Back-to-back powerful Pacific storm systems to impact the Pacific Northwest and northern California through the end of this week with heavy rain, flooding, strong winds, and higher elevation mountain snow. A strong, long-duration atmospheric river will accompany the Pacific storms, bringing excessive rainfall and flash flooding to southwest Oregon and northwest California through the week. Read More >
| Weather Glossary | |
| Terms used by meteorologists...weather observers and in weather forecasts. Compiled from several sources) | |
| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | |
| A | |
| ACID RAIN | Cloud or rain droplets containing pollutants, such as oxides of sulfur and nitrogen, to make them acidic. |
| ADIABATIC | Changes in temperature caused by the expansion (cooling) or compression (warming) of a body of air as it rises or descends in the atmosphere. |
| ADIABATIC PROCESS | The change of temperature of air without transferring heat. In an adiabatic process compression results in warming, and expansion results in cooling. |
| ADVECTION | The horizontal transport of air or atmospheric properties. Commonly used with temperatures, i.e., "warm air advection", or moisture, i.e.,"moisture advection". |
| ADVECTION FOG | A type of fog that results from the advection of moist air over a cold surface and the cooling of the air to its dew point that follows; this type of fog is most common in coastal regions. |
| ADVISORY | Issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life-threatening situations. |
| AGL | Above ground level. |
| AIR MASS | A large body of air having similar horizontal temperature and moisture characteristics. |
| AIR PARCEL | An imaginary small body of air that is used to explain the behavior of air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform throughout. |
| AIR PRESSURE | (atmospheric pressure) air pressure is the force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal. |
| ALBERTA CLIPPER | A small, fast-moving low-pressure system that forms in western Canada and travels southeastward into the United States. These storms, which generally bring little precipitation, generally precede an Arctic air mass. |
| ALTIMETER | An active instrument (see active system) used to measure the altitude of an object above a fixed level. |
| ALTIMETER SETTING | That pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea-level of an aircraft on the ground at the location for which the value was determined. |
| ALTITUDE | Height expressed as the distance above a reference point, which is normally sea level or ground level. |
| ALTOCUMULUS | Mid-altitude clouds with a cumuliform shape. |
| ALTOSTRATUS | Mid-altitude clouds with a flat sheet-like shape. |
| ANEMOMETER | An instrument that measures wind speed. |
| ANTICYCLONE | A large area of high pressure around which the winds blow clockwise in the Northern Hemisphere. |
| ANTICYCLONIC | Describes the movement of air around a high pressure, and rotation about the local vertical opposite the earth's rotation. This is clockwise in the Northern Hemisphere. |
| ANVIL | A flat, elongated cloud formation at the top of a thunderstorm. |
| ARCTIC AIR | A mass of very cold, dry air that usually originates over the Arctic Ocean north of Canada and Alaska. |
| ARCTIC HIGH | A very cold high pressure that originates over the Arctic Ocean. |
| ATMOSPHERE | The gaseous envelope surrounding the earth, composed primarily of nitrogen and oxygen. |
| ATMOSPHERIC PRESSURE | (also called air pressure or barometric pressure) The pressure asserted by the mass of the column of air directly above any specific point. |
| ATMOSPHERIC STABILITY | An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started. |
| AVN | Aviation Model generated every 12 hours by NCEP. |
| AWIPS | Advanced Weather Information Processing System. New NWS computer system integrating graphics, satellite and radar imagery. |
| B | |
| BACK DOOR COLD FRONT | A front that moves east to west in direction rather than the normal west to east movement. |
| BACK-BUILDING THUNDERSTORM | A thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain stationary or propagate in a backward direction. |
| BACK-SHEARED ANVIL | A thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe weather potential. |
| BACKING WIND | Wind which shifts in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). Backing winds with height are indicative of cold air advection (CAA). The opposite of veering winds. |
| BAROCLINIC ZONE | A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems. |
| BAROMETER | An instrument for measuring atmospheric pressure. |
| BAROMETRIC PRESSURE | The actual pressure value indicated by a pressure sensor. |
| BAROMETRIC TENDENCY | The amount and direction of change in barometer readings over a three-hour period. |
| BAROTROPIC SYSTEM | A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. |
| BLOCKING HIGH | A high pressure area (anticyclone), often aloft, that remains nearly stationary or moves slowly compared to west-to-east motion. It blocks the movement eastward movement of low pressure areas (cyclones) at its latitude. |
| BLOWING DUST | Reduction of visibility by winds blowing across dry ground with little or no foliage. |
| BOUNDARY LAYER | In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. |
| BOW ECHO | An accelerated portion of a squall line of thunderstorms, taking on bow configuration, created by strong downburst winds. |
| BOX (or WATCH BOX) | A severe thunderstorm or tornado watch. |
| BROKEN CLOUDS | Clouds which cover between 6/10 and 9/10 of the sky. |
| BUBBLE HIGH | A mesoscale area of high pressure, typically associated with cooler air from the rainy downdraft area of a thunderstorm or a complex of thunderstorms. A gust front or outflow boundary separates a bubble high from the surrounding air. |
| BULK RICHARDSON NUMBER (or BRN) |
A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development. |
| BUST | An inaccurate forecast, usually a situation in which significant weather is expected, but does not occur. |
| BWER | Bounded Weak Echo Region. (Also known as a vault.) Radar signature within a thunderstorm characterized by a local minimum in radar reflectivity at low levels which extends upward into, and is surrounded by, higher reflectivities aloft. This feature is associated with a strong updraft and is almost always found in the inflow region of a thunderstorm. It cannot be seen visually. |
| C | |
| CAA | Cold Air Advection |
| CALM | The absence of apparent motion in the air. |
| CAP (or CAPPING INVERSION) | A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. |
| CAPE | Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. |
| Cb | Cumulonimbus cloud |
