"absorption coefficient","A measure of the amount of radiant energy, incident normal to a planar surface, that is absorbed per unit distance or unit mass of substance."
"accessory cloud","A cloud which is dependent on a larger cloud system for development and continuance. Roll clouds, shelf clouds, and wall clouds are examples of accessory clouds."
"acid rain","Synonymous with acidic deposition. Both are somewhat inaccurate versions of the phrase "deposition of acidifying substances from the atmosphere." Acidity is measured as the concentration of hydrogen ion (H+), in logarithmic pH units. Solutions having pH < 7.0 are acidic, but "acid rain" is usually considered to have pH < 5.65 because of the natural influence of carbonic acid in precipitation."
"acid shock","Short-term event of great acidity that occurs in freshwater systems receiving intense pulses of acidic water when the snowpack melts."
"acidification","Increase over time in the content of acidity in a system, accompanied by decreases in the acid-neutralizing capacity due to alkalinity and decreases in calcium and magnesium content."
"active species","Trace constituent in the stratosphere that is directly involved in catalytic ozone destruction."
"adiabatic process","A thermodynamic change of state of a system such that no heat or mass is transferred across the boundaries of the system. In an adiabatic process, expansion always results in cooling, and compression in warming."
"advection","The predominately horizontal large-scale movement of air that causes changes in temperature or other physical properties. In oceanography, advection is the horizontal or vertical flow of sea water as a current. Transport of a property by fluid motion; in meteorology, referring to horizontal flow."
"aerosol","Particulate material, other than water or ice, in the atmosphere ranging in size from approximately 10E-3 to larger than 10E2 micrometers in radius. Aerosols are important in the atmosphere as nuclei for the condensation of water droplets and ice crystals, as participants in various chemical cycles, and as absorbers and scatterers of solar radiation, thereby influencing the radiation budget of the earth-atmosphere system, which in turn influences the climate on the surface of the Earth."
"agglomeration","In meteorology, the process by which precipitation particles grow larger by collision or contact with cloud or other precipitation particles."
"air pollution","One or more contaminants in concentrations or durations that are injurious to human, animal, plant, and property or that impair the enjoyment of life or property."
"airborne particulates","Total suspended matter found in the atmosphere as solid pieces or liquid droplets. Airborne particulates include windblown dust, emissions from industrial processes, smoke from the burning of wood and coal, and the exhaust of motor vehicles."
"airmass","A widespread body of the atmosphere that gains certain meteorological or polluted characteristics while set in one location. The characteristics can change as it moves."
"albedo","The fraction of the total solar radiation incident on a body that is reflected by it."
"albedo effect","Increased reflectivity of the ground surface following removal of the vegetation (desertification) thereby changing the heat balance of the surface-atmosphere system and, although unproven, assumed by some meteorologists to result in less uplift and less rain."
"altithermal period","A period of high temperatures, particularly the one from 8000 to 4000 BP (before the present era), which was apparently warmer in summers, as compared with the present, and with the precipitation zones shifted poleward.  More rainfall occurred in most of the subtropical United States and Scandinavia. Also called the hypsithermal period."
"annual flood series","Maximum instantaneous discharge in each year of record."
"Antarctic Circumpolar Current","The largest circulation feature in the ocean in terms of both its sustained transport and its length.  Whereas both the North Atlantic and North Pacific have distinct subpolar gyres, the southern hemisphere systems are masked by the circumglobal flow of the Antarctic Circumpolar Current, which is more similar to unbounded atmospheric flows.  Also know as the West Wind Drift in reference to its general orientation down the prevailing winds. The availability of a more or less continuous circumglobal channel at approximately 55ø-60ø S allows the current to connect the three major ocean basins -- Atlantic, Pacific, and Indian."
"Antarctic Convergence","Sharp oceanographic boundary between latitudes 50ø S and 60ø S in the southern ocean, separating colder and less saline waters around the Antarctic continent from warmer and denser waters to the north."
"anthropogenic","Literally, "giving rise to humanity," but more commonly used to signify "human-induced."  Usually used in the context of emissions that are produced as the result of human activities."
"anticyclone","An atmospheric high-pressure closed circulation with clockwise rotation in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, and undefined at the Equator."
"Arctic haze","A persistent winter diffuse layer in the Arctic atmosphere whose origin may be related to long-range transport of midlatitude continental man-made pollutants. Industrial air pollution transported to the Arctic from mid- and high-latitude sources; primarily composed of sulfur compounds, visible as a yellow-brown layer over most of the Arctic mainly during the months of February, March, and April. The haze, first observed in 1957, is a result of pollution from Europe and Asia that migrates north in the late winter and spring. It grew yearly until 1982, then began a decline that probably stems from Russia's increased reliance on natural gas instead of coal and oil, as well as tighter pollution controls in western Europe.  As of 1993, the haze had declined to about half its 1982 value."
"arid","Climate or region where the precipitation is barely sufficient to support vegetation. Aridity (also hyperaridity and semiaridity) may be defined by means of climatic diagrams or by various aridity indices (formulae involving precipitation, evapotranspiration, and temperature). Sometimes characterized as receiving either 80-150 mm or 200-350 mm annual precipitation, depending on the climatic regime."
"assimilation","Process of CO2 fixation by photosynthetic tissue."
"atmosphere (the)","The envelope of air surrounding the Earth and bound to it by the Earth's gravitational attraction. Studies of the chemical properties, dynamic motions, and physical processes of this system constitute the field of meteorology."
"atmosphere (unit)"," standard unit of pressure representing the pressure exerted by a 29.92-in. column of mercury at sea level at 45ø latitude and equal to 1000 g/cm2."
"atmospheric turbulence","A state of the flow of air in which apparently random fluctuations occur in the air's instantaneous velocities, often producing major deformations of the flow."
"atmospheric window","The spectral regions between 8.5 and 11.0 microns where the atmosphere is essentially transparent to longwave radiation."
"available soild moisture","That portion of the total water held in the soil that can be extracted for use by the plant, i.e., with a water potential usually above -1.5 mPe."
"backing winds","Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds. In storm spotting, a backing wind usually refers to the turning of a south or southwest surface wind with time to a more east or southeasterly direction. Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels."
"baroclinic waves","Atmospheric waves with wavelengths of about 1000 km which draw their energy from the energy accumulated due to the north-south global temperature differences."
"baroclinity","Horizontally varying density along an isobaric surface, mostly due to the horizontal temperature gradient."
"Benard cell","Convection cells that occur in unstable fluids, initially at rest."
"Bergeron process","Process by which ice crystals grow at the expense of supercooled water droplets, coexisting in a cloud with temperatures below 0ø C. Because vapor pressure over water is greater than over ice at a common temperature, if supercooled water is sufficiently available, ice grows at the expense of water."
"bimodal distribution","A size or mass spectrum that has two peaks of concentration."
"biome","Landscape unit with its own climate, flora, and fauna."
"bipole","Organized pattern of locations for cloud-to-ground lightning of positive and negative polarity."
"black stratus","Mist particles, produced in trade-wind clouds, that are too small to fall rapidly and that scatter light over a very narrow angle."
"boreal forest","Northern forest or taiga (Russian term), covering large areas in the subarctic but extending well north of the Arctic Circle, dominated by coniferous trees."
"boundary layer","Region where influence of boundary is important to flow solution -- generally a very thin layer."
"bow echo","A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation -- especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation."
"brightness temperature","Apparent temperature of an object based on its radiative brightness. It equals the thermometric temperature when the object emissivity equals 1."
"Bulk Richardson Number","A non-dimensional number relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 50 to 100 suggest environmental conditions favorable for supercell development. Abbreviated BRN."
"catalytic process","Chemical reaction or set of reactions that is enhanced by the presence of some trace species that is neither created nor destroyed in the overall process."
"chlorofluorocarbons","A family of inert nontoxic and easily liquified chemicals used in refrigeration, air conditioning, packaging, and insulation or as solvents or aerosol propellants. Because they are not destroyed in the lower atmosphere, they drift into the upper atmosphere where their chlorine components destroy ozone.  Abbreviated CFC. A compound containing only chlorine, fluorine, and carbon commonly used as a refrigerant, a cleaning agent, and a blowing agent for plastic foams; in the stratosphere CFCs decompose to release chlorine, which can catalyze ozone destruction."
"cirrus","High-level clouds (16,000 feet or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms."
"Clausius-Clapeyron relation","Physical law relating the temperature and the equilibrium vapor pressure (or vapor density) of water."
"clear slot","A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft."
"climate","The statistical collection and representation of the weather conditions for a specified area during a specified time interval, usually decades, together with a description of the state of the external system or boundary conditions.  The properties that characterize the climate are thermal (temperature of the surface air, water, land, and ice), kinetic (wind and ocean currents, together with associated vertical motions and the motions of air masses, aqueous humidity, cloudiness and cloud water content, groundwater, lake lands, and water content of snow on land and sea ice), and static (pressure and density of the atmosphere and ocean, composition of the dry air, salinity of the ocean, and the geometric boundaries and physical constants of the system). These properties are interconnected by the various physical processes such as precipitation, evaporation, infrared radiation, convection, advection, and turbulence."
