Meteorological Instrumentation and Measurements Glossary

Glossary of Terms and Acronyms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Note: Glossary terms are linked to the American Meteorological Society or other public source definitions when available. Linked items (in red) will open in a new window.

Absorbance: A logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls). Depending on the base of the logarithm a decadic and Napierian absorbance are used. Symbols: A, A₁₀, A_e. This quantity is sometimes called extinction, although the term extinction, better-called attenuance, is reserved for the quantity which takes into account the effects of luminescence and scattering as well.

Absorptance: The ratio of the radiant flux absorbed by a body to that incident upon it. Also called absorption factor.
Total absorptance refers to absorptance measured over all wavelengths.
Spectral absorptance refers to absorptance measured at a specified wavelength.

Actinic flux: The spherically integrated radiation flux in the earth's atmosphere that originates from the sun, including the direct beam and any scattered components.

This radiation is responsible for initiating the chemistry of the atmosphere.

Aerosol: A colloidal system in which the dispersed phase is composed of either solid or liquid particles, and in which the dispersion medium is some gas, usually air.

There is no clear-cut upper limit to the size of particles composing the dispersed phase in an aerosol, but as in all other colloidal systems, it is rather commonly set at 1 μm. Haze, most smokes, and some fogs and clouds may thus be regarded as aerosols. However, it is not good usage to apply the term to ordinary clouds with drops so large as to rule out the usual concept of colloidal stability. It is also poor usage to apply the term to the dispersed particles alone; an aerosol is a system of dispersed phase and dispersing medium taken together.

Albedo: The ratio of reflected flux density to incident flux density, referenced to some surface.

Albedos commonly tend to be broadband ratios, usually referring either to the entire spectrum of solar radiation, or just to the visible portion. More precise work requires the use of spectral albedos, referenced to specific wavelengths. Visible albedos of natural surfaces range from low values of ∼0.04 for calm, deep water and overhead sun, to < 0.8 for fresh snow or thick clouds. Many surfaces show an increase in albedo with increasing solar zenith angle.

Anemometer: The general name for instruments designed to measure either total wind speed or the speed of one or more linear components of the wind vector.
These instruments may be classified according to the transducer employed; those commonly used in meteorology include the cup, propeller, Pitot-tube, hot-wire or hot-film, and sonic anemometers.

Anisotropy: This refers to the nonisotropic part of the turbulence spectrum.

Atmospheric Window: A range of wavelengths over which there is relatively little absorption of radiation by atmospheric gases.

The major windows are the visible window, from ∼0.3 to ∼0.9 μm; the infrared window, from ∼8 to ∼13 μm; and the microwave window, at wavelengths longer than ∼1 mm. The infrared window loses much of its transparency under very humid conditions due to continuum absorption by water vapor, and can become completely opaque when clouds are present.

Attenuate: The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals.

Autocorrelation Function: The autocorrelation for variable lag.

Avogadro Constant: The number of molecules in one mole (approx. 6.022×10²³ mol^-1).

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Barometer: An instrument for measuring atmospheric pressure.

Two types of barometers are commonly used in meteorology: the mercury barometer and the aneroid barometer.

Blackbody: A hypothetical body that cannot be excited to radiate by an external source of electromagnetic radiation of any frequency, direction, or state of polarization except in a negligibly small set of directions around that of the source radiation.

The traditional definition of a blackbody—as one that absorbs all the radiation incident on it—is inadequate unless to this definition is added the requirement that the body be large compared with the wavelength of the incident radiation. The concept of radiation incident on a body is from geometrical (or ray) optics, which is never strictly valid (because all bodies are finite) and may break down completely when the body is small compared with the wavelength. This was recognized by Planck, but by almost no one who followed him. Although no strict blackbody exists, some bodies are approximately black over a limited range of frequencies, directions, and polarization states of the exciting radiation. See Planck's law, emissivity.

Boltzmann Constant: The ratio of the universal gas constant to the Avogadro constant (approx. 1.38065×10^-23 J K^-1).

Brightness Temperature: A descriptive measure of radiation in terms of the temperature of a hypothetical blackbody emitting an identical amount of radiation at the same wavelength.

The brightness temperature is obtained by applying the inverse of the Planck function to the measured radiation. Depending on the nature of the source of radiation and any subsequent absorption, the brightness temperature may be independent of, or highly dependent on, the wavelength of the radiation.

Brownian Motion: The rapid and chaotic motion of particles suspended in a fluid at rest as a consequence of fluctuations in the rate at which fluid molecules collide with the particles.

On average, the particles experience zero net force, but deviations from this average give rise to Brownian motion, so named because it reputedly was observed first in pollen grains by the botanist Robert Brown. Whether Brown actually observed what we now call Brownian motion has been questioned on the grounds that his grains were so large that their fluctuating motion would have been imperceptible. But even if Brownian motion can ever be shown not to have been observed by Brown, his name is likely to be forever attached to this motion, especially since mathematicians have generalized this term to a broad class of stochastic processes. Brownian motion is more than just a scientific curiosity, having played a key role in establishing the reality of molecules and in presenting a convincing argument for the kinetic theory of gases.

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Calibration: The process whereby the magnitude of the output of a measuring instrument (e.g., the level of mercury in a thermometer or the detected backscatter power of a meteorological radar) is related to the magnitude of the input force (e.g., the temperature or radar reflectivity) actuating that instrument.

Capacitance: The property of being able to collect a charge of electricity.

Capacitor: A device for accumulating and holding a charge of electricity, consisting of two equally charged conducting surfaces having opposite signs and separated by a dielectric.

Chemiluminescence: Emission of radiation resulting from a chemical reaction. The emitting species may be a reaction product or a species excited by energy transfer from an excited reaction product. The excitation may be electronic, vibrational or rotational; if the luminescence occurs in the infrared the expression infrared chemiluminescence is used.

Cloud: A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface.

Cloud differs from fog only in that the latter is, by definition, close (a few meters) to the earth's surface. Clouds form in the free atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest stratum of fog. For condensation to occur at a low degree of supersaturation there must be an abundance of cloud condensation nuclei for water clouds, or ice nuclei for ice-crystal clouds, at temperatures substantially above -40°C. The size of cloud drops varies from one cloud type to another, and within any given cloud there always exists a finite range of sizes. Generally, cloud drops (droplets) range from 1–100 μm in diameter, and hence are very much smaller than raindrops.

Any collection of particulate matter in the atmosphere dense enough to be perceptible to the eye, as a dust cloud or smoke cloud.

Colloidal System: An intimate mixture of two substances, one of which, called the dispersed phase (or colloid), is uniformly distributed in a finely divided state through the second substance, called the dispersion medium (or dispersing medium).

