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RADAR METEOROLOGICAL OBSERVATION.— An evaluation of the echoes that appear on the indicator of a weather radar, in terms of the orientation, coverage, intensity, tendency of intensity, height, movement, and unique characteristics of echoes that may be indicative of certain types of severe storms (such as hur-ricanes, tornadoes, or thunderstorms) and of anomalous propagation.

RADIATION.—(1) The process by which electromagnetic radiation is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space. This concept is to be distinguished from convection and conduction. (2) The process by which energy is propagated through any medium by virtue of the wave motion of that medium, as in the propaga-tion of sound waves through the atmosphere, or ocean waves along the water surface.

RADIATIONAL COOLING.—The cooling of Earth’s surface and adjacent air, accomplished (mainly at night) whenever Earth’s surface suf-fers a net loss of heat due to terrestrial radiation.

RADIATION FOG.—A major type of fog, produced over a land area when radiational cool-ing reduces the air temperature to or below its dew point.

RADIOSONDE.—A balloon-borne instru-ment for the simultaneous measurement and transmission of meteorological data.

RADIOSONDE OBSERVATION.—(com-monly contracted to raob) An evaluation in terms of temperature, relative humidity, and pressure aloft, of radio signals received from a balloon-borne radiosonde; the height of each mandatory and significant pressure level of the observation is computed from these data.

RAINBOW.—Any one of a family of circular arcs consisting of concentric colored bands, arranged from red on the inside to blue on the outside, which may be seenona‘‘sheet" of water drops (rain, fog, or spray).

RAWIN.—A method of winds-aloft observa-tion; that is, the determination of wind speeds and directions in the atmosphere above the station. It is accomplished by tracking a balloon-borne radar target or radiosonde transmitter with either radar or a radio direction-finder.

RAWINSONDE.—A method of upper-air observation consisting of an evaluation of the wind speed and direction, temperature, pressure, and relative humidity aloft by means of a balloon-borne radiosonde tracked by a radar or radio direction-finder. If radar is used for tracking, a radar target is also attached to the balloon. Thus, it is a radiosonde observation combined with a type of rawin observation.

RECURVATURE.—With respect to the mo-tion of severe tropical cyclones (hurricanes and typhoons), the change in direction from westward and poleward to eastward and poleward. Such "recurvature" of the path frequently occurs as the storm moves into middle latitudes.

REDUCTION.—In general, the transforma-tion of data from a "raw" form to some usable form. In meteorology, this often refers to the con-version 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 observing is that of station pressure to sea-level pressure.

REFLECTION.—The process whereby a sur-face of discontinuity turns back a portion of the incident radiation into the medium through which the radiation approached.

REFLECTIVITY.—A measure of the fraction of radiation reflected by a given surface; defined as the ratio of the radiant energy reflected to the total that is incident upon that surface. The reflec-tivity of a given surface for a specified broad spec-tral range, such as the visible spectrum or the solar spectrum, is referred to as albedo.

REFRACTION.—The process in which the direction of energy propagation is changed as the result of a change in density within the prop-agating medium, or as the energy passes through the interface representing a density discontinuity between two media.

RELATIVE VORTICITY.—The vorticity as measured in a system of coordinates fixed on Earth’s surface. Usually, only the vertical com-ponent of the vorticity is meant.

RESOLUTION.—The ability of an optical system to render visible separate parts of an ob-ject, or to distinguish between different sources of light.

RESULTANT WIND.—In climatology, the vectorial average of all wind directions and speeds for a given level at a given place for a certain period, as a month. It is obtained by resolving each wind observation into components from north and east, summing over the given period, obtaining the averages, and reconverting the average components into a single vector.

RETROGRADE.—The motion of an at-mospheric wave or pressure system in a direction opposite to that of the basic flow in which it is embedded.

RIDGE.—A elongated area of relatively high atmospheric pressure. The most common use of this term is to distinguish it from the closed cir-culation of a high; but a ridge may include a high, and a high may have one or more distinct ridges radiating from its center.

SCATTERING.—The process by which small particles suspended in a medium of a different in-dex of refraction diffuse a portion of the incident radiation in all directions.

SEA BREEZE.—A coastal local wind that blows from sea to land, caused by the temperature difference when the sea surface is colder than the adjacent land. Therefore, it usually blows on relatively calm, sunny, summer days; and alter-nates with the oppositely directed, usually weaker, nighttime land breeze.