"climate change","The long-term fluctuation in temperature, precipitation, wind, and all other aspects of the Earth's climate. External processes, such as solar-irradiance variations, variations of the Earth's orbital parameters (eccentricity, precession, and inclination), lithospheric motions, and volcanic activity, are factors in climatic variation. Internal variations of the climate system also produce fluctuation of sufficient magnitude and variability to explain observed climate change through feedback processes interrelating the components of the climate system."
"climate sensitivity","The magnitude of a climatic response to a perturbing influence. In mathematical modeling of the climate, the difference between simulations as a function of change in a given parameter."
"climate signal","A statistically significant difference between the control and disturbed simulations of a climate model. See climate sensitivity."
"climate system","The five physical components (atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere) that are responsible for the climate and its variations."
"climate variation","The change in one or more climatic variables over a specified time."
"climatic analog","A past climate situation in which changes similar to the present occurred. Used in making climatic projections."
"climatic anomaly","The deviation of a particular climatic variable from the mean or normal over a specified time."
"climatic optimum","The period in history from about 5000 to 2500 BP during which surface air temperatures were warmer than at present in nearly all regions of the world. In the Arctic region, the temperature rose many degrees, and in temperate regions the increase was 1.0ø-1.7ø C. In this period, glaciers and ice sheets receded greatly, and the melt-water raised sea level by about 3 meters."
"cloud","A visible mass of condensed water vapor particles or ice suspended above the Earth's surface. Clouds may be classified on their visual appearance, height, or form."
"cloud albedo","Reflectivity that varies from less than 10 to more than 90 percent of the insolation and depends on drop sizes, liquid water content, water vapor content, thickness of the cloud, and the sun's zenith angle. The smaller the drops and the grater the liquid water content, the greater the cloud albedo, if all other factors are equal."
"cloud condensation nuclei","Small particles of dust, smoke, or salt that serve as centers for condensation. Abbreviated CCN."
"cloud feedback","The coupling between cloudiness and surface air temperature in which a change in surface temperature could lead to a change in clouds, which could then amplify or diminish the initial temperature perturbation. For example, an increase in surface air temperature could increase the evaporation; this in turn might increase the extent of cloud cover. Increased cloud cover would reduce the solar radiation reaching the Earth's surface, thereby lowering the surface temperature. This is an example of negative feedback and does not include the effects of longwave radiation or the advection in the oceans and the atmosphere, which must also be considered in the overall relationship of the climate system."
"cloud street","A straight or curved line of convective clouds."
"ClOx","Collective formula for the inorganic component of atmospheric chlorine, including ClO, ClONO2, HOCl, and OClO species."
"coalescence","Fusing together; the process by which a large raindrop grows larger by absorbing smaller drops in its fall path."
"coefficient of variation","Standard deviation of the annual flows divided by the mean annual flow."
"cold front","The boundary between a mass of warm air and a mass of cold air that underuns and displaces it."
"comma echo","A thunderstorm radar echo which has a comma-like shape. It often appears during later stages in the life cycle of a bow echo."
"conduction","Transfer of heat from one place to another by molecular agitation and without movement of the medium."
"contrail","A streamer cloud of water drops or ice crystals that forms in the wake of an aircraft, resulting from condensation of moisture added to the air by the combustion of fuel."
"convection","Rotational flow of a fluid resulting from imbalances in density. This often occurs because the fluid below is heated and becomes less dense than the fluid above or because the fluid above is cooled and becomes more dense than the fluid below. Transfer of heat by physical movement of the heated medium from one place to another. Atmospheric or oceanic motions that are predominately vertical and that result in vertical transport and mixing of atmospheric or oceanic properties. Because the most striking meteorological features result if atmospheric convective motion occurs in conjunction with the rising current of air (i.e., updrafts), convection is sometimes used to imply only upward vertical motion."
"convective adjustment","A numerical procedure applied in many atmospheric models to approximate the vertical nonradiative heat transport. This procedure adjusts the lapse rate whenever necessary so that some prescribed critical lapse rate is never exceeded."
"convergence","The quasi-horizontal flow of a fluid toward a common destination from different directions. When waters of different origins come together at a point or along a line (convergence line), the denser water from one side sinks under the lighter water from the other side.  The ocean convergence lines are the polar, subtropical, tropical, and equatorial.  Also see divergence.  Movement of air on a plane toward a common point or common line (resulting in rising currents); the result of friction unbalancing the forces of geostrophic flow."
"Coriolis effect","The tendency for an object moving above the Earth to turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere relative to the Earth's surface. The effect arises because the Earth rotates and is not, therefore, an inertial reference frame."
"counter twilight","A colored border of 6ø-12ø in height above the eastern horizon a half hour before sunset showing transitions in orange, yellow, green, and blue."
"cryosphere","The portion of the climate system consisting of the world's ice masses and snow deposits, which includes the continental ice sheets, mountain glaciers, sea ice, surface snow cover, and lake and river ice. Changes in snow cover on the land surfaces are by and large seasonal and closely tied to the mechanics of atmospheric circulation. The glaciers and ice sheets are closely related to the global hydrologic cycle and to variations of sea level and change in volume and extent over periods ranging from from hundreds to millions of years."
"cumulonimbus","Cloud generally 10 or more kilometers tall, consisting of ice and water, appearing like towering, billowing cotton, generally with a white top (anvil) extending downstream from the cloud. Commonly called thunderstorm or thunderhead. Cloud turret can be white, gray, or very dark. Contains rain, supercooled water, graupel, ice crystals, and hail and is turbulent."
"cumulonimbus cloud","A convective cloud characterized by strong vertical development in the form of mountains or huge towers topped at least partially by a smooth, flat, often fibrous anvil. Also known colloquially as a "thunderhead.""
"cumulus","Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal."
"cyclone","A large-scale system of winds blowing around a center of low pressure; motion is counterclockwise in the Nothern Hemisphere, clockwise in the Southern Hemisphere."
"cyclonic","In the same sense as the component of the earth's rotation about the local vertical (anticyclonic, the opposite). In the northern hemisphere a streamline curving to the left has cyclonic curvature, and wind speed decreasing to the left of the current represents cyclonic shear. Taken together, curvature and shear constitute the vorticity, or rotation of a body of air about the vertical axis."
"dendrochronology","The dating of past events and variations in the environment and the climate by studying the annual growth rings of trees. The approximate age of a temperate forest tree can be determined by counting the annual growth rings in the lower part of the trunk. The width of these annual rings is indicative of the climatic conditions during the period of growth; wide annual rings signify favorable growing conditions, absence of diseases and pests, and favorable climatic conditions, while narrow rings indicate unfavorable growing conditions or climate."
"dendroclimatology","The use of tree growth rings as proxy climate indicators. Tree rings record responses to a wider range of climatic variables over a larger part of the Earth than any other type of annually dated proxy record."
"density","The mass per unit volume of a substance, commonly expressed in grams per cubic centimeter."
"derecho","(pronounced day-RAY-cho). A widespread and usually fast-moving windstorm associated with convection. Derechos can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across."
"desertification","The progressive destruction or degradation of vegetative cover especially in arid or semiarid regions bordering existing deserts. Overgrazing of rangelands, large-scale cutting of forests and woodlands, drought, and burning of extensive areas all serve to destroy or degrade the land cover. The climatic impacts of this destruction include increased albedo leading to decreased precipitation, which in turn leads to less vegetative cover; increased atmospheric dust loading could lead to decreased monsoon rainfall and greater wind erosion and/or atmospheric pollution. Continuous and sustained human-induced decline toward desert-like conditions brought about by deforestation, overcultivation, and overgrazing."
"dew point","A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant)."
"dew point","Temperature at which condensation begins on a surface; relative humidity is 100%"
"diffusion","Movement or spreading out of molecules, such as CO2 into a fluid or porous media, in such a way that concentrations tend to be equalized in all parts of the system."
"direct circulation","Circulation with rising warm and/or sinking cold air, converting potential to kinetic energy."
"dispersion (wave)","The spreading out of a wave train due to each wavelength traveling with its own velocity."
"dissociative recombination","Neutralization of a positive molecular ion by recombination with an electron to produce two neutral particles (atoms). These atoms may be in excited states."
"divergence","A horizontal flow of water, in different directions, from a common center or zone; it is often associated with upwelling. Also see convergence."
"Doppler radar","Radar configured such that the phase of the transmitted signal can be compared to the received signal so that a frequency shift can be measured. This is used to compute the component of scatter motion in the direction toward or away from the radar."
"downburst","A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder."
"downwelling","The process of accumulation and sinking of warm surface waters along a coastline. A change of air flow of the atmosphere can result in the sinking or downwelling of warm surface water. The resulting reduced nutrient supply near the surface affects ocean productivity and meteorological conditions of the coastal regions in the downwelling area. Compare upwelling."
"dry deposition","Influx from the atmosphere of gaseous or particulate chemicals, occurring in the intervals between precipitation events."
"dry line","A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or ven further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward."
"dual-Doppler solution","Method of deducing the three-dimensional winds within storms that requires at least two radars and an auxiliary relation. Usually, the requirement of continuity of air entering and leaving a volume is used."
"dust devil","A small whirlwind common in dry regions and made visible by the dust and debris picked up from the ground."