The dispersion medium may be a gas, a liquid, or a solid and the dispersed phase may also be any of these, with the exception of one gas in another. A system of liquid or solid particles colloidally dispersed in a gas is called an aerosol. A system of solid substance or water-insoluble liquid colloidally dispersed in liquid water is called a hydrosol. There is no sharp line of demarcation between true solutions and colloidal systems or between mere suspensions and colloidal systems. When the particles of the dispersed phase are smaller than about 10^-3 μm in diameter, the system begins to assume the properties of a true solution; when the particles dispersed are much greater than 1 μm, separation of the dispersed phase from the dispersing medium becomes so rapid that the system is best regarded as a suspension. According to the latter criterion, natural clouds in the atmosphere should not be termed aerosols; however, since many cloud forms apparently exhibit characteristics of true colloidal suspensions, this strict physico-chemical definition is often disregarded for purposes of convenient and helpful analogy. Condensation nuclei and many artificial smokes may be regarded as aerosols.

Core Sample: A sample of rock, soil, snow, or ice obtained by driving a hollow tube into the medium and withdrawing it with its contained sample or "core."

In general, the aim of core sampling is to obtain a specimen in its undisturbed natural state for subsequent analysis. The snow sampler is a hydrometeorological example of the type of instrument used to obtain core samples.

Critical Damping: The minimum amount of viscous damping that results in a displaced system returning to its original position without oscillation. Symbol: Cc

Current: Any movement of electric charge in space, by virtue of which a net transport of charge occurs as, for example (in atmospheric electricity), in a conduction current, convection current, or precipitation current.

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Damping: The more or less steady diminishing in time or space (or both) of the amplitude of any physical quantity.

In the atmosphere, for example, acoustic, hydrodynamic, and electromagnetic waves are damped.

Degrees of Freedom: In an unconstrained dynamic or other system, the number of independent variables required to specify completely the state of the system at a given moment.

Dewpoint: The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur.
When this temperature is below 0°C, it is sometimes called the frost point. The dewpoint may alternatively be defined as the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Isobaric heating or cooling of an air parcel does not alter the value of that parcel's dewpoint, as long as no vapor is added or removed. Therefore, the dewpoint is a conservative property of air with respect to such processes. However, the dewpoint is nonconservative with respect to vertical adiabatic motions of air in the atmosphere. The dewpoint of ascending moist air decreases at a rate only about one-fifth as great as the dry-adiabatic lapse rate. The dewpoint can be measured directly by several kinds of dewpoint hygrometers or it can be deduced indirectly from psychrometers or devices that measure the water vapor density or mixing ratio.

Dielectric Polymer: Dielectric electroactive polymers are materials in which actuation is caused by electrostatic forces between two electrodes which squeeze the polymer. Dielectric elastomers are capable of very high strains and are fundamentally a capacitor that changes its capacitance when a voltage is applied by allowing the polymer to compress in thickness and expand in area due to the electric field. This type of electroactive polymers typically requires a large actuation voltage to produce high electric fields (hundreds to thousands of volts), but very low electrical power consumption. Dielectric electroactive polymers require no power to keep the actuator at a given position. Examples are electrostrictive polymers and dielectric elastomers.

Direct Measurement: A measurement may be direct or indirect. When the instrument reports the measurement, as is the case for a thermometer or a barometer, that is a direct measurement. When you must measure something else and then infer the measurement, as is the case for a measurement of potential temperature for example, that is an indirect measurement because it is based on the direct measurements of pressure and temperature which are then used in a calculation. Indirect measurements are sometimes called derived measurements. Sometimes the distinction is blurred, as in the case of a voltage measured from a thermocouple to deduce temperature. This is usually considered a direct measurement of temperature even though the direct measurement is a voltage. Use of a calibration does not make a measurement indirect.

Disdrometer: An instrument that measures and records the sizes of raindrops.

A common type of disdrometer consists of a sensitive transponder that measures the momentum of individual drops as they fall onto an exposed horizontal surface. Size is determined from momentum through calibration, and the drop-size distribution is obtained by keeping a tally of the number of drops in different size categories that fall onto the surface in a given period of time.

Doppler Effect: (Also called Doppler shift.) "The change in the apparent time interval between two events which arises from the motion of an observer together with the finite velocity of information about the events" (Gill 1965).

Doppler effect is often used to mean frequency shifts (Doppler shift) of acoustic and electromagnetic waves because of relative motion between sources and observers. The relative magnitude of a Doppler shift is of order of the ratio of a characteristic speed (e.g., speed of a source) to a speed of propagation (e.g., speed of sound, speed of light). A shift to lower frequency (relative to a reference frequency) is sometimes called a red shift whereas a shift to higher frequency is sometimes called a blue shift, although no colorimetric meaning should be attached to these terms. Acoustic waves do not evoke sensations of color, nor do electromagnetic waves outside the visible spectrum, and even Doppler shifts of visible light are so small as to yield no visually perceptible color changes. According to classical theory, there is no frequency shift of electromagnetic radiation for motion of a transmitter perpendicular to the line between receiver and transmitter. But according to relativistic theory, even for this kind of motion there is a Doppler shift (transverse Doppler shift), although it is appreciably smaller than the longitudinal Doppler shift.

Drift, Brock and Richardson (2001): A source of error in instruments caused by physical changes in the instrument subsequent to calibration.

Dropsonde: A radiosonde with a parachute dropped from an airplane carrying receiving equipment for the purpose of obtaining an upper-air sounding during descent.

Dynamic: (Of a process or system) characterized by constant change, activity, or progress.

Dynamic Calibration: Calibration in which the input varies over a specific length of time and the output is recorded vs. time.

Dynamic Error, Brock and Richardson (2001): Dynamic error, due to changing input, is the sensor output, after static calibration has been applied, minus the impact at any given instant.

Dynamic Lag, Brock and Richardson (2001): Dynamic lag, due to changing input, is the time delay after the input reaches a certain level until the output, after static calibration has been applied, reaches the same level.

Dynamic Pressure: In engineering fluid mechanics, the kinetic energy, (1/2)ρV², of the fluid, where ρ is the density and V the speed.

This applies in cases where this quantity may be conveniently considered as adding to the static pressure; that is, the dynamic pressure at a given point is the difference between the static pressure at that point and the total pressure at the stagnation point of the same streamline. This concept must be distinguished from the hydrodynamic pressure, and the terminology is confusing in meteorological contexts.

Dynamic Range: A measure of the ability of an amplifier, transducer, receiver, or other kind of sensor to measure both weak and strong signals.

Specifically, the range of input levels, ordinarily expressed in decibels, over which the system can operate within some specified range of performance; for example, the range over which the response of the system is linear or approximately linear.

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E-folding Time: The time interval in which an exponentially growing quantity increases by a factor of e; it is the base-e analog of doubling time. This term is often used in many areas of science, such as in atmospheric chemistry, medicine and theoretical physics, especially when cosmic inflation is investigated.

Electromagnetic Radiation: Electromagnetic radiation is a form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter. This energy is then grouped into categories based on its wavelength into the electromagnetic spectrum, which is the ordered sequence of all known electromagnetic radiations, extending from the shortest cosmic rays through gamma rays, x-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other radio wavelengths. The observable properties and physical effects of various portions of the electromagnetic spectrum are of considerable importance in meteorology.

