.

AIR DENSITYThe mass density of a parcel of air expressed in units of mass per volume.

ALLARD'S LAWA basic equation in night visual range theory, relating the illuminance of a point source of light to distance and the transmissivity of the atmosphere.

ALTER SHIELDA type of wind shield used around the mouth of a precipitation gage to reduce the effect of wind on catch.

ANALOGPertaining to measurements or devices in which the output varies continuously (e.g., voltage or rotation signals); compare to digital.

ANEMOMETER A general term for instruments designed to measure the speed or force of the wind.

ANEROID BAROMETER A barometer which measures atmospheric pressure using one or a series of aneroid capsules.

ASOSAutomated Surface Observing System. A network of instrumented weather stations deployed primarily by the U.S. National Weather Service to make weather observations without observer involvement.

AWOSAutomated Weather Observing System. A Federal Aviation Administration specification describing an automatic weather station capable of making aviation weather observations without operator involvement.

BARA unit of pressure equal to 100 kilopascals.The millibar (1/1000 bar) is commonly used in aviation and meteorology. The pascal (Newton/meter² ) is the S.I. unit for pressure.

BAROGRAPHA continuous-recording barometer.

BAROMETERAn instrument for measuring the pressure of the atmosphere; the two principle types are aneroid and mercurial.

BAROMETRIC PRESSUREThe atmospheric pressure at a given point due to the gravitational force on the column of air above it.

BAUDA unit used in describing the rate of character transmission and equal to approximately one bit per second.

BCDBinary Coded Decimal. A code format in which decimal digits (0-9) are expressed as four digit binary numbers.

BIMETALLIC THERMOMETERA thermometer, the sensitive element of which consists of two metal strips which have different coefficients of expansion and are brazed together. The distortions of the system in response to temperature variations are used as a measure of temperature.

BOURDON TUBEClosed, curved, flexible tube of elliptic cross section which is deformed, according to type, by variations of atmospheric pressure or temperature and so provides a measurement of the particular parameter.

CEILINGThe height ascribed to the lowest layer of clouds or obscuring phenomena when it is reported as broken, overcast, or obscuration and not classified as thin or partial. The ceiling is termed unlimited when these conditions are not satisfied.

CEILOMETER Instrument used to measure cloud base height.

CELSIUSA temperature scale having the freezing point of pure water at 0° and the boiling point at 100° under standard sea level pressure.

CENTIGRADE TEMPERATURE SCALEThe older name for the Celsius temperature scale. The use of this name was officially abandoned by international agreement in 1948.

CLOUD BASEFor a given cloud or cloud layer, the lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles.

CLOUD HEIGHTThe height of the cloud base above the local terrain.

COMPASS POINTSThe cardinal points of the compass: N, NNE, NE, etc.

CONFORMAL COATINGA protective coating applied to circuits usually by spray deposition.

CROSSWINDA wind blowing perpendicular to the course of a moving object. Often used when referring to winds affecting ballistics.

CUP ANEMOMETERAnemometer which measures wind speed by the speed of rotation of 3 or 4 hemispherical or conical cups, each fixed to the end of a horizontal arm projecting from a vertical axis.

DAMPING RATIOA parameter used to describe the response of a wind vane to a sudden change in wind direction. It is defined as the ratio of the actual damping to the critical damping, where critical damping is that value of damping which gives the fastest transient response without overshoot.

DEAD BANDThe range through which the input may be varied without initiating a response; usually expressed as a percentage of full-scale range.

DELAY DISTANCEThe passage of air necessary over a wind vane to cause the vane to respond to 5O% of a step function change in wind direction.

DELTA TEMPERATUREThe difference between temperature measurements taken at two significant levels above the ground. Temperatures at 10 and 40 meters are commonly used.

DEWPOINT TEMPERATUREThe temperature to which a given parcel of air must be cooled at constant pressure and constant water vapor content in order for saturation to occur. Any further cooling usually results in the formation of dew or frost.

DIGITALPertaining to measurements or devices in which the output varies in discrete steps (e.g., on-off pulse signals); compare to analog.

DISTANCE CONSTANTThe passage of air necessary over a wind speed sensor to cause the sensor to respond to 63% of a step function change in wind speed.

DRYBULB TEMPERATUREThe actual temperature of the air, used for comparison with wet bulb temperature.

EEPROMElectrically erasable programmable read-only memory.

EPROMErasable programmable read-only memory.

ERRORThe algebraic difference between the indication by an instrument and the true value of the measured signal.

FAAThe Federal Aviation Administration. A branch of the U.S. Department of transportation responsible for regulating aviation operations within the U.S.

FAHRENHEITA temperature scale having the freezing point of pure water at 32° and the boiling point at 212° under standard sea level pressure.