| CELL | Convection in the form of a single updraft, downdraft, or updraft/downdraft couplet, typically seen as a vertical dome or tower as in a cumulus or towering cumulus cloud. A typical thunderstorm consists of several cells. |
| CELSIUS | A temperature scale in which zero is the freezing point of water and one hundred is the boiling point. |
| CHANCE | A 30, 40 or 50 percent chance of occurrence of measurable precipitation. |
| CEILING | The height of the lowest layer of clouds, when the sky is broken or overcast. |
| CIRRUS | High clouds above 18,000 feet, composed of ice crystals. |
| CLEAR | Sky condition of less than 1/10 cloud coverage. |
| CLEAR SLOT | A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. |
| CLIMATE | The historical record of average daily and seasonal weather events. |
| CLOSED LOW | A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly. |
| CLOUDY | The state of the sky when 7/10ths or more of the sky is covered by clouds. |
| COLD-ARI FUNNEL | A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes. |
| COLD FRONT | The boundary between a cold air mass that is advancing and a relatively warmer airmass. |
| COLD LOW | A low pressure system with cold air mass from near the surface to all vertical levels (also called a cold core low). |
| COLD POOL | A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air. |
| CONDENSATION | The process of gas changing to liquid. The process by which water vapor changes into water droplets and clouds. |
| CONFLUENCE | A pattern of wind flow in which air flows inward toward an axis oriented parallel to the general direction of flow. It is the opposite of difluence. Confluence is not the same as convergence. Winds often accelerate as they enter a confluent zone, resulting in speed divergence which offsets the (apparent) converging effect of the confluent flow. |
| CONVECTION | The transfer of heat within a the air by its movement. The term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. |
| CONVECTIVE OUTLOOK | A forecast containing the area(s) of expected thunderstorm occurrence and expected severity over the contiguous United States, issued several times daily by the SPC. |
| CONVECTIVE TEMPERATURE | The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding. |
| CONVERGENCE | An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region. |
| COOLING DEGREE DAY | A form of degree day used to estimate the required energy for cooling. one cooling degree day occurs for each degree the daily mean temperature is above 65 degrees Fahrenheit. |
| CORIOLIS FORCE | An apparent force caused by the rotation of the earth. In the Northern Hemisphere winds are deflected to the right, and in the Southern Hemisphere to the left. |
| CUMULONIMBUS | A vertically developed cumulus cloud, often capped by an anvil shaped cloud. Also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail or gusty winds. |
| CUMULUS CLOUD | A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower. A cloud less vertically-developed than a cumulonimbus cloud. |
| CUT OFF LOW | An area of low pressure aloft cut off from its associated jet stream. |
| CYCLOGENESIS | Development or intensification of a low-pressure center. |
| CYCLONE | An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere. Also the term used for a hurricane in the Indian Ocean and in the Western Pacific Ocean. |
| CYCLONIC CIRCULATION (or CYCLONIC ROTATION) |
Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. |
| D | |
| DEBRIS CLOUD | A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. |
| DECOUPLE | The tendency for the surface wind to become much lighter than wind above it at night when the surface temperature cools. |
| DEGREE DAY | A measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty-five degrees Fahrenheit. |
| DENDRITE | Hexagonal ice crystals with complex and often fernlike branches. |
| DENSE FOG | A fog in which the visibility is less than one-quarter mile. |
| DENSE FOG ADVISORY | Issued when fog is expected to reduce visibility to 1/4 mile or less over a widespread area. |
| DEW | Moisture that has condensed on objects near the ground, whose temperatures have fallen to the dew point temperature. |
| DEW POINT | The temperature to which the air must be cooled for water vapor to condense. |
| DIFLUENCE (or DIFFLUENCE) | A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence. |
| DIRTY RIDGE | Most of the time, upper-level ridges bring fairly clear weather as the storms are steered around the ridge. Sometimes, however, strong storms undercut the ridge and create precipitation. Ridges that experience this undercutting by storms are known as dirty ridges because of the unusual precipitation. |
| DISTURBANCE | A disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather. |
| DIURNAL | Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night). |
| DIVERGENCE | The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. |
| DOPPLER RADAR | A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It uses the Doppler effect to measure the velocity of particles suspended in the atmosphere. |
| DOWNBURST | A severe localized downdraft from a thunderstorm. |
| DOWNDRAFT | A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm. |
| DOWNSLOPE | Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions. |
| DOWNSTREAM | In the same direction as a stream or other flow, or toward the direction in which the flow is moving. |
| DRIZZLE | Small, slowly falling water droplets, with diameters between .2 and .5 millimeters. |
| DRY LINE | A line that separates very warm, moist air to the east from hot, dry air to the west. |
| DRY LINE BULGE | A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. |
| DRY PUNCH | A surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. |
| DRY SLOT | A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs. |
| DUST DEVIL | A small, rapidly rotating wind that is made visible by the dust, dirt, or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons. |
| DYNAMICS | Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere. |
| E | |
| EASTERLY WAVE | A wavelike disturbance in the tropical easterly winds that usually moves from east to west. Such waves can grow into tropical depressions. |
| ECMWF | European Center for Meteorology Weather Forecast model. |
| EL NINO | A major warming of the equatorial waters in the Pacific Ocean. El Nino events usually occur every 3 to 7 years, and are characterized by shifts in "normal" weather patterns. |
| ENTRANCE REGION | The region upstream from a wind speed maximum in a jet stream (jet max), in which air is approaching (entering) the region of maximum winds, and therefore is accelerating. This acceleration results in a vertical circulation that creates divergence in the upper-level winds in the right half of the entrance region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the right rear quadrant (or right entrance region) of the jet max. Severe weather potential sometimes increases in this area as a result. |
