Weather Glossary: R's

Fog produced results from the air near the ground being cooled to saturation by contact with the cold ground. The cooling of the ground results from night time loss of heat from the Earth to space (terrestrial radiation).

Favorable conditions for radiation fog are clear sky, little or no wind, and high relative humidity. It occurs in stable air and is primarily a night time or early morning phenomenon. As the Earth and the lower layers of the atmosphere warm during the day, air that was stable during the early morning hours may become unstable - at least in the lower levels.

For this reason, visibility usually improves as the temperature rises during the day. Mixing in the lower levels disperses the fog into a thicker layer, and eventually it evaporates into the warmer air.

When cloud layers form aloft over a radiation fog and retard heating from the sun, visibility improvement is very slow. It is also known as Ground Fog and Valley Fog.

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.

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.

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,...

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.

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 met:

1. A sustained wind average 15 mph (24 km/h) or greater,
2. Relative humidity less than or equal to 25%, and
3. A temperature of greater than 75°F (24°C).

The radar operator uses this radar product to determine the strength or the intensity of a precipitation target. In order for the radar to calculate the reflectivity, it sends out a small burst of energy. This energy strikes the small water particles located in the precipitation target.

For simplification sake, it is assumed that these water particles are evenly spread throughout the precipitation target. The more of these particles located in the precipitation target, the greater the return of energy returned back to the radar. One will see a greater reflectivity return from heavy rain than light rain.

Reflectivity is expressed in the units of dBZ where 'dB' stands for decibels and 'Z' stands for reflectivity. See dBZ and VIP.

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.

• An elongated area of relatively high atmospheric pressure; the opposite of trough.
• A line or wall of broken ice forced up by pressure. May be fresh or weathered.

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. Also, see exit region and left front quadrant.

A strong, narrow current of surface water that flows seaward through the surf into deeper water. Waves approaching the shoreline create a water buildup which results in a return flow. This return flow (rip current) transports the excess water into deeper waters.

Bubbles and debris usually float on the surface of the rip current. Although this current is extremely localized, they result in numerous deaths every year. These deaths are contributed to swimmers becoming exhausted by trying to swim against the rip current.

If you are a swimmer caught in a rip current, wade sideways parallel to the beach until you are out of its pull. Another means of escape for those who are good swimmers is to ride the current out beyond the surf zone where the rip current dissipates then swim to shore outside the effects of the narrow current. This phenomenon is sometimes mistakenly called an "undertow".

This is product is issued by the local National Weather Service when forecast points (those that have formal gauging sites and established flood stages) at specific communities or areas along rivers where flooding has been forecasted, is imminent, or is in progress.

Flooding is defined as the inundation of normally dry areas as a result of increased water levels in an established water course. The flood warning normally specifies crest information. It usually occurs 6 hours or later after the causative event and it is usually associated with widespread heavy rain and/or snow melt or ice jams.