Glossary

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

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Absolute humidity: The mass of water vapor per unit volume of air containing the water vapor; usually expressed as grams of water vapor per cubic meter of air.
Absolute instability: Property of an ambient air layer that is unstable for both saturated (cloudy) and unsaturated (clear) air parcels.
Absolute stability: Property of an ambient air layer that is stable for both saturated (cloudy) and unsaturated (clear)air parcels.
Absolute zero: The theoretical temperature at which a body does not emit electromagnetic radiation and all molecular activity ceases (usually some subatomic activity takes place); 0 K.
Absorption: The process whereby a portion of the radiation incident on an object is converted to heat.
Adiabatic process: Expansional cooling or compressional warming of air parcels in which there is no net heat exchange between the air parcels and the surrounding (ambient) air.
Advanced Weather Interactive Processing System (AWIPS): A computerized workstation that enables the National Weather Service meteorologists to integrate a variety of weather data and displays.
Advection fog: Ground-level clouds generated by the cooling of a mild, humid air mass as it travels over a relatively cool surface.
Aerosols: Tiny liquid or solid particles of various composition that occur suspended in the atmosphere.
Agroclimatic compensation: Poor growing weather and decreased crop yields in one area is offset to some extent by better growing weather and increased crop yields in other areas.
Air density: Mass per unit volume of air; about 1.275 kg per cubic meter at 0 ·C and 1000 millibars.
Air mass: A huge volume of air covering thousands of square kilometers that is relatively uniform in temperature and water vapor concentration.
Air mass advection: Horizontal movement of air or air masses from one place to another.
Air mass modification: Changes in the temperature, humidity, or stability of an air mass as it travels away from its source region.
Air pressure: The cumulative force exerted on any surface by the molecules composing air; usually expressed as the weight of a column of air per unit surface area.
Air pressure gradient: Change in air pressure with distance.
Air pressure tendency: Change in air pressure with time; on a surface weather map, the air pressure change over the prior 3 hours.
Albedo: The fraction or percent of radiation striking a surface that is reflected by that surface.
Altimeter: An aneroid barometer calibrated to read in altitude or elevation.
Altocumulus clouds (Ac): Middle clouds consisting of roll-like patches or puffs forming a wavy pattern.
Altocumulus lenticularis: A lens-shaped altocumulus cloud; a mountain-wave cloud generated by the disturbance of horizontal airflow by a prominent mountain range.
Altostratus clouds (As): Middle layer clouds that are uniformly gray or bluish white.
Aneroid barometer: A portable instrument that utilizes a flexible metal chamber and spring to measure air pressure; may be used as an altimeter.
Anomalies: Departures of temperature, precipitation, or other weather elements from long-term averages.
Antarctic Circle: Poleward of this latitude (66 degrees 33 minutes S) there are 24 hours of sunlight on the summer solstice and 24 hours of darkness on the winter solstice.
Antarctic ozone hole: A large area of significant stratospheric ozone depletion over the Antarctic continent that typically develops annually between late August and early October, and generally ends in mid-November. Ozone thinning is attributed to the action of chlorine (Cl) liberated from a group of chemicals known as chlorofluorocarbons (CFCs).
Anticyclone: A dome of air that exerts relatively high surface pressure compared with surrounding air; same as a high. Viewed from above in the Northern Hemisphere, surface winds in an anticyclone blow clockwise and outward.
Aphelion: The time of the year when the Earth is farthest from the sun (about 4 July).
Arctic air (a): A very cold and dry air mass that forms primarily in winter over the Arctic Basin, Greenland, and the northern interior of North America.
Arctic Circle: Poleward of this latitude (66 degrees 33 minutes N) there are 24 hours of sunlight at the summer solstice and 24 hours of darkness at the winter solstice.
Arctic high: Anticyclone originating in the source regions for cold, dry arctic air.
Atmosphere: A thin envelope of gasses (also containing suspended solid and liquid particles and clouds) that encircles the globe.
Atmospheric stability: Property of ambient air that either enhances (unstable) or suppresses (stable) vertical motion of air parcels; depends on the vertical temperature profile or sounding of the ambient air and whether air parcels are saturated (cloudy) or unsaturated (clear).
Atmospheric windows: Infrared wavelength bands within which there is little or no absorption by the major greenhouse gases (e.g., H20, CO2, O3).
Aurora australis: Southern hemisphere equivalent of the aurora borealis.
Aurora borealis: A luminous phenomenon in the night sky consisting of overlapping curtains of greenish-white light, sometimes fringed with pink, appearing in the ionosphere of high latitudes. It is caused by the emission of light by atoms and molecules that are excited by beams of electrons generated by a complex interaction between the solar wind and the Earth’s atmosphere.
Automated Surface Observing System (ASOS): Meteorological sensors that record and transmit atmospheric conditions automatically; a component of the modernization program of the National Weather Service.

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Back-door cold front: A surge of relatively cool air that advances from the east or northeast; usually along the North Atlantic coast.
Barograph: A recording instrument that provides a continuous trace of air pressure variation with time.
Barometer: An instrument used to monitor variations in air pressure. See also mercury barometer and aneroid barometer.
Beaufort scale: A scale of wind speed based originally on visual assessment of the effects of wind on seas.
Bergeron-Findeisen process: Precipitation formation in cold clouds whereby ice crystals grow at the expense of supercooled water droplets.
Blackbody: A perfect radiator or absorber; a material that absorbs 100% of the radiation striking it and emits the maximum possible radiation at all wavelengths.
Blizzard warning: Issued when falling or blowing snow is accompanied by winds of over 55 km (35 mi) per hour and reduced visibility.
Blocking system: A cutoff cyclone or anticyclone that blocks the usual west-to-east progression of weather systems.
Bowen ratio: For a moist surface, the ratio of heat energy used for sensible heating (conduction and convection) to the heat energy used for latent heating (evaporation of water or sublimation of snow). The Bowen ratio ranges from 0.1 for the ocean surface to more that 2.0 for deserts; negative values are also possible.
British thermal unit (Btu): The quantity of heat needed to raise the temperature of one pound of water one Fahrenheit degree (from 62 to 63 ·F).