FIRMWAREPrograms or instructions which are stored in read-only memory.

FOGA hydrometeor consisting of numerous minute water drop- lets and based at the surface; droplets are small enough to be suspended in the earth's atmosphere indefinitely.

FREEZING POINTTemperature of solidification of a liquid under given conditions.

HEADWINDA wind blowing in a direction opposite to the course of a moving object. Often used when referring to winds affecting ballistics.

HOT FILM ANEMOMETERAnemometer which measures wind speed by measuring the degree of cooling of a metal film heated by an electric current.

HUMIDITYWater vapor content of the air; may be expressed as absolute humidity, specific humidity, relative humidity, or mixing ratio.

HYGROTHERMOGRAPHInstrument resulting from the combination of a thermograph and a hygrograph and furnishing, on the same chart, simultaneous time recording of ambient temperature and humidity.

HYSTERESISThe difference noted in a sensor's output as a response to first an increasing, and then a decreasing, input signal of the same value. If Yi is the value of the output with an increasing input of value X, and Yd is the value of the output with decreasing input of value X, then the hysteresis can be defined as the maximum absolute value of the difference (Yi - Yd) for any value of X. This is frequently expressed as a percentage of the sensor's full scale range and usually is included as a component of total sensor error.

INSTRUMENTA term used to describe a sensor (or sensors), the associated transducer(s), and the data read-out or recording device.

ISOBARA line of equal or constant barometric pressure.

ISO-ELASTIC SPRINGA spring which is designed to achieve a fixed spring constant over a wide temperature range. Usually, this involves the use of an alloy with high nickel content such as Ni-Span C. It is common for these springs to be stress relieved at elevated temperature after forming.

KELVIN TEMPERATURE SCALEAn absolute temperature scale based upon the triple point of pure water defined as 273.16° K. The size of the degree is the same as on the Celsius scale, and the zero point is absolute zero.

KNOTA unit of speed, one nautical mile per hour or approximately 1.15 statute miles per hour. The nautical mile is closely related to the geographical mile which is defined as the length of one minute of arc on the earth's equator. By international agreement, the nautical mile is now defined as 1852 meters.

KOSCHMIEDER'S LAWA basic equation in daytime visual range theory, relating the apparent luminance of a distant black object, the apparent luminance of the background-sky above the horizon, and the extinction coefficient of the atmosphere.

LANGLEYA unit of energy per unit area, equal to 1 gramcalorie/cm² and commonly employed in radiation measurement.

LEEWARD:  Facing away from the wind.

LIDARLight detecting and ranging. A technique used to detect atmospheric constituents or related parameters such as atmospheric extinction coefficient. Light is produced in a modulated source and the resulting backscattered or reflected light is analyzed to quantify some property of the atmosphere.

LINEARITYThe maximum departure of an instrument's actual response curve from the "best fit" straight line response. It applies only to instruments with more or less linear response, and is usually stated as a percentage of full scale range.

LIQUID THERMOMETERThermometer in which the difference in the rates of expansion with temperature of a liquid and its receptacle is used as a measure of the temperature. The liquid used may be ethyl alcohol, toluene, petroleum, or mercury.

LOCAL VISUAL DISTANCEThe meteorological visual range, which can be estimated from the average extinction coefficient using the Koschmieder equation:  Lv =   3.9/(sigma)e    where Lv is the visual Range in meters and (sigma)e is the extinction coefficient in meters^-1.

LOW LEVEL WIND SHEAR:   A local variation in the wind direction or speed. This condition can present danger to aircraft, especially at landing, when a sudden shift from headwind to tailwind can cause a rapid loss of airspeed and lift.

MAXIMUM THERMOMETERThermometer used for measuring the highest temperature attained during a given interval of time; for example, a day.

MICROBAROGRAPHAn aneroid barograph designed to record atmospheric pressure changes of very small magnitude.

MILLIBARA unit of pressure equal to 100 Pascals.The Pascal (Newton/meter² ) is the S.I. unit for pressure.

MINIMUM THERMOMETERThermometer used for measuring the lowest temperature attained during a given interval of time; for example, a day.

MISTDrizzle or heavy fog.

MIXING RATIOIn a system of moist air, the dimensionless ratio of the mass of water vapor to the mass of dry air.

NAUTICAL MILE Closely related to the geographical mile which is defined as the length of one minute of arc on the earth's equator. By international agreement, the nautical mile is now defined as 1852 meters.

NEPHELOMETERAn instrument used to measure the scattering coefficient of an air sample caused by suspended particles. The measurement can be used to determine the visual range through the medium.