"dust veil index","A quantitative method developed by H. H. Lamb for comparing the magnitude of volcanic eruptions. The formulae use observations either of the depletion of the solar beam, temperature lowering in the middle latitudes, or the quantity of solid matter dispersed as dust. The reference dust veil index is 1000, assigned to the Krakatoa 1883 eruption, and the index is calculated using all three methods, where the information is available, for statistical comparison purposes. Abbreviated D.V.I."
"Earth shadow","The curved red-to-blue shadow seen in the east shortly after sunset."
"eccentricity","Measures how elliptical the earth-sun orbit is, with consequent effects on solar radiation received by the earth. If e is the eccentricity, then (1 + e)/(1 - e) is the ratio of the farthest and closest earth-sun distance."
"ecosystem","The interacting system of a biological community and its nonliving environmental surroundings."
"eddy","A circular movement of water or air that is formed where currents pass obstructions or between two adjacent currents that are flowing counter to each other,"
"effective temperature","Temperature of a planet resulting from the balance between incoming solar radiation and outgoing planetary surface-emitted infrared radiation; does not include the temperature enhancement due to absorption and reemission due to absorption and reemission of outgoing infrared radiation by greenhouse gases. The effective temperature of the earth is 253 K (-4ø F)."
"El Ni¤o","An irregular variation in ocean current that from January to March flows off the west coast of South America, carrying warm, low-salinity, nutrient-poor water to the south. It does not usually extend farther than a few degrees south of the equator, but occasionally it does penetrate beyond 12ø S, displacing the relatively cold Peru Current. The effects of this phenomenon are generally short-lived, and fishing is only slightly disrupted. Occasionally (in 1891, 1925, 1941, 1957-58, 1965, 1972-73, 1976, 1982-83), the effects are major and prolonged. Under these conditions, sea surface temperatures rise along the coast of Peru and in the equatorial eastern Pacific Ocean and may remain high for more than a year, having disastrous effects on marine life and fishing. Excessive rainfall and flooding occur in the normally dry coastal area of western tropical South America during these events. Some oceanographers and meteorologists consider only the major, prolonged events as El Ni¤o phenomena rather than the annually occurring weaker and short-lived ones."
"emissions","Materials (gases, particles, vapors, chemical compounds, etc.) that come out of smokestacks, chimneys, and tailpipes."
"emissivity","The ratio of the radiation emitted by a surface to that emitted by a black body at the same temperature. Ability of an object to emit thermal radiation, ranging from 0 for a nonemitter (perfect reflector) to unity for a perfect emitter (nonreflector)."
"energy balance models","An analytical technique to study the solar radiation incident on the Earth in which explicit calculations of atmospheric motions are omitted. In the zero-dimensional models, only the incoming and outgoing radiation is considered. The outgoing infrared radiation is a linear function of global mean surface air temperature, and the reflected solar radiation is dependent on the surface albedo. The albedo is a step function of the global mean surface air temperatures, and equilibrium temperatures are computed for a range of values of the solar constant. The one-dimensional models have surface air temperature as a function of latitude. At each latitude, a balance between incoming and outgoing radiation and horizontal transport of heat is computed. Abbreviated EBM."
"environment","The sum of all external conditions affecting the life, development, and survival of an organism."
"equatorial anomaly","Structure of the low-latitude ionosphere in which there are two electron density maxima, at about ñ 15ø magnetic dip latitude, rather than one maximum at the subsolar point."
"equivalent","Abberivation for mole-equivalent, calculated as the molecular or atomic weight multiplied by the number of charges on the ion. Equivalent units are required for charge-balance calculations."
"eustatic sea level variations","Changes in sea level that is a consequence of a change in the volume of the water in the global ocean."
"evapotranspiration","Discharge of water from the Earth's surface to the atmosphere by evaporation from bodies of water, or other surfaces, and by transpiration from plants."
"exogenic system","Earth's interacting envelopes: the hydrosphere (mostly the ocean), atmosphere, biosphere, reactive lithosphere (that part of the rocky crust within reach of the processes of weathering). The processes of deep seawater circulation at spreading ridges overlap with endogenic (within the earth) processes. Also relevant are extraterrestrial processes. Refractory problems such as mass extinction usually become arenas for competing theories drawn from the three sources."
"fallout","Settling and deposition of particulate matter, such as tephra and volcanic aerosols, out of an eruption plume and onto the earth's surface."
"feedback mechanisms","A sequence of interactions in which the final interaction influences the original one. Also see positive feedback and negative feedback."
"fog","Liquid particles less than 40 microns in diameter that are formed by condensation of vapor in air."
"fossil fuel","Any hydrocarbon deposit that can be burned for heat or power, such as petroleum, coal, and natural gas."
"front","Zone of sharp temperature contrasts between cold and warm air masses. Fronts slope toward cold air (with slopes of order 1:100) and are associated with marked cyclonic wind shear and vertical shear."
"frontogenesis","The process of producing large horizontal contrasts, principally in pressure and density."
"Gaia theory","Theory proposed by James Lovelock and Lynn Margulis according to which the biosphere and atmosphere are considered as a unified living entity, in a lifelike homeostatic equilibrium state, where feedback mechanisms correct all changes in the existing conditions."
"general circulation models","Hydrodynamic models of the atmosphere on a grid or spectral resolution that determine the surface pressure and the vertical distributions of velocity, temperature, density, and water vapor as functions of time from the mass conservation and hydrostatic laws, the first law of thermodynamics, Newton's second law of motion, the equation of state, and the conservation law for water vapor. Abbreviated GCM. Atmospheric general circulation models are abbreviated AGCM, while oceanic general circulation models are abbreviated OGCM."
"geophysical noise","Fluctuations in a geophysical parameter that are of a quasirandom nature and cannot be attributed to fluctuations in known causative phenomena."
"geosphere","The solid mass (lithosphere) of the Earth as distinct from the atmosphere and hydrosphere or all three of these layers combined."
"geostrophic flow","A type of movement where the Coriolis force balances exactly the horizontal pressure force."
"glacial maximum","The position or time of the greatest advance of a glacier (e.g., the greatest equatorward advance of Pleistocene glaciation)."
"glacial rebound","The isostatic adjustment of previously glaciated areas after glacial retreat (e.g., the uplift of Scandinavia after the most recent glaciation."
"glacier","A mass of land ice that is formed by the cumulative recrystallization of firn. A glacier flows slowly (at present or in the past) from an accumulation area to an ablation area. Some well-known glaciers are: the Zermatt, Stechelberg, Grinelwald, Trient, Les Diablerets, and Rhone in Switzerland; the Nigards, Gaupne, Fanarak, Lom, and Bover in Norway; the Wright, Taylor, and Wilson Piedmont glaciers in Antarctica; the Bossons Glacier in France; the Emmons and Nisqually glaciers on Mt. Ranier, Wasington; Grinnell glacier in Glacier National Park, Montana; the Dinwood glacier in the Wind River Mountains and the Teton glacier in Teton National Park, both in Wyoming; and many glaciers in the Canadian Rockies."
"glacier flow (ice flow)","The slow downward or outward movement of ice in a glacier caused by gravity."
"global energy budget","Balance of radiative and convective fluxes within the atmosphere and the surface."
"glory","A colored halo opposite the sun."
"graupel","A type of precipitation consisting of frozen cloud droplets that have clumped and formed pellets. Snow pellets, i.e., white, opaque, approximately round ice particles having a snowlike structure and 2-5 mm in diameter. Precipitation particles composed of ice and formed by the accretion and freezing of supercooled water droplets."
"greenhouse effect","A popular term used to describe the warming of the earth due to atmospheric constituents trapping the warmth radiated by the surface. The atmosphere of the earth keeps the surface about 30ø C warmer than it would by without the atmosphere. Earth's surface is heated by absorbing the radiation of wavelengths 0.4-4 microns arriving from the sun through the atmosphere. In turn, it emits thermal infrared radiation of relatively higher wavelengths of 4-100 microns, to acheive a steady state. Due to the presence of an atmosphere with radiatively active trace gases like water vapor, carbon dioxide, methane, nitrous oxide, ozone, and halocarbons, which have absorption bands in the wavelength region of the earth's thermal emission, a good part of which is trapped. This trapping of the earth's thermal radiation by the atmospheric trace gases, and the consequent heating of its surface and surrounding, is commonly known as the greenhouse effect."
"greenhouse forcing","Temperature enhancement due solely to the change in the atmospheric constituent of interest; also called trace-gas radiative forcing. Does not take into account various climate feedbacks, such as the greenhouse temperature enhancement associated with increased atmospheric water vapor in a warmer atmosphere. The water vapor feedback increases the greenhouse temperature forcing for a doubling of carbon dioxide by roughly 50-100% depending on the model; other feedbacks, including lapse rate feedback, increase the greenhouse temperature forcing by more than 100%."
"greenhouse gases","Those gases, such as water vapor, carbon dioxide, tropospheric ozone, nitrous oxide, and methane, that are transparent to solar radiation but opaque to longwave radiation, particularly between 8-18 microns. Also see greenhouse effect and trace gas. Atmospheric gases that have the property of absorbing and then reemitting outgoing infrared or heat radiation emitted by the surface. Important atmospheric greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs)."