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Calorie (cal): The amount of heat needed to raise the temperature of one gram of water one Celsius degree (from 14.5 to 15.5 ·C).
Centripetal force: An inward-directed force that confines an object to a curved path; the result of other forces.
Chinook wind: Air that is adiabatically compressed as it is drawn down the leeward slope of a mountain range. As a consequence, the air is warm and dry.
Chromosphere: Portion of the sun above the photosphere; consists of transparent ionized hydrogen and helium at 4000 to 40,000 ·C.
Cirrocumulus cloud (Cc): A high cloud composed of ice crystals that exhibits a wavelike pattern of small white puffs.
Cirrostratus cloud (Cs): A high, thin, layered cloud composed of ice crystals that forms a thin white veil over the sky.
Cirrus cloud (Ci): A high thin cloud occurring as silky strands and composed of ice crystals.
Climate: Weather of some locality averaged over some time period plus extremes in weather behavior observed during the same period or during the entire period of record.
Climatology: The study of climate and its controls and variability.
Cloud: A visible suspension of minute water droplets and/or ice crystals in the atmosphere above the Earth’s surface. Clouds differ from fog only in that the latter is, by definition, in contact with the Earth’s surface. Clouds form in the free atmosphere primarily as a result of condensation or deposition of water vapor in ascending air.
Cloud condensation nuclei (CCN): Tiny sold and liquid particles on which water vapor condenses.
Cloud street: Clouds aligned in rows due to strong vertical shear in horizontal wind speed or direction.
Cold air advection: Flow of air from relatively cool localities to relatively warm localities.
Cold clouds: Clouds composed of ice crystals or supercooled water droplets or a mixture of both which have temperatures below 0 ·C (32 ·F).
Cold front: A narrow zone of transition between relatively cold, dense air that is advancing and relatively warm, less dense air that is retreating.
Cold-core anticyclone: Shallow high-pressure systems that coincide with a dome of relatively cold, dry air.
Cold-core cyclone: An occluded low-pressure system that occupies a column of relatively cold air.
Cold-type occlusion: A front formed when a cold front overtakes a warm front and the air behind the front is colder than the air ahead of the front.
Collision-coalescence process: The growth of cloud droplets into raindrops within warm clouds; droplets merge upon impact.
Comma cloud: The pattern of cloudiness associated with a wave cyclone; the head of the comma stretches from the low center to the northwest and its tail follows along the cold front.
Compressional warming: A temperature rise that accompanies a pressure increase on a parcel of air, as when air parcels descend within the atmosphere.
Conceptual model: A model that describes the general functional relationship among components of a system.
Condensation: The process by which water vapor becomes a liquid.
Conditional stability: Property of an ambient air layer that is stable for unsaturated (clear) air parcels and unstable for saturated (cloudy) air parcels.
Conduction: The transfer of energy (electrical, heat) within and through a conductor by means of internal particle or molecular activity.
Continental drift: The slow movement of continents which are parts of gigantic plates, across the face of the globe.
Continental polar air (cP): Relatively dry air mass that develops over the northern interior of North America; it is very cold in winter and relatively mild in summer.
Continental tropical air (cT): Warm, dry air mass that forms over the subtropical deserts of the southwestern United States.
Contrails: Cloud-like streamers frequently seen forming behind aircraft flying in clear, cold, humid air.
Convection: Vertical air circulation in which cool air sinks and forces warm air to rise.
Convective condensation level (CCL): The altitude at which air parcels rising with the updraft of a convection cell reaches saturation; coincides with the base of cumuliform clouds.
Conveyor-belt model: A three-dimensional depiction of fronts and cyclones in terms of three interacting airstreams, often referred to as conveyor belts.
Cooling degree-day: A measure of the need for air conditioning when the average daily temperature is above 65 ·F (18 ·C); computed by subtracting 65 ·F from the average daily temperature in ·F.
Coriolis force: A deflective force arising from the rotation of the Earth on its axis; affects principally synoptic-scale and planetary-scale winds. Winds are deflected to the right of their initial direction in the Northern Hemisphere and to the left in the Southern Hemisphere.
Corona: Colored rings about the moon or sun; due to diffraction of light by spherical cloud droplets.
Cumuliform clouds: Clouds that exhibit significant vertical development; often produced by updrafts in convection currents.
Cumulonimbus clouds (Cb): Thunderstorm clouds that form as a consequence of deep convection in the atmosphere.
Cumulus clouds (Cu): Clouds that develop as a consequence of the updraft in convection currents; resemble huge puffs of cotton floating in the sky.
Cumulus congestus: An upward-building convective cloud with vertical development between those of a cumulus cloud and a cumulonimbus cloud.
Cumulus stage: Initial stage in the life cycle of a thunderstorm cell; consists of towering cumulus clouds with updraft throughout the system.
Cup anemometer: An instrument used to monitor wind speed. Wind rotates the cups, and that motion is calibrated in wind speed.
Cyclogenesis: The birth and development of a cyclone; a low-pressure system.
Cyclolysis: A process whereby a cyclone weakens; its central pressure rises and its winds slacken.
Cyclone: A weather system characterized by relatively low surface air pressure compared with the surrounding air; same as a low. Viewed from above, surface winds blow counterclockwise and inward in the Northern Hemisphere.

