Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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Radar - An instrument that uses microwaves to detect the presence of objects in the air, such as precipitation, by determining how much the microwaves are reflected and scattered by the objects.

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.)

Radiational Cooling - The process in which the earth's surface loses heat into outer space by emitting infrared radiation. This process is maximized on clear nights with little or no wind and a fresh snow cover.

Radiosonde - The instrument package that hangs from the bottom of a weather balloon. It transmits measurements of temperature, wind speed, wind direction, dew-point and atmospheric pressure back to the ground for use in meteorological analysis.

Rainbow - The rainbow is a lovely and familiar guest in our skies, especially during the summer months. Rainbows are created by the light rays from the sun passing through the water droplets of a passing storm. The sun must be at a fairly low angle in the sky, so usually the storm has already passed by to the east and the late day sun shines in from the west. The opposite can happen early in the morning and foretell an approaching storm - but early morning thunderstorms are rare in the Rocky Mountains.

As the light shines through the water droplets, the light is bent or refracted, similar to when light travels through the glass in a prism. The light moves more slowly through the prism or the raindrop and is therefore spread out into the various colors that make up visible light (Red, orange, yellow, green, blue, indigo and violet). The light is refracted once as it enters the drop, then is reflected off the back of the drop and finally refracted again as it exits the drop. Secondary rainbows are often seen and are the result of the light being reflected again off of the inside of the raindrop before it exits. The results of all of this optical bouncing around are the lovely rainbows that we enjoy.

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.

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.

Red Flag Warning
- A warning that is issued to alert forecasters of an ongoing or imminent critical fire weather pattern. When such a warning is issued there are many factors that can affect the warning. Some of the factors are the size of the fire and the winds around the fire. These warning are issued to inform the public about a possible threat and to keep people out of harms way.

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. See dBZ, VIP.

Relative Humidity - A measure of the water vapor in the air compared with the total amount of vapor needed to saturate the air at a given temperature. 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.

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) - 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. See also exit region, left front quadrant.

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. They can be signs of strong winds, but are not as destructive as a tornado. Typically in any thunderstorm, after the roll cloud and gusty winds come, heavy rain, lightning, thunder and small hail follow. Often the storm will be over in less than one hour. However, severe thunderstorms may produce large hail from the size of quarters to larger than baseballs. Large hail indicates a very powerful thunderstorm. Tornadoes often develop shortly after large hail occurs in a severe thunderstorm. Swirling clouds that protrude through the base of the thunderstorm are called "wall clouds". It is from the wall cloud that the tornado funnel will develop and drop to the ground. Interestingly, the wall cloud and tornado often forms in a part of the storm where little, if any, rain is falling. The sky may even be starting to clear in the distance behind the storm. This is still a very dangerous part of the storm - so, do not be fooled by a few breaks in the 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 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.

Rotor Cloud - Another aspect of strong winds along the Front range is the "rotor cloud". These clouds are just what their name implies - a rotating cloud - but they are not tornadoes. The tornado rotates on a vertical axis, but a rotor cloud spins like a pencil rolling off a desk. Go back to the analogy of the mountain stream. In certain areas, the water motion could develop into a standing rapid that has the water swirling over and over on itself. If you have been whitewater rafting, this is the one thing the guides warn you about! Winds descend from the mountaintops and may develop into a swirling band about halfway down from the top. Any moisture in this swirl of air will form into a cloud that rolls over and over, while staying at about the same location on the mountainside. Rotor clouds are of special concern to pilots as the descending air currents can trap a helicopteror airplane into a deadly fall - even at full power. Wintertime pilots are especially wary of rotors, and watch lenticulars very carefully as the same conditions can create the rotor clouds.

RUC - Rapid Update Cycle, a numerical model run at NCEP that focuses on short-term (up to 12 h) forecasts and small-scale (meso-scale) weather features. Forecasts are prepared every 3 hours for the contiguous United States.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society