"greenhouse or icehouse","Useful polarized concepts of the state of the planet. "Greenhouse" collects together global warmth, flattened thermal gradients, increased humidity, low relief, high sea level, and reduced vigor of oceanic circulation with a tendency toward enhanced anoxia in some parts of the ocean. "Icehouse" is essentially the reverse state in that assemblage of generalizations. It is important to add not-so-icecap-prone and icecap-prone, respectively, without implying necessarily that the world was ever ice-free."
"grounding line","The boundary between the area where an ice shelf or a glacier is floating on water and where it is in contact with the shore or underlying earth ("grounded")."
"Gulf Stream meander","A transient winding bend in the Gulf Stream. These bends intensify as the Gulf Stream merges into the North Atlantic and can break up into detached eddies at about 40ø N."
"gyre","Major circular flow patterns in the oceans. The wind-driven eastward- and westward-flowing equatorial currents are blocked by the continents and rotate slowly in a clockwise direction in the North Atlantic and Pacific Oceans and in a counterclockwise direction in the South Atlantic, South Pacific, and Indian Ocean."
"Hadley cell","A direct thermally-driven and zonally symmetric large-scale atmospheric circulation first proposed by George Hadley in 1735 as an explanation for the trade winds. It carries momentum, sensible heat, and potential heat from the tropics to the midlatitudes (30ø). The poleward transport aloft is complemented by subsidence in the subtropical high-pressure ridge and a surface return flow. The variability of this cell and the Walker cell is hypothesized to be a major factor in short-term climatic change."
"hail","Roundish ice mass consisting of concentric layers of ice formed on a mineral nucleus; hard, may be lumpy; diameters of 5 cm or more, though most of the hail measured at the earth's surface is less than 0.5 cm in diamter."
"half-life","Time after which half of the radioactive atoms initially present have decayed."
"halo","A colored ring, appearing 22ø or 46ø around the sun or moon, caused by refraction of light through ice crystals."
"halons","Compounds containing only carbon, bromine, and one or more other halogens (chlorine, fluorine). Completely halogenated hydrocarbons with at least one bromine atom, usually having long lifetimes and mostly used as fire retardants."
"heat flux (thermal flux)","The amount of heat that is transferred across a unit area in a unit of time."
"heat island effect","A "dome" of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement."
"heat sink","That portion of a thermodynamic system that absorbs unused heat."
"hertz","The unit of frequency equal to one cycle per second or 2 pi radians per second."
"heterogeneous chemistry","Chemical transformations involving reactants (including catalytic agents) in different phases. For example, ClNO3 + HCl(solid) ---> Cl2 + HNO3(solid) or ClNO3 + H20(solid) ---> HOCl + HNO3(solid)."
"HF","High frequency (~3-30 MHz)"
"Holocene","The most recent epoch of the Quaternary period, covering approximately the last 10,000 years."
"hook echo","A radar reflectivity pattern characterized by a hook-shaped extension of a thunderstorm echo, usually 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 tornado development."
"hurricane","A violent tropical cyclone storm of the western Atlantic. A tropical storm with pronounced rotary circulation, constant wind speed of 74 miles per hour (64 knots) or more."
"hydrologic budget","A quantitative accounting of all water volumes and their changes with time for a basin or area."
"hydrologic cycle","The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30 percent of the mid-latitudes' heat transport from the equatorial to polar regions."
"hydrometeor","Any form of precipitation (snow, rain, hail, etc.)."
"hydromite","An ice cylinder, like a cave stalagmite, formed by falling drops of water."
"hydrosphere","The aqueous envelope of the Earth, including the oceans, freshwater lakes, rivers, saline lakes and inland seas, soil moisture and vadose water, groundwaters, and atmospheric vapor."
"hydrostatic equation","In the vector equation of motion, the form assumed by the vertical component when all Coriolis, earth-curvature, frictional, and vertical-acceleration terms are considered negligible compared with those involving the vertical pressure force and the force of gravity. The error in applying the hydrostatic equation to the atmosphere for cyclonic-scale motions is less than 0.01%. In extreme situations, the strong vertical accelerations in thunderstorms and mountain waves can be 1% of gravity."
"hygroscopic","Absorbing or attracting moisture, as with salt crystals."
"ice age","A glacial epoch or time of extensive glacial activity. Also, as Ice Age, which refers to the latest glacial epoch, the Pleistocene Epoch. Periods characterized by very low temperature worldwide and advancing glaciers."
"ice and snow albedo","The reflectivity of ice and snow-covered surfaces. The albedo of freshly fallen snow may be as much as 90%, while older snow may have values of 75% or less. The larger the areal extent of snow and ice cover, the higher the albedo value. The surface albedo will also increase as a function of the depth of snow cover up to 13 cm and be unaffected by increased snow cover after reaching that depth."
"ice and snow albedo temperature feedback","Interactions that can be described as a theoretical concept of a feedback mechanism in which the interacting elements are the areal extent of polar ice and snow cover, the albedo of the polar region (dependent on areal extent of ice and snow), absorption of solar radiation (dependent on the albedo), temperature (dependent on the absorption of solar radiation), and the area of ice and snow cover (dependent on temperature). Less snowfall would mean more absorption of solar radiation, therefore a surface warming would occur. Climate modeling studies indicate an amplification effect (i.e., positive feedback) of the ice and snow albedo feedback on increased surface air temperatures caused by increases in the atmospheric concentrations of carbon dioxide."
"ice core","Deep drill into the earth's permanent glaciers revealing the history of the atmospheric gas and dust content, enabling deduction of former atmospheric temperatures."
"ice cover","During the present time, the extent, especially the thickness, of glacier ice on a land surface. Also the same as ice concentration, which is the ratio of an area of sea ice to the total are of sea surface within some large geographic area."
"ice islands","Tabular icebergs that have broken off the ice shelves of northern Ellesmere Island; can be up to 60 m thick and 30 km wide, drift with the pack ice, and have been known to last for as much as three decades."
"ice sheet","A glacier of considerable thickness and more than 50,000 sq. km in area. It forms a continuous cover of ice and snow over a land surface. An ice sheet is not confined by the underlying topography but spreads outward in all direction. During the Pleistocene Epoch, ice sheets covered large parts of North America and northern Europe but they are now confined to polar regions (e.g., Greenland and Antarctica).  Also called continental glacier."
"ice shelf","A sheet of very thick ice with a level or gently undulating surface. It is attached to the land on one side, but most of it is floating. On the seaward side, it is bounded by a steep cliff (ice front) two to 50 m or more above sea level. Ice shelves have formed along polar coasts (e.g., Antarctica and Greenland); they are very wide, with some extending several hundreds of kilometers toward the sea from the coastline. They increase in size from annual snow accumulation and seaward extension of land glaciers. They decrease in size from warming, melting, and calving."
"ice stream","Large, fast glaciers embedded in slow-moving ice. Flow velocities are a few hundred meters to kilometers per year."
"iceberg","A floating ice mass up to 100 km long and 200-300 M thick that has broken off (calved) from ice shelves, glaciers, or coastal ice cliffs into the ocean."
"incoherent scatter radar","Sensitive radar operating on a frequency well above the critical frequency that detects radio signals scattered from refractive index irregularities in the ionospheric and/or neutral atmosphere."
"incoming solar radiation","Electromagnetic radiation emitted by the sun that is incident on the earth's upper atmosphere. Most of the incoming solar radiation is in the spectral region 0.3-5 microns. The earth's atmosphere is largely transparent to the incoming solar radiation and permits most of it to reach the surface (the rest is reflected back to space by clouds and the surface). At the surface, the incoming solar radiation is absorbed and heats the surface."
"index of variability","Standard deviation of the annual flood series in the log domain."
"infrared emission","Light in the infrared region of the spectrum emitted by a heated sample."
"infrared radiation","Electromagnetic radiation lying in the wavelength interval from 0.7 microns to 1000 microns. Its lower limit is bounded by visible radiation, and its upper limit is bounded by microwave radiation. Most of the energy emitted by the Earth and its atmosphere is at infrared wavelengths. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The tri-atomic gases, such as water vapor, carbon dioxide, and ozone absorb infrared radiation and play important roles in the propagation of infrared radiation in the atmosphere. Abbreviated IR; also called longwave radiation."
"infrared spectroscopy","Interaction of infrared radiation with a sample, usually to excite vibrational modes of the material."
"insolation","The solar radiation incident on a unit horizontal surface at the top of the atmosphere; sometimes referred to as solar irradiance. The latitudinal variation of insolation supplies the energy for the general circulation of the atmosphere. Insolation depends on the angle of incidence of the solar beam and on the solar constant."
"instability","A layering of the atmosphere that tends toward vertical overturning. Property of a system such that certain distrubances or perturbations introduced into the steady state will increase in magnitude; tendency of an atmosphere to overturn, enhanced by some disturbance or perturbation; specifically when a parcel of air, being lifted, encounters ambient air cooler than itself and thereafter rises of its own accord."
