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Weather Glossary: R's

An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADAR = RAdio Detection And Ranging.
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 Cross Section
The area of a fictitious, perfect reflector of electromagnetic waves (e.g., metal sphere) that would reflect the same amount of energy back to the radar as the actual target (e.g., lumpy snowflake).
Radar Reflectivity
The sum of all backscattering cross-sections (e.g., precipitation particles) in a pulse resolution volume divided by that volume. The radar reflectivity can be related to the radar reflectivity factor through the dielectric constant term |K|2, and the radar wavelength.
Radial Velocity
The component of motion toward or away from a given location. As "seen" by Doppler radar, it is the component of motion parallel to the radar beam. (The component of motion perpendicular to the beam cannot be seen by the radar. Therefore, strong winds blowing strictly from left to right or from right to left, relative to the radar, cannot be detected.) Also known as Doppler velocity.
Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red).
Radiation Fog

Fog produced results from the air near the ground being cooled to saturation by contact with the cold ground. The cooling of the ground results from night time loss of heat from the Earth to space (terrestrial radiation).

Favorable conditions for radiation fog are clear sky, little or no wind, and high relative humidity. It occurs in stable air and is primarily a night time or early morning phenomenon. As the Earth and the lower layers of the atmosphere warm during the day, air that was stable during the early morning hours may become unstable - at least in the lower levels.

For this reason, visibility usually improves as the temperature rises during the day. Mixing in the lower levels disperses the fog into a thicker layer, and eventually it evaporates into the warmer air.

When cloud layers form aloft over a radiation fog and retard heating from the sun, visibility improvement is very slow. It is also known as Ground Fog and Valley Fog.

Radiation Inversion
It is a thermally produced, surface-based inversion formed by rapid radiational cooling of the Earth's surface at night. It does not usually extend above the lower few hundred feet. Conditions which are favorable for this type of inversion are: long nights, clear skies, dry air, little or no wind, and a cold or snow-covered surface. It is also called a Nocturnal Inversion.
Radiational Cooling
The cooling of the Earth's surface. At night, the Earth suffers a net heat loss to space due to terrestrial cooling. This is more pronounced when you have a clear sky.
Radioisotope Snow Gage
A snow water equivalent gage based on the absorption of gamma radiation by snow; this gage can measure up to 55 inches water equivalent with a 2 to 5 percent error.
A balloon-borne instrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station. These were originally tracked by theodolites, but they are now tracked by the Automatic Radiotheodolite Master Control Unit (ART).
Rain (RA)
Precipitation, either in the form of drops larger than 0.02" (0.5 mm), or smaller drops, which in contrast to drizzle, are widely separated. The following table shows how rainfall intensity is determined:
Intensity of rain based on rate of accumulation
Rainfall Intensity Rate-of-fall Visual Estimation
6-Minutes One Hour
Light < 0.01"
(< 0.25 mm)
Up to 0.10"
(Up to 2.5 mm)
From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
Moderate 0.01"
(0.25 to 0.76 mm)
(2.8 to 7.6 mm)
Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.
Heavy > 0.03"
(> 0.7 mm)
> 0.30"
(> 7.6 mm)
Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to the height of several inches is observed over hard surfaces.
Rain Foot
Slang for a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.
Rain-Free Base

A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area.

Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.

Rain Induced Fog

When warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues.

Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.

Rain Shadow
Areas of the leeward side of a mountain or mountain range which often receive much less rain than the windward side.
Rain Shield
It is a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye of the hurricane. The outer edge is well defined and its distance from the eye varies greatly from storm to storm. The wind, both sustained and peak gusts, keeps increasing as much as one moves through the rain shield toward the storm's eye.
An arc that exhibits in concentric bands the colors of the spectrum and is formed opposite the sun by refraction and reflection of the sun's rays in rain drops.
Distance from the radar antenna. The WSR-88D radar has a range for velocity products out to 124 nm (230 km) and reflectivity products out to 248 nm (460 km).
Range Folding

This occurs when the radar receives a signal return from a pulse other than the most recent pulse. In this case, the radar sends out a pulse (a short burst of energy). This pulse will continue to go in a straight line until it strikes a target.

When it strikes the target, a portion of the pulse will be back scattered towards the radar. If the target it strikes is well beyond the normal range of the radar, it will take longer for the back scattered energy to arrive back at the radar.

As a result, the radar will most likely have sent out another pulse in the same direction before the back scattered energy arrives back at the radar. Therefore, when the radar receives the back scattered energy, it will assume that it came from an object much closer to the radar and it will improperly locate the echo.

