A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #


see Rain

Radar Beam

the straight line that a radar pulse travels along. As the radar beam gets further away from the radar, it gets wider and wider. In order for a precipitation target to be detected by the radar, it must fill the entire radar beam; therefore, the radar will have a difficult time detecting small showers and thunderstorms at a great distance from the radar.

Radar Data Acquisition (RDA)

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 wideband data are transmitted to the RPG, which creates and disseminates end-user products.

Radar Data Processor II (RADAP II)

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

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. A 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.

Radar Product Generator (RPG)

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 used 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.

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 (at a right angle) to the radar beam has a radial velocity of zero.


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

Radiant Energy

the energy produced by any type of electromagnetic 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 (or Ground) Fog

fog formed when temperatures near the ground cool to near the dewpoint temperature. The ground cools because the sun does not warm it at night.

Radiation Laws

the four physical laws that 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 the air near the surface cool by emission of infrared radiation

Radiational Inversion

see Nocturnal Inversion

Radio Detection and Ranging (RADAR)

a radio device or system for locating an object by means of ultrahigh-frequency radio waves reflected from the object. These reflected waves are received, observed, and analyzed by the receiving part of the radio device in such a way that characteristics (such as distance and direction) of the object may be determined

Radio Waves

electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum. They have the longest wavelengths in this spectrum (from a few millimeters up to hundreds of miles).


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

Rain (RA)

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

Rain Droplet

larger than cloud droplets, they are approximately 0.5 mm (~0.02 in) or larger in diameter (up to around 5 mm or 0.2 in).

Rain Gauge

an instrument used to measure the quantity of rain that has fallen

Rain-Free Base

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


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

Rainfall Estimates

a series of NEXRAD products that employ a Z-R relationship to estimate surface rainfall totals from observed reflectivity.


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

Range Folding

one limitation of a Doppler radar. 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 values.

Range-Height Indicator (RHI)

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.


Radiosonde Observation (an upper-air observation)

Rapid Update Cycle (RUC) Model

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

Rapidly Intensifying

indicator for a maritime cyclone whose central pressure is dropping, or is expected to drop, at a rate of 1 mb per hour for 24 hours. See Bomb Cyclogenesis.


Remote Automated Weather Stations


Rocky Mountains






see Ridge




refers to the rapid retrieval, processing and transmission of data

Rear Flank Downdraft (RFD)

a region of dry air descending on the back side of, and wrapping around, a mesocyclone. It is often visible as a clear slot wrapping around the wall cloud. Scattered large precipitation particles (rain and hail) 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.

Rear Inflow Jet

associated with MCS structures such as bow echoes, it aids in creating a stronger cold pool (more cold, dense air that acts as a plow) and downdrafts. Strong straight-line winds can result with a strong rear inflow jet.




the process whereby radiation (or other waves) that hits a surface is directed back into the medium through which it traveled.


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).


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 (RH)

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





Residual Layer

the elevated portion of a convective boundary layer that remains after a stable boundary layer develops at the ground (usually in late afternoon or early evening) and cuts off convection.

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 that 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

Return Stroke

an electrical discharge that propagates upward along a lightning channel from the ground to the cloud after a stepped leader and a streamer meet.






Rhode Island

Ridge (RDG)

an elongated area of relatively high atmospheric pressure; the opposite of a trough. This term usually refers to an upper-level high pressure system.

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.

Rime Ice

an opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object.


Rio Grande

River Flooding

the rise of a river to an elevation such that the river overflows its natural banks, causing or threatening damage. It is on a longer timescale than flash flooding (on the order of days to months) and is usually more gradual.











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 often 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

Rossby Waves

a series of troughs and ridges on fairly horizontal surfaces in the major belt of upper tropospheric westerlies. The waves are thousands of kilometers long and have significant latitudinal amplitude.


















retard (or slow down)