Emission: With respect to radiation, the generation and sending out of radiant energy.

The emission of radiation by natural emitters is accompanied by a loss of energy and is considered separately from the processes of absorption or scattering.

Emission Spectrum: A plot of the emitted spectral radiant power (spectral radiant exitance) or of the emitted spectral photon irradiance (spectral photon exitance) against a quantity related to photon energy, such as frequency, 𝜈, wavenumber, 𝞼, or wavelength, λ. When corrected for wavelength-dependent variations in the equipment response, it is called a corrected emission spectrum.

Emissivity: The ratio of the power emitted by a body at a temperature T to the power emitted if the body obeyed Planck's radiation law.

Strictly, emissivity should be qualified by the frequency, direction, and even polarization state of the emitted radiation. This is recognized in qualifiers such as monochromatic (at a given frequency) as opposed to total (over a broad range of frequencies), and directional (the ratio for a particular direction) as opposed to hemispherical (the ratio for a hemisphere of directions). Contrary to a widespread misconception, the upper limit of emissivity is not 1. This upper limit is valid (approximately) only for bodies large compared with all relevant wavelengths.

Equilibrium: In mechanics, a state in which the vector sum of all forces, that is, the acceleration vector, is zero.
In hydrodynamics, it is usually further required that a steady state exist throughout the atmospheric or fluid model. The equilibrium may be stable or unstable with respect to displacements therefrom. See also hydrostatic equilibrium.
In thermodynamics, any state of a system that would not undergo change if the system were to be isolated.
Processes in an isolated system not in equilibrium are irreversible and always in the direction of equilibrium.

Excitation Spectrum: A plot of the spectral radiant exitance or of the spectral photon exitance against the frequency (or wavenumber, or wavelength) of excitation. When corrected for wavelength dependent variations in the excitation radiant power this is called a corrected excitation spectrum.

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Fetch

The distance upstream of a measurement site, receptor site, or region of meteorological interest, that is relatively uniform.

If a measurement site is located in the middle of a farm field with homogeneous land use, and if there are no changes to the land use and no obstructions such as trees or buildings immediately upstream of the site, then the site is said to have "large fetch". Large fetch is usually considered good if the measurements are to be representative of the atmosphere over the farm field. Similarly, measurements over a homogeneous forest could also have large fetch if there are no clearcuts or changes in the tree characteristics upstream of the measurement site.

(Also called generating area.) An ocean area where waves are generated by a wind having a constant direction and speed.

The length of the fetch area, measured in the direction of the wind in which ocean waves are generated. In many cases, the fetch is limited by the upwind distance to the coast.

First-Order System: Contains a single mode of energy storage.

Fluorescence: The emission of radiation associated with the relaxation of an atom or molecule from an excited energy level to a lower (usually ground state) level.
The emission can be in the visible or ultraviolet if an electronic transition is involved, or in the infrared if it is a vibrational transition.

Flux: The rate of flow of some quantity, often used in reference to the flow of some form of energy.

In the field of atmospheric turbulence and boundary layers, often used as a contraction for flux density; namely, the flow of a quantity per unit area per unit time. These fluxes can be defined in two forms: dynamic and kinematic. The dynamic flux of a quantity is the flow of that quantity per unit area per unit time, where often the word dynamic is assumed if it is not explicitly stated. The advantage of a kinematic flux is that it has units that are more easily measured by a conventional meteorological instrument. The units are usually a velocity (m s^-1) times a temperature (K), specific humidity (kgwater/kgair), or wind speed (m s^-1).

Frost point: See dewpoint.

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Gain: An increase or amplification.

There are two general uses of the term in radar meteorology: 1) antenna gain (or gain factor), which is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; and 2) receiver gain (or video gain), which is the amplification given a signal by the receiver.

Geostrophic: Referring to the balance, in the atmosphere, between the horizontal Coriolis forces and the horizontal pressure forces.

Gravimetric Hygrometry: A method using absorption of water vapor by a desiccant from a known volume of air.

The mass of water vapor is determined through the weighing of the drying agent before and after absorbing the vapor. This method is used mostly in national calibration laboratories for providing an absolute reference standard.

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Harmonic Oscillator: A physical system that, when displaced from equilibrium, experiences a restoring force proportional to the displacement. A harmonic oscillator that is displaced and then let go will oscillate sinusoidally. Examples from classical physics are a mass attached to a spring and a simple pendulum swinging through a small angle.

Herriott Cell: A multiple-pass spectroscopic absorption cell commonly used in spectroscopy to measure low-concentration components or to observe weak spectral features in gases or liquids.

Hodograph: In general (mathematics), the locus of one end of a variable vector as the other end remains fixed. A common hodograph (or hodogram) in meteorology represents the vertical distribution of the horizontal wind.

Homogeneity: The quality or state of being all the same or all of the same kind.

Humidity: Generally, some measure of the water vapor content of air.
The multiplicity of humidity measures is partly due to different methods of measurement and partly because the conservative measures (mixing ratio, specific humidity) cover an extremely wide dynamic range, as a result of the rapid variation of saturation vapor pressure with temperature.

Hydrometeors: Any product of condensation or deposition of atmospheric water vapor, whether formed in the free atmosphere or at the earth's surface; also, any water particle blown by the wind from the earth's surface.
Hydrometeors may be classified in a number of different ways, of which the following is one example: 1) liquid or solid water particles formed and remaining suspended in the air, for example, damp (high relative humidity) haze, cloud, fog, ice fog, and mist; 2) liquid precipitation, for example, drizzle and rain; 3) freezing precipitation, for example, freezing drizzle and freezing rain; 4) solid (frozen) precipitation, for example, snow, hail, ice pellets, snow pellets (soft hail, graupel), snow grains, and ice crystals; 5) falling particles that evaporate before reaching the ground, for example, virga; 6) liquid or solid water particles lifted by the wind from the earth's surface, for example, drifting snow, blowing snow, and blowing spray.

Hydrostatic Equation: The form assumed by the vertical component of the vector equation of motion when all Coriolis, earth curvature, frictional, and vertical acceleration terms are considered negligible compared with those involving the vertical pressure force and the force of gravity. Thus:

where p is the pressure, ρ the density, g the acceleration of gravity, and z the geometric height. For cyclonic-scale motions the error committed in applying the hydrostatic equation to the atmosphere is less than 0.01%. Strong vertical accelerations in thunderstorms and mountain waves may be 1% of gravity or more in extreme situations.

Hydrostatic Equilibrium: The state of a fluid with surfaces of constant pressure and constant mass (or density) coincident and horizontal throughout.
Complete balance exists between the force of gravity and the pressure force. The relation between the pressure and the geometric height is given by the hydrostatic equation. The analysis of atmospheric stability has been developed most completely for an atmosphere in hydrostatic equilibrium.

Hydrostatic Pressure: The pressure in a fluid in hydrostatic equilibrium.