| ENSO | El Nino-Southern Oscillation. |
| EQUILIBRIUM LEVEL (or EL) | On a sounding, the level above the level of free convection (LFC) at which the temperature of a rising air parcel again equals the temperature of the environment. |
| ETA | "Eta" (from Greek) model generated every 12 hours by NCEP. |
| EVAPORATION | The process of a liquid changing into a vapor or gas. |
| EXCESSIVE HEAT ADVISORY | Issued when the combined effect of high temperatures and high humidities result in daytime heat indices greater than or equal to 105 degrees F, and nighttime ambient temperatures greater than or equal to 80 degrees F persist for two days or longer. |
| EXIT REGION | The region downstream from a wind speed maximum in a jet stream (jet max), in which air is moving away from the region of maximum winds, and therefore is decelerating. This deceleration results in divergence in the upper-level winds in the left half of the exit region (as would be viewed looking along the direction of flow). This divergence results in upward motion of air in the left front quadrant (or left exit region) of the jet max. Severe weather potential sometimes increases in this area as a result. |
| F | |
| FAHRENHEIT | The standard scale used to measure temperature in the United States; in which the freezing point of water is thirty-two degrees and the boiling point is two hundred and twelve degrees. |
| FAIR | Describes weather in which there is less than 4/10ths of opaque cloud cover, no precipitation, and there is no extreme visibility, wind or temperature conditions. |
| FEW | A cloud layer that covers between 1/8th and 2/8ths of the sky. |
| FLANKING LINE | A line of cumulus connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest side of the storm. The cloud line has roughly a stair step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms. |
| FLASH FLOOD | A flood that occurs within a few hours (usually less than six) of heavy or excessive rainfall, dam or levee failure. |
| FLASH FLOOD WARNING | Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely. |
| FLASH FLOOD WATCH | Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent. |
| FLOOD STAGE | The level of a river or stream at which considerable inundation of surrounding areas will occur. |
| FLURRIES | Light snow falling for short durations. No accumulation or just a light dusting is all that is expected. |
| FOG | The visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface. Essentially a cloud whose base is at the earth's surface. |
| FREEZE | Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time. |
| FREEZE WARNING | Issued for the first major freeze of the winter season...for a major freeze during the growing season...and at other times when freezing temperatures are of such magnitude or duration that widespread damage to water pipes may occur. |
| FREEZING | The change in a substance from a liquid to a solid state. |
| FREEZING DRIZZLE | Drizzle that falls in liquid form and then freezes upon impact with the ground or an item with a temperature of 32 degrees Fahrenheit or less, possibly producing a thin coating of ice. Even in small amounts, freezing drizzle may cause traveling problems. |
| FREEZING FOG | A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog. |
| FREEZING LEVEL | The altitude in the atmosphere where the temperature equals 32F. |
| FREEZING RAIN | Rain which falls as liquid then freezes upon impact, resulting in a coating of ice on exposed objects. |
| FRONT | The transition zone between two distinct air masses. The basic frontal types are cold fronts, warm fronts, occluded fronts, and stationary fronts. |
| FROST | The covering of ice, due to condensed water vapor, that is formed on exposed surfaces whose temperature falls below freezing. |
| FUJITA SCALE | System developed by Dr. Theodore Fujita to classify tornadoes based on wind damage. Scale is from F0 for weakest to F5 for strongest tornadoes. |
| FUJIWHARA EFFECT | The Fujiwhara effect describes the rotation of two storms around each other. |
| FUNNEL CLOUD | A rotating, cone-shaped column of air extending downward from the base of a thunderstorm, but not in contact with the ground. When it reaches the ground it is then called a tornado. |
| G | |
| GEOSTATIONARY SATELLITE | A satellite that rotates at the same rate as the earth, thus remaining over the same spot above the equator. |
| GOES | Geostationary Operational Environmental Satellite. |
| GRADIENT | The time rate or spatial rate of change of an atmospheric property. |
| GRAUPEL | Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail. |
| GRAVITY WAVE | A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium. Waves on the ocean are examples of gravity waves. |
| GREENHOUSE EFFECT | The warming of the atmosphere by the trapping of earth's longwave radiation being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide. |
| GROUND FOG | Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog. |
| GUST | A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph. |
| GUST FRONT | The leading edge of the downdraft from a thunderstorm. |
| GUSTNADO | Gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. |
| H | |
| HAIL | Precipitation in the form of circular or irregular-shaped lumps of ice. |
| HALOS | Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds. |
| HAZE | Fine dry or wet dust or salt particles in the air that reduce visibility. |
| HEAT INDEX | An index that combines air temperature and humidity to give an apparent temperature (how hot it feels). |
| HEAT ISLAND | A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement. |
| HEATING DEGREE DAY | A form of degree day used to estimate the required energy for heating. One heating degree day occurs for each degree the daily mean temperature is below 65 degrees Fahrenheit. |
| HEAVY SNOW | Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours. |
| HELICITY | A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). |
| HIGH | The center of an area of high pressure, accompanied by anticyclonic and outward wind flow in the northern hemisphere. Also known as an anticyclone. |
| HIGH WIND WARNING | Issued whe non-convective synoptic-scale gradient or mesoscale post-convective wind speeds are sustained at greater than or equal to 40 mph sustained and/or wind gusts reach or exceed 58 mph. |
| HODOGRAPH | A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. |
| HOOK ECHO | A radar pattern sometimes observed in the southwest quadrant of a tornadic thunderstorm. Appearing like a fishhook turned in toward the east, the hook echo is precipitation aloft around the periphery of a rotating column of air 2-10 miles in diameter. |