NEWTONIAN TELESCOPEA reflecting type telescope with a 45° mirror, so that the primary image is observed through a hole in the side of the tube.

NWS:  The National Weather Service, administered by the U.S. Department of Commerce, and responsible for the collection of weather data, the routine production of weather forecasts, and the issuance of weather warnings within the U.S.

OPERATIONAL WEATHER LIMITSThe limiting values of ceiling, visibility and wind, or runway visual range, established as safety minima for aircraft landings and take-offs.

OSHAThe Occupational Safety and Health Administration, a regulatory office of the U.S. Department of Labor.

PARALLEL OUTPUTAn output which has a separate communication path (or wire) for each bit of a digital character This form of transmission makes each bit available simultaneously, and thus results in very fast communications.

PHENOLIC:  A plastic molding compound formed by the reaction of phenol and formaldehyde. It can be heavily reinforced or "filled" with glass fibers or other materials. Phenolics are known for their high impact strength, excellent wear characteristics, and dimensional stability over a wide temperature range.

PHOTOMETER Instrument that measures luminous intensity, luminous flux, light distribution or color.

POTENTIOMETER A variable resistor having three terminals and a movable wiper. Precision potentiometers can be used to create a variable resistance proportional to angular or linear displacement.

PRECIPITATIONAny and all forms of water particles, liquid or solid, that fall from the atmosphere and reach the surface.

PRESSURE ALTIMETERAn aneroid barometer with a scale graduated in altitude instead of pressure units.

PRESSURE TENDENCY(BAROMETRIC TENDENCY)  The change in barometric pressure within a specified period of time (typically 3 hours for meteorological observations).

PROMProgrammable read-only memory. Read-only memory which can be programmed by the user using a special hardware programmer.

PROTOCOLA set of rules or conventions used to standardize data transfer between devices.

PSYCHROMETERInstrument consisting of a wet-bulb and a dry-bulb thermometer and used to measure the water vapor content of the air.

PYRANOMETERInstrument which measures diffuse and direct solar radiation.

RADIOSONDEA balloon-borne instrument for the simultaneous measurement and transmission of meteorological data. It includes transducers for the measurement of pressure, temperature, and humidity, a modulator, a switching mechanism, and a radio transmitter.

RAIN GAGEInstrument for measuring the depth of water from precipitation that is assumed to be distributed over a horizontal, impervious surface and not subject to evaporation.

RAMRandom-Access Memory.The memory of a computer which can be read and written into at any location without passing through preceding locations.

RANGEThe interval between the lower and upper measuring limits of an instrument e.g., a thermometer with a range of -35° to 50° C (compare to span).

RAWINSONDEA method of upper air observation consisting of an evaluation of the wind speed and direction, temperature, pressure,and humidity aloft by means of a balloon-borne radiosonde tracked by radar or a radio theodolite.

REPEATABILITYThe degree of agreement between a sensor's output values in response to the same input value when this same input condition is presented multiple times. The repeatability is usually expressed as a percentage of full scale range (i.e. 1% repeatability).

RESOLUTIONThe smallest change in the parameter being measured that causes a detectable change in the output of the instrument.

RESPONSEThe value of the quantity being measured as indicated or otherwise provided by a measuring instrument.

RESPONSE TIMEThe time required for an instrument to register a designated percentage (frequently 90%) of a step change in the variable being measured.

RMSRoot Mean Square.This notation is used frequently with error analysis. In that context, it is the square root of the arithmetic mean of the squares of the deviations of the individual calibration points from the theoretical or ideal response.

ROMRead-Only Memory. Any type of memory which cannot be readily rewritten. A memory that cannot be altered in the normal use of a computer; usually used to store information permanently, such as firmware programs.

RS 232A specification of the Electronic Industries Association defining a standard serial data interface. A standard interface between a computer input/output port and a peripheral device.

RS 422A protocol similar to RS 232 which makes use of differential transmission to provide high speed data transmission over significantly longer distances.

RS 485A protocol similar to RS 232 which permits data interchange on multi-drop networks of up to 32 nodes using a single twisted pair cable. In order for this protocol to be used, each device on a network must have some level of intelligence in order to establish orderly data transfer over a single path.

RUNWAY VISUAL RANGE (RVR) The range over which the pilot of an aircraft on the center line of a runway can see the runway surface markings or the runway lights.

SCALEThe array of indicating marks and figures in relation to which the position of an index is observed (e.g., a scale plate on a recorder).

SCATTERING COEFFICIENTA measure of the attenuation due to scattering of light as it traverses a medium containing scattering particles.

SENSITIVITYThe ratio of the output of an instrument to the input (i.e. -gain).

SENSORThe part of a measuring instrument which responds directly to changes in the environment.

