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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • radar - (Coined word for radio detection and ranging.) An electronic instrument used for the detection and ranging of distant objects of such composition that they scatter or reflect radio energy. A radar consists of a transmitter, receiver, antenna, display, and associated equipment for control and signal processing. The most common radars are monostatic radars, which use the same antenna for both transmission and reception. These radars depend on backscattering to produce a detectable echo from a target. Bistatic radars have the transmitter and its antenna at one location and the receiver and its antenna at a remote location. These radars depend upon forward scattering to produce a detectable signal. Radio energy emitted by the transmitter and focused by the antenna of a monostatic radar propagates outward through the atmosphere in a narrow beam. Objects lying in the path of the beam reflect, scatter, and absorb the energy. A small portion of the reflected and scattered energy, called the target signal, travels back along the same path through the atmosphere and is intercepted by the receiving antenna. The time delay between the transmitted signal and the target signal is used to determine the distance or slant range of the target from the radar. The direction in which the focused beam is pointing at the instant the target signal is received (i.e., the azimuth and elevation angles of the antenna) determine the direction and height of the target. This information is presented visually as echoes on different types of radar displays. Because hydrometeors scatter radio energy, weather radars, operating in certain radar frequency bands, can detect the presence of precipitation and other weather phenomena at distances up to several hundred kilometers from the radar, depending upon meteorological conditions and the type of radar. MST radars and wind profilers, which operate at longer wavelengths than weather radars, are able to detect echoes from optically clear air that are caused by spatial fluctuations of refractivity. Additional information provided by a radar about a target may include the radial velocity or rate of change of range, as measured by a Doppler radar, or the depolarizing characteristics of the target, as measured by a polarimetric radar.

  • radiation - 1. The process by which electromagnetic radiation is propagated through free space. The propagation takes place at the speed of light (3.00 x 108 m s-1 in vacuum) by way of joint (orthogonal) oscillations in the electric and magnetic fields. This process is to be distinguished from other forms of energy transfer such as conduction and convection. 2. Propagation of energy by any physical quantity governed by a wave equation. 3. See alpha ray, beta ray.

  • radiational cooling - In meteorology, the result of radiative cooling of the earth's surface and adjacent air. Radiational cooling occurs, as is typical on calm, clear nights, whenever the longwave emission from the surface is not balanced by significant amounts of absorbed shortwave radiation or downwelling longwave from the atmosphere above the surface, and there are no nonradiative sources of sufficient energy to make up the difference.

  • rain - Precipitation in the form of liquid water drops that have diameters greater than 0.5 mm, or, if widely scattered, the drops may be smaller. The only other form of liquid precipitation, drizzle, is to be distinguished from rain in that drizzle drops are generally less than 0.5 mm in diameter, are very much more numerous, and reduce visibility much more than does light rain. For observing purposes, the intensity of rainfall at any given time and place may be classified as 1) "light," the rate of fall varying between a trace and 0.25 cm (0.10 in.) per hour, the maximum rate of fall being no more than 0.025 cm (0.01 in.) in six minutes; 2) "moderate," from 0.26 to 0.76 cm (0.11 to 0.30 in.) per hour, the maximum rate of fall being no more than 0.076 cm (0.03 in.) in six minutes; 3) "heavy," over 0.76 cm (0.30 in.) per hour or more than 0.076 cm (0.03 in.) in six minutes. When rain gauge measurements are not readily available to determine the rainfall intensity, estimates may be made according to a descriptive system set forth in observing manuals.

  • rainband - The complete cloud and precipitation structure associated with an area of rainfall sufficiently elongated that an orientation can be assigned. The band is either nonconvective or only weakly convective.

  • rainfall - The amount of precipitation of any type (including the liquid equivalent of frozen hydrometeors); usually taken as that amount measured by means of a rain gauge (thus a small, varying amount of direct condensation is included). A more accurate term would be precipitation or precipitation amount. However, the broad use of "rainfall" is firmly established in meteorology, especially in hydrologic and climatological literature. Its best utilization would confine it to liquid precipitation, and so would provide a distinction between precipitation immediately accessible to soil and streams and that delayed in storage as snow or ice on the earth's surface.

  • range - 1. 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. 2. The distance between two objects, usually an observation point and an object under observation. See slant range. 3. A maximum distance attributable to some process, as in visual range or the range of an aircraft. 4. The difference between high and low water in a tidal cycle. Ranges greater than 4 m are sometimes termed macrotidal and those less than 2 m are termed microtidal. Intermediate ranges are termed mesotidal. 5. In radar, lidar, and sodar, the radial distance measured outward from the location of the transmitter; ordinarily the distance to a target.