| HUMIDITY | The amount of water vapor in the atmosphere. (See relative humidity). |
| HURRICANE | A severe tropical cyclone with sustained wind speeds in excess of 74 mph (64 knots). |
| I | |
| ICE PELLETS | Precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (5 mm), or less. There are two main types. Hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice. |
| INDIAN SUMMER | An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather. |
| 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). |
| INSTABILITY | A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation. |
| INVERSION | An increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. |
| ISENTROPIC LIFT | Lifting of air that is traveling along an upward-sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation. |
| ISENTROPIC SURFACE | A two-dimensional surface containing points of equal potential temperature. |
| ISOBAR | A line of equal barometric pressure on a weather map. |
| ISODROSOTHERM | A line of equal dew point temperature. |
| ISOHYET | A line of equal precipitation amounts. |
| ISOPLETH | General term for a line of equal value of some quantity. Isobars, isotherms, etc. all are examples of isopleths. |
| ISOTACH | A line of equal wind speed. |
| ISOTHERM | A line of equal temperature on a weather map. |
| J | |
| JET STREAK | A local wind speed maximum within a jet stream. |
| JET STREAM | Strong winds concentrated within a narrow band in the atmosphere. The jet stream often "steers" surface features such as fronts and low pressure systems. |
| K | |
| KELVIN TEMPERATURE SCALE | A temperature scale in which 0 degrees is the point at which all molecular motion ceases (absolute zero). |
| KNOT | One nautical mile per hour (1.15 mph). |
| L | |
| LAKE EFFECT | The effect of a lake (usually a large one) in modifying the weather near the shore and down wind. It is often refers to the enhanced rain or snow that falls downwind from the lake. This effect can also result in enhanced snowfall along the east coast of New England in winter. |
| LAMINAR | 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. |
| LA NINA | A cooling of the equatorial waters in the Pacific Ocean. |
| LANDSPOUT | A tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts. |
| LAPSE RATE | The change in temperature with altitude in the atmosphere. |
| LATENT HEAT | The heat energy that must be absorbed when a substance changes from solid to liquid and liquid to gas, and which is released when a gas condenses and a liquid solidifies. |
| LAYER | An array of clouds and/or obscurations whose bases are at approximately the same level. |
| LEFT FRONT QUADRANT (or LEFT EXIT REGION) |
The area downstream from and to the left 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. |
| LEFT MOVER | A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm. |
| 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 if thunderstorms develop they are more likely to be stronger. |
| LIFTING | The forcing of air in a vertical direction by an upslope in terrain or by the movement of a denser air mass. |
| LIFTING CONDENSATION LEVEL | The level in the atmosphere where a lifted air parcel reaches its saturation point, and as a result, the water vapor within condenses into water droplets. |
| LIGHTNING | An electrical discharge from a thunderstorm. |
| LIKELY | In probability of precipitation statements, the equivalent of a 60 or 70 percent chance. |
| LOADED GUN (SOUNDING) | 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. |
| 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. |
| LOW | The center of an area of low pressure, accompanied by cyclonic and inward wind flow in the northern hemisphere. Also known as a cyclone. |
| LOW-LEVEL JET | A region of relatively strong winds in the lower part of the atmosphere. |
| M | |
| MACROBURST | Large thunderstorm downbursts with a 2.5 mile diameter or greater outflow of damaging winds lasting 5 to 20 minutes. |
| MEAN TEMPERATURE | The average of a series of temperatures taken over a period of time, such as a day or a month. |
| MEDIUM RANGE | In forecasting, (generally) three to seven days in advance. |
| MEASURABLE | Precipitation of 0.01" or more. |
| MEDIUM RANGE | In forecasting, (generally) three to seven days in advance. |
| MERIDIONAL FLOW | A type of atmospheric circulation pattern in which the north and south component of motion is unusually pronounced. Opposite of zonal flow. |
| MESOCYCLONE | The rotating updraft in a supercell thunderstorm. |
| MESOHIGH | A mesoscale high pressure area, usually associated with MCSs or their remnants. |
| 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. |
| MESONET | A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes. |
| MESOSCALE | Size scale referring to weather systems smaller than synoptic-scale systems but larger than single storm clouds. Horizontal dimensions generally range from around 50 miles to several hundred miles. Squall lines are an example of mesoscale weather systems. |
| MESOSCALE CONVECTIVE COMPLEX (MCC) |
A large mesoscale convective system, 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. |
| MESOSCALE CONVECTIVE SYSTEM (MCS) |
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. |
| METAR | A weather observation near ground level. It may include date and time, wind, visibility, weather and obstructions to vision, sky condition, temperature and dew point, sea level pressure, precipitation amount and other data used for aircraft operations. |
| METEOROLOGY | The study of the atmosphere and atmospheric phenomena. |
| MICROBURST | A strong localized downdraft less than 2.5 miles in diameter from a thunderstorm. Peak gusts last from 2 to 5 minutes. |
| MILLIBAR | A unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars. |
| MIST | Consists of microscopic water droplets suspended in the air which produce a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog. |
| MIXING | Air movements (usually vertical) that make the properties of the air with a parcel homogeneous. It may result in a lapse rate approaching the moist or dry adiabatic rate. |
| MODEL | A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions. |
| MODERATE RISK (of severe thunderstorms) |
Severe thunderstorms are expected to affect between 5 and 10 percent of the area. |
| MOISTURE ADVECTION | Transport of moisture by horizontal winds. |
| 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. |
| MOS | Model Output Statistics. |
| MRF | Medium Range Forecast model generated every 12 hours by NCEP. |
| MSL | Mean sea level. |
| MSLP | Mean sea level pressure. |
| MUGGY | Colloquially descriptive of warm and especially humid weather. |
