Weather Glossary - R



Radio Detection And Ranging. An electronic instrument used to detect precipitation by the ability of rain droplets (and other hydrometeors) to reflect microwaves (radio waves) back to a receiver.

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, can not be detected.)


The process by which is energy is propagated through any medium by virtue off the wave motion of that medium.

Radiation fog

Occurs on calm, clear nights when a moist layer at the surface is cooled to its dew point.

Radiational cooling

The cooling of the earth?s surface and the adjacent air due to outgoing radiation.


Precipitation of liquid water that falls from stratiform cloud, with diameter greater than 0.5mm. It is generally steadier than showers. Precipitation (from stratiform cloud) with droplets less than 0.5mm is called drizzle.

Rain day

A rain day occurs when a daily rainfall of at least 0.2 mm is recorded.

Rain foot

[Slang] A horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. A rain foot (bulging to the right in this image) 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. See HP storm, LP storm and supercell.

Rain gauge

An instrument used to measure the amount of rain that has fallen.

Rain shadow

A region on the leeward side of a mountain or similar barrier where precipitation is less than on the windward side. For example, eastern TAS is in the rain shadow of the central plateau receives far less rain than western TAS.


The total liquid product of precipitation or condensation from the atmosphere, as received and measured in a rain gauge.

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.

Reference Climate Station

A climatological station, the data of which are intended for the purpose of determining climatic trends. This requires long periods (not less than thirty years) of homogeneous records, where human-influenced environmental changes have been and/or are expected to remain at a minimum. Ideally the records should be of sufficient length to enable the identification of secular (lasting for ages) changes of climate.


A radar term referring to the ability of a radar target to return energy. Reflectivity is used to derive echo intensity, and to estimate precipitation intensity and rainfall rates. See VIP.

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.


(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 at mid-latitudes.

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).


An elongated area of relatively high atmospheric pressure. Generally associated with light winds and clear weather.

Right Exit Region

(or Right Front Quadrant) The area downstream 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 seed maximum. See also entrance region, left rear quadrant.

Right Front Quadrant

See Right Exit Region

Right mover

A thunderstorm which moves to the right relative to the steering winds, and to other nearby thunderstorms; often the southern part of a splitting storm. See also left mover.

Roll cloud

A low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or 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.

In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary.


(or Rope Funnel) A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.

Rope Cloud

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.

Rossby waves

The movement of ridges and troughs in the upper wind patterns, primarily the jet stream.

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