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Dalton’s law: A scientific law that states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each constituent gas.
Deposition: Process by which water changes phase directly from a vapor into a solid without first becoming a liquid.
Dew: Water droplets formed by condensation of water vapor on a relatively cold surface.
Dewpoint: Temperature to which air must be cooled (at constant pressure and constant water vapor content) to achieve saturation (if above 0 ·C or 32 ·F).
Diffraction corona: See corona.
Diffuse insolation: Solar radiation that is scattered or reflected by atmospheric components (clouds, for example) to the Earth’s surface.
Direct insolation: Solar radiation that is transmitted directly through the atmosphere to Earth’s surface without interacting with atmospheric components.
Dissipating stage: The final phase in the life cycle of a thunderstorm cell; features downdrafts throughout the system and vaporization of clouds.
Divergence: A wind pattern whereby there is a net outflow of air from some point.
Doldrums: An east-west belt of light and variable surface winds where the trade winds of the two hemisphere converge.
Doppler effect: A shift in the frequency of an electromagnetic or sound wave due to the relative movement of the source or the observer.
Doppler radar: Weather radar that determines the velocity of targets (precipitation, dust particles) moving directly toward or away from the radar unit based on the difference in frequency (or phase) between the outgoing and returning radar signal.
Downburst: A strong and potentially destructive thunderstorm downdraft; depending on size, classified as either a microburst or a macroburst.
Drizzle: A form of liquid precipitation consisting of water droplets less than 0.5 mm (0.02 in) in diameter; falls from low stratus clouds.
Dropwindsonde: A small instrument package equipped with a radio transmitter that is dropped from an aircraft and measures vertical profiles of temperature, pressure, relative humidity, and wind.
Dry adiabatic lapse rate: Rising unsaturated (clear) air parcels cool at the rate of about 10 Celsius degrees per 1000 m of uplift (or 5.5 Fahrenheit degrees per 1000 ft).
Dry line: A boundary between warm, dry air and warm, humid air in the southeast sector of a mature midlatitude cyclone; likely site for severe thunderstorm development.
Dust devil: A swirling mass of dust triggered by intense solar heating of dry surface areas.

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Eddy viscosity: Frictional resistance arising from eddies (irregular whirls) within a fluid such as air or water.
El Niño: An anomalous warming of surface ocean waters in the eastern tropical Pacific; accompanied by suppression of upwelling off the coasts of Peru and Ecuador. (More info - Midwest Climate: El Niño)
Electromagnetic radiation: Energy transfer in the form of waves that have both electrical and magnetic properties; occurs even in a vacuum.
Electromagnetic spectrum: Range of radiation types arranged by wavelengths or by frequency or both.
Emissivity: A measure of how close a radiating object approximates a blackbody, a perfect radiator.
ENSO (El Niño/Southern Oscillation): An episode of anomalously high sea-surface temperatures in the equatorial and tropical eastern Pacific; associated with large-scale swings in surface air pressure between the western and central tropical Pacific. (More info - Midwest Climate: El Niño)
Equinoxes: The first days of spring and autumn when day and night are of equal length at all latitudes (except the poles) and the noon sun is directly over the equator.
Evaporation: The process by which water changes phase from a liquid to a vapor at a temperature below the boiling point of water.
Evapotranspiration: Vaporization of water through direct evaporation from wet surfaces plus the release of water vapor by vegetation.
Expansional cooling: A temperature drop that accompanies a pressure reduction on an air parcel, as when air parcels ascend within the atmosphere.
Eye wall: A circle of cumulonimbus clouds surrounding the eye of a mature hurricane.

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Fair-weather bias: The observation that fair weather days outnumber stormy days almost everywhere.
Flash flood: A sudden rise in river or stream levels causing flooding. (More info - Living with Weather: Floods)
Forced convection: Convection aided by topographic uplift or converging surface winds.
Free convection: Convection triggered by intense solar heating of Earth’s surface.
Freezing rain: Supercooled raindrops that freeze on contact with cold surfaces.
Friction: The resistance an object encounters as it comes into contact with other objects; in fluids known as viscosity.
Friction layer: The zone of the atmosphere between the Earth’s surface and an altitude of about 1,000 m (3,280 ft), where most frictional resistance is confined.
Front: A narrow zone of transition between air masses of contrasting density, that is, air masses of different temperature or different water vapor concentration or both.
Frontal fog: A cloud formed when precipitation falls from relatively warm air aloft into a wedge of relatively cool air near the Earth’s surface and raises the vapor pressure in the cool air to saturation; occurs either just ahead of a warm front or just behind a cold front. (More info - Living with Weather: Fog)
Frontogenesis: The development or strengthening of a front.
Frontolysis: The dissipation or weakening of a front.
Frost: Ice crystals that are formed by deposition of water vapor on a relatively cold surface.
Frost point: The temperature to which air must be cooled at constant pressure to achieve saturation at or below 0·C (32 ·F).
F-scale (Fujita scale): A tornado intensity scale developed by T. Theodore Fujita that rates tornadoes from F0 to F5 on the basis of rotational wind speed estimated from property damage.
Funnel cloud: A tornadic circulation extending below cloud base but not reaching the ground; made visible by a cone-shaped cloud.

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Gamma radiation: Electromagnetic radiation having very short wavelength and great penetrating power.
Geologic time: A span of millions or billions of years in the past.
Geostationary Operational Environmental Satellite (GOES): Geostationary satellite designed to monitor weather systems.
Geostationary satellite: A satellite that orbits the Earth at the same rate and in the same direction as the Earth rotates so that the satellite is always directly over the same point (the sub-satellite point) on Earth’s equator. The altitude of the satellite’s orbit is about 35,790 km (22,240 mi).
Geostrophic wind: Unaccelerated horizontal wind that flows along a straight path parallel to isobars or contours above the friction layer; results from a balance between the horizontal pressure gradient force and the Coriolis effect.
Glacial climate: Conditions favorable to the initiation and growth of glacial ice.
Global radiative equilibrium: The balance between net incoming solar radiation and infrared radiation emitted to space by the Earth-atmosphere system.
Global water budget: Balance sheet for the inputs and outputs of water to and from the various global water reservoirs; shows a net flow of water from land to sea.
Glory: Concentric rings of color about the shadow of an observer’s head that appear on top of a cloud situated below the observer. A glory is caused by the same optics as a rainbow plus diffraction.
Gradient wind: A theoretical horizontal wind that blows parallel to curved isobars or contours above the friction layer.
Granules: A network of huge, irregularly shaped convective cells in the sun’s photosphere.
Graphical model: A compilation or display of data in a form (e.g., a map) that can be readily useful.
Graupel: Ice pellets, generally 2 to 5 millimeters in diameter, formed in a cloud when supercooled water droplets collide and freeze on impact; a form of frozen precipitation. Sometimes applied to snow pellets.
Green flash: A brilliant green color that occasionally appears on the upper limb of the sun as it rises or sets.
Greenhouse effect: Although nearly transparent to solar radiation, the atmosphere is much less transparent to infrared radiation. Terrestrial infrared radiation is absorbed and radiated primarily by water vapor and, to a lesser extent, by carbon dioxide and other trace gases, thereby slowing the loss of heat to space from the Earth-atmosphere system.
Greenhouse gases: Infrared-absorbing gases that contribute to the greenhouse effect in the Earth-atmosphere system. The main greenhouse gas is water vapor; others are carbon dioxide, ozone, methane, and nitrous oxide.
Ground clutter: Reflection of radar signals by fixed objects such as buildings on the Earth’s surface.
Gust front: Leading edge of a mass of relatively cool gusty air that flows out of the base of a thunderstorm cloud (downdraft) and spreads along the ground well in advance of the parent thunderstorm cell; a mesoscale cold front.