  • RASS - Abbreviation for radio acoustic sounding system.

  • raw - Colloquially descriptive of uncomfortably cold weather, usually meaning cold and damp, but sometimes cold and windy.

  • rawin - A method of winds-aloft observation; that is, the determination of wind speeds and directions in the atmosphere above a station. It is accomplished by tracking a balloon-borne radar target, responder, or radiosonde transmitter with either radar or a radio direction finder. With a radio direction finder, the height data must be supplied by other means, normally by concurrent radiosonde observation. With radar, if height data are not otherwise supplied, the slant range must be recorded in addition to the angles of elevation and azimuth.

  • ray - An imaginary bundle of propagating electromagnetic or acoustic energy, the lateral dimensions of which are negligible. It is impossible to isolate a ray. Nevertheless, rays are useful conceptual devices if used with a knowledge of their limitations. For example, the rainbow can be described by imagining sunlight incident on a raindrop to be subdivided into many rays, each of which obeys the laws of (specular) reflection and refraction. Because rays do not exist, ray optics (or geometrical optics) is an approximation.

  • reach - 1. A specified portion of a stream channel, commonly taken between two stream gauging stations, but may be taken between any two specified endpoints. 2. In hydraulic and sediment transportation calculations, a reach of stream is a specified length of a stream channel used for computational purposes such as flood routing.

  • reduction - In general, the transformation of data from a "raw" form to some usable form. In meteorology, this often refers to the conversion of the observed value of an element to the value that it theoretically would have at some selected or standard level, usually mean sea level. The most common reduction in weather observing is that of station pressure to sea level pressure. Temperature is sometimes reduced to a sea level value, mostly in climatological work. Most calculations of this sort are based upon approximate actual atmospheric conditions or on the standard atmosphere.

  • reflection - A change of direction and possibly amplitude of an electromagnetic, acoustic, or any other wave propagating in a material medium, as a consequence of spatial variation in the properties of the medium. In specular (mirrorlike) reflection, the spatial variation is abrupt (on the scale of the wavelength), as at an interface between water and air. Specular reflection is described by the law of reflection, according to which incident and reflected waves lie in the plane of incidence, defined by the normal to the interface and the direction of the incident wave, and make the same angle of incidence with this normal. Specular reflection is distinguished from refraction in that the direction of propagation of the reflected wave has a component opposite the direction of the incident wave. Although the law of (specular) reflection is often a good approximation, it is not exact: Diffuse reflection in directions not accounted for by the law of reflection always accompanies specular reflection because matter is not homogeneous on all scales. Light reflected by a cloud illuminated by sunlight is an example of diffuse reflection. Reflection may also refer to the change of direction of a beam of particles, in the broadest sense of this term.

  • regime - See rainfall regime, Rossby regime.

  • relative humidity - The ratio of the vapor pressure to the saturation vapor pressure with respect to water. This quantity is alternatively defined by the World Meteorological Organization as the ratio of the mixing ratio to the saturation mixing ratio. These two definitions yield almost identical numerical values. Relative humidity is usually expressed in percent and can be computed from psychrometric data. Unless specified otherwise, relative humidity is reported with respect to water rather than ice because most hygrometers are sensitive to relative humidity with respect to water even at subfreezing temperatures, and because the air can easily become supersaturated with respect to ice, which would require three digits in coded messages for relative humidity with respect to ice.

  • residual - (Also called error of estimate.) In general, the difference between any quantity and an approximation to it; in particular, the difference (y - Y) between any random variable y and its regression function Y. See regression.

  • resolution - 1. The degree to which nearly equal values of a quantity can be discriminated. 2. The smallest measurable change in a quantity. 3. The least value of a measured quantity that can be distinguished. 4. A formal inference rule permitting computer programs to reason logically. 5. The ability of an optical system to render visible separate parts of an object or to distinguish between different sources of light.

  • restoring - A technique used in numerical modeling, particularly ocean modeling, in which predicted variables are slowly changed toward prescribed values. The prescribed values are typically based on observations. An arbitrary time constant determines the degree to which the predicted variables can deviate from the prescribed values.

  • RF - Abbreviation for radio frequency.

