Glossary of Meteorological Terms (R)

R

Rabal

A method of winds aloft observation essentially the same as a pilot balloon observation except the height data is derived from the radiosonde observation rather than from assumed ascension rates.

Radar

Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back. See Doppler radar, NEXRAD.

Radarsonde

A system in which radar techniques are used to determine the range, elevation, and azimuth of a radar target carried aloft by a radiosonde, so that wind data may be obtained along with the other meteorological data.

Radar theodolite

A radar which is used to obtain the azimuth, elevation, and slant range of an airborne target.

Radar wind system

Apparatus in which radar techniques are used to determine the range, elevation, and azimuth of a balloon-borne target, to computer upper-air wind data. It is a type of rawin system.

Radiance

In radiometry, a measure of the intrinsic radiant intensity emitted by a radiator in a given direction.

Radiant energy

The energy of any type of electromagnetic radiation. Also called radiation.

Radiant-energy thermometer

An instrument which determines the black-body temperature of a substance by measuring its thermal radiation.

Radiation

Emission or transfer of energy in the form of electromagnetic waves or particles.

Radiation pattern

A diagram showing the intensity of the radiation field in all directions from a transmitting radio or radar antenna at a given distance from the antenna.

Radiation shield

A device used on certain types of instruments to prevent unwanted radiation from affecting the measurement of a quantity. Also called solar radiation shield.

Radiator

Any source of radiant energy, especially electromagnetic energy.

Radio atmometer

An instrument designed to measure the effect of sunlight on evaporation from plant foliage. It consists of a porous clay atmometer whose surface has been blackened so that it absorbs radiant energy.

Radio direction-finder

An instrument for determining the direction from which radio waves approach a receiver. It may consist of a manually operated direction indicator, or it may use a servo system to position the antenna automatically in the direction of the incident waves. This instrument may be used in place of a theodolite for observation of a radiosonde. Also called radio theodolite, radio goniometer.

Radio frequency (RF)

The RF waves emanating from an antenna are generated by the movement of electrical charges in the antenna. Electromagnetic waves can be characterized by a wavelength and a frequency. The wavelength is the distance covered by one complete cycle of the electromagnetic wave, while the frequency is the number of electromagnetic waves passing a given point in one second. The frequency of an RF signal is usually expressed in terms of a unit called the "hertz" (abbreviated Hz). One Hz equals one cycle per second. One megahertz (MHz) equals one million cycles per second.

Radio theodolite

An instrument for determining the direction from which radio waves approach a receiver. It may consist of a manually operated direction indicator, or it may use a servo system to position the antenna automatically in the direction of the incident waves.

Radioactive snow gauge

A device which automatically records the water equivalent of snow on a given surface as a function of time. A small sample of a radioactive salt is placed in the ground in a shielded collimator which directs a beam of radioactive particles upwards. A Geiger-Müller counting system located above the snow level measures the amount of depletion of radiation caused by the presence of snow.

Radiometer

An instrument for measuring radiant energy. See actinometer, Dines radiometer, photometer, Tulipian radiometer.

Radiosonde

A balloon-borne instrument for the simultaneous measurement and transmission of meteorological data. It includes transducers for the measurement of pressure, temperature, and humidity; a modulator for the conversion of the output of the transducers to a quantity which controls a property of the radio frequency signal; a selector switch which determines the sequence in which the parameters are to be transmitted; and a transmitter which generates the radio-frequency carrier.

Radiosonde balloon

A balloon used to carry a radiosonde aloft, considerably larger than pilot balloons or ceiling balloons.

Radiosonde commutator

A component of a radiosonde consisting of a series of alternate electrically conducting and insulating strips. As these are scanned by a contact the radiosonde transmits temperature and humidity signals alternately. The contact may be a baroswitch as in the Diamond-Hinman radiosonde, or may be motor driven.

Radiosonde modulator

That part of an audio-modulated radiosonde consisting of the baroswitch, the sensing elements, the reference elements, and the relay.

Radiosonde observation

An evaluation of upper air temperature, pressure, and humidity from radio signals received from a balloon-borne radiosonde.

Radiosonde recorder

An instrument, located at the surface observing station, which is used to record the data presented by a radiosonde aloft.

Radiosonde transmitter

The component of the radiosonde which includes the modulating blocking oscillator and the radio-frequency carrier oscillator.

Radiosonde-radiowind system

Apparatus consisting of (a) standard radiosonde and radiosonde ground equipment to obtain upper-air data on pressure, temperature, and humidity, and (b) a self-tracking radio direction-finder to provide the elevation and azimuth angles of the radiosonde so that wind vectors may be obtained. It is a type of rawinsonde system.