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Haboob: A dust or sandstorm caused by the downdraft of a desert thunderstorm.
Hadley cell: Thermally-driven air circulation in tropical and subtropical latitudes of both hemispheres resembling a huge convective cell with rising air near the equator and sinking air in the subtropical anticyclones.
Hail: Precipitation in the form of nearly spherical or jagged chunks of ice; often characterized by internal concentric layering. Hail is associated with thunderstorm cells that have strong updrafts and relatively great moisture content. (More info - Living with Weather: Hail)
Hailstreak: Accumulation of hail in a long narrow path along the ground.
Hair hygrometer: An instrument designed to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.
Heat: The total kinetic energy of the atoms or molecules composing a substance.
Heat equator: The latitude (about 10 degrees N) of highest mean annual surface air temperature.
Heating degree-day: A measure of space heating needs on days when the average outdoor air temperature falls below 65 ·F (18 ·C); computed by subtracting the day’s average temperature from 65 ·F.
Heat Index: The heat index or “apparent temperature” is an approximation of how hot it “feels” for a given combination of air temperature and relative humidity. Generally, higher relative humidity values at the same temperature feel warmer or more stressful because of less evaporative cooling when people perspire.
Heterosphere: The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.
Homosphere: The atmosphere up to 80 km (50 mi) in which the proportions of principal gases, such as oxygen and nitrogen, are constant.
Hook echo: A distinctive radar pattern that often indicates the presence of a severe thunderstorm cell and perhaps tornadic circulation.
Horse latitudes: Areas of calm winds associated with subtropical anticyclones; near 30 degrees latitude in both hemispheres.
Hot-wire anemometer: An instrument that measures wind speed based on the rate of heat loss to air flowing by a sensor.
Hurricane: An intense warm-core oceanic cyclone that originates in tropical latitudes; called a typhoon in the western Pacific Ocean. Sustained winds are 119 km (74 mi) per hr or higher.
Hygrograph: An instrument that provides a continuous trace of relative humidity variations with time.
Hygroscopic nuclei: Tiny particles of matter that have a special chemical affinity for water molecules, so that condensation may take place on these nuclei at relative humidities under 100 percent.
Hypothesis: A proposed explanation for some observation or phenomenon which is tested through the scientific method.
Hypsithermal: An extended mild episode between about 5000 and 7000 years ago when global mean temperature was somewhat higher (perhaps 1 Celsius degree) than at present.

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Ice pellets: Frozen raindrops that bounce on impact with the ground; also called sleet.
Ice-forming nuclei: Tiny particles that promote the formation of ice crystals at temperatures well below freezing; include freezing nuclei and deposition nuclei.
Infrared radiation (IR): Electromagnetic radiation at wavelengths shorter than microwaves and longer than visible red light; emitted by most objects on Earth.
Insolation: The incoming solar radiation that reaches the Earth’s surface.
Interglacial climate: Conditions that favor the melting of glacial ice (if present).
Intertropical convergence zone (ITCZ): Discontinuous belt of thunderstorms paralleling the equator and marking the convergence of the Northern and Southern Hemisphere surface trade winds.
Inverse square law: Intensity of radiation decreases as the inverse square of distance traveled.
Ionosphere: The region of the upper atmosphere from 80 to 900 km (50 to 600 mi) that contains a relatively high concentration of ions (electrically charged particles).
Isobars: Lines on a map joining localities reporting the same air pressure.

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Jet streak: An area of accelerated air flow within a jet stream.
Jet stream: A narrow, variable band of very strong, predominantly westerly air currents encircling the globe several miles above the earth. There are typically two or three jet streams in each of the northern and southern hemispheres.

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Kinetic energy: The energy within a body that is a result of its motion.

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La Niña: A period of strong trade winds and unusually low sea-surface temperatures (SSTs) in the central and eastern tropical Pacific; opposite of El Niño. (More info - Midwest Climate: El Niño & La Niña)
Lake breeze: A relatively cool mesoscale surface wind directed from a lake toward land in response to differential heating between land and lake; develops during daylight hours.
Land breeze: A relatively cool mesoscale surface wind directed from land to sea or land to lake in response to differential cooling between land and a body of water; develops at night.
Latent heat of fusion: Heat released when water changes phase from liquid to solid; 80 calories per gram.
Latent heat of melting: Heat required to change the phase of water from solid to liquid; 80 calories per gram.
Latent heat of vaporization: Heat required to change the phase of water from liquid to vapor; 540 to 600 calories per gram, depending on the temperature of the water.
Latent heating: Transport of heat from one place to another within the atmosphere as a consequence of phase changes of water. Heat is supplied for evaporation and sublimation of water at the Earth’s surface, and heat is released during condensation and deposition (cloud formation) within the atmosphere.
Law of energy conservation: Energy is neither created nor destroyedbut can change from one form to another; same as the first law of thermodynamics.
Law of reflection: The angle of incident reflection is to equal to the angle of reflected radiation.
Lee-wave clouds: Lens-shaped clouds that form in the crests of a standing wave downwind from a prominent mountain range.
Lifting condensation level (LCL): The altitude to which air must be lifted so that expansional cooling leads to condensation (or deposition) and cloud development; corresponds to the base of clouds.
Lightning: A flash of light produced by an electrical discharge in response to the buildup of an electrical potential between cloud and Earth’s surface, or between different portions of the same cloud.
Lightning detection network (LDN): System that provides real-time information on the location and severity of lightning strokes.
Little Ice Age: The period from about 1550 to 1850 when average global temperatures were lower, and alpine glaciers increased in size and advanced down mountain valleys.