Hygrometer: Any instrument that measures the water vapor content of the atmosphere.
There are six basically different means of transduction used in measuring this quantity and hence an equal number of types of hygrometers. These are 1) the psychrometer, which utilizes the thermodynamic method; 2) the class of instruments that depends upon a change of physical dimensions due to the absorption of moisture; 3) those that depend upon condensation of moisture; 4) the class of instruments that depend upon the change of chemical or electrical properties due to the absorption of moisture; 5) the class of instruments that depend upon the diffusion of water vapor through a porous membrane; and 6) the class of instruments that depend upon measurements of the absorption spectra of water vapor.

Hygrometry: The study that treats the measurement of the humidity of the atmosphere and other gases.

Hysteresis: When an instrument reports different values when approaching a measurand from below vs. from above, this is called hysteresis. A common cause of hysteresis occurs when an instrument has a characteristic time response that produces a time lag in a measurement, which will produce a value too low when approaching the measurand from previous lower measurands. In this case, the hysteresis will be eliminated if the measurements are continued for a time long compared to the characteristic time response of the instrument. However, this is not the only cause of hysteresis. For example, a measurement from a caliper with gears may exhibit hysteresis that results from "lash" or "play" in the gears.

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Ideal-Gas Law: A physical law describing the relationship of the measurable properties of an ideal gas, where P (pressure) × V (volume) = n (number of moles) × R (the gas constant) × T (temperature in Kelvin). It is derived from a combination of the gas laws of Boyle, Charles, and Avogadro. Also called universal gas law.

Impaction (Impactor): A general term for instruments that sample particles suspended in the atmosphere by impaction.
This instrument is based on the principle that particles in an airstream will continue in a straight line due to their inertia when the flow of the air bends sharply; if a surface to which they can adhere is present, they will strike it and may stick.

Impedance: The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

In situ: An in situ measurement is a measurement that is made "in place," or at the location of the measurand. Such a measurement is in contrast to a "remote" measurement, where the measurement is made using properties that are sensed at a distance. For example, temperature can be measured in situ by using a liquid-in-glass thermometer or remotely by sensing the emitted infrared radiation from the object.

Indirect Measurement: See "Direct measurement."

Incident: An incident ray is a ray of light that strikes a surface. The angle between this ray and the perpendicular or normal to the surface is the angle of incidence. For more information, see this information from Wallace and Hobbs ( )
http://cup.aos.wisc.edu/453/2016/readings/Atmospheric_Science-Wallace_Hobbs.pdf

Intensive State Variable: An intensive variable is one which does not depend on system size (like temperature, pressure, or density).

Interference: The superposition of two or more waves resulting in an amplitude of the composite wave not necessarily the algebraic sum of the amplitudes of each of its components.

Interferometry: The use of interference phenomena for purposes of measurement.
In radar, one use of interferometric techniques is to determine the angle of arrival of a wave by comparing the phases of the signals received at separate antennas or at separate points on the same antenna. Another interferometric application is to shape and steer the beams of phased- array antennas by adjusting the phases of the different elements of the array.

Internal Boundary Layer: A layer within the atmosphere bounded below by the surface, and above by a more or less sharp discontinuity in some atmospheric property.

Internal boundary layers are associated with the horizontal advection of air across a discontinuity in some property of the surface (e.g., aerodynamic roughness length or surface heat flux) and can be viewed as layers in which the atmosphere is adjusting to new surface properties.

Internal Standard: A substance, added to a specimen, presumed not otherwise to be present in the specimen, which after addition acts similarly to but does not interfere with the sought-for substance. By monitoring a signal due to the internal standard simultaneously with that of the analyte, signal changes due to incomplete sample uptake or due to variations in sample transport can be compensated.

Inversion: In meteorology, a departure from the usual decrease or increase with altitude of the value of an atmospheric property; also, the layer through which this departure occurs (the "inversion layer"), or the lowest altitude at which the departure is found (the "base of the inversion").

Irradiance: (Or radiant flux density.) A radiometric term for the rate at which radiant energy in a radiation field is transferred across a unit area of a surface (real or imaginary) in a hemisphere of directions.

In general, irradiance depends on the orientation of the surface. The radiant energy may be confined to a narrow range of frequencies (spectral or monochromatic irradiance) or integrated over a broad range of frequencies. Irradiance follows from radiance but not, in general, vice versa. The photometric equivalent of irradiance is illuminance, obtained by integrating spectral irradiance times luminous efficiency over the visible spectrum.

Isothermal: Of equal or constant temperature, with respect to either space or time.

Isotropic: Having the same properties in all directions.
Obtained by combining the Greek iso, meaning alike or same, and tropos, meaning turning. Its antonym is anisotropic.

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Kelvin Temperature Scale: (Abbreviated K; also called absolute temperature scale, thermodynamic temperature scale). An absolute temperature scale independent of the thermometric properties of the working substance.

On this scale, the difference between two temperatures T1 and T2 is proportional to the heat converted into mechanical work by a Carnot engine operating between the isotherms and adiabats through T1 and T2. A gas thermometer utilizing a perfect gas has the same temperature scale. For convenience the Kelvin degree is identified with the centigrade degree. The ice point in the Kelvin scale is 273.16 K.

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Lambert’s Cosine Law: A law governing the angular dependence of emitted or reflected radiation from an idealized surface.

The radiation emitted from or reflected by a surface obeying Lambert's law is unpolarized and has a radiance that is constant with angle, or isotropic. Such surfaces are variously termed Lambertian or diffuse surfaces, reflectors, or emitters. They may also be termed perfectly diffusing radiators or reflectors. This law is sometimes called Lambert's cosine law to distinguish it from the Bouguer–Lambert law.

Latent Heat: The specific enthalpy difference between two phases of a substance at the same temperature.

The latent heat of vaporization is the water vapor specific enthalpy minus the liquid water specific enthalpy. When the temperature of a system of dry air and water vapor is lowered to the dewpoint and water vapor condenses, the enthalpy released by the vapor heats the air–vapor– liquid system, reducing or eliminating the rate of temperature reduction. Similarly, when liquid water evaporates, the system must provide enthalpy to the vapor by cooling. The latent heat of fusion is the specific enthalpy of water minus that of ice and the latent heat of sublimation is the specific enthalpy of water vapor minus that of ice. The latent heats of vaporization, fusion, and sublimation of water at 0°C are, respectively,
It is common to see an expression like "release of latent heat." In other thermodynamic terms in this glossary, such expressions are avoided in favor of others using enthalpy and temperature, which are measurable quantities.

Law of Propagation of Uncertainty: This law, in its general form, can be applied when covariance or correlation exists between the various inputs.

Limit of Detection: The limit of detection, expressed as the concentration, c_L, or the quantity, q_L , is derived from the smallest measure, x_L , that can be detected with reasonable certainty for a given analytical procedure. The value of x_L is given by the equation

where x_bi is the mean of the blank measures, sbi is the standard deviation of the blank measures, and k is a numerical factor chosen according to the confidence level desired.

Linear Ordinary Differential Equations (ODE): A differential equation that is linear in the dependent variable and derivatives thereof.