"Internal energy in the form of heat released when a change of state occurs from gas to liquid or liquid to solid. Heat is required to bring about a change of state from solid to liquid or liquid to gas.",""
"inversion","An anomaly in the normal positive lapse rate; usually refers to a thermal inversion, in which temperature increases rather than decreases with height."
"ion","An atom or molecule that has an electric charge."
"ionosphere","Ionized gas (plasma) that surrounds the earth from approximately 90 to 600 km in height."
"irradiance","The total radiant flux received on a unit area of a given real or imaginary surface. Also called the radiant flux density."
"isobar","Line of constant barometric pressure, usually on a map at a given height. (An isobaric surface is a three-dimensional surface of constant pressure.)"
"isopynic","A line on a chart that connects all points of equal or constant density."
"isostatic adjustment","The process whereby lateral transport at the Earth's surface from erosion or deposition is compensated for by movements in a subcrustal layer to maintain equilibrium among units of varying masses and densities. Also called isostatic compensation."
"isostatic sea level change","Change in relative sea level associated with a local radial displacement of the surface of the solid earth as, for example, may occur elastically in the near field of an earthquake source or viscously in response to glacial loading or unloading."
"isotach","Line or surface on which velocities are constant."
"isotherm","A line on a chart that connects all points of equal or constant temperature."
"jet stream","A fast-moving ribbon of air that occurs near the top of the troposphere; width is 25-100 km, thickness 1-3 km, and velocity 99-300 km per hour. Strong zonal current of air, usually near the 500-mbar constant pressure surface in each hemisphere, that encircles the earth. Referred to as the jet stream because of its high concentration and great speed, often up to 500 km/h. Current of strongest wind (order 50 m/s) surmounting a region of strong tropospheric baroclinity and with pronounced decreases of wind toward its lateral flanks."
"Joule heating","Heating in the ionosphere below ~140 km height due to the interaction of ionospheric currents and the ambient electric field."
"katabatic winds","Winds produced when gravity pulls cold, dense air from, fort example, the Antarctic plateau downslope to the coast."
"L shell","Surface of the geomagnetic field given by the longitudinal drift of an L line along which trapped charged particles bounce between northern and southern hemisphere. The L value is a length, in units of earth radius, which reduces to the equatorial radius of a field line in the case of a dipole field."
"La Ni¤a","Conditions that are complementary to El Ni¤o -- cold surface waters off Peru and intense trades over the Pacific."
"laminar flow","Smooth, nonturbulent flow of a gas or liquid."
"lapse rate","The rapidity with which temperature decreases with altitude. The normal lapse rate is defined to be 3.6ø F per 1000 ft, and the wet adiabatic lapse rate varies between 2ø and 5ø F per 1000 ft."
"latent heat","Energy transferred from the earth's surface to the atmosphere through the evaporation and condensation processes."
"latent heat flux","Energy associated with the exchange of water vapor between ocean and atmosphere; amount (or rate) of surface evaporation per unit area per unit time expressed in energy units."
"Le Chatlier's principle","When an external force is applied to an equilibrium system, the system adjusts to minimize the effect of the force."
"lidar","Light detection and ranging, a family of laser-based remote-sensing techniques which involve sending a powerful laser beam into the atmosphere and collecting and analyzing the backscattered signal."
"lightning","Electrical discharge in thunderstorms caused by the separation of electric charge of positive and negative polarity."
"limb","The edge of a celestial body as it appears to an observer; the edge of the sun's disk, for example."
"lithosphere","The component of the Earth's surface comprising the rock, soil, and sediments. It is a relatively passive component of the climate system, and its physical characteristics are treated as fixed elements in the determination of climate. Semirigid upper platelike layer of the earth. Its thickness is near zero at oceanic ridge axes, but it increases with age to about 100 km. Oceanic lithosphere consists of a crustal layer ~5 km thick of basaltic composition underlain by mantle rocks. Continental lithosphere consists of a layer of crustal rock 30-60 km thick of low density and variable composition underlain by mantle rocks. Layer of greater strength than the underlying asthenosphere for deformationat geological rates. It includes the crust and part of the uppermost mantle and is of the order of 100 km in thickness beneath the continents. Earth's cool outer rigid layer, including the crust, separated from the earth's deeper layers by a relatively weak zone, the asthenosphere. The lithosphere is of negligible thickness at some ocean ridges and may be as thick as 200 km or more beneath continents."
"Little Ice Age","A cold period that lasted from about A.D. 1550 to about A.D. 1850 in Europe, North America, and Asia. This period was marked by rapid expansion of mountain glaciers, especially in the Alps, Norway, Ireland, and Alaska. There were three maxima, beginning about 1650, about 1770, and 1850, each separated by slight warming intervals."
"long-range transport","Transport of pollutants within a moving air mass for a distance greater than 100 km."
"longwave radiation","The radiation emitted in the spectral wavelength greater than 4 microns corresponding to the radiation emitted from the Earth and atmosphere. It is sometimes referred to as "terrestrial radiation" or "infrared radiation," although somewhat imprecisely. Compare shortwave radiation."
"magnetosphere","Entire plasma envelope that surrounds the earth as constrained by the earth's dipole magnetic field and the interaction of this envelope with the solar wind, and with the interplanetary magnetic field."
"mammatus clouds","Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well."
"Maunder minimum","The period from 1654 to 1714 when it was believed that there were no sunspots. It is now thought that there were some sunspots during that time but fewer than those counted after 1800."
"mean sea level","The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent water that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated MSL and referred to simply as "sea level," serves as the reference surface for all  altitudes in upper atmospheric studies."
"Mediterranean cllmate","A climate characterized by dry summers and wet winters, often found along coasts lying about 40ø from the equator. Much of California and much of the Mediterranean region of Europe have this kind of climate."
"Milankovitch theory","An astronomical theory formulated by the Yugoslav mathematician Milutin Milankovitch that associates climate change with fluctuations in the seasonal and geographic distribution of insolation determined by periodic variation of the Earth's eccentricity and obliquity and the longitude of perihelion."
"mist","Liquid particles 40 to 500 microns in diameter that are formed by condensation of vapor in air."
"mixed-phase region","Zone in the atmosphere that contains water substance in all three of its phases (vapot, liquid, and solid) and whose temperature is gerneally between -40øC and 0ø C."
"MMED","Mass Median Equivalent aerodynamic Diameter; where the equivalent aerodynamic diameter of a particle is the diameter of a sphere of density 1 g cm-3 that has the same falling velocity."
"modeling","An investigative technique that uses a mathematical or physical representation of a system or theory that accounts for all or some of its known properties. Models are often used to test the effects of changes of system components on the overall performance of the system."
"monsoon","A name for seasonal winds, first applied to the winds over the Arabian Sea that blow for six months from the northeast and for six months from the southwest. The term has been extended to similar winds in other parts of the world (i.e., the prevailing west to northwest winds of summer in Europe have been called the "European monsoon"). The primary cause for these seasonal winds is the much greater annual variation of temperature over large land areas compared with neighboring ocean surfaces, causing an excess of pressure over the continents in winter and a deficit in summer, but other factors, such as topography of the land, also have an effect. The monsoons are strongest in the southern and eastern sides of Asia, but also occur along the coasts of tropical regions wherever the planetary circulation is not strong enough to inhibit them.  Term originally coined by Arab mariners in reference to the seasonally shifting winds in the Indian Ocean. It has now come to denote climatic systems anywhere in which rainfall is markedly seasonal and in which the moisture comes mainly in the warm season."
"monsoonal circulation","Atmospheric circulation induced by differential land-ocean heating. In summer, the flow is from the ocean to land; in winter it is from land to ocean, but with modification by the effects of the earth's rotation."
"negative feedback","An interaction that reduces or dampens the response of the system in which it is incorporated."
"net radiation","Absorbed minus emitted radiation. On a global average the earth's surface and atmosphere, respectively, absorb 170 W/m2 and 60 W/m2 solar radiation and emit 70 and 160 W/m2 as infrared radiation. Thus the net radiative gain of 100 W/m2 by the earth equals the loss by the atmosphere."
"NEXRAD","Next Generation Doppler Radar, the network of approximately 144 10-cm.-wavelength radars that form a component of the National Weather Service's radar storm identification network."
"normal modes of vibration","Natural resonances on the coupled periodic displacements of atoms in a material about their equilibrium positions."
"nucleation","The beginning of the process of droplet or ice-crystal formation; homogenous nucleation occurs when there is no foreign particle to act as a nucleus."
"obliquity","Measures the tilt of the earth's axis of rotation with respect to the plane of the earth-sun orbit; provides the mid and high latitudes with seasons."
"occluded front","A complex front that forms when a cold front overtakes a warm front and cuts off warm-sector air from the surface."
"ocean mixing","Processes that involve rates of advection, upwelling/downwelling, and eddy diffusion and that determine how rapidly excess atmospheric carbon dioxide can be taken up by the oceans."
"ocean-atmosphere interactions","Interactions that are invoked to explain why certain phenomena, such as the Southern Oscillation, are attributable to changes in the ocean from a meteorological perspective and are attributable to changes in atmospheric conditions from an oceanographic perspective."
"opacity","The degree of obscuration of light; for example, a glass window has almost 0% opacity, whereas a concrete wall has 100% opacity."