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Macroburst: A downburst (strong downdraft) that affects a path longer than 4.0 km (2.5 mi).
Magnetosphere: Regions of the upper atmosphere encompassed by the Earth’s magnetic field; deflected by solar wind into a teardrop-shaped cavity.
Mammatus clouds: Clouds that form on the underside of a thunderstorm anvil and exhibit pouchlike, downward protuberances; may indicate turbulent air.
Maritime polar air (mP): Cool, humid air masses that form over the cold ocean waters of the North Pacific and North Atlantic.
Maritime tropical air (mT): Warm, humid air masses that form over tropical and subtropical oceans.
Mature stage: The middle and most intense phase in the life cycle of a thunderstorm cell; begins when precipitation reaches Earth’s surface and is characterized by both updrafts and downdrafts.
Maunder minimum: A 70-year period from 1645 to 1715 when sunspots were rare.
Medieval warm period: A relatively mild episode between about 950 and 1250 A.D.
Mercury barometer: A mercury-filled tube used to measure air pressure; the standard barometric instrument, which features great precision.
Meridional flow pattern: Flow of westerlies in a series of deep troughs and sharp ridges; westerlies exhibit considerable amplitude.
Mesocyclone: A vertical column of cyclonically rotating air that develops in the updraft of a severe thunderstorm cell; an early stage in the development of a tornado.
Mesopause: Narrow zone of transition between the mesosphere below and the thermosphere above; the top of the mesosphere.
Mesoscale convective complex (MCC): A nearly circular organized cluster of many interacting thunderstorm cells covering an area of many thousands of square kilometers.
Mesosphere: The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 kilometers above Earth’s surface.
Meteorology: The scientific study of the atmosphere and atmospheric processes.
Microburst: A downburst that affects a path on the ground that is 4.0 km (2.5 mi) or shorter.
Microscale system: The smallest spatial subdivision of atmospheric circulation, such as a tornado.
Microwave: Electromagnetic radiation having wavelengths in the 0.1 to 300 mm ranges; used in weather radar.
Midlatitude westerlies: Planetary-scale prevailing west-to-east winds in the mid- and upper-troposphere between about 30 and 60 degrees of latitude.
Mie scattering: Produced by spherical particles having the same diameter as the wavelength of visible light; light is scattered equally at all wavelengths.
Milankovich cycles: Systematic changes in three elements of Earth-sun geometry: precession of the solstices and equinoxes, tilt of Earth’s rotational axis, and orbital eccentricity; affects the seasonal and latitudinal distribution of incoming solar radiation and influences climatic fluctuations over tens to hundreds of thousands of years.
Mirage: A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher that it actually is, it is called a superior mirage. When an object appears lower that it actually is, it is called an inferior mirage.
Mist: Very thin fog in which visibility is greater than 1.0 km (0.62 mi).
Mixing ratio: Mass of water vapor per mass of dry air; expressed as grams per kilogram.
Moist adiabatic lapse rate: A variable rate of cooling applicable to saturated (cloudy) air parcels that are ascending within the atmosphere. This rate is less than the dry adiabatic lapse rate because some of the expansional cooling is compensated for by the release of latent heat that accompanies condensation or deposition of water vapor.
Molecular viscosity: Frictional resistance arising from the interaction of molecules composing a fluid such as air or water.
Monsoon active phase: A generally cloudy period with frequent deluges of rain.
Monsoon dormant phase: A generally sunny and hot period that interrupts rainy monsoon episodes.
Mountain breeze: A shallow, gusty downslope flow of cool air that develops at night in some mountain valleys.
Mountain-wave clouds: Stationary clouds situated downwind of a prominent mountain range and formed as a consequence of the disturbance of the wind by the mountain range.

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Nacreous clouds: Rarely seen clouds that form in the upper stratosphere; may be composed of ice crystals or supercooled water droplets. Also called mother-of-pearl clouds.
National Centers for Environmental Information (NCEI): Umbrella organization for the Center for Weather and Climate and the Center for Coasts, Oceans, and Geophysics. These centers are responsible for hosting and providing public access to oceanic, atmospheric and geophysical data.
National Climatic Data Center (NCDC): Located in Asheville, North Carolina, this agency houses archives of climatic data of the United States. This agency name changed in 2015 to become the Center for Weather and Climate, one of the National Centers for Environmental Information (NCEI).
National Hurricane Center (NHC): Agency, located in south Florida, that is responsible for forecasting tropical storms and hurricanes along the East and Gulf coasts and the eastern Pacific Ocean.
National Oceanographic and Atmospheric Administration (NOAA): The administrative unit within the U.S. Department of Commerce that oversees the National Weather Service (NWS).
National Weather Service (NWS): The agency of NOAA responsible for weather data acquisition, data analysis, forecast dissemination, and storm watches and warnings.
Neutral air: An air layer in which an ascending or descending air parcel always has the same temperature as its surroundings.
Newton’s first law of motion: An object at rest or in straight-line, unaccelerated motion remains that way unless acted upon by a net external force.
Newton’s second law of motion: A net force is required to cause a unit mass of a substance to accelerate (or decelerate); force=mass x acceleration.
Nimbostratus (Ns): Low, gray, layered clouds that resemble stratus clouds but are thicker and yield more substantial precipitation.
Noctilucent clouds: Wavy, thin clouds resembling cirrus, but usually bluish-white or silvery; best seen at high latitudes just before sunrise or just after sunset. These rare clouds, which occur in the upper mesosphere, may be composed of ice deposited on meteoric dust.
Normals: The arithmetic mean of a climatological element computed over three consecutive decades.
Norwegian cyclone model: The original description of the structure and life cycle of a midlatitude low-pressure system, first proposed during World War Iby researchers at the Norwegian School of Meteorology at Bergen.
Numerical model: One or more mathematical expressions that approximate the behavior of a system such as the Earth-atmosphere system.
NWS Cooperative Observer Network: Consists of more than 8000 weather stations across the United States that record data for hydrologic, agricultural, and climatic purposes.

