W

Source edition 1965. This document is under construction. Formulas, symbols, and tables are being worked. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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walk-around bottle: A personal supply of oxygen for the use of crewmembers when temporarily disconnected from the craft's system.
walled plain: See lunar crater, note.
wander: Short for apparent wander.
waning moon: The moon between full and new when its visible part is decreasing. See phases of the moon.
warhead: Originally the part of a missile carrying the explosive, chemical, or other charge intended to damage the enemy. By extension, the term is sometimes used as synonymous with payload or nose cone .
warmup time: The time interval required for a gyro to reach specified performance from the instant that it is energized.
water: Dihydrogen oxide (molecular formula H20). The word is used ambiguously to refer to the chemical compound in general and to its liquid phase; when the former is meant, the term water substance is often used.; Water is distinguished from other common terrestrial substances in existing in all three phases at atmospheric temperatures and pressures (see ice, water vapor). The phase changes, are of great significance in many geophysical processes. The same is true of the large specific heat of liquid water and ice relative to both land surface and atmosphere. Water's complex absorption spectrum gives rise to the greenhouse effect.
waterfall effect = Lenard effect.
water-flow pyrheliometer: An absolute pyrheliometer, developed by C. G. Abbot, in which the radiation-sensing element is a blackened water calorimeter.; It consists of a cylinder blackened on the interior and surrounded by a special chamber through which water flows at a constant rate. The temperatures of the incoming and outgoing water, which are monitored continuously by thermometers, are used to compute the intensity of the radiation. This instrument is used by the Smithsonian Institution as its standard instrument.
water substance: See water.
water suit: A g-suit in which the fluid used in the interlining is a liquid, thereby automatically approximating the required hydrostatic pressure gradient under acceleration.
water vapor: Water (H2O) in gaseous form. Also called aqueous vapor . See vapor.; The amount of water vapor present in a given gas sample may be expressed in a number of ways. See absolute humidity, mixing ratio, dewpoint, relative humidity, specific humidity, vapor pressure.
water-vapor absorption: The absorption of certain wavelengths of infrared radiation by atmospheric water vapor. See absorption band.; The water-vapor absorption spectrum is composed of bands near 1.4, 1.8, 2.7, and 6.3 microns and a series of bands beginning at 11 microns and growing stronger with increasing wavelength.
watt (abbr w, W): The unit of power in the MKSA system; that power which produces energy at the rate of 1 joule per second.
wave: A disturbance which is propagated in a medium in such a manner that at any point in the medium the quantity serving as measure of disturbance is a function of the time, while at any instant the displacement at a point is a function of the position of the point.; Any physical quantity that has the same relationship to some independent variable (usually time) that a propagated disturbance has, at a particular instant, with respect to space, may be called a wave.
wave equation: The partial differential equation of the form where is usually a function of the position and time coordinates; ² is the Laplacian operator; t is the time; and C² is a constant. Also called equation of wave motion . See wave, gravity wave.; The general solution to this equation is any function defined over a plane, the phase front, moving perpendicular to itself at the speed c.
wave filter: A transducer for separating waves on the basis of their frequency. It introduces relatively small loss to waves in one or more frequency bands and relatively large loss to waves of other frequencies. Also called filter .
waveform: The graphical representation of a wave, showing variation of amplitude with time.
wave front = phase front.
waveguide: A system of boundaries capable of guiding wave.
wave interference: The phenomenon which results when waves of the same or nearly the same frequency are superposed; characterized by a spatial or temporal distribution of amplitude of some specified characteristic differing from that of the individual superposed waves. Also called interference .
wavelength: In general, the mean distance between maximums (or minimums) of a roughly periodic pattern. Specifically, the least distance between particles moving in the same phase of oscillation in a wave disturbance.; The wavelength is measured along the direction of propagation of the wave, usually from the midpoint of a crest (or trough) to the midpoint of the next adjoining crest (or trough). It is related to frequency f and phase speed v by λ = v / f , where λ is wavelength. The reciprocal of wavelength is the wave number.
wavelets: See Huygens wavelets.
wave motion: The oscillatory motion of the particles of a medium caused by the passage of a wave, produced by forces external to the medium, but propagated through the medium by internal forces. Wave motion per se involves no net translation of the medium.; Various types of oscillation are found in natural wave motions. Among the simplest are the linear oscillation parallel to the direction of propagation of a longitudinal wave, the linear oscillation perpendicular to the direction of propagation of a transverse wave, and the orbital motion produced by the passage of a progressive gravity wave.