SEA-BREEZE FRONT.—A sea breeze that forms out over the water, moves slowly toward the coast and then moves inland quite suddenly. Often associated with the passage of this type of sea breeze are showers, a sharp wind shift from seaward to landward, and a sudden drop in temperature. The leading edge of such a sea breeze is sometimes called the sea-breeze front.

SEA LEVEL.—The height or level of the sea surface.

SEASON.—A division of the year according to some regularly recurrent phenomena, usually astronomical or climatic. Astronomical seasons extend from an equinox to the next solstice (or vice versa). Climatic seasons are often based on precipitation (rainy and dry seasons).

SECONDARY CIRCULATION.—Atmos-pheric circulation features of synoptic scale.

SECONDARY FRONT.—A front that forms within a baroclinic cold air mass that itself is separated from a warm air mass by a primary frontal system. The most common type is the secondary cold front.

SHEAR.—The variation (usually the direc-tional derivative) of a vector field along a given direction in space. The most frequent context for this concept is wind shear.

SHEAR LINE—A line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line; a line of maximum horizontal wind shear.

SHORT WAVE.—With regard to at-mospheric circulation, a progressive wave in the horizontal pattern of air motion with dimensions of synoptic scale, as distinguished from a long wave.

SHORT-WAVE RADIATION.—A term used loosely to distinguish radiation in the visible and near-visible portions of the electromagnetic spec-trum (roughly 0.4 to 1.0 micron in wavelength) from long-wave radiation.

SIBERIAN HIGH.—A cold-core high-- pressure area that forms over Siberia in winter, and which is particularly apparent on mean charts of sea-level pressure.

SINGULAR POINT.—In a flow field, a point at which the direction of flow is not uniquely determined, hence a point of zero speed, e.g., a col.

SMOOTHING.—An averaging of data in space or time, designed to compensate for ran-dom errors or fluctuations of a scale smaller than that presumed significant to the problem at hand; the analysis of a sea-level weather map smoothes the pressure field on a space-scale more or less systematically determined by the analyst by tak-ing each pressure as representative not of a point but of an area about the point.

SOLAR CONSTANT.—The rate at which solar radiation is received outside Earth’s at-mosphere on a surface normal to the incident radiation, and at Earth’s mean distance from the Sun.

SOLSTICE.—(1) Either of two points on the Sun’s apparent annual path where it is displaced farthest, north or south, from Earth’s equator. The Tropic of Cancer (north) and Tropic of Capricorn (south) are defined as the parallels of latitude that lie directly beneath a solstice. (2) Popularly, the time at which the Sun is farthest north or south; the "time of the solstice." In the Northern Hemisphere, the summer solstice falls on or about 21 June, and the winter solstice on or about 22 December. The reverse is true in the southern latitudes.

SOUNDING.—In meteorology, the same as upper-air observation.

SPECIFIC HEAT.—The heat capacity of a system per unit mass. That is, the ratio of the heat absorbed (or released) by unit mass of the system to the corresponding temperature rise (or fall).

SPECIFIC HUMIDITY.—In moist air, the ratio of the mass of water vapor to the total mass of the system. For many purposes it may be approximated by the mixing ratio. 

SPECULAR REFLECTION.—Reflection in which the reflected radiation is not diffused; reflection as from a mirror.

SPIRAL BAND.—Spiral-shaped radar echoes received from precipitation areas within intense tropical cyclones. They curve cyclonically in toward the center of the storm and appear to merge to form the wall around the eye of the storm.

SQUALL LINE.—Any non-frontal line or narrow band of active thunderstorms.

STANDARD ATMOSPHERE.—A hypothet-ical vertical distribution of atmospheric temperature, pressure, and density which, by international agreement, is taken to be represent-ative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc. The air is assumed to obey the perfect gas law and the hydrostatic equation, which, taken together, relate temperature, pressure, and density variations in the vertical. It is further assumed that the air contains no water vapor and that the acceleration of gravity does not change with height.

STEERING CURRENT.—A basic fluid flow that exerts a strong influence upon the direction of movement of disturbances embedded in it.

STEERING LEVEL.—A level, in the at-mosphere, where the velocity of the basic flow bears a direct relationship to the velocity of move-ment of an atmospheric disturbance embedded in the flow.

STORM.—Any disturbed state of the at-mosphere, especially as affecting Earth’s surface, and strongly implying destructive or otherwise unpleasant weather. Storms range in scale from tornadoes and thunderstorms, through tropical cyclones, to widespread extratropical cyclones.