| MULTICELL CLUSTER 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. |
| MULTIVORTEX 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. |
| N | |
| NCDC | National Climatic Data Center. Located in Asheville, North Carolina, the agency that archives climatic and forecast data from the National Weather Service. |
| NCEP | National Centers for Environmental Prediction. Central computer and communications facility of the National Weather Service; located in Washington, DC. |
| 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. |
| NEXRAD | NEXt Generation RADar. A NWS network of about 160 Doppler radars being installed nationwide. |
| NHC | National Hurricane Center. The office of the National Weather Service in Miami that is responsible for tracking and forecasting tropical cyclones. |
| NOAA | National Oceanic and Atmospheric Administration. A branch of the U.S. Department of Commerce, NOAA is the parent organization of the National Weather Service. |
| NOAA WEATHER WIRE (NWWS) | A computer dissemination network that sends National Weather Service products to the media and public. |
| NOAA WEATHER RADIO (NWR) | Continuous, 24 hour-a-day VHF broadcasts of weather observations and forecasts directly from National Weather Service offices. A special tone allows certain receivers to alarm when watches or warnings are issued. |
| NOCTURNAL | Related to nighttime, or occurring at night. |
| NOR'EASTER | A low-pressure disturbance forming along the South Atlantic coast and moving northeast along the Middle Atlantic and New England coasts to the Atlantic Provinces of Canada. It usually causes strong northeast winds with rain or snow. Also called a Northeaster or Coastal Storm. |
| NORMAL | The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years. |
| NOWCAST | A short-term weather forecast, generally out to six hours or less. |
| NSSL | The National Severe Storms Laboratory. |
| NUCLEUS | A particle of any nature upon which molecules of water or ice accumulate. |
| NUMERICAL FORECASTING | Forecasting the weather through digital computations carried out by supercomputers. |
| NWP | Numerical Weather Prediction. |
| NWS | National Weather Service. |
| O | |
| OCCLUDED FRONT | A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion. |
| OMEGA | A term used to describe vertical motion in the atmosphere. The "omega equation" used in numerical weather models is composed of two terms, the "differential vorticity advection" term and the "thickness advection" term. Put more simply, omega is determined by the amount of spin (or large scale rotation) and warm (or cold) advection present in the atmosphere. On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results. |
| OROGRAPHIC UPLIFT | The vertical forcing of air by terrain features such as hills or mountains. This can create orographic clouds and/or precipitation. |
| OUTFLOW | Air that flows outward from a thunderstorm. |
| OVERRUNNING | A condition that exists when a relatively warm air mass moves up and over a colder and denser air mass on the surface. The result is usually low clouds, fog and steady, light precipitation. |
| OVERCAST | Sky condition when 9/10 or 10/10 of the sky is covered. |
| OVERSHOOTING TOP | A 'bubble' of cloud sticking up above the anvil of a thunderstorm, due to a vigorous updraft within the storm. |
| OZONE | A form of oxygen containing 3 molecules, usually found in the stratosphere, and responsible for filtering out much of the sun's ultraviolet radiation. |
| P | |
| PARTLY CLOUDY | Sky condition when between 3/10 and 7/10 of the sky is covered. Used more frequently at night. |
| PARTLY SUNNY | Similar to partly cloudy. Used to emphasize daytime sunshine. |
| PATCHES | Used with fog to denote random occurrence over relatively small areas. |
| PENDANT ECHO | Radar signature generally similar to a hook echo, except that the hook shape is not as well defined. |
| POLAR VORTEX | A circumpolar wind circulation which isolates the Antarctic continent during the cold Southern Hemisphere winter, heightening ozone depletion. |
| POP | Probability Of Precipitation. Probability forecasts are subjective estimates of the chances of encountering measurable precipitation at some time during the forecast period. |
| 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. |
| POSITIVE-TILE 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. |
| 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 mb. |
| PRECIPITATION | Liquid or solid water molecules that fall from the atmosphere and reach the ground. |
| PRESSURE | The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure. |
| PRESSURE CHANGE | The net difference between pressure readings at the beginning and ending of a specified interval of time. |
| PRESSURE FALLING RAPIDLY | A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more. |
| PRESSURE GRADIENT | The rate of decrease of pressure with distance at a fixed level. |
| PRESSURE GRADIENT FORCE | Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure. |
| PRESSURE RISING RAPDILY | An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more. |
| PRESSURE TENDENCY | The character and amount of atmospheric pressure change during a specified period of time, usually the 3-hour period preceding an observation. |
| PRESSURE UNSTEADY | A pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement. |
| PREVAILING WESTERLIES | Winds in the middle latitudes (approximately 30 degrees to 60 degrees) that generally blow from west to east. |
| PREVAILING WIND | The direction from which the wind blows most frequently in any location. |
| PROFILER | 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. |
| PSYCHROMETER | An instrument used for measuring the water vapor content of the atmosphere. It consists of two thermometers, one of which is an ordinary glass thermometer, while the other has its bulb covered with a jacket of clean muslin which is saturated with distilled water prior to use. |
| 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. |
| 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 precipitation. |
| Q | |
| QPF | Quantitative Precipitation Forecast |
| R | |
| RADAR | An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. RADAR = RAdio Detection And Ranging. |
| RADIATION | Energy emitted in the form of electromagnetic waves. Radiation has differing characteristics depending upon the wavelength. Radiation from the Sun has a short wavelength (ultra-violet) while energy re-radiated from the Earth's surface and the atmosphere has a long wavelength (infra-red). |
| RADIATION FOG | Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as ground fog. |
| RADIATIONAL COOLING | Cooling process of the Earth's surface and adjacent air, which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction, so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere. |
