Weather Terms and Definitions

This page provides some basic weather terms and definitions which may appear on our site. At the end of some terms are links to additional information.

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Weather FAQ:

Weather FAQ:
Inflow Bands

  • Inflow Bands (or Feeder Bands)
  • Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity.
  • Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone.

Weather FAQ:
Inflow Jets

  • Local jets of air near the ground flowing inward toward the base of a tornado.

Weather FAQ:
Inflow Notch

  • A radar signature characterized by an indentation in the reflectivity pattern on the inflow side of the storm.
  • The indentation often is V-shaped, but this term should not be confused with V-notch.
  • Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).

Weather FAQ:
Inflow Stinger

Weather FAQ:

  • Incoming solar radiation. Solar heating; sunshine.

Weather FAQ:

  • The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted.
  • Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.
  • See Lifted Index and Figure 6.

Weather FAQ:

  • Generally, a departure from the usual increase or decrease in an atmospheric property with altitude.
  • Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs.
  • An inversion is present in the lower part of a cap.
  • See Figure 6.

Weather FAQ:
Isentropic Lift

  • Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs.
  • Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.

Weather FAQ:
Isentropic Surface

Weather FAQ:

  • line connecting points of equal pressure.

Weather FAQ:

  • A line connecting points of equal dew point temperature.

Weather FAQ:

  • A line connecting points of equal precipitation amounts.

Weather FAQ:

  • General term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths.

Weather FAQ:

  • A line connecting points of equal wind speed.

Weather FAQ:

  • A line connecting points of equal temperature.


Weather FAQ:

  • Joules per square centimeter, a measurement of solar energy. A joule is the unit of energy in the meter-kilogram-second system of units, equal to 107 ergs or approximately 0.7375 foot-pounds.

Weather FAQ:
Jet Max

  • Jet Max (or Speed Max or Jet Streak)
  • A point or area of relative maximum wind speeds within a jet stream.

Weather FAQ:
Jet Streak

Weather FAQ:
Jet Stream

  • Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds.
  • The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams).
  • A jet stream at low levels is known as a low-level jet.


Weather FAQ:

  • Crop coefficient, available from local agricultural advisory services. To find the evapotranspiration for a given crop, multiply the reference evapotranspiration by the K-factor.

  Wikipedia: K-Factor

Weather FAQ:


Weather FAQ:

  • Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

Weather FAQ:

Weather FAQ:
Langley (unit)

  • Langley (Ly) is a measurement of solar energy. One langley is equal to one gram-calorie per square centimeter. A gram-calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius.

  Wikipedia: Langley (unit)

Weather FAQ:
Lapse Rate

  • The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.
  • See Figure 6.

Weather FAQ:

Weather FAQ:
Left Front Quadrant

Weather FAQ:
Left Mover

Weather FAQ:
LEWP - Line Echo Wave Pattern

  • A bulge in a thunderstorm line producing a wave-shaped "kink" in the line (Figure 4).
  • The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo.
  • Severe weather potential also is increased with storms near the crest of a LEWP.

Weather FAQ:
Lifted Index (or LI)

  • A common measure of atmospheric instability.
  • Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level.
  • Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.
  • However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.
  • See Figure 6.

Weather FAQ:
Loaded Gun (Sounding)

  • [Slang] A sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.
  • See Figure 6.

Weather FAQ:
Longwave Trough

  • A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration.
  • Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time.
  • Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

Weather FAQ:
Low-Level Jet (or LLJ)

  • A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).
  • The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

Weather FAQ:
LP Storm (or LP Supercell)

  • Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation.
  • Visually similar to a classic supercell, except without the heavy precipitation core (Figure 5).
  • LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation.
  • They are capable of producing tornadoes and very large hail.
  • Radar identification often is difficult relative to other types of supercells, so visual reports are very important.
  • LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

Weather FAQ:
LSR - Local Storm Report

  • A product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events.


Weather FAQ:
Mammatus Clouds

  • Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil).
  • Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well.
  • See Figure 3, Figure 5 and Figure 7.

