Pepper Ridge North Valley's

Weather Glossary/Terminology Page

This Weather Glossary, features an alphabetical listing of over 950 weather-related terms, phrases and abbreviations covering the environmental sciences. The terms with *asterisks before them are related to Storm Spoting and Chasing. The purpose of this glossary to aid the general public in better understanding weather related terms and related NWS products.

Haboob - The name comes from the Arabic word habb, meaning "wind." A Haboob is a giant duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, the wall of dust sometimes reachese 3000ft tall, and travels a 40mph+, they frequently form here in Arizona from the gust front outflow boundary of summer thunderstorms. Haboobs are most frequent in Southwestern North America during the months of May through September, with the most frequent occurrence in late June, but they can occur in any month, and are particularly severe during periods drought.

Hail - Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls, lumps or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones and usually rangesin size from that of a small pea to the size of cherries. Hail occurs most frequently during the summer when thunderstorm activity is at a peak. Extensive damage in the agricultural industry is caused each year by hailstorms .It is reported as "GR" in an observation and on the METAR. Small hail and/or snow pellets is reported as "GS" in an observation and on the METAR.

Halo - In ages past, the huge rings or haloes around the sun or the moon were thought to portend everything from storms to great personal disasters. It is the ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight. The most commonly observed is a halo that forms at a 22 degrees radius, although another one at 46 degrees radius may also be seen. On occasion, only two bright spots on either side of the sun can be seen. These are known as sun dogs and are caused when the ice crystals occur in a certain uniform arrangement.

Haze - A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an bluish or yellowish tinge or opalescent appearance. It is reported as "HZ" in an observation and on the METAR.

HEAT - A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.

HEAT Balance - The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).

HEAT Exhaustion - Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The body has difficulty in cooling the body. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT Index - The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature. For an example, check out the heat index chart.

HEAT Island Effect - This refers to the apparent increase in average mean surface temperatures in urban areas that is the result of increase urbanization with asphalt and concrete jungles retaining the incoming insolation and solar radiation received by the earth during the day and releasing it slower at night resulting in warmer overnight low temperatures especially durning the summertime.

HEAT Lightning - Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.

HEAT Stroke - Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

HEAT Wave - A period of abnormally and uncomfortably hot and usually humid weather. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states with 90 degree plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.

HEATING Degree Day - One heating degree day is given for each degree that the daily mean temperature is below 65 degrees Fahrenheit. It is used as an indication of fuel consumption. Refer to degree day or cooling degree day.

Helicity - A property of a moving fluid, such as air, which represents the potential for helical flow (flow that follows a corkscrew pattern) 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). Atmospheric helicity is computed from the vertical wind profile of the lower atmosphere (usually from the surface up to 3 km), and is measured relative to the motion of a storm, it is used to forecast the formation of mesocyclones. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.

High Clouds - A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus, cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

High Latitudes - The latitude belt roughly between 60 and 90 degrees North and South. Also referred to as the polar region.

HIGH Pressure System - An area of relative of high atmosphericpressure that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. Also known as an anticyclone, it is the opposite of an area of low pressure or a cyclone.

High Risk (of severe thunderstorms) - Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. (Also See slight risk, moderate risk,or the convective outlook.)

Hoar Frost - Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is brought to saturation by cooling.

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. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc...).

Hook Echo - A radar reflectivity pattern characterized or observed in a thunderstorm, that appears like a fish hook. This hook-shaped extension of a thunderstorm echo, usually appears in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornadic storms and possible tornado development. However, hook echoes and tornadoes do not always accompany each other.

Horizon - One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth. The geographic horizon, also called the apparent horizon, is the distant line along which earth and sky appear to meet. This is the usual concept of horizon and is used in weather observing. The local horizon is the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions, such as mountains.

Horse Latitudes - Located between 30 degrees North and South in the vicinity of the equator, this area typically has calm or light and variable winds. Another name for the equatorial trough, the Intertropical Convergence Zone (ITCZ), or the doldrums.

*HP Storm or HP Supercell - A High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.

Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous. See bear's cage.

Hudson Bay Low - An area of low pressure over or near the Hudson Bay area of Canada that often introduces cold air to the north central and northeast United States.