| RADIOSONDE | An instrument attached to a weather balloon that transmits pressure, humidity, temperature, and winds as it ascends. |
| RAIN | Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle. |
| RAIN-FREE BASE | A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft. Tornadoes most commonly develop (1) from wall clouds that are attached to the rain-free base, or (2) from the rain-free base itself. This is particularly true when the rain-free base is observed to the south or southwest of the precipitation shaft. |
| RAINBOW | Optical phenomenon when light is refracted and reflected by moisture in the air into concentric arcs of color. |
| RAWINSONDE | A balloon that is tracked by radar to measure wind speeds and wind directions in the atmosphere. |
| REFLECTIVITY | Radar term referring to the ability of a radar target to return energy; used to estimate precipitation intensity and rainfall rates. |
| REFRACTION | The bending of light as it passes through areas of different density, such as from air through ice crystals. |
| RELATIVE HUMIDITY | The amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated. (Expressed as a percentage). |
| 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. |
| 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). |
| RFC | River Forecast Center. The Arkansas-Red Basin River Forecast Center is located in Tulsa, OK. |
| RIDGE | An elongated area of high pressure at the surface or aloft. |
| RIGHT ENTRANCE REGION (or RIGHT READ 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. |
| 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.) |
| ROLL CLOUD | A relatively rare, low-level horizontal, tube-shaped accessory cloud completely detached from the cumulonimbus base. When present, it is located along the gust front and most frequently observed on the leading edge of a line of thunderstorms. The roll cloud will appear to be slowly "rolling" about its horizontal axis. Roll clouds are not and do not produce tornadoes. |
| ROPE (or ROPE FUNNEL) | A narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. |
| ROPE CLOUD | In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. |
| 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. |
| RUC | 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. |
| S | |
| SCATTERED CLOUDS | Sky condition when between 1/10 and 5/10 of the sky is covered. |
| SCUD CLOUDS | Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow. |
| SEA-LEVEL PRESSURE | The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level. |
| SEVERE THUNDERSTORM | A strong thunderstorm with wind gusts in excess of 58 mph (50 knots) and/or hail with a diameter of 3/4" or more. |
| SEVERE THUNDERSTORM WARNING | Issued when thunderstorms are expected to have wind gusts to 58 mph or above or hail 3/4 inch or more in diameter. |
| SEVERE THUNDERSTORM WATCH | Issued when conditions are favorable for the development of severe thunderstorms in and close to a defined area. |
| SHELF CLOUD | Long, wedge-shaped clouds associated with the gust front. Shelf clouds indicate the downdraft or outflow of a thunderstorm. |
| SHEAR (WIND SHEAR) | Variation in wind speed and/or direction over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances. |
| SHORT-FUSE WARNING | A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less. |
| SHORTWAVE (SHORTWAVE TROUGH) | A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave. |
| SHOWER | Precipitation that is intermittent, in space, time, or intensity. |
| SKY CONDITION | The state of the sky in terms of such parameters as sky cover, layers and associated heights, ceiling, and cloud types. |
| SKY COVER | The amount of the sky which is covered by clouds or obscurations in contact with the surface. |
| SLEET | A type of frozen precipitation, consisting of small transparent ice pellets. |
| SLIGHT RISK (of severe thunderstorms) |
Severe thunderstorms are expected to affect between 2 and 5 percent of the area. A slight risk generally implies that severe weather events are expected to be isolated. |
| SLING PSYCHROMETER | A psychrometer in which the wet and dry bulb thermometers are mounted upon a frame connected to a handle. The psychrometer may be whirled by hand in order to provided the necessary ventilation. |
| SLIGHT CHANCE | In probability of precipitation statements, usually equivalent to a 20 percent chance. |
| SNOW | Frozen precipitation composed of ice particles in complex hexagonal patterns. |
| SNOW FLURRIES | Light snow showers, usually of an intermittent nature with no measurable accumulation. |
| SOUNDING | A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location ( example). Soundings are used extensively in weather forecasting, e.g., to determine instability, locate temperature inversions etc. |
| SOUTHERN OSCILLATION | A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. |
| SPC | Storm Prediction Center. Located in Norman, OK. This office is responsible for monitoring and forecasting severe convective weather in the continental U.S. This includes the issuance of Tornado and Severe Thunderstorm Watches. |
| SPEED SHEAR | The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere. |
| SPIN-UP | A small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms. |
| SQUALL LINE | A non-frontal band or line of thunderstorms. |
| STABILITY | An indication of how easily a parcel of air is lifted. If the air is very stable it is difficult to make the parcel rise. If the air is very unstable the parcel may rise on its own once started. |
| STABLE AIR | Air with little or no tendency to rise, that is usually accompanied by clear dry weather. |
| STATEMENT | Provides the public with information concerning the status of existing warnings. |
| STATION INDENTIFIER | A group of four alphabetic characters used to identify a location that makes weather observations. |
| STATION PRESSURE | The pressure that is read from a barometer but is not adjusted to sea level. |
| STATIONARY FRONT | A transition zone between air masses, with neither advancing upon the other. |
| STEERING WINDS (Steering Currents) |
A prevailing synoptic scale flow which governs the movement of smaller features embedded within it. |
| STORM TRACK | The path that a low pressure area follows. |
| STORM-RELATIVE | Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity. |
| STORM-SCALE | Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale, mesoscale. |
| STRAIGHT LINE WINDS | Thunderstorm winds most often found with the gust front. They originate from downdrafts and can cause damage which occurs in a "straight line", as opposed to tornadic wind damage which has circular characteristics. |
| STRATIFORM | Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. |