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Occluded front: A front formed when a cold front overtakes and merges with a warm front; represents the final stage in the life cycle of a midlatitude cyclone.
Orographic lifting: The forced rising of air up the slopes of a hill or mountain.
Outgassing: Release of gasses to the atmosphere from hot, molten rock during volcanic activity; thought to be the origin of most atmospheric gases.
Overrunning: The process whereby less dense air displaces more dense air by flowing up and over the denser air; occurs along a warm front.
Ozone shield: Ozone (O3) within the stratosphere that filters out potentially lethal intensities of ultraviolet radiation (UV) from the sun.

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Parhelia: Two bright spots of light appearing on either side of the sun; each is separated from the sun by an angle of 22 degrees. Parhelia are caused by refraction of sunlight by ice crystals; also called mock suns and sundogs.
Penumbra: Outer, lighter area of a sunspot.
Perihelion: The time of the year when the Earth’s orbital path brings it closest to the sun (about 3 January).
Persistence: Tendency for weather episodes to continue for some period of time.
Photodissociation: The process by which radiation breaks down molecules into their smallest components.
Photosphere: The visible surface of the sun.
Photosynthesis: The process whereby plants use sunlight, water, and carbon dioxide to manufacture their food and generate oxygen as a byproduct.
Physical model: A miniaturized version of a real system.
Plages: Small, bright, relatively hot spots on the sun.
Planetary albedo: The fraction (or percent) of solar radiation that is scattered and reflected back into space by the Earth-atmosphere system.
Planetary-scale systems: The largest spatial scale of atmospheric circulation; includes the global wind belts and semipermanent pressure systems.
Polar amplification: The tendency of a major temperature change to increase in magnitude with latitude.
Polar front: Transition zone between cold polar easterlies and mild midlatitude westerlies.
Polar front jet stream: A corridor of strong westerlies in the upper troposphere between the midlatitude tropopause and the polar tropopause and directly over the polar front.
Polar high: Cold anticyclone that originates in a source region for continental polar air.
Polar-orbiting satellite: A satellite in relatively low orbit that travels near the geographical poles on meridional trajectories.
Poleward heat transport: Flow of heat from tropical to middle and high latitudes in response to latitudinal imbalances in radiational heating and cooling. Poleward heat transport is accomplished primarily by air mass exchange, but also by storms, and ocean currents.
Precipitable water: The depth of water produced when all the water vapor in a column of air is condensed; usually the column of air reaches from Earth’s surface to the “top” of the atmosphere.
Precipitation: Water in solid or liquid form that falls toEarth’s surface from clouds.
Pressure gradient force: A force operating in the atmosphere that accelerates air parcels away from regions of high pressure toward regions of low pressure in response to an air pressure gradient.
Primary air pollutant: Substances that become pollutants immediately upon entering the atmosphere.
Pyranometer: The standard instrument for measuring solar radiation incident on a horizontal surface; calibrates the temperature response of a special sensor in units of radiation flux.

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Radar echo: Microwaves scattered or reflected by distant rain or snow back to a receiver where they are displayed as bright spots on a cathode ray tube.
Radiation fog: A ground-level fog formed by nocturnal radiational cooling of a humid air layer so that its relative humidity approaches 100 %; sometimes called ground fog.
Radio waves: Long-wavelength, low-frequency electromagnetic waves.
Radiosonde: A small balloon-borne instrument package equipped with a radio transmitter that measures vertical profiles (soundings) of temperature, pressure, and humidity in the atmosphere.
Rain: A form of precipitation consisting of liquid water droplets having diameters between 0.5 and 5.0 mm (0.02 and 0.2 in).
Rain gauge: A device – usually a cylindrical container – for collecting and measuring rainfall.
Rain shadow: A region situated downwind of a high mountain barrier and characterized by descending air and, as a consequence, a relatively dry climate.
Rainbow: An arc of concentric colored bands formed by refraction and internal reflection of sunlight by raindrops. Observer must be looking at a distant rain shower with the sun at his/her back.
Rawinsonde: A radiosonde tracked from the ground by a direction-finding antenna to measure variations in horizontal wind direction and wind speed with altitude.
Rayleigh scattering: Caused by spherical particles whose diameter is much smaller than the wavelength of scattered radiation; responsible for the blue of the daytime sky.
Reflection: The process whereby a portion of the radiation that is incident on a surface is reflected by that surface.
Refraction: The bending of a light ray as it passes from one transparent medium to another (from air to water, for example). Bending is due to the differing speeds of light in the two media.
Relative humidity: A measure of how close air is to saturation at a specific temperature, always expressed as a percentage.
Roll cloud: A low, cylindrically-shaped and elongated cloud occurring behind a gust front; associated with but detached from a cumulonimbus cloud.
Rossby waves: Series of long-wavelength troughs and ridges that characterize the planetary-scale westerlies as they encircle the globe; also called long waves.