Humbolt Current - Also known as the Peru Current, this ocean current flows northward along the western side of South America, offshore Chile and Peru. There is considerable upwelling of the colder subsurface waters due to the prevailing southerly winds. Dominate weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the weather pattern known as El Nino.

Humidity - Is the a measure of amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity and specific humidity.

Hurricane - (also known as (Typhoon, Tropical Cyclones, Willy- Willies)) - The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean. In the North Atlantic and Eastern Pacific, the hurricane season is from May to November, but the majority of storms occur in August, September and October.

Hurricane Warning - A formal advisory issued by forecasters at the National Hurricane Center when they have determined that a tropical storm or hurricane may threaten a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.

Hurricane Watch - A formal advisory issued by forecasters at the National Hurricane Center when they have determined that a tropical storm or hurricane may threaten a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.

Hydrometeor - Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

Hydrologic Cycle - Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.

Hydrology - The study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface.

Hydrosphere - Considered as the water portion of the earth's surface. Part of the geosphere.

Hygrograph - An instrument that records the hygrometer's measure of water vapor.

Hygrometer - An instrument that measures the water vapor content of the atmosphere. ie(Humidity) See the psychrometer as an example.

HypoThermia - Occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.

Ice - The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail, are examples.

Ice Crystals - Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust". It is reported as "IC" in an observation and on the METAR.

Ice Fog - Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm conditions in the polar latitudes, and may produce a halo around the sun or moon.

Ice Jam - An accumulation of broken river ice caught in a narrow channel, frequently producing local flooding. Primarily occurs during a thaw in the late winter or early spring.

Icelandic Low - A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. It is strongest during a Northern Hemisphere winter and early spring, centered over Iceland and southern Greenland, and is the dominate weather feature in the area. During the summer, it is weaker, less intense, and might divide into two parts, one west of Iceland, the other over the Davis Strait between Greenland and Baffin Island. Then the Azores or Bermuda High becomes the dominate weather feature in the North Atlantic.

Ice Pellets - Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. They have a diameter of 0.2 inches (5 mm) or less. They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets. It is reported as "PE" in an observation and on the METAR. Also known as sleet.

Ice Storm - A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems.

Icicle - Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows.

Icing - The forming or depositing of ice on an object. See glaze.

Impulse- See upper level system.

Inches Of Mercury (Hg) - The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. See barometric pressure. First divised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

Indian Summer - A period of abnormally warm weather in mid to late autumn with mildclear skies and cool nights. This usually occurs after a period of relatively cool or cold weather and the first frost normally precedes this warm spell.

*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.

InfraRed - The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vapor, ozone, and carbon dioxide are capable of absorbing or transmitting infrared radiation. Also referred to as IR.

*Inflow Jets - Local jets of air near the ground flowing inward toward the base of a tornado.

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).

*Inflow Stinger - A beaver tail cloud with a stinger-like shape.

InSolation - Incoming Solar radiation or heating received at the earth's surface The name is derived from INcoming SOLar radiATION. Solar heating; sunshine.

Instability - The state of equilibrium in which a parcel of air when displaced has a tendency for air parcels to accelerate and move further away from its original position; especially, the tendency to accelerate upward after being lifted. It is the condition of the atmosphere when spontaneous convection and severe weather can occur. Air parcels, when displaced vertically, will accelerate upward, often forming cumulus clouds and possibly thunderstorms. The greater the instability, the greater the potential for severe thunderstorms. Refer to absolute instability and unstable. Contrast with stable air. See lifted index.

Instrument Flight Rules (IFR) - Refers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. IFR conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles. See MVFR and VFR.

Instrument Shelter - A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation.

Intermountain HIGH - An area of high pressure that occurs during the winter between the Rocky Mountains and the Sierra-Cascade ranges. It blocks the eastward movement of Pacific cyclones. Also called Plateau High or Great Basin High.

International Date Line - The line of longitude located at 180 degrees East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.

InterTropical Convergence Zone (ITCZ) - An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10 degrees North and South of the equator. It is a broad area of low pressure where both the Coriolis force and the low-level pressure gradient are weak, occasionally allowing tropical disturbances to form. It fluctuates in location, following the sun's rays, so that during the Northern Hemisphere summer, the ITCZ moves northward over the southern North Atlantic and southern Asia.