| STRATOCUMULUS | Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. |
| STRATOSPHERE | The layer of the atmosphere above the troposphere, where temperature increases with height. |
| STRATUS | Flat low level clouds. |
| STRIATIONS | Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. |
| SUBLIMATION | The change from ice directly to water vapor or from water vapor to ice with out going through the liquid water phase. |
| SUBSIDENCE | Sinking air that is associated with warming air and little cloud formation. |
| SUBTROPICAL JET | The branch of the jet stream that is found in the lower latitudes. |
| SUCTION VORTEX (sometimes SUCTION SPOT) |
A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices. |
| SUPERCELL | A highly organized thunderstorm with a rotating updraft, known as a mesocyclone. It poses an inordinately high threat to life and property. Often produces large hail, strong winds, and tornadoes. |
| SUPERCOOLED WATER | Water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing point. |
| SUPERSATURATION | The condition which occurs in the atmosphere when the relative humidity is greater than 100 percent. |
| SURFACE PRESSURE | The pressure that is read from a barometer but is not adjusted to sea level. |
| SUSTAINED WINDS | The wind speed obtained by averaging the observed values over a one minute period. |
| SWEAT INDEX | Severe Weather ThrEAT index. A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds. |
| SYNOPTIC CHART | Chart showing meteorological conditions over a region at a given time; weather map. |
| SYNOPTIC SCALE (LARGE SCALE) | Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale. |
| T | |
| TAF | A weather forecast for aircraft operations at an airport. |
| TELECONNECTION | A strong statistical relationship between weather in different parts of the globe. For example, there appears to be a teleconnection between the tropics and North America during El Ni�o. |
| THERMAL | Small rising column of air due to surface heating. |
| THERMODYNAMICS | In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability. |
| THETA-E (or Equivalent Potential Temperature) |
The temperature a parcel of air would have if a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. |
| THETA-E RIDGE | An axis of relatively high values of theta-e. Severe weather and excessive rainfall often occur near or just upstream from a theta-e ridge. |
| THUNDER | The sound wave produced as a lightning stroke heats the air causing it to rapidly expand. |
| THUNDERSTORM | A storm with lightning and thunder, produced by a cumulonimbus cloud, and usually associated with gusty winds, heavy rain, and sometimes hail and tornadoes. |
| TILTED STORM or TILTED UPDRAFT |
A thunderstorm or cloud tower which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development. |
| TOPOGRAPHY | Generally, the lay-out of the major natural and man-made physical features of the earth's surface. Bridges, highways, trees, rivers and fields are all components that make up this topography. |
| TORNADO | A violently rotating column of air below the base of a thunderstorm, and in contact with the ground. A tornado does not require the visible presence of a condensation funnel cloud. |
| TORNADO ALLEY | The area of the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River Valleys. Although no state is entirely free of tornadoes, they are most frequent in the Plains area between the Rocky Mountains and Appalachians. |
| TORNADO FAMILY | A series of tornadoes produced by a single supercell, resulting in damage path segments along the same general line. |
| TORNADO WARNING | Issued when there is likelihood of a tornado within the given area based on radar or actual sighting. It is usually accompanied by conditions indicated for Severe Thunderstorm Warning. |
| TOTOAL-TOTALS INDEX | A stability index and severe weather forecast tool, equal to the temperature at 850 mb plus the dew point at 850 mb, minus twice the temperature at 500 mb. |
| TOWERING CUMULUS | A large cumulus cloud with great vertical development, usually with a cauliflower-like appearance, but lacking the characteristic anvil shaped top of a Cb. (Often shortened to "towering cu," and abbreviated TCU.) |
| TRACE | Precipitation amounts less than 0.01". |
| TRADE WINDS | Persistent low-level tropical winds that blow from the subtropical high pressure centers towards the equatorial low. |
| TRIPLE POINT | The intersection point between two boundaries (dry line, outflow boundary, cold front, warm front etc.), often a focus for thunderstorm development. |
| TROPICAL DEPRESSION | Tropical mass of thunderstorms with a cyclonic wind circulation and winds between 20 and 34 knots. |
| TROPICAL DISTURBANCE | An organized mass of tropical thunderstorms, with a slight cyclonic circulation and winds less than 20 knots. |
| TROPICAL STORM | An organized cyclone in the tropics with wind speed between 35 and 64 knots. |
| TROPOPAUSE | The boundary between troposphere and the stratosphere. It is usually characterized by an abrupt change in temperature with height from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). |
| TROPOSPHERE | The lowest layer of the atmosphere where the temperature decreases with height. Most of earth's weather occurs in this layer. |
| TROUGH | An elongated area of low pressure at the surface or aloft. |
| TURBULENCE | Disrupted flow in the atmosphere that produces gusts and eddies. |
| TVS | Tornadic Vortex Signature. Doppler radar signature in the radial velocity field indicating intense, concentrated rotation - more so than a mesocyclone. |
| U | |
| UKMET | United Kingdom forecast model. |
| ULTRAVIOLET RADIATION | The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition. |
| UNSTABLE AIR | Air that rises easily and can form clouds and rain. |
| UPDRAFT | A small-scale current of rising air. This is often associated with cumulus and cumulonimbus clouds. |
| UPPER LEVEL SYSTEM | A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. |
| UPSLOPE FLOW | Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable. |
| UPSTREAM | Toward the source of the flow, or located in the area from which the flow is coming. |
| UTC | Coordinated Universal Time. The time in the zero degree meridian time zone. |
| UVI | Ultraviolet Index |
| UVV | Upward Vertical Velocity. |
| V | |
| VAPOR PRESSURE | The pressure exerted by water vapor molecules in a given volume of air. |
| VARIABLE CEILING | A ceiling of less than 3,000 feet which rapidly increases or decreases in height by established criteria during the period of observation. |
| VEERING WIND | Wind which changes in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). Veering winds with height are indicative of warm air advection (WAA). |
