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.
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.
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.
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.
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.
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.
(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.)
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
a miniature radio transmitter that is carried by an unmanned balloon aloft with instruments for the simultaneous measurement and transmission of meteorological data
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).
an instrument used to measure the quantity of rain that has fallen
in radar meteorology, the straight-line distance from the radar
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.
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)
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
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.
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.
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).
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
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
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
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.
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.
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