SERIAL OUTPUTA digital data output in which the characters are sent one bit at a time over a single communication path.

SLING PSYCHROMETERPsychrometer to which a small chain or rotary handle is attached so that the observer can rotate the instrument rapidly to properly ventilate the thermometer bulbs.

SNOW BRIDGINGAn effect noticed primarily in wet snow conditions when snow clings to the sides of a precipitation gage and gradually accumulates until the gage orifice is capped with accumulated snow. This effect can be minimized by using large collectors, and wind screens around the gages.

SOFTWAREThe programs and instructions which direct a computer.

SOLID STATEA device which is able to control current without the use of moving parts or vacuum tubes.

SPANThe absolute value of the differences between the upper and lower limits of an instrument's range.

SPECIFIC HUMIDITYIn a system of moist air, the dimensionless ratio of the mass of water vapor to the total mass of the system.

SPLAYED TAILA type of wind vane having a split or V-shaped tail. The apex orients itself into the direction of the wind.

STATIC PRESSURE VENTA vent used with pressure sensors to reduce the effect of wind on the pressure inlet. It is normally mounted remotely and connected to the sensor using airtight tubing.

STATUTE MILEA unit of distance equal to 5280 feet. It is sometimes referred to as a land mile.

SWITCHING POWER SUPPLYA power supply which achieves its output regulation by means of one or more active power handling devices which are alternately placed in the "off" or "on" states. It is more efficient than linear supplies which vary the conduction of power devices to achieve output regulation.

SYNCHROA motor like device containing a rotor and a stator and capable of converting an angular position into an electrical signal, or an electrical signal into an angular position. When several synchros are correctly connected, all of the rotors will align themselves in the same angular position. This is useful, since one synchro whose angular position is forced to change, can drive another synchro to indicate the angular change.

TEMPERATURE COEFFICIENTA factor relating the response characteristics of a device with changes in the ambient temperature.

THERMOGRAPHThermometer used to give a graphic record of the time variations of temperature.

THERMOMETERInstrument used in the measurement of temperature.

THERMOSTATA device used to switch electrical current at a selectable set-point temperature.

THRESHOLDThe smallest input value to a sensor which will cause the sensor to respond. Commonly used with mechanical wind sensors to describe the wind speed necessary to cause the anemometer and wind vane to turn.

TIME CONSTANTThe time required for an instrument to register 63.2% of an instantaneous change in the measured parameter.

TOTALIZING ANEMOMETERAnemometer in which the sensor rotation is transmitted to a mechanical counter which directly integrates the air movement past the instrument.

TOWNSEND SUPPORTA common support used to fixture maximum and minimum thermometers. It is designed to hold the thermometers at the proper angles for measuring, and it also simplifies resetting of the thermometer markers.

TRANSMISSIVITYA measure of luminous flux remaining in a light beam after it has passed through a specified distance of the atmosphere.

TRANSMISSOMETERAn instrument which measures the transmissivity of the atmosphere between two points for the determination of visual range.

VIRTUAL TEMPERATURETemperature to which absolutely dry air would have to be brought in order for it to have the density as moist air considered at the same pressure.

VISIBILITYThe greatest distance at which it is possible with the unaided eye to recognize a prominent dark object against the horizon sky. At night, it is defined as the greatest distance at which a moderately intense, unfocused light source can be seen on the horizon.

VISIBILITY SENSORGeneral term for an instrument used to make direct measurements of visual range or measurements of the physical characteristics of the atmosphere which determine the visual range.

VISUAL RANGEThe maximum distance, usually horizontally, at which a given object or light source is just visible under particular conditions of transmittance and background luminance.

WET BULB TEMPERATUREThe temperature of the wet bulb thermometer at equilibrium with a constant flow of ambient air at a rate of 2.5 to 10.0 meters per second.

WET BULB THERMOMETER A thermometer with a muslin-covered bulb which is moistened; used to measure wet-bulb temperature.

WIND DIRECTIONThe direction from which the wind is blowing. Usually measured in degrees azimuth.

WIND GUSTThe peak momentary wind velocity within a given interval of time.

WIND PASSAGEThe distance or length of flow of the air past a point during a given interval of time.

WIND ROSEA flower-like diagram indicating the relative frequencies of different wind directions for a given station and period of time.

WIND SPEEDThe rate of wind movement in units of distance/time.

WIND VANEAn instrument used to indicate wind direction.

WIND VECTORA component of the wind (often using Cartesian coordinates; i.e. X and Y wind vectors). The term can also apply to the resultant wind vector which is sometimes drawn as an arrow with length proportional to wind speed.

WIND VELOCITYA vector term which includes both wind speed and wind direction.

WINDWARDSituated on the side from which the wind blows.

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