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Saffir-Simpson Hurricane Intensity Scale: Scale based on central pressure, and specifying a range of wind speed, height of storm surge, and damage potential; 1 is minimal, 5 is most intense.
Santa Ana wind: A hot, dry chinook-type wind that blows from the desert plateaus of Utah and Nevada downslope toward coastal southern California.
Saturation mixing ratio: Maximum concentration of water vapor in a given volume of air at a specific temperature.
Saturation vapor pressure: The maximum vapor pressure in a sample of air at a specific temperature.
Scattering: The process by which small particles disperse radiation in all directions.
Scientific method: A systematic form of inquiry that involves observation, speculation, and formulation and testing of hypotheses.
Scientific model: An approximation or simulation of a real system that omits all but the most essential variables of the system.
Scientific theory: A hypothesis that is widely accepted by the scientific community.
Sea breeze: A relatively cool mesoscale surface wind directed from sea toward land in response to differential heating of land and sea; develops during daylight hours.
Second law of thermodynamics: All systems tend toward disorder.
Secondary air pollutant: Pollutants generated by chemical reactions occurring within the atmosphere.
Semipermanent pressure systems: Persistent cyclones and anticyclones that are components of planetary-scale circulation. These pressure cells exhibit some seasonal changes in location and surface pressures.
Sensible heating: The transport of heat from one location or object to another via conduction, convection or both.
Severe thunderstorm: Thunderstorms accompanied by locally damaging surface winds, frequently lightning, or large hail. (More info - Living with Weather: Thunderstorms)
Shelf cloud: A low, wedge-shaped, and elongated cloud that occurs along a gust front; associated with and attached to a cumulonimbus cloud.
Short waves: Relatively small short-wavelength ripples (troughs and ridges) superimposed on long waves in the planetary-scale westerlies; they propagate with the air flow through long waves in the middle and upper troposphere.
Single-station forecasts: Weather forecasts based on observations at one location.
Snow: A type of precipitation consisting of an assemblage of ice crystals in the form of plates, columns and flakes.
Snow grains: Frozen form of precipitation consisting of opaque particles of white ice having diameters less than 1 mm; originates in the same way as drizzle.
Snow pellets: Frozen form of precipitation consisting of soft spherical (or sometimes conical) particles of opaque, white ice having diameters of 2 to 5 mm. They often break up when striking a hard surface and differ from snow grains in being softer and larger.
Solar altitude: The angle of the sun 90 degrees or less above the horizon.
Solar constant: The flux of solar radiational energy falling on a surface that is positioned at the top of the atmosphere and oriented perpendicular to the solar beam when Earth is at its average distance from the sun.
Solar wind: A stream of charged subatomic particles (mainly protons and electrons) flowing into space from the sun.
Solstice: A time during the year when the sun is at its maximum poleward location relative to the Earth (23 degrees, 30 minutes, North or South); the first days of summer and winter.
Soundings: Continuous altitude measurements that provide profiles of such variables as temperature, humidity, and wind speed.
Southern oscillation: Opposing swings of surface air pressure between the central and western tropical Pacific Ocean.
Specific heat: The amount of heat required to raise the temperature of 1 gram of a substance by 1 Celsius degree.
Specific humidity: The mass of water vapor per mass of air containing the water vapor; usually expressed as grams of water per kilogram of humid air.
Split flow pattern: Westerlies to the north have a wave configuration that differs from that of westerlies to the south.
Sp`rer minimum: A period of reduced sunspot activity between about 1450 and1550 A.D.
Squall line: A line of intense thunderstorm cells parallel to and ahead of a fast-moving well-defined cold front.
Stable air layer: Air layer characterized by a vertical temperature profile such that air parcels return to their original altitudes following any upward or downward displacement.
Standard atmosphere: Mean vertical profiles of temperature, pressure, and density within the atmosphere.
Station model: A conventional representation on a weather map using standard symbols of weather conditions at some locality.
Stationary front: A nearly stationary narrow zone of transition between contrasting air masses; winds blow parallel but in opposite directions on either side of the front.
Steam fog: The general name for fog produced when cold air comes in contact with relatively warm water surface; has the appearance of rising streamers. (More info - Living with Weather: Fog)
Stefan-Boltzmann law: The total energy radiated by a blackbody at all wavelengths is directly proportional to the fourth power of the absolute temperature (in kelvins) of the body.
Storm surge: A rise in sea level along a shore caused primarily by strong onshore winds and, to a lesser extent, low air pressure associated with a storm (often a hurricane); may be responsible for considerable coastal erosion and flooding.
Stratiform clouds: Layered clouds, such as altostratus, often produced by air overrunning.
Stratocumulus (Sc): Low clouds consisting of large, irregular puff or rolls arranged in a layer.
Stratopause: Transition zone between the stratosphere and the mesosphere.
Stratosphere: The atmosphere’s thermal subdivision situated between the troposphere and mesosphere and the primary site of ozone formation. Air temperature in the lower portion of the stratosphere is constant with altitude, and then the temperature increases with altitude to the stratopause.
Stratus (St): Low clouds that occur as a uniform gray layer stretching from horizon to horizon. They may produce drizzle, and where they intersect the ground, they are classified as fog.
Stüve thermodynamic diagram: A graphical representation of air temperature, pressure, and humidity.
Sublimation: The process whereby water changes phase from a solid into a vapor without first becoming liquid.
Subpolar low: High-latitude, semipermanent cyclones marking the convergence of planetary-scale surface southwesterlies of midlatitudes with surface northeasterlies of polar latitudes; Icelandic low and Aleutian low are examples.
Subtropical anticyclones: Semipermanent warm-core, high-pressure systems centered over subtropical latitudes of the Atlantic, Pacific, and Indian Oceans.
Subtropical jet streams: A zone of unusually strong winds aloft situated between the tropical tropopause and the midlatitude tropopause.
Sundog: A colored luminous spot produced by refraction of light by ice crystals that appear on either side of the sun. Also called parhelia.
Sunspots: Relatively large, dark blotches that appear on the face of the sun.
Supercell thunderstorm: A relatively long-lived, large and intense cell characterized by an exceptionally strong updraft; may produce a tornado. (More info - Living with Weather: Thunderstorms)
Synoptic-scale systems: Weather phenomena operating at the continental or oceanic spatial scale; includes migrating cyclones and anticyclones, air masses and fronts.