Weather FAQ:
MCC - Mesoscale Convective Complex

  • A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night.
  • The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:
  • Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more.
  • Duration: Size criteria must be met for at least 6 hours.
  • Eccentricity: Minor/major axis at least 0.7.
  • MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest.
  • During peak intensity, the primary threat shifts toward heavy rain and flooding.

Weather FAQ:
MCS - Mesoscale Convective System

  • A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more.
  • MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others).
  • MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

Weather FAQ:
MED - Minimal Erythemal Dose

  • Minimal erythemal dose (MDE), a measurement of UV dose. Erythema is the reddening of the skin due to capillary congestion. Sunburn is among the most common forms of erythema.

  Wikipedia: Sunburn

Weather FAQ:
Medium Range

  • In forecasting, (generally) three to seven days in advance.

Weather FAQ:
Meridional Flow

  • Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced.
  • The accompanying zonal (east-west) component often is weaker than normal.
  • Compare with zonal flow.

Weather FAQ:

  • A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm).
  • The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.
  • Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration.
  • Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

Weather FAQ:

  • A mesoscale high pressure area, usually associated with MCSs or their remnants.

Weather FAQ:

  • Mesolow (or Sub-synoptic Low)
  • A mesoscale low-pressure center.
  • Severe weather potential often increases in the area near and just ahead of a mesolow.

Weather FAQ:

  • MesoMap is a combination of the terms "mesonet" and "map".
  • In meteorology, "mesonet" is a network of weather stations used to monitor weather over a regional area.
  • Such stations gather data like temperature, pressure, humidity, wind speed/direction, cloud conditions, and precipitation.
  • A "map" is a graphical representation of some area.
  • Thus, a weather MesoMap is a graphical representation of observing stations (usually surface stations) designed to display regional scale weather information. This allows an observer to gain a perspective of weather phenomena otherwise unobtainable.

  WeatherBonk/Google MesoMap

Weather FAQ:

  • A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.

Weather FAQ:

  • Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems.
  • Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems.

Weather FAQ:
Metars (or Aviation Routine Weather Report)

  • Acroymn for METeorological Aerodrome Report (or from the french term MÉTéorologique Aviation Régulière). It is the primary observation code used in the North America to satisfy requirements for reporting surface meteorological data.
  • Minimum reporting requirments includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting.
  • METAR reports typically come from airports or permanent weather observation stations. Reports are typically generated once an hour; if conditions change significantly, however, they can be updated in special reports called SPECIs. Some reports are encoded by automated airport weather stations located at airports, military bases, and other sites.
  • Some locations still use augmented observations, which are recorded by digital sensors, encoded via software, and then reviewed by certified weather observers or forecasters prior to being transmitted. Observations may also be taken by trained observers or forecasters who manually observe and encode their observations prior to transmission.
  • Victoria International Airport (CYYJ) is this site's primary METAR. Victoria International Airport is also the weather most often quoted by weather sources for the Vancouver Island south island area (but can be over an hour old!).

  Wikipedia: Metar

Weather FAQ:

  • A small, concentrated downburst affecting an area less than 4 kilometers (about 2.5 miles) across.
  • Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

Weather FAQ:
Mid-Level Cooling

  • Local cooling of the air in middle levels of the atmosphere (roughly 8 to 25 thousand feet), which can lead to destabilization of the entire atmosphere if all other factors are equal.
  • Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.

Weather FAQ:
MilliBar (unit)

  • Millibar, a unit of pressure equal to one-thousandths of a bar. One bar equals 105 pascals, 105 newtons per square meter, or 106 dynes per square centimeter. One millibar equals one hectopascal.

  Wikipedia: MilliBar (unit)

Weather FAQ:
Moderate Risk

Weather FAQ:
Moisture Advection

  • of moisture by horizontal winds.

Weather FAQ:
Moisture Convergence

  • A measure of the degree to which moist air is converging into a given area, taking into account the effect of converging winds and moisture advection.
  • Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.

Weather FAQ:
Morning Glory

  • An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable.
  • Morning glories result from perturbations related to gravitational waves in a stable boundary layer.
  • They are similar to ripples on a water surface; several parallel morning glories often can be seen propagating in the same direction.

Weather FAQ:
MRF - Medium-Range Forecast model

  • One of the operational forecast models run at NCEP.
  • The MRF is run once daily, with forecast output out to 240 hours (10 days).