The existence of a wealth of mathematical techniques and tables for the treatment of linear equations guarantees that a physical problem representable by such an equation is very much easier to solve and understand than a nonlinear one. The advection terms in the fundamental equations of hydrodynamics are not linear (strictly, they are "quasi-linear"), and much of dynamic meteorology has been an attempt to circumvent this difficulty.

Linearity: The property of being linear.

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Manometer: Although this term can refer to any device that measures pressure in a gas or fluid, the term usually refers to a U-shaped type of fluidic barometer where the difference in height in the two arms reflects the difference in pressure at the ends of the open arms.

Measurand: The quantity or characteristic to be measured by an instrument, usually with specification of the conditions of measurement like the location and other physical aspects affecting the quantity being measured.

Metadata: Information that provides enhanced knowledge of the content of data. For example, the format description of a METAR is the metadata about the meteorological data being observed and reported. Literally, metadata are data about data.

Metastable: In physics, metastability is a stable state of a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is only slightly pushed, it will settle back into its hollow, but a stronger push may start the ball rolling down the slope. Bowling pins show similar metastability by either merely wobbling for a moment or tipping over completely. A common example of metastability in science is isomerisation. Higher energy isomers are long lived as they are prevented from rearranging to their preferred ground state by (possibly large) barriers in the potential energy.

Microgram

One microgram (µg) is a measure of mass equal to:

one thousandth of a milligram (1 x 10^-3 mg)
one millionth of a gram (1 x 10^-6 g)
one billionth of a kilogram (1 x 10^-9 kg)

Milligram

One milligram (mg) is a measure of mass equal to:

one thousand micrograms (1 x 10³ µg)
one thousandth of a gram (1 x 10^-3 g)
one millionth of a kilogram (1 x 10^-6 kg)

Mixing Ratio: The ratio of the mass of a variable atmospheric constituent to the mass of dry air.
If not otherwise indicated, the term normally refers to water vapor. For many purposes, the mixing ratio may be approximated by the specific humidity. Either r or w is commonly used to symbolize water vapor mixing ratio, with r used for thermodynamic terms in this glossary. In terms of the pressure p and vapor pressure e, the mixing ratio r is
.

Moment: The product of a distance and another parameter. The moment may be about a point, line, or plane; if the parameter is a vector, the moment is the vector product of the vector distance from the point, line, or plane, into the parameter. Thus, the moment of the momentum of a fluid parcel per unit volume about an axis is r × ρu, where r is the vector from axis to the parcel, ρ the density, and u the velocity vector of the parcel; this is also called the angular momentum. The moment of a force F about an axis is r × F, called the torque. The second moment of a parameter is the moment of the first moment, and so on, for higher moments. 2) By analogy, in statistical terminology, the mean value of a power of a random variable. The symbol μn′ (or ν_n) is used for a raw moment as distinguished from the corresponding central moment μn taken about the mean μ. Thus the raw moments are where E(xn) is the expected value of the variate x to the nth power. In particular, μ₀′ ≡ 1 and μ₁′ ≡ ν₁ ≡ μ. The central moments are where E[(x - μ)ⁿ] is the expected value of the nth power of the deviation of the variate from its mean. In particular, μ₀ ≡ 1, μ₁ ≡ 0, μ₂ ≡ σ², where σ² is the variance.

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Nano-resolution, Schaad (2016): A resolution of parts-per-billion.

Nonlinearity: Not a linear function of the relevant variables.

Numerical Modeling: Solutions are obtained by assigning discrete values to temporal and spatial derivatives in order to convert the governing differential equations into algebraic equations that can be solved by using computational methods. Because computational resources are finite, no one technique is ideal for all applications. Some models define the equations on very fine spatial intervals. This approach furnishes solutions that are very accurate, but that span only small spatial regions (spatial scales of a few meters, at present). At the other extreme, some models span entire ocean basins by using large spatial intervals (hundreds of kilometers). Here, approximation of unresolved motions is a crucial and difficult issue. Similar trade-offs must be made with respect to temporal solutions. Numerical models also differ in the equations and boundary conditions that are employed. The most general model commonly used in oceanography includes momentum conservation via the incompressible Navier–Stokes equations with the Boussinesq approximation, mass conservation via the incompressibility condition, and equations expressing conservation of heat energy and salt (e.g., Gill 1982). For large-scale applications, the hydrostatic approximation is usually made. The vertical coordinate may be the geometric height, or a convenient substitute such as density, pressure, logarithm of pressure, or potential temperature. Surface boundary conditions generally express fluxes of momentum, heat, and freshwater from the atmosphere. Basin-scale models use boundary conditions that approximate the effects of bottom topography. Smaller-scale models typically specify periodic conditions at the side boundaries and an energy radiation condition at the bottom.

Nyquist Frequency: (Also called turnover frequency.) The highest frequency that can be determined in a Fourier analysis of a discrete sampling of data.

If a time series is sampled at interval Δt, this frequency is 1/2Δt cps.

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Oscillation: A swinging, as of a pendulum.

Often applied to periodic motion or variation in time of any quantity, although may mean any more or less regular variation between fixed bounds. Fluctuation is more suggestive of irregular variation. Vibration is a near synonym except that oscillation is applied to variations in space as well as time. The amplitude of a damped oscillation steadily decreases. Oscillations are said to be forced or free according to whether the oscillating system is or is not acted upon by an external force, although what constitutes such a force is a matter of convention. An ordinary pendulum is acted on by the external force of gravity, and yet the pendulum probably would be described as undergoing free oscillation.

As used by Sir Gilbert Walker, a single number, empirically derived, that represents the distribution of pressure and temperature over a wide ocean area.
Basically, the process is one of weighting pressure and temperature values for selected island and coastal stations, and algebraically combining them. These numbers were originally employed in correlations with single station values. Three such "oscillations" were derived: the North Atlantic Oscillation; the North Pacific Oscillation; and the Southern Oscillation.

Ozonesonde: A radiosonde equipped with an instrument to measure the atmospheric concentration of ozone (O₃).

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Partial Pressure: The pressure that a component of a gaseous mixture would have if it alone occupied the same volume at the same temperature as the mixture.

Pearson Correlation: A measure of the strength of a linear association between two variables and is denoted by r.

Photodissociation: Fragmentation of a molecule into two or more components, which may or may not be charged, as a consequence of absorption of a photon (interaction with electromagnetic radiation).

Photolysis: The process by which a chemical species undergoes a chemical change as the result of the absorption of a photon of light energy.

Piezoelectric [Wikipeidia definition]: Piezoelectricity is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat.

Planck’s Law: The distribution law of photon energies for radiation in equilibrium with matter at absolute temperature T: where E is photon energy, c is the free-space speed of light, h is Planck's constant, and k is Boltzmann's constant.

The integral of this function between any two energies is the contribution to the total irradiance from all photons with energies in this range. This law is mathematically similar and physically analogous to the Maxwell–Boltzmann distribution for the kinetic energies of (ideal) gas molecules in equilibrium. One of the salient differences between these two distributions is that the number of gas molecules within a container is conserved whereas the number of photons is not, increasing with the absolute temperature of the container. Within an enclosure the walls of which are opaque (but not necessarily black) and at temperature T, the distribution of photon energies is given by Planck's radiation law.