Radome

A structural, weatherproof enclosure that protects a radar system or antenna and is constructed of material that minimally attenuates the electromagnetic signal transmitted or received by the antenna. (from Radome Services LLC)

Rain

Precipitation composed of liquid water drops more than 0.5 mm in diameter, falling in relatively straight, but not necessarily vertical, paths. Compare to drizzle.

Rain gauge

Instrument for measuring the depth of water from precipitation that is assumed to be distributed over a horizontal, impervious surface and not subject to evaporation.

Rain gauge shield

A device which surrounds a rain gauge and acts to maintain horizontal flow in the vicinity of the funnel so that the catch will not be influenced by eddies generated near the gauge. See Alter shield, Nipher shield, Wild fence, Wyoming shield.

Rain-intensity gauge

An instrument which measures the instantaneous rate at which rain is falling on a given surface. Also called a rate-of-rainfall gauge.

Rain shadow

A region of sharply reduced precipitation on the lee side of an orographic barrier, as compared with regions upwind of the barrier.

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 seen on a "sheet" of water drops (rain, fog, spray). The most common of the many possible is the primary rainbow. The second most common is the secondary rainbow, seen outside the primary bow and having the reverse spectral sequence. Tertiary and higher order bows are exceedingly rare due to their low luminosity. Supernumerary rainbows are seen fairly often just within the primary bow.

Raindrop spectrograph

An instrument which automatically determines the size distribution of raindrops.

RAM

Random Access Memory. The memory of a computer which can be read and written into at any location without passing through preceding locations.

Ram penetrometer

A cone-tipped metal rod designed to be driven downward into deposited snow or firn. The measured amount of force required to drive the rod a given distance is an indication of the physical properties of the snow or firn.

Random

Eluding precise prediction, completely irregular.

Random error

The inherent imprecision of a given process of measurement, the unpredictable component of repeated independent measurements of the same object under sensibly uniform conditions.

Range

The interval between the lower and upper measuring limits of an instrument, i.e. a thermometer with a range of -35 to 50°C. Compare to span.

Rankine temperature scale

An absolute temperature scale with the degree of the Fahrenheit scale and the zero point of the Kelvin scale. The freezing point of water equals 491.67°. The boiling point of water equals 671.67°. The temperature scale is named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859.

Raob

Contraction for radiosonde observation.

RASS

Abbreviation for radio acoustic sounding system.

Rate-of-rainfall gauge

Same as rain-intensity gauge.

Rawin

A method of winds aloft observation accomplished by tracking a balloon-borne radar target or radiosonde with either radar or a radio theodolite.

Rawinsonde

A method of upper air observation consisting of an evaluation of the wind speed and direction, temperature, pressure, and humidity aloft by means of a balloon-borne radiosonde tracked by radar or a radio theodolite.

Rawin target

A special type of radar target, usually a corner reflector, tied beneath a free balloon and designed to be an efficient reflector of radio energy.

RAWS

Remote Automated Weather Station. A network of weather stations positioned throughout the U.S. that collect, store, and forward data hourly via satellite to a computer system located at the National Interagency Fire Center in Boise, Idaho.

RDF

Radio Direction Finder.

Reach

A definite portion of a stream channel, commonly taken between two gauging stations, but may be taken between any two specified points.

Real-time

The actual time during which physical events take place.

Reaumur temperature scale

A scale with the ice point at zero degrees and the boiling point of water at 80 degrees, with pressure of one atmosphere.

Receiver

(1) The initial component or the sensing element of a measuring system. For example, the receiver of a rain gauge is the funnel which captures the rain and the receiver of a thermoelectric thermometer is the measuring thermocouple. (2) An instrument used to detect the presence of and to determine the information carried by electromagnetic radiation, i.e. a radio receiver.

Recording potentiometer

An instrument which automatically records the voltage applied to it, as a function of time.

Recording rain gauge

A rain gauge which automatically records the amount of precipitation collected, as a function of time.

Reduction

In general, the transformation of data from a "raw" form to some useable form. In meteorology, this often refers to the conversion of the observed value of an element to the value which it would theoretically have at some selected or standard level. The most common reduction in weather observing is that of station pressure to sea level pressure.

Reflected solar radiation (reflected global radiation)

Upward-directed solar radiation, reflected by the earth’s surface and the atmosphere.

Reflecting nephoscope

Same as mirror nephoscope.

Reflection rainbow

A rainbow formed by light rays which have been reflected from an extended water surface. Not to be confused with a reflected rainbow whose image may be seen in a still body of water. The center of a reflection rainbow is at the same elevation as the sun but in the opposite part of the sky.

Reflector

In general, any object that reflects incident energy. Usually it is a device designed for specific reflection characteristics.

Reflectometer

Downward-facing pyranometer used for measuring reflected solar radiation.

