Subj : Wx Terms (R)(2)
To : All
From : Daryl Stout
Date : Sat Aug 19 2017 09:29 am
Rain Gauge
An instrument for measuring the quantity of rain that has fallen.
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
An area of reduced precipitation on the lee side of a mountain barrier
caused by warming of air and dissipation of cloudiness as air descends
the barrier.
Rain Shield
In a hurricane, a solid or nearly solid area of rain that typically
becomes heavier as one approaches the eye. 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.
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.
Rainbow
A luminous arc featuring all colors of the visible light spectrum
(red, orange, yellow, green, blue, indigo, and violet). It is created
by refraction, total reflection, and the dispersion of light. It is
visible when the sun is shining through air containing water spray or
raindrops, which occurs during or immediately after a rain shower. The
bow is always observed in the opposite side of the sky from the sun.
Rainfall
The amount of precipitation of any type, primarily liquid. It is usually
the amount that is measured by a rain gauge. Refer to rain for rates of
intensity and the quantitative precipitation for forecasting.
Rainfall Estimates
A series of NEXRAD products that employ a Z-R relationship to produce
accumulations of surface rainfall from observed reflectivity.
Range
Distance from the radar antenna. The WSR-88D radar has a range for
velocity products out to 124 nm and reflectivity products out to 248 nm.
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
100-1000 meter intervals. A "radial" of radar data is composed of
successive range gates, out to the maximum unambiguous range.
Range Height Indicator
The RHI is 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.
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
but at different ranges.
Range Unfolding
Process of removing range ambiguity in apparent range of a multitrip
target on the radar.
Rankine Vortex
Velocity profile for a symmetric circulation in which the inner core is
in solid rotation and tangential winds outside the core vary inversely
with radial distance from the center.
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
Rapidly Intensifying
Indicator for a maritime cyclone whose central pressure is dropping, or
is expected to drop, at a rate of 1 MB per hour for 24 hours.
Rawinsonde
A radiosonde that is tracked to measure winds.
Rawinsonde Observation
A radiosonde observation which includes wind data.
RAWS
Remote Automated Weather Stations
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.
RCKY
Rocky Mountains
RCMD
Recommend
RCV
Receive
RDG
ridge
RDS
Radius
Reach
In hydrologic terms, 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.
Real-Time
Refers to the rapid retrieval, processing and transmission of data.
Rear Flank Downdraft
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.
Receiver
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.
Reconnaissance Code
An aircraft weather reconnaissance code that has come to refer primarily
to in-flight tropical weather observations, but actually signifies any
detailed weather observation or investigation from an aircraft in flight.
Record Event Report
This non-routine narrative product is issued by the National Weather
Service to report meteorological and hydrological events that equal or
exceed existing records.
Recreation Report
This National Weather Service 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
Used especially in reference to the recurrence of physical parameters
every 27 days (the rotation period of the sun).
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 be met:
1) a sustained wind average 15 mph or greater
2) relative humidity less than or equal to 25 percent and
3) a temperature of greater than 75 degrees F.
In some states, dry lightning and unstable air are criteria. A Fire
Weather Watch may be issued prior to the Red Flag Warning.
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