Polydisperse: Of, relating to, or characterized by or as particles of varied sizes in the dispersed phase of a disperse system.

Precipitation Amount: Depth to which precipitation in liquid form would cover a horizontal projection of the earth's surface, in the absence of abstractions.

It is typically expressed in millimeters, equivalent to liters per square meter. Snowfall is also measured by the depth of fresh snow covering a horizontal surface.

Precipitation Intensity: The rate of precipitation, usually expressed in millimeters or inches per hour.

Precision: The quality of being exactly defined.

Sometimes indicated by the minimum number of significant digits required for an adequate representation of a quantity. Not the same as accuracy but often confused as such. A measurement having small random error is said to have high precision; a measurement having small systematic error or bias is said to have high accuracy.

Pressure: A type of stress characterized by uniformity in all directions.

As a measurable on a surface, the net force per unit area normal to that surface exerted by molecules rebounding from it. In dynamics, it is that part of the stress tensor that is independent of viscosity and depends only upon the molecular motion appropriate to the local temperature and density. It is the negative of the mean of the three normal stresses. The concept of pressure as employed in thermodynamics is based upon an equilibrium system, where tangential forces vanish and normal forces are equal.

In meteorology, commonly used for atmospheric pressure.

In mechanics, same as stress.

Principle of Superposition: Any of several physical laws that the resultant of similar vector quantities at a point is a function of the sum of the individual quantities, especially the law that the displacement at a point in a medium undergoing simple harmonic motion is equal to the sum of the displacements of each individual wave.

Provenance: The place of origin or earliest known history of something.

Psychrometers: An instrument used to measure humidity. It consists of two thermometers exposed side by side, one of which (the dry bulb) is an ordinary glass thermometer, while the other (the wet bulb) has its bulb covered with a jacket of clean muslin that is saturated with distilled water prior to an observation.
The temperature measured by the wet-bulb thermometer is generally lower (due to evaporation of water from the wet bulb) than that measured by the dry bulb. The difference in the temperatures is a measure of the humidity of the air; the lower the ambient humidity, the greater the rate of evaporation and, consequently, the greater the depression of the wet-bulb temperature. The size of the wet-bulb depression is related to the ambient humidity by the psychrometric formula.

Pyroelectric: A detector (based on the temperature dependence of ferroelectricity in some crystals) which produces an electrical signal proportional to the energy flux on the collector surface.

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Radiance: A radiometric term for the rate at which radiant energy in a set of directions confined to a unit solid angle around a particular direction is transferred across unit area of a surface (real or imaginary) projected onto this direction.

Unlike irradiance, radiance is a property solely of a radiation field, not of the orientation of the surface. The SI units of radiance are W m^-2 sr^-1. In general, radiance depends on time, position, and direction as well as frequency (monochromatic or spectral radiance) or range of frequencies. Irradiance for any surface is the integral of radiance over a hemisphere of directions above or below that surface. The photometric equivalent of radiance is luminance, obtained by integrating spectral radiance weighted by luminous efficiency over the visible spectrum.

Radiant Flux: (Sometimes called hemispheric flux.) Radiant energy per unit time passing some specified area from one side; units are watts (W).

Radiosonde: An expendable meteorological instrument package, often borne aloft by a free-flight balloon, that measures, from the surface to the stratosphere, the vertical profiles of atmospheric variables and transmits the data via radio to a ground receiving system.

Raman Scattering: When light encounters molecules in the air, the predominant mode of scattering is elastic scattering, called Rayleigh scattering. This scattering is responsible for the blue color of the sky; it increases with the fourth power of the frequency and is more effective at short wavelengths. It is also possible for the incident photons to interact with the molecules in such a way that energy is either gained or lost so that the scattered photons are shifted in frequency. Such inelastic scattering is called Raman scattering.

Random Errors: The inherent imprecision of a given process of measurement; the unpredictable component of repeated independent measurements on the same object under sensibly uniform conditions.

Range: The difference between the maximum and minimum of a given set of numbers; in a periodic process it is twice the amplitude, that is, the wave height.

Rawinsonde: An upper-air sounding that includes determination of wind speeds and directions.

Historically, wind data were obtained by tracking a balloon-borne radiosonde with a radio direction finder. Contemporary methods include measuring position or radiosonde velocity from GPS or Loran radio navigation signals.

Recovery Factor: The ratio of the actual temperature difference across the boundary layer at any longitudinal station to the temperature rise that would result if the fluid were brought to rest adiabatically from the velocity existing just outside the boundary layer at that station.

Recovery Temperature: The adiabatic value of local air temperature on each portion of the aircraft surface due to incomplete recovery of the kinetic energy.

Reliably: In a consistently good or accurate way.

Remote Sensing: A method of obtaining information about properties of an object without coming into physical contact with that object.

Repeatability: The 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).

Repeatedly: Done, made, or said again and again.

Representativeness: Property of an air mass that is typical of the air mass as a whole and thus may be used in air mass analysis.

Resistance: In general, any force that tends to oppose motion.

Same as drag.

In electricity, the opposition offered by a substance to the passage of an electric current; the reciprocal of conductance.

By virtue of the resistance, a portion of the electrical energy is converted into heat.

Resistor: A device designed to introduce resistance into an electric circuit.

Resolution: The smallest measurable change in a quantity.

Response Time: In general, the time interval required for a system exposed to some discontinuous change of environment to undergo the fraction (1 - e^-1), or about 63%, of the total change of state that it would exhibit after an infinitely long time.

Responsivity: Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, responsivity measures the electrical output per optical input. The responsivity of a photodetector is usually expressed in units of either amperes or volts per watt of incident radiant power.

Richardson Number: The dimensionless ratio of buoyant suppression of turbulence to shear generation of turbulence. It is used as a dynamic stability measure to determine if turbulence will exist.

Root-mean-square (RMS) Error: (Abbreviated rmse; also called standard error of estimate, residual standard deviation.) The positive square root of the mean-square error.

It is equal to the standard error only when the mean error is zero.

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Saturation Vapor Pressure: The vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase.

Second-Order System: Has two predominant energy stores.

Selectivity: When an analytical method is selective for the analyte, analyzing samples is a relatively simple task. For example, a quantitative analysis for glucose in honey is relatively easy to accomplish if the method is selective for glucose, even in the presence of other reducing sugars, such as fructose. Unfortunately, few analytical methods are selective toward a single species.

Sensing Element: (Or sensor.) The component of an instrument that converts an input signal into a quantity that is measured by another part of the instrument.

Sensitivity: The ratio of the output of an instrument to the input.

Signal-to-Noise Ratio: (Abbreviated SNR.) A ratio that measures the information content of a signal, usually defined as the ratio of the power of the signal unaffected by noise to the power of the noise.

