RADAP II (RAdar DAta Processor II) - equipment attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, and accumulated rainfall.

RADAR - acronym for RAdio Detection And Ranging; a radio device or system for locating an object by means of ultrahigh-frequency radio waves reflected from the object and received, observed, and analyzed by the receiving part of the device in such a way that characteristics (as distance and direction) of the object may be determined

Radar Meteorology - branch of meteorology that uses radars for weather observations and forecasts.

Radar Mosaic - a radar product that combines information from multiple radars to give a regional or national view of reflectivity or precipitation. An individual NEXRAD radar is limited to a range of about 200 miles. Typically, a mosaic product is produced for regions spanning several hundreds to several thousands of miles. Mosaic products are produced by vendors external to the NEXRAD system.

Radial Velocity - the component of motion toward or away from a given location. As detected by Doppler radar, it is the component of motion parallel to the radar beam. An object moving perpendicular to the radar beam has a radial velocity of zero.

Radiance - a measure of the intensity of the radiant energy flux emitted by a body in a given direction

Radiant Energy - the energy produced by any type of electromagnetic radiation

Radiation - (1) the process by which radiated energy moves through space or material media; (2) energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields (e.g., visible light, x-rays, microwaves, radio waves, infrared radiation, ultraviolet waves, cosmic rays, etc.)

Radiation Laws - the four physical laws which fundamentally describe the behavior of blackbody radiation: Kirchhoff's law, Planck's law, Stefan-Boltzmann law and Wien's displacement law

Radiational Cooling - the process by which the surface of the earth and its adjacent air cool by emission of infrared radiation

Radiosonde - a miniature radio transmitter that is carried by an unmanned balloon aloft with instruments for the simultaneous measurement and transmission of meteorological data

Rain - precipitation in the form of liquid water drops with diameters greater than 0.5 mm

Rainbow - an arc of concentric colored bands, arranged from red to blue, which may be seen on a "sheet" of water drops (rain, fog, spray)

Rain-Free Base - a dark, horizontal cloud base with no visible precipitation beneath it; it typically marks the location of the thunderstorm updraft

Rain Gauge - an instrument for measuring the quantity of rain that has fallen

Rainfall Estimates - a series of NEXRAD products that employ a Z-R relationship to produce accumulations of surface rainfall from observed reflectivity.

Range - in radar meteorology, the straight-line distance from the radar

Range Folding - a limitation of a pulsed Doppler radar whereby an echo at a far range (beyond the radar's maximum unambiguous range) could be misinterpreted as being at a closer range. Range folding hampers accurate velocity determinations.

RDA - an acronym for Radar Data Acquisition. The RDA is the hardware component of the NEXRAD system that consists of the radar antenna, transmitter, receiver, tower, and controlling computer. The RDA collects the unprocessed, analog voltages from the radar antenna and converts the signal to base reflectivity , base velocity, and spectrum width (in polar coordinate form). These "wide-band" products are transmitted to the RPG, which creates and disseminates end-user products.

Real-Time - refers to the rapid retrieval, processing and transmission of data

Rear Flank Downdraft (or RFD) - a region of dry air descending on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) at the interface between the clear slot and wall cloud may show up on radar as a hook or pendant; thus the presence of a hook or pendant may indicate the presence of an RFD.

Reflection - the process whereby radiation (or other waves) incident upon a surface is directed back into the medium through which it traveled.

Reflectivity - a radar term referring to the ability of a radar target to return energy; used to derive echo intensity and to estimate precipitation intensity and rainfall rates

Reflectivity Factor - the result of a mathematical equation (called the Weather Radar Equation) that converts the analog power (in Watts) received by the radar antenna into a more usable quantity. The reflectivity factor (denoted by Z) takes into account several factors, including the distance of a target from the radar, the wavelength of the transmitted radiation, and certain assumptions about the kind and size of targets detected by the radar. The reflectivity factor ranges over several orders of magnitudes, so it is usually expressed on a logarithmic scale called dBZ (decibels of reflectivity).

Refraction - the process whereby radiation experiences a change in direction as a result of a change in density of the medium or media through which it travels

Relative Humidity - a measure of the water vapor content of the air at a given temperature; the amount of moisture in the air as compared with the amount that the air could contain at the same temperature, expressed as a percentage

Retrogression (or Retrograde Motion) - movement of a weather system in a direction opposite to that of the basic flow in which it is embedded; usually refers to a closed low or a longwave trough which moves westward

Return Flow - south winds on the back (west) side of an eastward-moving surface high pressure system

Return Period - recurrence interval; a statistical parameter used in frequency analysis as a measure of the average time interval between the occurrence of a given quantity and that of an equal or greater quantity

RHI - an acronym for Range-Height Indicator. The RHI is a radar display in which the radar scans vertically, with the antenna pointing at a specific azimuth or radial. NEXRAD does not support RHI, but the PUP software allows the NEXRAD operator to construct a vertical cross-section using data from multiple scans of the radar.

Ridge - an elongated area of relatively high atmospheric pressure; the opposite of a trough

Right Mover - A thunderstorm that moves appreciably to the right relative to the main steering winds and to other nearby thunderstorms. Right movers typically are associated with a high potential for severe weather.

Roll Cloud - a low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front); roll clouds are completely detached from the thunderstorm base or other cloud features

Rope (or Rope Funnel) - a narrow, often twisted condensation funnel usually associated with the decaying stage of a tornado

Rope Stage - the dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel); damage still is possible during this stage

RPG - the acronym for Radar Product Generator. The RPG is the computer in the NEXRAD system that receives polar-coordinate base radar data from the RDA and processes these data into end-user products. Algorithms are utilized for pattern-recognition, rainfall estimation, computation of VIL and other products. The RPG communicates these products to end-users. A specific subset of available products is always generated for the NIDS vendors for distribution outside of the NWS, DoD, and FAA. Other products are generated by the RPG upon request from a PUP.

RUC (Rapid Update Cycle) - a numerical model simulation conducted at NCEP that focuses on short-term (up to 12 hours) forecasts and mesoscale weather features; forecasts are prepared every 3 hours for the contiguous United States