Inversion - Generally a departure from the usual increase or decrease of an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, an increase in temperature with increasing altitude, which is a departure from the usual decrease of temperature with height. An inversion is present in the lower part of a cap.

Ionosphere - A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is included as part of the thermosphere.

IsalloBar - The line of equal change in atmospheric pressure during a certain time period. It marks the change in pressure tendency.

Isentropic Lift - Lifting of air that is traveling along an upward-sloping isentropic surface.

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.

Isentropic Surface - A two-dimensional surface containing points of equal potential temperature.

IsoBar - The line drawn on a weather map connecting points of equal barometric pressure.

IsodrosoTherm - The line drawn on a weather map connecting points of equal dew point temperature.

IsoHel - A line drawn through geographic points having equal duration of sunshine or another form of solar radiation during a specified time period.

IsoHyet - The line drawn through geographic points recording equal amounts of rainfall during a given time or for a given of storm..

IsoPleth - A line connecting equal points of value of some quantity. Also called an isoline. Examples:Isobars, Isotherms, etc. all are examples of isopleths.

IsoTach - A line connecting equal points of wind speeds.

IsoTherm - The line of equal or constant air temperature. If something is isothermal, it is of equal or constant temperature with respect to either time or space.

January Thaw - A period of mild weather popularly supposed to recur each year in late January.

Jet A fast-moving wind current surrounded by slower moving air. See "Jet Stream"

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

Jet Streak (Same as Jet Max;) - A point or local area ("streak") of relative maximum wind speeds within a jet stream

Jet Stream (abbrev. JSTR) - An area of relatively strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Normally referring to horizontal, high-altitude winds that flow in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. It is marked by a concentration of isotherms and strong vertical shear. 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). Various types include the arctic, the low level jet, the polar, and the subtropical jets.

Jet Stream Cirrus - A loose term for filamentous cirrus that appears to radiate from a point in the sky, and exhibits characteristics associated with strong vertical wind shear, such as twisted or curved filaments.

Jet Wind Speed Profile - A vertical wind speed profile characterized by a relatively narrow current of high winds with slower moving air above and below. A large wind (speed) shear occurs above and below the jet axis.

Jetty - In hydrologic terms, a structure (e.g.; a pier, or mole of wood or stone) extending into a "sea, lake, or river to influence the current or tide or to protect a harbor

Japanese Meteorological Agency (JMA) - The branch of the Japanase government responisible for handling Meteorological forecasts and warnings.

Jokulhlaup - In hydrologic terms, an Icelandic term meaning glacier dammed lake outburst flood.

Joules (J\/kg) - The unit of measure, Joules per kilogram, unit commonly used to represent CAPE and CIN.

Juvenile Water - In hydrologic terms, water formed chemically within the earth and brought to the surface in intrusive rock.

Katabatic Wind - A wind that is created by air flowing downhill. When this air is warm, it may be called a foehn wind, and regionally it may be known as a Chinook or Santa Ana. When this air is cold or cool, it is called a drainage wind, and regionally it may be known as a mountain breeze or glacier wind. The opposite of an anabatic wind.

KataFront - A front where the warm air descends the frontal surface (except in the low layers of the atmosphere).

KELVIN Temperature Scale - A temperature scale with the freezing point of -273 degrees K (Kelvin) and the boiling point of +273 degrees K. It is used primarily for scientific purposes. Also known as the Absolute Temperature Scale. Proposed in 1848 by William T. Kelvin, 1st Baron of Largs (1824-1907), Irish-born Scottish physicist and mathematician.

K Index - The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer.

Kilopascal - The internationally recognized unit used by the Atmospheric Environment Service for measuring atmospheric pressure.

Knot - A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.

*Knuckles - [Slang], For the lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not mammatus clouds. See also cumuliform anvil, anvil rollover.

Lake Effect Snow - Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.

*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.

Land Breeze - A diurnal coastal breeze that blows offshore, from the land to the sea. It is caused by the temperature difference when the sea surface is warmer than the adjacent land. Predominate during the night, it reaches its maximum about dawn. It blows in the opposite direction of a sea breeze.