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Teleconnection: A linkage between weather changes occurring in widely separated regions of the globe.
Temperature: A measure of the average kinetic energy of the individual atoms or molecules composing a substance.
Temperature gradient: Temperature change with distance.
Terminal velocity: Constant downward-directed speed of a particle within a fluid due to a balance between gravity and fluid resistance.
Thermal inertia: Resistance to a change in temperature.
Thermal wind: A component of geostrophic wind that arises from a horizontal air temperature gradient.
Thermograph: A recording instrument that provides a continuous trace of temperature variations with time.
Thermometer: An instrument for measuring temperature.
Thermosphere: The outermost thermal subdivision of the atmosphere in which the air temperature increases with altitude.
Thunder: Sound accompanying lightning; produced by violent expansion of air due to intense heating by a lightning discharge.
Thunderstorm: A mesoscale weather system produced by strong convection currents that reach to great altitudes within the troposphere. Consists of cumulonimbus clouds accompanied by lightning and thunder and often, locally heavy rainfall (or snowfall) and gusty surface winds. (More info - Living with Weather: Thunderstorms)
Tipping-bucket rain gauge: A device that collects rainfall in increments of 0.01 in. by containers that alternately fill, tip and empty.
Tornado: A small mass of air that whirls rapidly about an almost vertical axis; made visible by clouds and by dust and debris sucked into the system. (More info - Living with Weather: Tornadoes)
Tornado alley: Region of maximum tornado frequency in North America; a corridor stretching from central Texas northward into Oklahoma, Kansas and Nebraska, and eastward into central Illinois and Indiana,
Trade wind inversion: An elevated stable air layer that occurs on the eastern flank of a subtropical anticyclone.
Trade winds: Prevailing planetary-scale surface winds in tropical latitudes; blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
Transpiration: The process by which water vapor escapes from plants through leaf pores.
Triple point: The point of occlusion where cold, warm, and occluded fronts all come together.
Tropic of Cancer: A solstice position of the sun with a latitude 23 degrees 27 minutes N.
Tropic of Capricorn: A solstice position of the sun with a latitude of 23 degrees, 30 minutes S.
Tropical depression: An early stage in the development of a hurricane; sustained winds are at least 37 km (23 mi) per hour but less than 63 km (39 mi) per hour.
Tropical disturbance: A region of convective activity over tropical seas with a detectable center of low pressure; the initial stage in the development of a hurricane.
Tropical storm: A tropical cyclone having wind speeds of 63 to 118 km (39 to 73 mi) per hour; a storm at pre-hurricane stage.
Tropopause: Zone of transition between the troposphere below and the stratosphere above; the top of the troposphere.
Troposphere: Lowest thermal subdivision of the atmosphere in which air temperature normally drops with altitude; the site of most weather.
Turbulence: Irregular, apparently random motions of a fluid such as air or water.

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Ultraviolet radiation: Short-wave, energetic electromagnetic radiation emitted by the sun. Much of the solar ultraviolet radiation is absorbed in the stratosphere, where it is involved in the formation and destruction of ozone.
Umbra: Central dark area of a sunspot.
Universal Coordinated Time (UTC): A worldwide time reference used for synchronizing weather observations; the time at 0 degrees longitude, the prime meridian.
Unstable air layer: An air layer characterized by a vertical temperature profile such that air parcels accelerate upward or downward and away from their original altitudes.
Upslope fog: Ground-level cloud formed as a consequence of the expansional cooling of humid air that is forced to ascend a mountain slope.
Upwelling: The upward circulation of cold, nutrient-rich bottom water toward the ocean surface.

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Valley breeze: A shallow, upslope flow of air that develops during daylight hours within mountain valleys.
Vapor pressure: The portion of the total air pressure exerted by the water vapor component of air.
Variables of state: Temperature, pressure and density of air.
Vertical wind shear: Change in wind speed or direction with increasing altitude.
Viscosity: Friction within fluids such as air and water.
Visible radiation (light): Electromagnetic radiation having wavelengths in the range of about 0.40 (violet) to 0.70 (red) micrometers.

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Wall cloud: Local lowering of a cumulonimbus cloud associated with a humid updraft; may develop into a mesocyclone and tornado.
Warm air advection: The flow of air from a relatively warm locality to a relatively cool locality.
Warm clouds: Clouds composed of liquid water droplets which have a temperatures above 0·C (32·F).
Warm front: A narrow zone of transition between relatively warm air that is advancing and relatively cool air that is receding
Warm-core anticyclone: High-pressure systems occupying a thick column of subsiding warm, dry air. Subtropical anticyclones are examples.
Warm-core cyclone: A surface, synoptic-scale stationary cyclone that develops as a consequence of intense solar heating of a large, relatively dry geographical area; same as a thermal low.
Warm-type occlusion: A front formed when a cold front overtakes and merges with a warm front and the air behind the front is warmer than the air ahead of the front.
Wave frequency: Number of crests or troughs of a wave that pass a given point in a specified period of time, usually 1 second.
Weather: The state of the atmosphere at some place and time described in terms of such variables as temperature, cloudiness, precipitation and wind.
Weather Forecast Offices (WFOs): Offices of the National Weather Service charged with providing regional weather reports and forecasts.
Weather warning: Warning issued when hazardous weather is observed or imminent.
Weather watch: Issued when hazardous weather is considered possible based on current or anticipated atmospheric conditions.
Weighing-bucket rain gauge: A device that is calibrated so that the weight of cumulative rainfall is recorded directly in terms of millimeters or inches.
Wet-bulb depression: On a psychrometer, the difference in readings between the wet-bulb thermometer and the dry-bulb thermometer; used to determine relative humidity.
Wien’s displacement law: The higher the temperature of a radiating object, the shorter is the wavelength of maximum radiation intensity; applies to blackbodies.
Wind: Air in motion relative to the Earth’s surface. (More info: Living with Weather: Winds)
Wind shear: The change in wind speed or direction with time or distance.
Wind vane: An instrument used to monitor wind direction by always pointing into the wind.
Windchill: equivalent temperature (WET) - An air temperature index that attempts to gauge the sensible heat loss from exposed skin resulting from the combined effect of low air temperature and wind.
World Meteorological Organization (WMO): Coordinates weather data collection and analysis by more than 178 member nations and territories; based in Geneva, Switzerland
World Weather Watch Programme (WWW): International weather-monitoring network coordinated by the World Meteorological Organization.

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X, Y, Z

X-rays: Highly energetic short-wavelength electromagnetic radiation.
Younger Dryas: A relatively cold period from about 12,900 to 11,600 year ago.
Zonal flow pattern: Flow of the planetary-scale westerlies almost directly from west to east; westerlies exhibit little amplitude