STORM SURGE.—(also called storm tide) An abnormal rise of the sea along a shore as the result, primarily, of storm winds. 

STRATOSPHERE.—The atmospheric shell above the troposphere and below the mesosphere. It extends, therefore, from the tropopause to the height where the temperature begins to increase in the 20- to 25-km region.

STREAMLINE.—A line whose tangent at any point in a fluid is parallel to the instantaneous velocity of the fluid at that point.

SUBGRADIENT WIND.—A wind of lower speed than the gradient wind required by the ex-isting pressure gradient and centrifugal force.

SUBLIMATION.—The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase.

SUBSIDENCE.—A descending motion of air in the atmosphere, usually with the implication that the condition extends over a rather broad area.

SUBSIDENCE INVERSION.—A temp-erature inversion produced by the adiabatic warming of a layer of subsiding air. This inversion is enhanced by vertical mixing of the air layer below the inversion.

SUBTROPICAL HIGH.—One of the semi-permanent highs of the subtropical high-pressure belt. They appear as centers of action on mean charts of surface pressure. They lie over oceans and are best developed in summer.

SUBTROPICAL HIGH-PRESSURE BELT.—One of the two belts of high atmospheric pressure that are centered, in the mean, near 30°N and 30°S latitudes.

SUNSPOT.—A relatively dark area on the surface of the Sun, consisting of a dark central umbra surrounded by a penumbra, which is intermediate in brightness between the umbra and the surrounding photosphere.

SUPERADIABATIC LAPSE RATE.—An environmental lapse rate greater than the dry-adiabatic lapse rate, such that potential temperature decreases with height.

SUPERCOOLING.—The reduction of temperature of any liquid below the melting point of that substance’s solid phase, that is, cooling beyond its nominal freezing point.

SUPERGRADIENT WIND.—A wind of greater speed than the gradient wind required by the existing pressure gradient and centrifugal force.

SUPERIOR AIR.—An exceptionally dry mass of air formed by subsidence and usually found aloft but occasionally reaching Earth’s sur-face during extreme subsidence processes.

SUPERIOR MIRAGE.—A spurious image of an object formed above its true position by abnormal refractive conditions; opposite of mirage.

SUPERSATURATION.—The condition ex-isting in a given portion of the atmosphere (or other space) when the relative humidity is greater than 100 percent, that is, when it contains more water vapor than is needed to produce saturation with respect to a plane surface of pure water or pure ice.

SURFACE BOUNDARY LAYER.—That thin layer of air adjacent to Earth’s surface, extending up to the so-called anemometer level (the height above the ground at which an anemometer is exposed; usually 10 meters to 100 meters. 

SURFACE CHART.—(also called surface map, sea-level chart, sea-level pressure chart) An analyzed synoptic chart of surface weather obser-vations. It shows the distribution of sea-level pressure (positions of highs, lows, ridges, and troughs) and the location and nature of fronts and air masses. Often added to this are symbols of occurring weather phenomena, analysis of pressure tendency (isallobars), indications of the movement of pressure systems and fronts, and perhaps others, depending on the use of the chart.

SURFACE INVERSION.—A temperature in-version based at Earth’s surface; that is, an in-crease of temperature with height beginning at ground level.

SURFACE OF DISCONTINUITY.—A sur-face separating two fluids across which there is a discontinuity of some fluid property, such as density, velocity, etc., or of some derivative of one of these properties in a direction normal to the interface. An atmospheric front is represented ideally by a surface of discontinuity of velocity, density, temperature, and pressure gradient; the tropopause is represented ideally by a surface of discontinuity of, for example, the derivatives: lapse rate and wind shear.

SYNOPTIC.—In general, pertaining to or af-fording an overall view. In meteorology, this term has become somewhat specialized in referring to the use of meteorological data obtained simultaneously over a wide area for the purpose of presenting a comprehensive and nearly instan-taneous picture of the state of the atmosphere.

SYNOPTIC CHART.—In meteorology, any chart or map on which data and analyses are presented that describe the state of the atmosphere over a large area at a given moment in time.

SYNOPTIC SCALE.—The scale of the migratory high- and low-pressure systems (or cyclonic waves) of the lower troposphere, with wavelengths of 1,000 to 2,500 km.

SYNOPTIC SITUATION.—The general state of the atmosphere as described by the major features of synoptic charts.

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