wave number (symbol ): The reciprocal of wavelength; the number of waves per unit distance in the direction of propagation; or, sometimes 2π times this quantity.; In spectroscopy, wave number is usually expressed in reciprocal centimeters, as 100,000 cm^-1 (100,000 per centimeter).
wave of translation: A wave in which the individual particles of the medium are shifted in the direction of wave travel, as ocean waves in shoal waters; in contrast with an oscillatory wave, in which only the form advances, the individual particles moving in closed orbits, as ocean waves in deep water.
wave speed = phase velocity.
wave theory of light: See electromagnetic radiation.
wave train: A limited series of waves caused by a periodic disturbance of short duration, e.g., the radiofrequency waves in a single pulse, or a succession of pulses themselves.
wave velocity = phase velocity.
web: The wall of a grain or propellant with an internal cavity.
weber (abbr wb): The unit of magnetic flux; the magnetic flux which, linking a circuit of one turn, produces in it an electromotive force of 1 volt as it is reduced to zero at a uniform rate in 1 second.
Weber-Fechner law: An approximate psychophysical law relating the degree of response or sensation of a sense organ and the intensity of the stimulus. The law asserts that equal increments of sensation are associated with equal increments of the logarithm of the stimulus, or that the just noticeable difference in any sensation results from a change in the stimulus which bears a constant ratio to the value of the stimulus. Also called Weber law .; The Weber-Fechner law is applied to the detection of contrast in the problem of visual range, as well as to many other psychophysical problems.
Weber law = Weber-Fechner law.
weight (symbol w ): 1. The force with which a body is attracted toward the earth.; 2. The product of the mass of a body and the acceleration acting on a body.; In a dynamic situation, the weight can be a multiple of that under resting conditions. Weight also varies on other planets in accordance with their gravity.
weight flow rate (symbol ): Mass flow rate multiplied by gravity, or = ( dm/dt ) g = mg Where m is mass and t is time; usually expressed in pounds per second.
weightlessness: 1. A condition in which no acceleration, whether of gravity or other force, can be detected by an observer within the system in question.; Any object failing freely in a vacuum is weightless, thus an unaccelerated satellite orbiting the earth is weightless although gravity affects its orbit. Weightlessness can be produced within the atmosphere in aircraft flying a parabolic flightpath.; 2. A condition in which gravitational and other external forces acting on a body produce no stress, either internal or external, in the body.
welding: Joining two or more pieces of metal by applying heat, pressure, or both, with or without filler material to produce a localized union through fusion or recrystallization across the interface.; The thickness of the filler material is much greater than the capillary dimensions encountered in brazing.
wet: To come in contact with, and flow across (a surface, body, or area) - said of air or other fluid.
wet emplacement: A launch emplacement that provides a deluge of water for cooling the flame bucket, the rocket engines, and other equipment during the launch of a missile. See flame deflector, dry emplacement.
wet-fuel rocket = liquid rocket.
whistler: A radiofrequency electromagnetic signal generated by some lightning discharges.; This signal apparently propagates along a geomagnetic line of force and often bounces several times between the Northern and Southern Hemispheres. Its name derives from the sound heard on radio receivers.
whistling meteor: Name applied to a radio meteor when a detection system is used in which the presence of the meteor is indicated by a rapidly changing audiofrequency radio signal.; The maximum reflection of a radio signal from a radio meteor occurs when the ion column is perpendicular to the line from the column to the transmitter-receiver. During the approach of the meteor to this position, the Doppler effect causes a change in the frequency of the reflected signal. When the reflected signal frequency is then combined with the transmitter frequency, the difference between the transmitted and reflected frequencies produces an audiofrequency beat. The audiofrequency beat, when amplified and fed to a loudspeaker, allows the meteors to be heard as a high-pitched whistle which rapidly falls to zero frequency as the meteor trail becomes normal to the line of sight.
white body: A hypothetical body whose surface absorbs no electromagnetic radiation of any wavelength, i.e., one which exhibits zero absorptivity for all wavelengths; an idealization exactly opposite to that of the black body. See gray body.; In nature, no true white bodies are known. Most white pigments possessing high reflectivity for visible radiation are fairly good absorbers in the infrared; hence, they are not white bodies in the sense of the radiation theory. However, the term white body is used for physical objects with respect to a particular wavelength interval.
white noise: A sound or electromagnetic wave whose spectrum is continuous and uniform as a function of frequency.
white room: A clean and dust-free room used for assembly and repair of precise mechanisms such as gyros.