Landfall - The point at which a tropical cyclone's eye first crosses a land mass.

*Landspout - A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud (visually) or mesocyclone(on radar). It may be observed beneath cumulonimbus or towering cumulus clouds (often as no more than a dust whirl),and is essentially the land equivalent of a waterspout.

La Nina - This is the opposite effect of El Nino this term to refer to a cooling of coastal waters Equatorial Pacific and it usually occurs after El Nino event. It has come to be used as a term for abnormal cooling events which usually have the opposite effects of a El Nino year, the Southern U.S. usually has drier than normal precipitation and this can lead to droughts. The US government has a very interesting site dedicated to La Nina

Lapse Rate - The decrease or rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height and is a sign of instability and a steepening lapse rate implies that destabilization is occurring.. See absolute instability.

Latent Heat - The energy released or absorbed during a change of state. Examples include condensation and sublimation.

Latitude - The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

Lee/Leeside/LEEward - The side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore, protected from the direct force of the wind. The opposite of windward.

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. See also entrance region, right rear quadrant.

*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. See also right mover.

Lenticular Cloud - A cloud species which has elements resembling smooth lenses or almonds and more or less isolated. These clouds are caused by a wave wind pattern created by the mountains. They are also indicative of down-stream turbulence on the leeward side of a barrier.

Level Of Free Convection (LFC) - The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

LEWP - Line Echo Wave Pattern - A bulge in a thunderstorm line producing a wave-shaped "kink" in the line. 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.

Lifted Index (LI) - A commom measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level(around 18,000 feet, usually) and comparing it to the actual temperature at that altitude. Positive values indicate more stable air and negative values indicate instability , the more negative number, 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.

Lifting Condensation Level (LCL) - The height at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.

Lightning - A sudden and visible discharge of electricity produced in a thunderstorm, creating a flash of light generated by the flow of electrons between opposite charged parts of a cumulonimbus cloud. This can occur within a cloud, between clouds, from the cloud to air, or from the cloud to the ground. For examples, see heat lightning and ball lightning.

Lightning Flash - Refers to the sudden and visible discharge of electricity produced in a thunderstorm, creating a flash of light generated by the flow of electrons between opposite charged parts of a cumulonimbus cloud.

Lightning Stroke - Refers to the shimmer effect of a lightning flash This caused by the multiple strokes of lightning within a single flash or bolt.

Light Waves - That part of the electromagnetic spectrum that contains visible light. The colors, from longest wave length to shortest, are red, orange, yellow, green, blue, indigo, and violet (ROY G. BIV). See visible light.

Line Echo Wave Pattern (LEWP) - A bulge in a line of thunderstorms, producing a wave-shaped in the line. It is seen as a scalloped radar echo signature and is often associated with severe weather.

Lithometeor - Atmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze.

Lithosphere - The solid, outer portion of the earth's crust coupled to the rigid upper mantle. Part of the geosphere.

Loaded Gun(Sounding)- [Slang], For 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.

Longitude - The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean Time.

Long Wave Trough - A wave or troughin the prevailing westerly flow aloft which is characterized by a large length and and (usually) long duration or amplitude. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. A long wave trough moves slowly and is persistent. Being associated with major cold troughs and warm ridges,their position and intensity govern weather patterns (e.g., hot/cold, wet/dry) over a period of day or weeks. 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)

Low Clouds - A term used to signify clouds with bases below 6,000 feet and are of a stratiform or a cumuliform variety. Stratiform clouds include stratus and stratocumulus. Cumuliform clouds include cumulus and cumulonimbus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

Low Latitudes - The latitude belt between 30 and 0 degrees North and South of the equator. Also referred to as the tropical or torrid region.

Low Level Jet (LLJ) - Is A region of relatively strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. It is often amplified at night. Specifically, it often refers to the southerly wind over the US Plains states during 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. Aslo see the jet stream.

Low Pressure System - An area of a relative lowpressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. See closed low, cold low, and cut-off low for further examples. Also known as a cyclone

*LP Storm (or LP Supercell) - A 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. 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.

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

Lunar Eclipse - An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color.

Mackeral Sky - The name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. The rippled effect gives the appearance of fish scales.