A multiple-trip return appears at the difference of the true range and a multiple of the unambiguous range, i.e., R_displayed = R_true - n * R_max, where n = 0,1,2,...

Range Gate
The discrete point in range along a single radial of radar data at which the received signal is sampled. Range gates are typically spaced at 100-1,000-meter intervals. A "radial" of radar data is composed of successive range gates, out to the maximum unambiguous range.
Range Normalization
A receiver gain function in the radar which compensates for the effect of range (distance) on the received power for an equivalent reflectivity.
Range Resolution
The ability of the radar to distinguish two targets along the same radial, it is approximately ½ the pulse length.
Range Unfolding
Process of removing range ambiguity in apparent range of a multitrip target on the radar.
An acronym for Radiosonde Observation. See Radiosonde.
Rapid Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours.
Rating Curve
A graph showing the relationship between the stage, usually plotted vertically (Y-axis) and the discharge, usually plotted horizontally (X-axis).
Rating Table
A table of stage values and the corresponding discharge for a river gauging site.
A balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere.
Rayleigh Scattering
Changes in directions of electromagnetic energy by particles whose diameters are 1/16 wavelength or less. This type of scattering is responsible for the sky being blue.
The distance between two specific points outlining that portion of the stream, or river for which the forecast applies. This generally applies to the distance above and below the forecast point for which the forecast is valid.
Rear Flank Downdraft (or RFD)

A region of dry air subsiding 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. See supercell.

The electronic device which detects the backscattered radiation, amplifies it and converts it to a low-frequency signal which is related to the properties of the target.
Recession Constant
Constant used to reduce the API value daily in the API method of estimating runoff.
Record Report
This nonroutine narrative product is issued by the National Weather Service to report meteorological and hydrological events that equal or exceed existing records.
Recreation Report
This product is used to relay reports on conditions for resorts and recreational areas and/or events. This report may also contain forecast information. Reports for recreational areas and resorts are often routine products, typically for a season, but possibly year-round.
Recurrence Interval
The average amount of time between events of a given magnitude. For example, there is a 1% chance that a 100-year flood will occur in any given year.
Red Flag
This a fire weather program which highlights the onset of critical weather conditions conducive to extensive wildfire occurrences.
Red Flag Warning

A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance.

Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period, if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to met:

  1. A sustained wind average 15 mph (24 km/h) or greater,
  2. Relative humidity less than or equal to 25%, and
  3. A temperature of greater than 75°F (24°C).
In some states, dry lightning and unstable air are criteria.
Red Watch or Red Box
Slang for a tornado watch.
Reflection of Waves
The process whereby waves bounce off a steep shoreline or structure rather than refracting or breaking, as they would in shallower waters. Reflected waves interact with oncoming waves to create confused sea conditions. See Refraction of Waves.

The radar operator uses this radar product to determine the strength or the intensity of a precipitation target. In order for the radar to calculate the reflectivity, it sends out a small burst of energy. This energy strikes the small water particles located in the precipitation target.

For simplification sake, it is assumed that these water particles are evenly spread throughout the precipitation target. The more of these particles located in the precipitation target, the greater the return of energy returned back to the radar. One will see a greater reflectivity return from heavy rain than light rain.

Reflectivity is expressed in the units of dBZ where 'dB' stands for decibels and 'Z' stands for reflectivity. See dBZ and VIP.

Changes in the direction of energy propagation as a result of density changes within the propagating medium. In weather terms, this is important on determining how a radar beam reacts in the atmosphere.
Refractive Index
A measure of the amount of refraction. Numerically equal to the ratio of wave velocity in a vacuum to a wave speed in the medium, i.e., n=c/v where: v is actual speed, and c is speed of light in a vacuum.
Refraction of Waves
The change in the direction of movement of waves which encounter shallow water. See Reflection of Waves.
Regulatory Floodway
Some maps show an area where construction regulations require special provisions to account for this extra hazard. This is a regulatory floodway.
Relative Humidity

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature.