Weather FAQ:
Multi-Cell(ular) Thunderstorm

  • A thunderstorm consisting of two or more cells, of which most or all are often visible at a given time as distinct domes or towers in various stages of development.
  • Nearly all thunderstorms (including supercells) are multi-cellular, but the term often is used to describe a storm which does not fit the definition of a supercell.

Weather FAQ:

  • Multiple-vortex (or Multi-vortex) Tornado
  • A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other.
  • Multiple-vortex tornadoes can be especially damaging. See suction vortex.

Weather FAQ:

  • [Slang] A thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.


Weather FAQ:
NCEP - National Centers for Environmental Prediction

  • The modernized version of NMC.

Weather FAQ:
Negative-Tilt Trough

  • An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest).
  • A negative-tilt trough often is a sign of a developing or intensifying system.

Weather FAQ:
NEXRAD - NEXt-Generation Weather RADar

  • Technologically-advanced weather radar being deployed to replace WSR-57 and WSR-74 units.
  • NEXRAD is a high-resolution Doppler radar with increased emphasis on automation, including use of algorithms and automated volume scans.
  • NEXRAD units are known as WSR-88D.

Weather FAQ:
NGM - Nested Grid Model

  • One of the operational forecast models run at NCEP. The NGM is run twice daily, with forecast output out to 48 hours.

Weather FAQ:
NMC - National Meteorological Center

  • National Meteorological Center, with headquarters near Washington D.C. is now known as NCEP.

Weather FAQ:
NOAA - National Oceanographic and Atmospheric Administration

  • NOAA is an agency that enriches life through science. NOAA's reach goes from the surface of the sun to the depths of the ocean floor as NOAA works to keep citizens informed of the changing environment around them.
  • From daily weather forecasts, severe storm warnings and climate monitoring to fisheries management, coastal restoration and supporting marine commerce, NOAA’s products and services support economic vitality and affect more than one-third of America’s gross domestic product.
  • NOAA’s dedicated scientists use cutting-edge research and high-tech instrumentation to provide citizens, planners, emergency managers and other decision makers with reliable information they need when they need it.
  • NOAA's roots date back to 1807, when the Nation’s first scientific agency, the Survey of the Coast, was established. Since then, NOAA has evolved to meet the needs of a changing country.NOAA maintains a presence in every state and has emerged as an international leader on scientific and environmental matters.
  • NOAA’s mission touches the lives of every North American and NOAA is proud of its role in protecting life and property and conserving and protecting natural resources.

Weather FAQ:

  • Nighttime, or nightly, or during the night
  • Related to actions which are completed in the course of a calendar night, and which typically recur every calendar night (e.g., diurnal temperature rises during the day, and nocturnal temperature falls at night).

Weather FAQ:

  • A short-term weather forecast, generally out to six hours or less.

Weather FAQ:
NSSFC - National Severe Storms Forecast Center

  • The National Severe Storms Forecast Center is located in Kansas City MO and is now known as SPC.

Weather FAQ:
NSSL - National Severe Storms Laboratory

  • The National Severe Storms Laboratory (or NSSL) is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma.
  • NSSL investigates all aspects of severe weather to improve severe weather warnings and forecasts in order to save lives and reduce property damage. Research areas include weather radar, automated algorithm detection tools for use with weather radar, and basic tornado research to understand how tornadoes form.
  • NSSL scientists developed the first Doppler weather radar, and have since contributed to the development of NEXRAD, as well as research mobile radar systems.
  • NSSL also works with the Storm Prediction Center to help verify and improve severe weather forecasting.

  Wikipedia: National Severe Storms Laboratory

Weather FAQ:
NWP - Numerical Weather Prediction.

  • Numerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. While the first efforts to accomplish this were done in the 1920's, it wasn't until the advent of the computer that it was feasible to do in real-time.
  • Manipulating the huge datasets and performing the complex calculations necessary to do this on a resolution fine enough to make the results useful requires the use of some of the most powerful supercomputers in the world.
  • A number of forecast models, both global and regional in scale, are run to help create forecasts for nations worldwide. Use of model ensemble forecasts helps to define the forecast uncertainty and extend weather forecasting farther into the future than would otherwise be possible.