MacroBurst - A large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds.

Macroscale - The meteorological scale for obtaining weather information covering an area ranging from the size of a continent to the entire globe.

Magnetic Poles - Either of the two points on the earth's surface where the magnetic meridians converge. They are not aligned with the geographical poles, but shift and do not lie exactly opposite of the other.

MammatoCumulus - An obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. Although they do not cause severe weather, they often accompany storms.

*Mammatus Clouds - The newer term used to describe the 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.

Mare's Tail - The name given to thin, wispy cirrus clouds composed of ice crystals that appear as veil patches or strands, often resembling a horse's tail.

Marginal Visual Flight Rules (MVFR) - Refers to the general weather conditions pilots can expect at the surface. VFR stands for Visual Flight Rules and MVFR means Minimum or Marginal Visual Flight Rules. MVFR criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility. Contrast with IFR.

Marine Wind Warnings - Weather reports and advisories for ships and fishing interests;

Small Craft Warning - issued if winds are forecast to be in the range of 20 to 33 knots inclusive.
Gale Warning - issued if winds are forecast to be in the range of 34 to 47 knots inclusive.
Storm Warning - issued if the winds are forecast to be in the range of 48 to 63 knots inclusive.
Hurricane Force Wind Warning - issued for winds of 64 knots or greater.

Maritime Air Mass - An air mass influenced by the sea. It is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. For example, mP is an air mass that is maritime polar in nature. Also known as a marine air mass.

Maximum - The greatest value attained by a function, for example, temperature, pressure, or wind speed. The opposite of minimum.

Maximum/Minimum thermometer - a thermometer that marks the lowest temperature (minimum) or highest temperature (maximum) since the previous reading (usually 1 day).

Mean Sea Level - The average height of the sea surface water level. For the United States, it is computed by averaging the levels of all tide stages over a nineteen year period, determined from hourly height readings measured from a fix, predetermined reference level. It is used as a basis for determining elevations, as the reference for all altitudes in upper air measurements, and as the level above which altitude is measured by a pressure altimeter for aviation. Often referred to as MSL. See sea level.

Mean Temperature - The average of temperature readings taken over a specified amount of time. Often the average of the maximum and minimum temperatures.

Measured Ceiling - ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. See variable ceiling.

Medium Range Forecast - In weather forecasting, (generally) refers to a forecast that is issued three to seven days in advance.

Melting Level - The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere. Refer to a bright band.

Melting Point - The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.

Mercurial Barometer - An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

Meridional Flow - A Large-scale atmospheric circulation in which the north and south, or meridional, component of motion is (i.e., longitudinal, or along a meridian) is unusually pronounced. This weakens the accompanying zonal (east-west) component zonal flow. Compare with zonal flow.

*Mesocyclone - A area of rotation of storm size that may be found on the southwest part of a supercell. Its area of circulation is usually 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).

Meso High - A small mesoscale, concentrated area of high pressure that may be created by the cold outflow and rain-cooled air from thunderstorms. It often forms a pseudo cold front or squall line on its leading edges. It is often associated with MCSs or their remnants. Also known as a bubble high.

Meso Low (or Sub-synoptic Low) - A small mesoscale low pressure center the size of an individual thunderstorm. Its presence may increase severe weather potential, in the area near and just ahead of a mesolow.

A Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.

Mesonet - A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes. Also see mobile mesonet.

Mesoscale - The scale of meteorological phenomena that includes MCCs, MCSs, and squall lines. It is a size scale used to indentify these weather systems may cover fifty to several hundreds of miles. Smaller phenomena are classified as storms, while larger are classified as synoptic-scale.

*Mesoscale Convective Complex (MCC) - A large mesoscale convective system (MCS) which is generally round or oval-shaped, and covers an area about the size of the state of Ohio or Iowa and lasts at least 6 hours. Forming during the afternoon and evening, this is the peak time for associated severe weather. The complex normally reaches its peak intensity at night when heavy rainfall and flooding become the primary threat.

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.

*Mesoscale Convective System (MCS) - A large organized convective weather system comprised of a number of large individual thunderstorms, that have becomed organized. It normally persists for several hours and may be rounded or linear in shape. MCSs include weather systems such as tropical cyclones, squall lines, and MCCs (among others). This term is often used to describe a cluster of thunderstorms that does not meet the criteria of size, shape, or duration a Mesoscale Convective Complex (MCC).