  • ribut - Sharp, short squalls during comparatively calm winds from May to November in Malaya.

  • ridge - 1. (Sometimes called wedge.) In meteorology, an elongated area of relatively high atmospheric pressure, almost always associated with and most clearly identified as an area of maximum anticyclonic curvature of wind flow. The locus of this maximum curvature is called the ridge line. Sometimes, particularly in discussions of atmospheric waves embedded in the westerlies, a ridge line is considered to be a line drawn through all points at which the anticyclonically curved isobars or contour lines are tangent to a latitude circle. The most common use of this term is to distinguish it from the closed circulation of a high (or anticyclone); but a ridge may include a high (and an upper-air ridge may be associated with a surface high) and a high may have one or more distinct ridges radiating from its center. The opposite of a ridge is a trough. 2. Also used as reference to other meteorological quantities such as equivalent potential temperature, temperature, and mixing ratio. That is, an elongated area of relatively high values of any particular field emanating from a maximum. 3. In oceanography, a linear accumulation of broken ice blocks projecting upward, formed by ice deformation, often at the edge of a floe. A ridge is distinguished from a hummock by being much longer than it is wide. The term ridge is often used to describe an entire ridged ice feature, in which case the portion above the water line is termed the sail and the portion below the water line is termed the keel.

  • ridge aloft - Same as upper-level ridge.

  • rig - Same as Michael-riggs.

  • rime - A white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water drops as they impinge upon an exposed object. It is denser and harder than hoarfrost, but lighter, softer, and less transparent than glaze. Rime is composed essentially of discrete ice granules and has densities as low as 0.2-0.3 g cm-3. Glaze is generally continuous but with some air pockets and has much higher densities. Factors that favor rime formation are small drop size, slow accretion, a high degree of supercooling, and rapid dissipation of latent heat of fusion. The opposite effects favor glaze formation. Both rime and glaze occur when supercooled water drops strike an object at a temperature below freezing. Such formation on terrestrial objects constitutes an ice storm; on aircraft, it is called aircraft icing (where rime is known as rime ice). Either rime or glaze may form on snow crystals, droxtals, or other ice particles in the atmosphere. When such a deposit is wholly or chiefly of rime, snow pellets result; when most or all of the deposit is glaze, ordinary hail or ice pellets result. The alternating clear and opaque layers of some hailstones represent glaze and rime, deposited under varying conditions around the growing hailstone. See also hard rime, soft rime.

  • rip - The agitation of water caused by the interaction of water currents or by a rapid current setting in over an irregular bottom; for example, a tide rip.

  • ripe - Descriptive of snow that is in a condition to discharge meltwater. Ripe snow usually has a coarse crystalline structure, a snow density near 0.5 kg m-3, and a temperature near 0°C.

  • ROB - Abbreviation for radar observation; aviation weather communications code word. See radar meteorological observation.

  • rocket - A vehicle designed or adapted for high-altitude research. The ideal meteorological research rocket has a low thrust and long burning time to avoid high acceleration while attaining high altitude, has a large and recoverable payload, and is a stable platform for experimental equipment.

  • roll - Transverse oscillation of a ship about its longitudinal axis. See pitch, yaw, ship motion.

  • rolls - 1. Overturning, quasi-two-dimensional circulations parallel to mean wind in the layer they occupy in which individual particles move downwind in a helical motion. In the atmosphere, boundary layer rolls usually consist of alternating counterrotating helices, aligned nearly parallel to the mean boundary layer wind. When clouds are present, they form over the upwelling parts of the roll circulation. Called cloud streets, they are a good measure of roll wavelength. Typically, cloud streets are spaced at about two to three times the depth of the rolls, although larger spacings are not unusual. Several mechanisms have been proposed for forming rolls; formation and maintenance of atmospheric rolls is thought to involve both buoyancy and shear effects. In some cases, rolls are thought to result from the action of gravity waves on the boundary layer. Rolls occur in the convective boundary layer and have been observed with both stronger winds and midboundary layer wind maxima. They have also been observed for lighter winds with weaker buoyant forcing. 2. The overturning motion that results from breaking Kelvin-Helmholtz waves.

  • rule - A law or regulation that governs behavior, actions, or operations. In rule-based systems, only those rules with true antecedents are used. For example, a rule that begins "IF the temperature is less than 0°C . . . " is ignored whenever the measured temperature is 0°C or higher. See IF-THEN rule, production rule, antecedent, consequent.

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