26 results. (Showing 1 - 20)

1. radar
An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADAR = RAdio Detection And Ranging.

2. RAdar DAta Processor II (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.

3. 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.)

4. radiosonde
An instrument attached to a weather balloon that transmits pressure, humidity, temperature, and winds as it ascends.

5. rain
Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle.

6. 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.

7. 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.

8. rainbow
Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color.

9. Rapid Update Cycle (RUC )
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.

10. Rear Flank Downdraft (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.

11. Red Watch or Red Box
[Slang], a tornado watch.

12. 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.

13. Relative humidity
An estimate of the amount of moisture in the air relative to the amount of moisture that the air can hold at a specific temperature. For example, if it's 70oF near dawn on a foggy summer morning, the relative humidity is near 100%. During the afternoon the temperature soars to 95oF and the fog disappears. The moisture in the atmosphere has not changed appreciably, but the relative humidity drops to 44% because the air has the capacity to hold much more moisture at a temperature of 95oF than it does at 70oF. But even when the relative humidity is "low" at 44%, it's a very humid day when the temperature is 95oF. For this reason, a better measure of comfort is dew point.

14. 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.

15. 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).

16. Ridge
An elongated area of relatively high atmospheric pressure; the opposite of trough.

17. 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.

18. 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.

19. Right Rear Quadrant

20. river recreation statement
Commonly used to provide forecasts of the pool and outflow for various reservoirs across a Hydrologic Service Area.