Mesosphere - The layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures drop rapidly with increasing height. It extends between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

Metar - Acroymn for METeorological Aerodrome Report. It is the primary observation code used in the United States 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.

Meteorologist - A person or a scientist who studies meteorology. There are many different paths within the field of meteorology. For example, one could be a research meteorologist, radar meteorologist, climatologist, or (my personal favourite) operational meteorologist.

Meteorology - The science and study of the physics, chemistry, and dynamics of the atmosphere and atmospheric phenomena. Or the study of effects of the atmosphere upon the Earth's surface, the oceans, and life in general. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few.

Microbarograph - A instrument designed to continuously record a barometer's reading. Or a instrument that continuosly records very small changes in atmospheric pressure.

*Microburst - A severe localized downburst wind blasting down from a thunderstorm. It covers an area less than 2.5 miles (4 kilometers) in diameter and is of short duration, usually less than 5 minutes. Although on rare occasions they have been known to last up to 6 times that long. Also see downburst.

Microscale - The smallest scale of meteorological phenomena, such as turbulence, with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain.

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.

Middle Clouds - A term used to signify clouds with bases between 6,000 and 18,000 feet. At the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. Altocumulus, altostratus, and nimbostratus are the main types of middle clouds. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.

Middle Latitudes - The latitude belt roughly between 35 and 65 degrees North and South. Also referred to as the temperate region.

Millibar (MB) - The standard unit of measurement for atmospheric pressure used by the National Weather Service. One millibar is equivalent to 100 newtons per square meter. Standard surface pressure is 1,013.2 millibars.

Minimum - The least value attained by a function, for example, temperature, pressure, or wind speed. The opposite of maximum.

Mist - A collection of microscopic water droplets suspended in the atmosphere. It tends to produce a thin greyish veil over the landscape. It does not reduce visibility as much as fog and is often confused with drizzle.

Mixed Layer - The layer of the water that is mixed through wave action or thermohaline convection.

Mixed Precipitation - Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.

Mobile Mesonet - A group of vehicles equipped with weather observation stations, usually used for research. Also see mesonet.

Moderate Risk (of severe thunderstorms) - Severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See , high risk, slight risk, convective outlook.

Moist Adiabat - The lifting of saturated air, or air that contains water vapor. As a parcel lifts, it cools to its saturation temperature and the relative humidity is then 100 percent. Further cooling results in condensation. When air is moist and is lifted adiabatically, then the temperature decreases at a rate of 0.55 degrees Celsius per 100 meters (2-3 degrees Fahrenheit per 1,000 feet). Contrast with a dry adiabat.

Moisture - Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

Moisture Advection - The 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.

Monsoon - The name is derived from the word mausim, Arabic for season. It refers to the seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is primarily associated with the seasonal reversals of the wind direction, such as the one that takes place along the shores of the southern India, especially in the Arabian Sea, when winds switch from, a dry northeastly direction to a more e southwesterly direction bringing a increase in moisture and copious rains. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States. As monsoons have come to be better understood, the definition has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia and Africa and the adjaacent seas and oceans. It is within these regions that the most vigorous and dramatic cycles of weather events on the earth takes place.

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.

Mountain Breeze - A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite direction of a valley breeze.

Mountain Wave - A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. Also called a standing wave or a lee wave.

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).

Mud Slide - Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.

Muggy - A subjective term for warm and excessively humid weather.

*Multicell(ular) Thunderstorm - A thunderstorm consisting, of two or more single-cell storms, 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.

*Multiple-vortex or Multi Vortex Tornado - A tornado which has two or more condensation funnels or debris cloudsare present at the same time, often rotating around a common center or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.

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

Nadir - The point on any given observer's celestial sphere diametrically opposite of one's zenith.

National Center For Atmospheric Research (NCAR) - A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. For further information, contact NCAR, located in Boulder, Colorado.

National Centers For Environmental Prediction (NCEP) - As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. For further information, contact the NCEP.