| VERTICAL SHEAR | The rate of change of wind speed or direction, with a given change in height. |
| VERTICALLY-STACKED SYSTEM | A low-pressure system, usually a closed low or cutoff low, which is not tilted with height, i.e., located similarly at all levels of the atmosphere. |
| VICINITY | A proximity qualifier used to indicate weather phenomena observed between 5 and 10 statute miles of the usual point of observation but not at the station. |
| VIL | Vertically-Integrated Liquid water. A property computed by RADAP II and WSR-88D units that takes into account the three-dimensional reflectivity of an echo. The maximum VIL of a storm is useful in determining its potential severity, especially in terms of maximum hail size. |
| VIRGA | Precipitation falling from the base of a cloud and evaporating before it reaches the ground. |
| VIRTUAL TEMPERATURE | The temperature a parcel of air would have if the moisture in it were removed and its specific heat was added to the parcel. |
| VISIBILITY | The horizontal distance an observer can see and identify a prominent object. |
| VORT MAX | (Short for vorticity maximum), a center, or maximum, in the vorticity field of an airmass. |
| VORTICITY | A measure of the amount of "spin" (rotation) and "shear" in the atmosphere. |
| VORTEX | An atmospheric feature that tends to rotate. It has vorticity and usually has closed streamlines. |
| W | |
| WAA | Warm Air Advection |
| WALL CLOUD | An isolated lowering of a cloud that is attached to the rain-free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm. |
| WARM FRONT | A boundary between a warm air mass that is replacing a cooler air mass. |
| WARNING | Issued when a particular hazard is "imminent" or already occurring (e.g., tornado warning, flash flood warning). |
| WATCH | Forecast issued in advance to alert the public of the possibility of a particular hazard (e.g., tornado watch, flash flood watch). |
| WATER EQUIVALENT | The liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow. |
| WATER VAPOR | Water substance in a gaseous state that comprises one of the most important of all the constituents of the atmosphere. |
| WATERSPOUT | A column of rotating air over a body of water (i.e., a tornado over the water). |
| WAVE | In meteorology any pattern identifiable on a weather map that has a cyclic pattern, or, a small cyclonic circulation in the early stages of development that moves along a cold front. |
| WAVE CREST | The highest point in a wave. |
| WAVE TROUGH | The lowest point in a wave. |
| WAVELENGTH | Physical distance of one period (wave repeat). |
| WEATHER BALLOON | Large balloon filled with helium or hydrogen that carries a radiosonde (weather instrument) aloft to measure temperature pressure and humidity as the balloon rises through the air. It is attached to a small parachute so that when the balloon inevitably breaks, the radiosonde doesn't hurtle back to earth dangerously quickly. |
| WEATHER SYNOPSIS | A description of weather patterns affecting a large area. |
| WEDGE (or WEDGE TORNADO) | A large tornado with a condensation funnel that is at least as wide (horizontally) at the ground as it is tall (vertically) from the ground to cloud base. |
| WFO | Weather Forecast Office. Oklahoma has 2 WFOs...one located in Tulsa and the other in Norman. |
| WIND | Air in motion relative to the surface of the earth. |
| WIND ADVISORY | Issued for sustained winds between 24 to 39 mph...or...sustained winds of less than 25 miles per hour but frequent gusts between 25 and 39 miles per hour. |
| WIND ALOFT | The wind speeds and wind directions at various levels in the atmosphere above the area of surface. |
| WIND CHILL | The additional cooling effect resulting from wind blowing on bare skin. The wind chill is based on the rate of heat loss from exposed skin caused by the combined effects of wind and cold. The (equivalent) wind chill temperature is the temperature the body "feels" for a certain combination of wind and air temperature. |
| WIND CHILL FACTOR | The apparent temperature which describes the cooling effect on exposed skin by the combination of temperature and wind, expressed as the loss of body heat. Increased wind speed will accelerate the loss of body heat. The formula to calculate wind chill is: WC=.0817(3.71 V^.5 + 5.81 - .25 v)(T-91.4)+91.4 where V=wind speed in MPH and T=temperature F. |
| WIND CHILL ADVISORY | Issued when wind chill values of minus 20 degrees or colder are expected which will cause exposed flesh to become quickly frostbitten or frozen. |
| WIND CHILL WARNING | Issued when life-threatening wind chills of -50F or colder are expected. |
| WIND DIRECTION | The direction from which the wind is blowing. |
| WIND SHEAR | The change of wind speed or direction with distance or height. |
| WIND SPEED | The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, or gust). |
| WIND VANE | An instrument that determines the direction from which a wind is blowing. |
| WINTER STORM WATCH | A significant winter storm may affect your area, but its occurrence, location and timing are still uncertain. A winter storm watch is issued to provide 12 to 36 hours notice of the possibility of severe winter weather. A watch will often be issued when neither the path of a developing winter storm nor the consequences of the weather event are as yet well defined. Ideally, the winter storm watch will eventually be upgraded to a warning when the nature and location of the developing weather event becomes more apparent. A winter storm watch is intended to provide enough lead time so those who need to set plans in motion can do so. |
| WINTER STORM WARNING | Issued when when a variety of hazardous conditions are forecast to occur across the area, or when there is difficulty in determining the type of conditions which will predominate. A warning is used for winter weather conditions posing a threat to life and property. |
| WINTER WEATHER ADVISORY | Issued when one to 3 inches of new snow is expected or icing which makes driving and walking hazardous. Tree branches and power lines may become ice-coated but damage or breakage is not expected. An advisory can also be issued when a variety of events are forecast to occur across an area...or the type of event is difficult to specify.
An advisory for less than one inch may be issued if the time of year or other circumstances warrant special caution in the opinion of the forecaster. |
| WSR-88D | Weather Surveillance Radar - 1988 Doppler; NEXRAD unit. |
| Z | |
| ZONAL FLOW (Zonal Wind) | Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. |
| ZULU TIME | Same as UTC, Universal Coordinated Time. Is is called Zulu because Z is often appended to the time to distinguish it from local time. |
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