"optical depth","In calculating the transfer of radiant energy, the mass of an absorbing or emitting material lying in a vertical column of unit cross-sectional area and extending between two specified levels. Also, the degree to which a cloud prevents light from passing through it; the optical thickness then depends on the physical constitution (crystals, drops, and/or droplets), the form, the concentration, and the vertical extent of the cloud."
"order of magnitude","Ten times; thus, an increase of two orders of magnitude is 10 x 10, or 100 times."
"orographic","Pertaining to mountain ranges; thus, a mountain or hill that causes air to rise over it is an orographic barrier."
"outgoing infrared radiation","Electromagnetic radiation emitted by the earth's surface sue to heating by incoming solar radiation. Most of the outgoing infrared radiation is in the spectral region 4-80 microns. The earth's atmosphere is not transparent to this outgoing radiation as greenhouse gases absorb and reemit these wavelengths. The downward-directed component of the reemitted infrared radiation results in an additional heating of the earth's surface and raises the surface temperature from 235 K (-49ø F), which is the "effective" temperature of the earth, to 288 K (59ø F). This additional heating is called the greenhouse temperature enhancement."
"ozone","A molecule made up of three atoms of oxygen. In the stratosphere, it occurs naturally and it provides a protective layer shielding the Earth from ultraviolet radiation and subsequent harmful health effects on humans and the environment. In the troposphere, it is a chemical oxidant and major component of photochemical smog. Ozone has a characteristic, pungent odor familiar to most persons because ozone is formed when electrical apparatus produces sparks in air. Ozone is irritating to mucous membranes and toxic to human beings and lower animals.  The U.S. Occupational Safety and Health Administration standards for industrial workers exposed to ozone on a daily basis limit ozone concentrations to 0.1 ppm on the average, with a maximum of 0.3 ppm for short exposures.  Near the Earth's surface the concentration is usually 0.02-0.03 ppm in country air, and less in cities except when there is smog (when concentrations may reach 0.5 ppm or more for brief periods). Maximum concentration -- 5 x 10E12 per cm3, more than 1000 times the normal peak level at the surface --  occurs at an altitude of 19 mi (30 km)."
"ozone hole","Extraordinary springtime ozone destruction. A phenomenon occurring primarily over Antarctica and first reported in 1985."
"ozone layer","Layer in the stratosphere with high concentrations of ozone. Generally between 20 and 30 km."
"pack ice","Ice produced when the ocean freezes. This ice is broken into floes of various sizes and is compressed into pressure ridges by winds and ocean currents. Floes are separated from one another by open water leads and larger open water bodies called polynyas."
"parhelic circle","A halo around the sun."
"parhelion","A bright spot on the 22ø and 46ø haloes, horizontal with the sun."
"particulate matter","Very small pieces of solid or liquid matter, such as particles of soot, dust, aerosols, fumes, or mists."
"perihelion","Time of the closest earth-sun approach (presently occurs in early January). Aphelion is the time of the farthest earth-sun approach (presently occurs in early July)."
"pH","Inverse measure of the hydrogen ion concentration. pH of pure water is 7."
"photochemical smog","Air pollution caused by chemical reactions among various substances and pollutants in the atmosphere."
"photodissociation","Splitting of a molecule into component parts due to absorption of a photon."
"photoelectron","Electron ejected from an atom or molecule through the photoionization process."
"photoexcitation","Excitation of an electron into an energy level above the ground state configuration by absorption of a photon by an atom or molecule."
"photoionization","Removal of an electron from an atom or molecule by absorption of a photon."
"photolysis","Breakdown of chemical compounds through the action of sunlight."
"photosynthesis","The manufacture by plants of carbohydrates and oxygen from carbon dioxide and water in the presence of chlorophyll and sunlight as the energy source. Oxygen and water vapor are released in the process. Photosynthesis is dependent on favorable temperature and moisture conditions as well as on the atmospheric carbon dioxide concentration. Increased levels of carbon dioxide can increase net photosynthesis in many plants."
"phytoplankton","That portion of the plankton community comprised of tiny plants (e.g., algae, and diatoms)."
"pileus","A cloud formed in saturated air above a convective tower and separated from it."
"planetary albedo","The fraction (approximately 30%) of incident solar radiation that is reflected by the earth-atmosphere system and returned to space, mostly by backscatter from clouds in the atmosphere."
"planetary boundary layer","The transition region between the turbulent surface layer and the normally nonturbulent free atmosphere. This region is about 1 km in thickness and is characterized by a well-developed mixing generated by frictional drag as the air masses move over the Earth's surface. This layer contains approximately 10% of the mass of the atmosphere. Also called the atmospheric boundary layer or frictional layer; in air pollution literature, it is also called the mixing height -- pollutants released near the surface are quickly mixed up to the top of the PBL. Abbreviated PBL. The PBL height varies over a wide range (several tens of meters to several kilometers), particularly over land surfaces, and exhibits strong diurnal variations. The PBL height over land reaches its maximum value of the order of 1 km (range 0.2-5 km) in the late afternoon and its minimum value of the order of 100 m (range 20-500 m) before midnight. The lowest 10% of the PBL is called the surface layer (abbrev. SL), and is the portion most directly influenced by the surface."
"planetary waves","Atmospheric waves with wavelengths the order of the radius of the earth (~6000 km) and larger."
"plankton","Passively floating or weakly motile aquatic plants (phytoplankton) and animals (zooplankton)"
"plasma","Medium composed of free electrons, ions, and neutral atoms."
"polar stratospheric clouds","Clouds formed in the stratosphere of high latitudes during the winter months. Until 1986 the PSCs were thought to consist mainly of water ice, the individual particles growing by accretion of the H2O on nuclei of hydrated H2SO4. However, the measured opacities of the PSCs were smaller than one would expect on the hypothesis that the PSCs were composed mainly of H2O. This fact and the solid state thermodynamic properties of the PSCs led to the recent (1986) suggestion that HNO3 might be the dominant constituent of PSCs, with significant quantities of HCl (~10E-2 mole fraction). The presence of HNO3 and HCl in the solid phase are important in promoting the heterogeneous reactions. A consequence of this is the reduction of the gas phase concentrations of odd nitrogen, HCl and H2O."
"polarized light","Light that vibrates only in one plane."
"positive feedback","An interaction that amplifies the response of the system in which it is incorporated."
"potential temperature","Temperature that an air parcel at a given height would have at the surface if it were displaced from that height to the surface by a adiabatic process. In a compressible fluid, the temperature that would by attained by a fluid parcel upon adiabatic expansion or compression (i.e., without the addition or removal of heat) from actual to a reference pressure (these reference pressures are 1000 hPa in the atmosphere and the surface in the ocean)."
"precession","Motion of the earth's axis of rotation about a perpendicular to the earth-sun orbital plane, in response to the earth rotation and the combined gravitational forces of the sun, moon, and planets. Affects the relationship of the winter and summer seasons to the earth-sun distance during an annual orbit."
"precipitation","Any or all forms of liquid or solid water particles that fall from the atmosphere and reach the Earth's surface. It includes drizzle, rain, snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. The ratio of precipitation to evaporation is the most important factor in the distribution of vegetation zones. Precipitation is also defined as a measure of the quantity, expressed in centimeters or milliliters of liquid water depth, of the water substance that has fallen at a given location in a specified amount of time."
"pressure gradient","The rate of decrease in atmospheric pressure per unit of horizontal distance; used to indicate relative pressures on the earth's surface, and measured in the direction of most rapid decrease."
"profiler","Multibeam fixed-antenna Doppler radar that points nearly vertically and measures the wind up to at least 15 km height. Operates from 900 to 50 MHz."
"propagating modes","Wavelike motions that transport energy away from the excitation  or source region."
"quantum yield","Efficiency of conversion of light energy to chemical energy."
"radar","Radio Detection and Ranging.  Developed largely during World War II, it is the use of focused transmitted electromagnetic waves to identify scatterers in the atmosphere. From the direction of the receiving antenna and the time between transmission and receiving, direction and range are found."
"radiation balance","The difference between the absorbed solar radiation and the net infrared radiation. Experimental data show that radiation from the earth's natural surfaces is rather close to the radiation from a black body at the corresponding temperature; the ratio of the observed values of radiation to black body radiation is generally 0.90-1.0."
"radiative equilibrium","Balance of the absorbed solar flux and emitted terrestrial infrared flux of a planet viewed from space."
"radiative transfer","Transfer of electromagnetic radiation in planetary atmospheres by means of scattering, absorption, and emission."
"radiative-convective models","Therodynamic models that determine the equilibrium temperature distribution for an atmospheric column and the underlying surface, subject to prescribed solar radiation at the top of the atmosphere and prescribed atmospheric composition and surface albedo. Submodels for the transfer of solar and terrestrial radiation, the heat exchange between the earth's surface and atmosphere, the vertical redistribution of heat within the atmosphere, the atmospheric water vapor content and clouds are included in these one-dimensional models. Abbreviated as RCM."
"radiatively active gases","Gases that absorb incoming solar radiation or outgoing infrared radiation, thus affecting the vertical temperature profile of the atmosphere. Most frequently being cited as being radiatively active gases are water vapor, CO2, methane, nitrous oxide, chlorofluorocarbons, and ozone."