National Climatic Data Center (NCDC) - The agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations. For further information, contact the NCDC, located in Asheville, North Carolina.

National Hurricane Center (NHC) - A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific. For further information, contact the NHC, located in Miami, Florida.

National Meteorlogical Center (NMC) - The division of the National Weather Service that produces, processes, handles, and distributes meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations. For further information, contact the NMC, located in Camp Spring, Maryland.

National Oceanic And Atmospheric Administration (NOAA) - A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources. For further information, contact the NOAA, located in Silver Spring, Maryland.

National Severe Storms Forecast Center (NSSFC) - The division of the National Weather Service that maintains a watch for thunderstorm activity on the lower forty-eight states. They are responsible for preparing thunderstorm outlooks and issuing tornado and severe thunderstorm watches. The Center is located in Kansas City, Missouri.

National Severe Storms Laboratory (NSSL) - A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms. For further information, contact the NSSL, headquartered in Norman, Oklahoma.

National Weather Service (NWS) - A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. For further information, contact the NWS.

Nautical Mile - A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles. Refer to a sea mile.

Nautical Twilight - The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars. See twilight.

Neap Tide - A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or three-quarters full. Compare with a spring tide.

Negative Vorticity Advection - Occurs when the rotation of the atmosphere advects lower values of vorticity into an area. This is created by divergence aloft. Contrast with positive vorticity advection.

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.

Newhall Winds - The local name for winds blowing downward from desert uplands through the Newhall Pass southward into the San Fernando Valley, north of Los Angeles.

Newton - The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.

NeXRad - Acronym for NEXt Generation Weather RADar. A network of Technologically-advanced Doppler radars implemented in the United States between 1992 and 1996, it detects the location and intensity of precipitation out to a range of 143 miles from the radar site. NEXRAD replaces the 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. The NEXRAD Doppler radar is highly sensitive and can detect precipitation from very light rain and snow up to the strongest thunderstorms with accuracy and detail. Sometimes, however, the radar's extreme sensitivity will cause ground clutter and other non-precipitation echoes to be displayed in the vicinity of the radar site.

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.

Night - The period of the day between dusk and dawn.

Nimbostratus - This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

Nitrogen (N2) - A colorless, tasteless, ordorless gas that is the most abundant constituent of dry air. It comprises 78.09% of the atmosphere.

NMC - National Meteorological Center, with headquarters near Washington D.C.; now known as NCEP.

N.0.A.A. - An acronym for the National Oceanic and Atmospheric Administration, the governmental body for the United States Weather Service. The Weather Serviceis responsible for issusing weather forecasts, watches and warnings to help serve and protect the U.S. citizens and property

Noctilucent Clouds - Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

Nocturnal - Related to nighttime, or occurring at night.

Nocturnal Thunderstorms - Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.

Nor'Easter - A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

Normal - The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. The usual average is over 30 years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

North Pacific High - A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Comparable systems are the Azores High and the Bermuda High.

Northern Lights - see Aurora Borealis

Nowcast - A short-term weather forecast for expected conditions in the next few hours, generally they are out to six hours or less.

NSSFC - The acronym for National Severe Storms Forecast Center, in Kansas City MO; now known as SPC and located in Norman, OK.

NSSL - The acronym for National Severe Storms Laboratory, in Norman OK.

NWP - Numerical Weather Prediction. (See Numerial Forecasting).

NWS - The acronym for National Weather Service

Numerial Forecasting - The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. These models are run on high-speed computers at the National Meteorological Center.

• A Comprehensive Glossary of Weather Terms for Storm Spotters: by Michael Branick NOAA/NWSFO Norman OK
  


• Everything Weather - The Essential Guide to the Whys and Wonders of Weather.
  


• Famighetti, Robert (ed.). The World Almanac and Book of Facts (1996). Mahwah, New Jersey. Funk & Wagnalls Corporation, 1995.
  


• Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports. National Weather Service, U.S. Department of Commerce, 1996.
  


• More Weather Terms from Environment Canada
  


• American Meteorology Society, 1990: Glossary of Meteorology. American Meteorological Society Press, Boston.
  


• National Weather Service Observing Handbook No. 7, Surface Weather Observations and Reports. U.S. Department of Commerce, 1996.