Aeronautical Terms beginning with R
Radar altitude
The altitude of an aircraft determined by radar-type radio altimeter; thus the actual distance from the nearest terrain or water feature encompassed by the downward directed radar beam. For all practical purposes, it is the“actual” distance above a ground or inland water surface or the true altitude above an ocean surface.
Range attenuation
Reduction of radar power density because of distance from the antenna. It occurs in the outgoing beam at a rate proportional to 1/(range²) The return signal is also attenuated at the same rate.
RADAR (contraction 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. Since hydrometeors can scatter radio energy, weather radars, operating on certain frequency bands, can detect the presence of precipitation, clouds, or both.
Radar beam
The focused energy radiated by radar similar to a flashlight or searchlight beam.
Radarsonde observation
A rawinsonde observation in which winds are determined by radar tracking a balloon-borne target.
Radiation
The emission of energy by a medium and transferred, either through free space or another medium, in the form of electromagnetic waves.
Radiation fog
Fog characteristically resulting when radiational cooling of the earth’s surface lowers the air temperature near the ground to or below its initial dew point on calm, clear nights.
Radiosonde
A balloon-borne instrument for measuring pressure, temperature, and humidity aloft.
Radiosonde observation
A sounding made by the instrument.
Rain
A form of precipitation; drops are larger than drizzle and fall in relatively straight, although not necessarily vertical, paths as compared to drizzle which falls in irregular paths.
Rain shower
Precipitation from a cumuliform cloud; characterized by the suddenness of beginning and ending, by the rapid change of intensity, and usually by rapid change in the appearance of the sky; showery precipitation in the form of rain.
Range-height indicator (RHI) scope
A radar indicator scope displaying a vertical cross section of targets along a selected azimuth.
RAOB
A radiosonde observation.
Rawin
A rawinsonde observation.
Rawinsonde observation
A combined winds aloft and radiosonde observation. Winds are determined by tracking the radiosonde by radio direction finder or radar.
Refraction
In radar, bending of the radar beam by variations in atmospheric density, water vapor content, and temperature.
Relative humidity
The ratio of the existing amount of water vapor in the air at a given temperature to the maximum amount that could exist at that temperature; usually expressed in percent.
Remote scope
In radar meteorology a “slave” scope remoted from weather radar.
Resolution
Pertaining to radar, the ability of radar to show discrete targets separately, i.e., the better the resolution, the closer two targets can be to each other, and still be detected as separate targets.
Range resolution
The ability of radar to distinguish between targets on the same azimuth but at different ranges.
Ridge (also called ridge line)
In meteorology, an elongated area of relatively high atmospheric pressure; usually associated with and most clearly identified as an area of maximum anticyclonic curvature of the wind flow (isobars, contours, or streamlines).
Rime icing (or rime ice)
The formation of a white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water droplets as they impinge upon an exposed aircraft.
Rocketsonde
A type of radiosonde launched by a rocket and making its measurements during a parachute descent; capable of obtaining soundings to a much greater height than possible by balloon or aircraft.
Roll cloud
A dense and horizontal roll-shaped accessory cloud located on the lower leading edge of a cumulonimbus or less often, a rapidly developing cumulus; indicative of turbulence. Sometimes improperly called rotor cloud.
Rotor cloud
A turbulent cloud formation found in the lee of some large mountain barriers, the air in the cloud rotates around an axis parallel to the range; indicative of possible violent turbulence. Sometimes improperly called roll cloud.
Runway temperature
The temperature of the air just above a runway, ideally at engine and/or wing height, used in the determination of density altitude; useful at airports when critical values of density altitude prevail.
Runway visibility
The meteorological visibility along an identified runway determined from a specified point on the runway; may be determined by a transmissometer or by an observer.
Runway visual range
An instrumentally derived horizontal distance a pilot should see down the runway from the approach end; based on either the sighting of high intensity runway lights or on the visual contrast of other objects, whichever yields the greatest visual range.
Relative vorticity
Vorticity of the air relative to the Earth, disregarding the component of vorticity resulting from Earth’s rotation.
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