  Wikipedia: Numerical Weather Prediction

Weather FAQ:
NWS - National Weather Service

  • The National Weather Service (NWS), once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration (NOAA) of the United States government.
  • It is tasked with providing "weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy." This is done through a collection of national and regional centers, and more than 122 local weather forecast offices (WFOs).

  Wikipedia: National Weather Service


Weather FAQ:
Occluded Mesocyclone

  • A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.

Weather FAQ:

  • Related to, or caused by, physical geography (such as mountains or sloping terrain).

Weather FAQ:
Orographic Lift

  • Lifting of air caused by its passage up and over mountains or other sloping terrain.

Weather FAQ:
Orphan Anvil

  • [Slang] An anvil from a dissipated thunderstorm, below which no other clouds remain.

Weather FAQ:
Outflow Boundary

  • A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature.
  • Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.
  • New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.).
  • See triple point.

Weather FAQ:

  • Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. The latter area is known as a weak-echo region, or WER.
  • The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).
  • See Figure 2.

Weather FAQ:

  • A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. Embedded thunderstorms sometimes develop in such a pattern; severe thunderstorms (mainly with large hail) can occur, but tornadoes are unlikely.
  • Overrunning often is applied to the case of warm air riding up over a retreating layer of colder air, as along the sloping surface of a warm front. Such use of the term technically is incorrect, but in general it refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.

Weather FAQ:
Overshooting Top

  • Overshooting Top (or Penetrating Top)
  • A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm.
  • A persistent and/or large overshooting top (anvil dome) often is present on a supercell.
  • A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm or a cyclic storm.
  • See Figure 3, Figure 5, and Figure 7.


Weather FAQ:
PDS Watch

Weather FAQ:
Pendant Echo

  • Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

Weather FAQ:
Penetrating Top

Weather FAQ:
Popcorn Convection

  • [Slang] Showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.
  • Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

Weather FAQ:
Positive Area

  • The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel.
  • See Figure 6. Positive area is a measure of the energy available for convection.
  • See CAPE.

Weather FAQ:
Positive CG

  • A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge.
  • Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG.
  • Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.

Weather FAQ:
Positive-Tilt Trough

  • An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast).
  • A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Weather FAQ:
Potential Temperature

  • The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 millibar.

Weather FAQ:
PPINE - Plan Position Indicates No Echoes

  • Plan position indicates no echoes, referring to the fact that a radar detects no precipitation within its range.

Weather FAQ:

  • An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile.
  • Profilers operate on the same principles as Doppler radar.

Weather FAQ:
Pseudo-Cold Front

Weather FAQ:
Pseudo-Warm Front

Weather FAQ:
Pulse Storm

  • A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather.
  • These storms generally are not tornado producers, but often produce large hail and/or damaging winds.
  • See overshooting top, cyclic storm.

Weather FAQ:
PVA - Positive Vorticity Advection

  • Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air.
  • PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.



Weather FAQ:
RADAP II - RAdar DAta Processor 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.

Weather FAQ:
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.

Weather FAQ:
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.

Weather FAQ:
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.
  • See Figure 3, Figure 5 and Figure 7.

Weather FAQ:
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 Figure 7.

Weather FAQ:
Red Watch (or Red Box)

  • [Slang] A tornado watch.

Weather FAQ:

  • 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.

Weather FAQ:
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.
  • See humidity.

Weather FAQ:
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.

Weather FAQ:
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).

Weather FAQ:
Right Entrance Region (or Right Rear Quadrant)

Weather FAQ:

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

Weather FAQ:
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.

Weather FAQ:
Right Rear Quadrant

Weather FAQ:
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.

Weather FAQ:
Rope (or Rope Funnel)

Weather FAQ:
Rope Cloud

  • In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary.

Weather FAQ:
Rope Stage

Weather FAQ:
RUC - Rapid Update Cycle

  • 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.

Most of the definitions on this page were borrowed from El Dorado Weather

The "Weather FAQ:" icons on this page are ©
Some of the definitions on this page were borrowed from

Some of the definitions on this page were borrowed from Davis Instruments