RADAP II - RAdar DAta Processor II, attached to some WSR-57
and WSR-74 radar units. It automatically controls the tilt sequence
and computes several radar-derived quantities at regular intervals,
including VIL, storm tops, accumulated rainfall, etc.
Radial Velocity - 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.)
Rain Foot - [Slang], 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
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 Fig. 7, supercell.
Red Watch or Red Box - [Slang], a tornado watch.
Reflectivity - 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.
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.
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).
Right Entrance Region (or Right Rear Quadrant) -
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. See also exit region, left front
Ridge - An elongated area of relatively high atmospheric
pressure; the opposite of trough.
*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, splitting storm.
Right Rear Quadrant - see Right Entrance Region.
*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.
*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
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
Rapid Update Cycle, a numerical model run at NCEP that
focuses on short-term (up to 12 h) forecasts and small-scale (mesoscale)
weather features. Forecasts are prepared every 3 hours for the
contiguous United States.