"radiometer","Instrument that measures visible, infrared, or microwave radiation at one or more frequencies."
"radiosonde","A balloon-borne instrument for the simultaneous measurement and transmission of meteorological data up to a height of approximately 30,000 m (100,000 ft). The height of each pressure level of the observation is computed from data received via radio signals."
"Raman lidar","Light detection and ranging technique, based on the Raman effect. The characteristic vibrational or rotational Raman lines of gas-phase molecules in the atmosphere are monitored to provide a three-dimensional map of their concentration profiles.  Rotational Raman lines of O2 and N2 molecules give a useful measure of the atmospheric temperature profile. Vibrational Raman lidar is particularly useful for monitoring SO2, CO2, and water vapor, and can be used to measure the concentration and spatial distribution in volcanic plumes."
"Raman scattering","Scattering of visible light accompanied by a shift in wavelength due to interaction with molecular vibration."
"Raman spectroscopy","Analysis of Raman scattered light to give information on vibrational modes of the sample."
"reflectivity","The ratio of the energy carried by a wave that is reflected from a surface to the energy of a wave incident on the surface."
"refraction","Wave bending due to the spatial variation of wave speed in the medium."
"regions or latitude bands","Tropical regions situated between 20ø N and 20ø S exhibit two main ecosystems, the rain forest and the savannah; the boreal regions, primarily located between 55ø and 65ø are mostly covered by the taige (conifer forest); the temperate regions between 40ø and 55ø display the most perturbed ecosystems (fields and various conifer and deciduous trees)."
"relative humidity","The amount of water in the air, expressed as a percentage of the maximum amount that the air could hold at a given temperature."
"reservoir","Any natural or artificial holding area used to store, regulate, or control a substance."
"reservoir species","Trace constituent in the stratosphere that is unreactive toward ozone but that can be chemically converted to molecules that are catalytically active toward ozone."
"ridge","Line or wall of broken ice forced upward by pressure. The submerged volume of broken ice under a ridge, forced downward by pressure, is termed an ice keel."
"riming","Process of freezing of supercooled water droplets when they come into contact with ice particles."
"rocketsonde","A rocket-borne instrument for measurement and transmission of upper-air meteorological data in the lower 76,000 m (250,000) ft) of the atmosphere, especially that portion inaccessible to radiosonde techniques."
"roll cloud","A low, horizontal tube-shaped arcus cloud associated with a thunderstorm gust front (or sometimes with a cold front). Roll clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features, thus differentiating them from the more familiar shelf clouds. Roll clouds usually appear to be "rolling" about a horizontal axis, but should not be confused with funnel clouds."
"rolls","Organized helical large eddies intrinsic to planetary boundary layer flow."
"roughness length","Length scale associated with the sea state; a measure of the efficiency of air-sea coupling."
"runoff ratio","Proportion of precipitation which becomes runoff, usually calculated on an annual basis and expressed as a percentage."
"saturation","A condition in the atmsophere corresponding to 100% relative humidity; super saturation is a condition corresponding to greater than 100% relative humidity."
"saturation vapor pressure","Pressure exerted by molecules of vapor (waper) when air is saturated and in equilibrium with respect to water or ice. Saturated vapor pressure is slightly, but significantly, greater when measured over water than over ice."
"Schaefer point","The temperature of -40ø C (-40ø F) at which ice crystals form spontaneously without the need of nuclei."
"scud","Small, ragged, low cloud fragments (they are also called fractus) that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow."
"sea surface temperature","The temperature of the layer of seawater (approximately 0.5 m deep) nearest the atmosphere."
"seasonal runoff regime","Distribution of flow in a stream throughout the year."
"seasonal variation","The change in a set of meteorological parameters averaged over three months. Seasonal variation is the largest climatic variation, and temperature is the most frequently observed meteorological parameter. Often, monthly averaged data are grouped into seasons, according to the prescribed definition."
"semiarid","Climate or region where the precipitation is sufficient to support steppe, scrub, or dry savanna vegetation types. Variously characterized as receiving a mean annual precipitation between 300-400 and 700 or even 800 mm in summer rainfall areas and between 200-250 and 450-500 mm in winter rainfall regimes, at Mediterranean and tropical latitudes."
"sensible heat","The excess radiative energy that has passed from the Earth's surface to the atmosphere through advection, conduction, and convection processes."
"sensible heat flux","Thermal energy (enthalpy) per unit area per unit time that is exchanged between ocean and atmosphere."
"sferics","Natural atmospheric fluctuations of the electromagnetic fields, caused mainly by thunderstorm activity, in the range of 1-10E5 Hz."
"shelf cloud","A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms). Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn."
"shock wave","Moving wave of strong compression that usually travels faster than sound in the medium."
"shortwave radiation","The radiation received from the sun and emitted in the spectral wavelengths less than 4 microns. It is also called solar radiation."
"signal-to-noise ratio","A quantitative measure of the statistical detectability  of a signal, expressed as a ratio of the magnitude of the signal relative to the variability. For first detection of a CO2-induced climate change, the model signal is the mean change or anomaly in some climatic variable, usually surface air temperatures, attributed by a numerical model to increased concentrations of carbon dioxide. Observed noise is the standards deviation or natural variability computed from observations of that variable and adjusted for sample size, autocorrelation, and time averaging."
"smog","Air pollution associated with oxidants."
"smoke","Particles suspended in air after incomplete combustion of materials."
"soft mode","Molecular vibrational mode for which the atomic displacements track the structural changes followed during a displacive phase transition. The frequency of the soft mode goes to zero at the transition temperature or pressure."
"solar constant","The rate at which solar energy is received just outside the Earth's atmosphere on a surface that is normal to the incident radiation and at the mean distance of the Earth from the Sun. The current value is 0.140 watt/cm2.  See also irradiance. Flux of incoming solar radiation incident at the top of Earth's atmosphere. The solar constant is about 1.4 x 10E6 erg/cm2/s or about 1367 W/m2."
"solar cycle","The periodic change in sunspot numbers. It is the interval between successive minima and is about 11.1 years on average."
"solar wind","Energetic plasma and embedded magnetic fields thrown out from the sun affecting all parts of the solar system."
"Southern Oscillation","A large-scale atmospheric and hydrospheric fluctuation centered in the equatorial Pacific Ocean. It exhibits a nearly annual pressure anomaly, alternately high over the Indian Ocean and high over the South Pacific. Its period is slightly variable, averaging 2.33 years. This variation in pressure is accompanied by variations in wind strengths, ocean currents, sea surface temperatures, and precipitation in the surrounding areas.  El Ni¤o occurrences are associated with the phenomenon. Irregular interannual fluctuation between El Ni¤o and La Ni¤a conditions that in the atmosphere has a scale larger than the tropical Pacific and extends into the Indian Ocean."
"squall line","A line along which converging air currents bring about cumulonimbus activity, with resulting heavy, squally precipitation."
"standing wave cloud","A cloud formed at the crest of a wave in the airstream caused by a mountain peak or ridge. Wind blows through the cloud while the cloud remains stationary."
"steric sea level change","Eustatic sea level change associated with a change in the volume occupied by fixed water mass caused by change in temperature or salinity."
"stratocumulus","Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet."
"stratosphere","The region of the upper atmosphere extending from the tropopause (8 to 15 km altitude) to about 50 km. The thermal structure is determined by its radiation balance and is generally very stable with low humidity. Region of the earth's atmosphere from about 15 to 50 km above the surface. The stratosphere contains only about 15% of the total mass of the atmosphere but about 90% of the total amount of ozone in the atmosphere.  Stratospheric ozone absorbs biologically lethal solar ultraviolet radiation (200-300 nm) and shields the surface."
"stratospheric warming","Disturbance of the winter polar, middle atmosphere lasting for several days and characterized by a warming of the stratosphere (13-50 km altitude) by some tens of degrees."
"stratus","A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus."
"sublimation","The conversion of ice directly from solid to vapor, or from vapor to solid. (For the latter purpose, this term is being supplanted by the term ice condensation)."
"sunspot","A relatively dark, sharply defined region on the solar disk, marked by an umbra approximately 2000K cooler than the effective photospheric temperature, surrounded by a less dark but also sharply bounded penumbra. The average spot diameter is about 3700 km, but can range up to 245,000 km. Most sunspots are found in groups of two or more, but they can occur singly. Sunspots are cyclic, with a period of approximately 11 years. The quantitative description of sunspot activity is called the Wolf sunspot number, denoted, R. The Wolf sunspot number is also referred to as Wolfer sunspot number, Zurich relative sunspot number, or relative sunspot number."
"supercooled cloud","CLoud in which the water droplets, though at temperatures below freezing, still remain in the liquid state."
"supernumerary bows","Additional bows inside the primary rainbow caused by diffraction effects in drops of different sizes."
"superoxide ion","A reduced form of dioxygen (O2-) that is an anion free radical."
"surface air temperature","The temperature of the air near the surface of the Earth, usually determined by a thermometer in an instrument shelter about 2 m above the ground. The true daily mean, obtained from a thermograph, is approximated by the mean of 24 hourly readings and may differ by 1.0 C from the average based on minimum and maximum readings. The global average surface air temperature is 15 C."