As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest know position is not necessarily a reasonable representation of the cyclone's movement.
A manmade facility for the storage, regulation and controlled release of water.
Reservoir Surface Area
The surface area of a reservoir when filled to the normal pool or water level.
Reservoir Volume
The volume of a reservoir when filled to normal pool or water level.
The degree to which a radar distinguishes detail in a spatial pattern.
Response Time
The amount of time in which it will take a watershed to react to a given rainfall event.
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 referring 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 flow over the central and eastern United States typically results in a return of moist air from the Gulf of Mexico (or the Atlantic Ocean).
Rex Block
A blocking pattern where there is an upper level high located directly north of a closed low.
RFC (River Forecast Center)
Centers that serve groups of Weather Service Forecast offices in providing hydrologic guidance and is the first echelon office for the preparation of river and flood forecasts and warnings.
Ribbon Lightning
Appears to be a broad stream of fire. A succession of strokes, each blown a bit to the side of the previous strokes by wind, but striking so fast that all the strokes are seen at once as a ribbon-like flash.
  • An elongated area of relatively high atmospheric pressure; the opposite of trough.
  • A line or wall of broken ice forced up by pressure. May be fresh or weathered.
Ridge Ice
Ice piled haphazardly one piece over another in the form of ridges or walls.
Right Entrance Region (or Right Rear Quadrant)

The area upstream from and to the right of an upper-level jet max (as would be viewed looking along the direction of flow).

Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. Also, see exit region and left front quadrant.

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. (Supercells often are right movers.) See left mover and splitting storm.
Right Rear Quadrant
see Right Entrance Region.
Rime Ice
It is a rough, milky, opaque ice formed by the instantaneous freezing of small supercooled droplets as they strike the aircraft. The fact that droplets maintain their nearly spherical shape upon freezing and thus trap air between them gives the ice its opaque appearance and makes it porous and brittle.


Rip Current (Run Out)

A strong, narrow current of surface water that flows seaward through the surf into deeper water. Waves approaching the shoreline create a water buildup which results in a return flow. This return flow (rip current) transports the excess water into deeper waters.

Bubbles and debris usually float on the surface of the rip current. Although this current is extremely localized, they result in numerous deaths every year. These deaths are contributed to swimmers becoming exhausted by trying to swim against the rip current.

If you are a swimmer caught in a rip current, wade sideways parallel to the beach until you are out of its pull. Another means of escape for those who are good swimmers is to ride the current out beyond the surf zone where the rip current dissipates then swim to shore outside the effects of the narrow current. This phenomenon is sometimes mistakenly called an "undertow".

Riparian Zone
A stream and all the vegetation on its banks.
River Basin
Drainage area of a river and its tributaries.
River Flood Statement (FLS)
This product is used by the local National Weather Service to update and expand the information in the River Flood Warning. This statement may be used in lieu of a warning if flooding is forecasted, imminent, or existing and it presents no threat to life or property. The statement will also be used to terminate a River Flood Warning.
River Flood Warning (FLW)

This is product is issued by the local National Weather Service when forecast points (those that have formal gauging sites and established flood stages) at specific communities or areas along rivers where flooding has been forecasted, is imminent, or is in progress.

Flooding is defined as the inundation of normally dry areas as a result of increased water levels in an established water course. The flood warning normally specifies crest information. It usually occurs 6 hours or later after the causative event and it is usually associated with widespread heavy rain and/or snow melt or ice jams.

River Flooding
The rise of a river to an elevation such that the river overflows its natural banks causing or threatening damage.
River Gage
A device for measuring the river stage.
River Gage Datum
The arbitrary zero datum elevation which all stage measurements are made from.
River Ice Statement (RVI)
A public product issued by the RFC's containing narrative and numeric information on river ice conditions.
River Recreation Statement (RVR)
A statement released by the NWS to inform river users of current and forecast river and lake conditions. These statements are especially useful for planning purposes.
River Statement (RVS)
A NWS product issued to communicate notable hydrologic conditions which do not involve flooding, i.e., within river bank rises, minor ice jams, etc.
River Summary (RVA)
A NWS summary of river and/or crest stages for selected forecast points along the river. It may also contain ice information.
River System
All of the streams and channels draining a river basin.
Rockfill Dam
An embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment.
A graduated staff used in determining the difference in elevation between two points. The two most common types of rods are the Philadelphia Rod, graduated in feet and hundredths of a foot, and a California Rod, graduated in feet, inches, and eighths of an inch.
Roll Cloud
A low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds.
Rolled Filled Dam
An embankment dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment.
Rope (or Rope Funnel)
A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.
Rope Cloud
In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.
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.
Rotten Ice
Ice in an advanced stage of disintegration.
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 mountain range.
The methods of predicting the attenuation of a flood wave as it moves down the course of a river.
RUC - Rapid Update Cycle
A numerical model run at NCEP that focuses on short-term (up to 12 hours) forecasts and small-scale weather features. Forecasts are prepared every 3 hours for the contiguous United States.
That part of precipitation that flows toward streams on the surface of the ground or within the ground. Runoff is composed of base flow and surface runoff.