Site Selection: Meteorological observing stations are designed so that representative measurements (or observations) can be taken according to the type of station involved.

Span, Brock and Richardson (2001): The algebraic difference between the upper and lower range limits.

Solid angle: In geometry, a solid angle (symbol: Ω) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The point from which the object is viewed is called the apex of the solid angle, and the object is said to subtend its solid angle from that point.

Stability: The characteristic of a system if sufficiently small disturbances have only small effects, either decreasing in amplitude or oscillating periodically; it is asymptotically stable if the effect of small disturbances vanishes for long time periods.

A system that is not stable is referred to as unstable, for which small disturbances may lead to large effects. Some authors also distinguish a neutral or marginally stable case, in which disturbances do not vanish, but also do not grow without bound. Classically, stability was defined only with respect to systems in equilibrium. More recently it has been extended to apply to evolving systems, for which an unstable disturbance leads to an evolution that becomes uncorrelated with the undisturbed evolution. From this standpoint stability and predictability can be equated.

Standard Deviation: The positive square root σ of the variance σ².

This is a measure of the scatter or spread in a series of observations.

Standard Error in the Mean: The standard error of the mean is the standard deviation of the sampling distribution of the mean. The formula for the standard error of the mean in a population is:

where σ is the standard deviation and N is the sample size. When computed in a sample, the estimate of the standard error of the mean is:

Standard Uncertainty: Uncertainty of the result of a measurement expressed as a standard deviation.

State Variables: Those variables that uniquely define the current state of a system and whose integral around a closed path is identically zero.

Static: Pertaining to or characterized by a fixed or stationary condition.

Static Sensitivity, Brock and Richardson (2001): Slope of the transfer equation or curve. It is a measure of the sensitivity of a sensor in the static sense. Also the derivative of the raw output with respect to the input.

Stefan-Boltzmann Law: One of the radiation laws, which states that the amount of energy radiated per unit time from a unit surface area of an ideal blackbody is proportional to the fourth power of the absolute temperature of the blackbody.

The law is written where E is the emittance of the blackbody, σ the Stefan–Boltzmann constant, and T the absolute temperature of the blackbody. This law was established experimentally by Stefan and was given theoretical support by thermodynamic reasoning due to Boltzmann. This law may be deduced by integrating Planck's law over the entire frequency spectrum.

Stokes Number: The Stokes number (Stk), named after George Gabriel Stokes, is a dimensionless number characterizing the behavior of particles suspended in a fluid flow. The Stokes number is defined as the ratio of the characteristic time of a particle (or droplet) to a characteristic time of the flow or of an obstacle, or where t₀ is the relaxation time of the particle (the time constant in the exponential decay of the particle velocity due to drag), u₀ is the fluid velocity of the flow well away from the obstacle and l₀ is the characteristic dimension of the obstacle (typically its diameter). A particle with a low Stokes number follows fluid streamlines (perfect advection), while a particle with a large Stokes number is dominated by its inertia and continues along its initial trajectory.

Supercooled Liquid: Liquid water at temperatures below the nominal freezing point of pure water [~0°C (32°F) for atmospheric-relevant pressure]. This usually refers to cloud droplets, which often remain liquid for a long time at temperatures below 0°C (32°F). The actual freezing temperature of a supercooled cloud droplet depends on its size and composition and may be as low as several tens of degrees below 0°C (32°F).

Supersaturation: The condition with the actual water vapor pressure e is greater than the saturation water vapor pressure e_s(T) at a given temperature. High values of supersaturation are rarely observed in clouds because of the presence of condensation nuclei, which enhance the phase change to liquid water.

Systematic Error: The part of the total error that is not random and so cannot be reduced by repeated measurements. It is the difference between the value of a measurand and the mean that would result from an infinite number of measurements of that measurand carried out under consistent conditions. Systematic error usually arises from instrumental bias that can be corrected by calibration.

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Temperature: Temperature is a measurable characteristic of objects or substances that reflects our sense of hot vs. cold. The most important property of temperature is that energy will move from a hotter to a colder object if the objects are in thermal contact, and therefore equality of temperature is a requirement for two objects to be in thermal equilibrium. Temperature is an intensive state variable for a gas like air. That is, it is a variable that is not proportional to the quantity of substance and can be one of a small set of such variables that provide complete specification of the state of the gas. Temperature is best understood in quantitative terms in connection with heat capacity, because (in proper units) the change in temperature will be equal to the energy that flows to or from the object divided by its heat capacity. The preceding requirements have led to consistent scales for temperature (e.g., the Kelvin scale) and units for heat capacity (energy per unit change in temperature). For an ideal gas (closely approximated by air), the temperature is the ratio of internal energy to the heat capacity at constant volume. The mean kinetic energy per molecule in a gas at equilibrium is proportional to the temperature if temperature is expressed using an absolute scale like the Kelvin scale.

Tethersonde: A radiosonde attached to a fixed or tethered balloon.
The balloon is usually larger than a balloon used for upper-air soundings, and the tether usually limits the sounding to the boundary layer. The radiosonde is typically moved up and down the tether to get multiple, high-resolution profiles of the boundary layer.

Theodolite: An optical instrument, similar to a surveyors's transit telescope, used to visually track a radiosonde balloon and determine its azimuth and elevation angles while in flight.

Thermal Inertia: The degree of slowness with which the temperature of a body approaches that of its surroundings and which is dependent upon its absorptivity, its specific heat, its thermal conductivity, its dimensions, and other factors.

Thermistor: A device with electrical resistance that varies markedly and monotonically and that possesses a negative temperature coefficient of resistivity.

The thermistors commonly used in meteorology are composed of solid semiconducting materials with resistance that decreases 4% per °C. They are constructed in a variety of sizes and may be obtained with thermal time constants of a millisecond or less. Meteorological applications include thermometers, anemometers, and bolometers.

Thermocouple: A temperature-sensing element that converts thermal energy directly into electrical energy.

In its basic form it consists of two dissimilar metallic electrical conductors connected in a closed loop. Each junction forms a thermocouple. One thermocouple is maintained at a known temperature (usually 0°C or a measured temperature) and the other thermocouple is used to measure the unknown temperature. The signal voltage is a function of the temperature, and the smooth curve can be handled with a simple linear fit over a moderate temperature range. Different materials have different curves. Popular thermocouples (and change in voltage per °C) include iron- constantan (50 mv per °C), copper-constantan (38 mv per °C), and various platinum alloys. Thermocouples are also important in home furnaces to detect the pilot light or that the fuel has ignited. A chain of thermocouples, called a thermopile, can be used as a power supply if a source of heat and cold is available.

Thermopile: A transducer for converting thermal energy directly into electrical energy.

It is composed of pairs of thermocouples that are connected either in series or in parallel. The output voltage of N pairs of series-connected thermocouples is N times the voltage developed by a single pair, while the current developed by N pairs of parallel connected thermocouples is N times the current developed by a single pair. Thermopiles are used in thermoelectric radiation instruments when the output of a single pair of thermocouples is not large enough.