Wien displacement constant: See Wien law.
Wien displacement law = Wien law.
Wien distribution law: A relation, derived on purely thermodynamic reasoning by Wien, between the monochromatic emittance of an ideal black body and that body's temperature, J_λ/T⁵ = f(λ, T) where J_λ is the monochromatic emittance (emissive power) of a black body at wavelength λ and absolute temperature T, and f(λ, T) is a function which cannot be determined purely on classical thermodynamic grounds. Compare Wien law.
Wien law: One of the radiation laws which states that the wavelength of maximum radiation intensity for a black body is inversely proportional to the absolute temperature of the radiating black body: λ_m = b/T where λ_m is the wavelength of maximum intensity; b is a constant; and T is the absolute temperature. The Wien displacement constant b is equal to 0.28978 centimeter-degree. Also called Wien displacement law .; This law, established experimentally by Wien in 1896, describes the manner in which the wavelength of maximum radiation shifts toward shorter values as the temperature of a radiator rises. It is to be distinguished from Wien distribution law which describes the variation with temperature of the intensity of emission at any wavelength. Wien displacement law is used to compute the color temperature of a radiator by insertion of its wavelength of peak intensity into the above equation to compute T.
wind axis: Any one of a system of mutually perpendicular reference axis established with respect to the undisturbed wind direction about an aircraft or similar body. See axis, sense 2.
window: 1. Any device introduced into the atmosphere for producing an appreciable radar echo, usually for tracking some airborne device or as a tracer of wind.; 2. A World War II code name for a type of radar-jamming device employed to confuse the operators of enemy radars (also referred to by the code names of rope, chaff , and clutter ).; One type of window consists of packages containing thousands of small strips of paperbacked tinfoil which may be dropped from aircraft and balloons, ejected from rockets, and carried within balloons. The packages burst open upon ejection, scattering the tinfoil widely, producing a radar echo which looks like a small shower or a tight formation of aircraft on plan-position-indicator scopes.; 3. Any gap in a linear continuum, as atmospheric windows , ranges of wavelengths in the electromagnetic spectrum to which the atmosphere is transparent, or firing windows , intervals of time during which conditions are favorable for launching a spacecraft on a specific mission.
wind shear: See barotropic model.
wind tunnel: A tubelike structure or passage, sometimes continuous, together with its adjuncts, in which a high-speed movement of air or other gas is produced, as by a fan, and within which objects such as engines or aircraft, airfoils, rockets (or models of these objects), etc., are placed to investigate the airflow about them and the aerodynamic forces acting upon them.; Tunnels are designated by the means used to produce the gas flow, as hot shot tunnel, arc tunnel, blow down tunnel; by the speed range, as supersonic tunnel, hypersonic tunnel; or by the medium used, as plasma tunnel, light gas tunnel.
wind-tunnel balance: A device or apparatus that measures the aerodynamic forces and moments acting upon a body tested in a wind tunnel.
winter solstice: 1. That point on the ecliptic occupied by the sum at maximum southerly declination. Sometimes called December solstice, first point of Capricornus .; 2. That instant at which the sun reaches the point of maximum southerly declination, about December 22.
wireless: Sometimes used as the equivalent of radio, particularly in British terminology.
wire link telemetry: Telemetry in which no radio link is used. Also called hard wire telemetry .
Wolf number = relative sunspot number.
Wolf-Wolfer-Wolfest number = relative sunspot number.
word: In electronic computers, an ordered set of characters which is the normal unit in which information may be stored, transmitted, or operated upon within a computer.
word rate: In computer operations, the frequency derived from the elapsed period between the beginning of transmission of one word and the beginning of transmission of the next word.
work (symbol W ): Energy resulting from the motion of a system against a force and existing only during the process of energy conversion.
work function: The energy required for an electron to escape a solid surface. See Helmholtz function.; In ion engines, the work function of the ionizer must be greater than the ionization potential of the neutral atoms in the propellant gas.
working fluid: A fluid (gas or liquid) used as the medium for the transfer of energy from one part of a system to another part.
World Geographic Reference System: A geographic reference system for the world, used in the Air Force for aircraft position reports and target designation, and for the control and direction of air units engaged in air defense, air-sea rescue, and tactical air operations.; The short title for this system is georef.
write: In computer terminology, record.

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