"surface albedo","The fraction of solar radiation incident on the Earth's surface that is reflected by it. Reflectivity varies with ground cover, and during the winter months it varies greatly with the amount of snow cover (depth and areal extent). Roughness of terrain, moisture content, solar angle, and angular and spectral distribution of ground-based irradiations are other factors affecting surface albedo."
"terminal velocity","The maximum velocity at which a falling body moves through a medium, such as air. Pull of gravity balanced by friction resisting force."
"terrestrial radiation","The total infrared radiation emitted by the Earth and its atmosphere in the temperature range of ~200-300K. Because the Earth is nearly a perfect radiator, the radiation from its surface varies as the fourth power of the surface's absolute temperature. Terrestrial radiation provides a major part of the potential energy changes necessary to drive the atmospheric wind system and is responsible for maintaining the surface air temperatures within limits for habitability."
"terrogenic effects","Structures in the ionosphere attributable to the electrical (conductivity) structure of the ground."
"thermal wind relation","Theoretical relationship in dynamical meteorology stating that the horizontal gradient in atmospheric temperature is proportional to the vertical gradient in horizontal wind."
"thermosphere","Region of the earth's upper atmosphere where temperature either remains constant or increases with height."
"tornado","A localized, violently destructive circular windstorm characterized by a long, funnel-shaped cloud."
"trace gas","A minor constituent of the atmosphere. The most important trace gases contributing to the greenhouse effect are water vapor, carbon dioxide, ozone, methane, ammonia, nitric acid, nitrous oxide, ethylene, sulfur dioxide, nitric oxide, dichlorofuoromethane or CFC-12, trichlorofluoromethane or CFC-11, methyl chloride, carbon monoxide, and carbon tetrachloride. Atmospheric gases other than O2, N2, and CO2; their average volume mixing ratio is less than 1 ppmv. Any atmospheric constituent that is present in small amounts (less than 0.01%)."
"trade winds","Winds that in each hemisphere blow diagonally westward toward the equator within about 30ø latitude of the equator. They result from a zone of high pressure forming between about 20ø and 30ø from the equator, where air that has risen near the equator builds up."
"transient tracers","Chemical elements (often radiative) or compounds that have finite lifetimes. Atmospheric testing of nuclear weapons from the mid-1950s to the early 1960s released large quantities of radionuclides to the to the atmosphere. Atmosphere-ocean exchange processes have transferred some of these elements to the oceans. Studying the behavior and distribution of these specific isotopes and other chemical tracers in the ocean will provide information on: residence time of the water and its dissolved components in gyres, basins, etc.; the mode and rate of formation and the subsequent spreading rates of specific water types, such as the polar water of the Norwegian and Greenland Seas; deep-ocean circulation and ocean-mixing processes, such as advection and upwelling; and the flux of anthropogenic carbgon dioxide into the ocean through its correlation with several different transient tracers."
"transpirational water","Water that passes through the xylem cells through the stomata and raised pores of the leaf."
"trapped modes","Wavelike motions that remain trapped near the excitation or source region."
"trend","Tendency for sustained increases or decreases in concentrations. Sometimes the term is also applied to seasonal and cyclic behavior."
"tropical climate","A climate in which the average annual temperature is in the range of 18ø to 20øC (64 to 68 F) or higher. Most tropical climates lie within about 30ø of the equator."
"tropical depression","A tropical disturbance with rotary circulation at surface, highest constant wind speed 38 mph (33 knots)."
"tropical disturbance","A moving area of thunderstorms in the Tropics that maintains its identity for twenty-four hours or more. A common phenomenon in the tropics."
"tropical storm","A tropical depression with distinct rotary circulation, constant wind speed ranges from 39-73 mph (34-63 knots)."
"tropopause","The boundary between the troposphere and the stratosphere (about 8 km in polar regions and about 15 km in tropical regions), usually characterized by an abrupt change of lapse rate. The regions above the troposphere have increased atmospheric stability than those below. The tropopause marks the vertical limit of most clouds and storms."
"troposphere","The inner layer of the atmosphere below about 15 km, within which there is normally a steady decrease of temperature with increasing altitude. Nearly all clouds form and weather conditions manifest themselves within this region, and its thermal structure is caused primarily by the heating of the Earth's surface by solar radiation, followed by heat transfer by turbulent mixing and convection. Lowermost part of the atmosphere, dominated by turbulent movements providing a good mixing of the airborne constituents (its maximum height is ~ 12 km near the equator). Lowest region of the earth's atmosphere, extending from the surface to about 15 km above the surface. About 85% of the total mass of the stratosphere is found in the troposphere. The troposphere is the atmospheric region in which weather and clouds occur. Almost all of the water vapor in the atmosphere is in the troposphere. Hence, the troposphere is the most important atmospheric region from a climate perspective."
"turbidity",""Muddiness" in the atmosphere or ocean, cazused by large amounts of suspended matter."
"typhoon","A tropical cyclone of the western Pacific Ocean and the China Sea."
"undersun","A mirrorlike image of the sun, reflected by high concentrations of floating ice crystals; appears at the same angle below the horizon as sun is above it."
"UV-A","Portion of the solar spectrum from 320 to 400 nm."
"UV-B","Portion of the solar spectrum from 280 to 320 nm."
"vapor","The gaseous phase of substances that are liquid or solid at atmospheric pressure (e.g., steam)."
"veering winds","Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation."
"ventilation","Rate (proportional to the surface wind speed) at which the sensible and latent heat fluxes occur."
"vertical moisture profile","Distribution of moisture at different altitudes above a given area at a certain time."
"virga","Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a downburst."
"virga","Falling precipitation, usually ice, that evaporates before reaching the ground."
"virtual temperature","Temperature at which a dry air parcel would have been the same density as a moist parcel at the same pressure. Since water vapor is lighter than air, the virtual temperature is always greater than or equal to the actual temperature."
"volatiles","Elements that are soluble in magmas at elevated pressure and temperature that evolve as gas from magmas during ascent and eruption at the earth's surface. They include sulfur, water, carbon dioxide, chlorine, and fluorine."
"wadi","Seasonal African river."
"Walker cell","A zonal circulation of the atmosphere confined to equatorial regions and driven principally by the oceanic temperature gradient. In the Pacific, air flows westward from the colder, eastern area to the warm, western ocean, where it acquires warmth and moisture and subsequently rises. A return flow aloft and subsidence over the eastern ocean complete the cell."
"wall cloud","A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation. However, not all wall clouds rotate. Rotating wall clouds usually develop before strong or violent tornadoes, by anywhere from a few minutes up to nearly an hour. Wall clouds should be monitored visually for signs of persistent, sustained rotation and/or rapid vertical motion."
"warm front","A weather front along which an advancing mass of warm air rises over a retreating mass of cold air."
"water vapor","Water present in the atmosphere in gaseous form; the source of all forms of condensation and precipitation. Water vapor, clouds, and carbon dioxide are the main atmospheric components in the exchange of terrestrial radiation in the troposphere,  serving as a regulator of planetary temperatures via the greenhouse effect. Approximately 50 percent of the atmosphere's moisture lies within about 1.84 km of the earth's surface, and only a minute fraction of the total occurs above the tropopause."
"water vapor feedback","A process in which an increase in the amount of water vapor increases the atmosphere's absorption of longwave radiation, thereby contributing to a warming of the atmosphere. Warming, in turn, may result in increased evaporation and an increase in the initial water vapor anomaly. This feedback, along with carbon dioxide, is responsible fro the greenhouse effect and operates virtually continuously in the atmosphere."
"water year","Twelve-month period beginning at the end of the annual low-flow period."
"water-use efficiency","Ratio of the number of moles of carbon dioxide fixed in assimilation to the number of moles of water transpired."
"watershed","Total expanse of terrain from which water flows into a water body or stream."
"waterspout","In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a towering cumulus or cumulonimbus cloud. Waterspouts are most common over tropical or subtropical waters. The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation. But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface."
"waterspout","A tornado over a body of water, with characteristic pendant funnel hanging from a cumulonimbus cloud."
"weather","The instantaneous state of the global atmosphere-ocean-cryosphere system."
"weighting function","Vertical distribution of the intensity of radiation received by a spaceborne radiometer, that is, what levels in the atmosphere the radiometer is sensitive to."
"west-wind drift","The eastward-flowing ocean current near Ihe North or South Pole that is created by Ihe major ocean gyres and the reinforcing westerly winds."
"westerlies","Winds that flow toward the northeast in each hemisphere between about 30ø and 60ø from the equator. These result from the same high-pressure zone that produces the trade winds, which flow in the opposite direction."
"wet deposition","Flux of chemicals to the surface of the earth during a precipitation event. Compare with dry deposition."
"Wilson cloud chamber","An experimental chamber in which supersaturated vapor is forced to condence on nuclei."
"wind shear","A change in wind velocity as altitude changes."
"wind stress","Force on the ocean (per unit area) associated with the momentum that is exchanged between the ocean and atmosphere."
"zonally-averaged models","Statistical-dynamical or energy-balance models in which only the latidudinally averaged quantities are determined and the effects of the longitudinally varying transports are determined parametrically. Abbreviated ZAM."