Threshold, Brock and Richardson (2001): Instruments with internal friction require an input to increase from zero to some finite value, called the threshold, before responding.

Time Constant: (Also called lag coefficient.) Generally, the time required for an instrument to indicate a given percentage of the final reading resulting from an input signal; the relaxation time of an instrument.

In the general case for instruments such as thermometers, with responses exponential in character to step changes in an applied signal, the time constant is equal to the time required for the instrument to indicate 63.2% of the total change, that is, the time to respond to all but 1/e of the original signal change.

Total Retention Time: The volume of mobile phase entering the column between sample injection and the emergence of the peak maximum of the sample component of interest, or the corresponding time. It includes the hold-up volume (time):

where F_c is the mobile phase flow rate at column temperature.

Trace

In general, an unmeasurable (less than 0.01 in.) quantity of precipitation.
An insignificantly small quantity.
The record made by any self-registering instrument.

Thus, one may speak of the barograph trace, the hygrothermograph trace, etc

Traceable Standard: Property of a measurement result whereby the result can be related to a stated reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.

Transducer: A device for converting energy from one form to another.
For example, a thermocouple transduces heat energy into electrical energy.

Transfer Curve: The relationship between the input and the output of a system, subsystem, or equipment in terms of the transfer characteristics.

Transfer-Function Diagram: A diagram that represents graphically the interconnection relationships between elements of an electronic system.

Transient Event: An impermanent time-dependent response to a change in external forcing.

Type A Evaluation: Uncertainty estimates using statistics (usually from repeated readings)

Type B Evaluation: Uncertainty estimates from any other information. This could be information from past experience of the measurements, from calibration certificates, manufacturer’s specifications, from calculations, from published information, and from common sense.

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Uncertainty: The standard deviation of a sufficiently large number of measurements of the same quantity by the same instrument or method.

Hence, the noncorrectable part of the inaccuracy of an instrument; it represents the limit of measurement precision. The uncertainty of an instrument is caused by the unpredictable effects upon its performance of such factors as friction, backlash, and electronic noise.

Universal Gas Constant: The constant of proportionality R needed by the ideal-gas law PV = nRT. (approx. 8.314 J mol^-1 K^-1).

Upper Limit of Measurement: The highest value of the air quality characteristic which can be measured by an instrument; its variations, caused for example by instability, are expected to lie within specified limits. The difference between the lower detection limit and the upper limit of measurement constitutes the dynamic range of the instrument.

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Virtual Temperature: (Also called density temperature.) The virtual temperature T_v = T(1 + r_v/ ε)/(1 + r_v), where r_v is the mixing ratio and ε is the ratio of the gas constants of air and water vapor, ≈ 0.622.

The virtual temperature allows the use of the dry-air equation of state for moist air, except with T replaced by T_v. Hence the virtual temperature is the temperature that dry air would have if its pressure and density were equal to those of a given sample of moist air. For typical observed values of r_v, the virtual temperature may be approximated by T_v = (1 + 0.61 r_v) T. Some authors incorporate the density increment due to liquid or solid water into virtual temperature, in which case the definition becomes T_v = T(1 + r_v/ε)/(1 + r_v + rl) ≈ T(1 + 0.61r_v - r_l), where r_l is the liquid or liquid plus solid water mixing ratio.

Voltage: Electromotive force or potential difference expressed in volts.

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Water Vapor: Water substance in vapor form; one of the most important of all constituents of the atmosphere.
Its amount varies widely in space and time due to the great variety of both "sources" of evaporation and "sinks" of condensation that provide active motivation to the hydrologic cycle. Approximately half of all of the atmospheric water vapor is found below 2-km altitude, and only a minute fraction of the total occurs above the tropopause. Water vapor is important not only as the raw material for cloud and rain and snow, but also as a vehicle for the transport of energy (latent heat) and as a regulator of planetary temperatures through absorption and emission of radiation, most significantly in the thermal infrared (the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as absolute humidity, mixing ratio, dewpoint, relative humidity, specific humidity, and vapor pressure.

Wetting [Wikipedia definition]: Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wetting deals with the three phases of materials: gas, liquid, and solid. It is now a center of attention in nanotechnology and nanoscience studies due to the advent of many nanomaterials in the past two decades.

Wien Displacement Law: A radiation law that is used to relate the wavelength of maximum emission from a blackbody inversely to its absolute temperature.

It is expressed as: where λ_m is the wavelength of maximum emittance in microns, and T is the blackbody temperature in Kelvin.

Wind Field: In its restricted physical sense, any physical quantity that varies in three-dimensional space (and possibly time), usually continuously except possibly on surfaces or curves.
Field quantities often satisfy partial differential equations. An example of a scalar field is the temperature T(x, y, z, t) at time t at each point (x, y, z) of a solid body; an example of a vector field is the (local) velocity field v(x, y, z, t) in a fluid, the separate parts of which are in motion relative to each other. The continuity of these fields is a mathematical fiction, obtained by averaging over volumes containing many atoms or molecules but still small on a macroscopic scale.

Wind Rose: Any one of a class of diagrams designed to show the distribution of wind direction experienced at a given location over a considerable period; it thus shows the prevailing wind direction.
The most common form consists of a circle from which eight or sixteen lines emanate, one for each compass point. The length of each line is proportional to the frequency of wind from that direction, and the frequency of calm conditions is entered in the center. Many variations exist; some indicate the range of wind speeds from each direction; some relate wind directions with other weather occurrences.

Wind Profiler: (Also called wind profiler radar, wind profiling radar.) A radar that is used to measure vertical profiles of the wind.

In general the term is applied to Doppler radars operating in the VHF–UHF band (30 MHz– 3 GHz) that determine the wind by measuring the line-of-sight Doppler shift of scattered signals (Bragg scattering) from refractive index fluctuations caused by turbulence. The turbulent scattering structures are assumed to be moving with the same average velocity as the wind. The three-dimensional wind vector is determined by using the beam swinging technique of pointing the radar beam in at least three different directions. Another type of wind profiler radar that operates in the VHF–UHF band uses a technique called spaced antenna drift (SAD). SAD radars use a single vertical-beam transmitting antenna and three or more horizontally spaced, vertical-beam receiving antennas. The horizontal wind is determined from the cross correlation of the received echoes and the vertical wind is determined from the Doppler shift of the echoes. In addition to measuring the wind vector, wind profiler radars can also determine several other atmospheric quantities from the power, mean Doppler shift, and Doppler spectral width of the returned signal. These quantities include the strength of turbulence (parameterized by the refractive index structure constant C_n²), the eddy dissipation rate, atmospheric stability, momentum flux, virtual temperature and heat flux (using the RASS technique), and precipitation rates and drop- size distributions (from scatter from hydrometeors). Doppler lidars and acoustic sounders (sodars) can also be used as wind profilers. The scatterers for lidars are aerosols (for wavelengths around 10 μm) and molecules (for wavelengths less than 1 μm). Acoustic refractive index fluctuations caused by turbulence provide the scattering mechanism for sodars.

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