Reference operating conditions

The range of operating conditions of a device within which operating influences are negligible. The range is usually narrow. Reference operating conditions are the conditions under which reference performance is stated and the base from which the values of operating influences are determined. See normal operating conditions, operating conditions.

Register

The writing component of a recording instrument.

Relative evaporation

See evaporative opportunity.

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.

Repeatability

The closeness of agreement among a number of consecutive output values measuring the same input value under the same operating conditions, approaching from the same direction. Usually measured as nonrepeatability but expressed as repeatability, a percentage of span.

Reproducibility

The closeness of agreement among measurements of the same value of the same quantity where the individual measurements are made under different defined conditions, i.e. by different methods or with different measuring instruments.

Resistance thermometer

A type of electrical thermometer in which the thermal element is a substance whose electrical resistance varies with the temperature. Such thermometers can be made with very short time constants and are capable of highly accurate measurements.

Resolution

The smallest change in the environment that causes detectable change in the indication of an instrument. Compare to sensitivity.

Response

The value of the quantity measured, as indicated or otherwise provided by a measuring instrument.

Response time

The time required for an instrument to register a designated percentage (frequently 90%) of a step change in the variable being measured.

Retreater

A defective maximum thermometer of the liquid-in-glass type in which the mercury flows too freely through the constriction. Such a thermometer will indicate a maximum temperature that is too low.

Reversing thermometer

A mercury-in-glass thermometer which records the temperature upon being inverted and retains its reading until being returned to the first position.

RF

Abbreviation for radio frequency.

Ridge

An elongated area of relatively high pressure. Usually associated with and most clearly identified as an area of maximum anticyclonic curvature of the wind flow. The opposite of a trough.

Rime

An accumulation of granular ice tufts on the windward sides of exposed objects that is formed from supercooled fog or cloud and built out directly against the wind.

River basin

The total area drained by a river and its tributaries. Same as watershed.

River forecast

A forecast of the expected stage or discharge at a specified time, or of the total volume of flow within a specified interval of time, at one or more points along a stream.

River gauge

A device for measuring the river stage. Also called stream gauge.

River stage

See stage.

RMS

Root Mean Square. This notation is used frequently with error analysis. In that context, it is the square root of the arithmetic mean of the squares of the deviations of the individual calibration points from the theoretical or ideal response.

Robitzsch actinograph

A pyranometer developed by M. Robitzsch. Its design utilizes three bimetallic strips which are exposed horizontally at the center of a hemispherical glass bowl. The outer strips are white reflectors and the center strip is a blackened absorber. The bimetals are joined in such a manner that the pen of the instrument deflects in proportion to the difference in temperature between the black and white strips and is thus proportional to the intensity of the received radiation.

Rocketsonde (meteorological rocket)

A rocket designed primarily for routine upper air observations in the lower 250,000 feet of the atmosphere, especially that portion inaccessible to balloons (above 100,000 feet).

Rockoon

A high-altitude sounding system consisting of a small solid-propellant research rocket carried aloft by a large plastic balloon. The rocket is fired near the maximum altitude of the balloon flight.

ROM

Read Only Memory. A memory that cannot be altered in normal use of a computer, Usually used to store information permanently, such as firmware programs.

ROMAN

Real-Time Observations Monitor and Analysis Network. A web-based weather observations monitor linking RAWS, airport observations, and other miscellaneous observations to a single user interface.

Rotating multicylinder

An instrument consisting of a series of graduated cylinders possessing selective collection efficiencies. It is used for the measurement of quantities relating to the size distribution of cloud droplets.

Rotation anemometer

A type of anemometer in which the rotation of an element serves to measure the wind. Rotation anemometers are divided into two classes; those in which the axis of rotation is horizontal, such as the windmill anemometer, and those in which the axis of rotation is vertical, such as the cup anemometer.

RS232

A standard interface between a computer input/output port and a peripheral device. Signal properties including time duration, voltage, and current, are specified by the Electronic Industries Association.

RS422

A protocol similar to RS232 which makes use of differential transmission to provide high speed data transmission over significantly longer distances.

RS485

A protocol similar to RS232 which permits data interchange on multidrop networks of up to 32 nodes using a single twisted pair cable. In order for this protocol to be used, each device on a network must have some level of intelligence in order establish orderly data transfer over a single path.

Runoff

The portion of the precipitation on the land which ultimately reaches the streams, especially the water from rain or melted snow that flows over the surface.

Runway visibility

The visibility along an identified runway, determined from a specified point on the runway with the observer facing in the same direction as a pilot using the runway. Compare to runway visible range.

Runway visual range (RVR)

The maximum distance along the runway at which the runway lights are visible to a pilot at touchdown. Runway visual range may be determined by an observer located at the end of the runway, facing in the direction of landing, or by means of a transmissometer installed near the end of the runway.