Auxiliary SpillwayA spillway, usually located in a saddle or depression in the reservoir rim which leads to a natural or excavated waterway, located away from the dam which…
auxinA group of plant growth regulators (natural or synthetic) which stimulate cell division, enlargement, apical dominance, root initiation, and flowering.
auxin-cytokinin ratioThe relative proportion of auxin to cytokinin present in plant tissue culture media.
auxotrophA mutant cell or micro-organism lacking one metabolic pathway present in the parental strain, and that consequently will not multiply on a minimal medium…
AVADSAutomated Vacancy Announcement Distribution System.
AvailabilityA reflection of the form and location of nutritional elements and their suitability for absorption.
Available capacityThe amount of water held in the soil that is available to the plants. See water holding capacity.
Available HeatThe amount of heat energy that may be converted into useful energy from a fuel.
AvalancheA mass of snow, rock, and/or ice falling down a mountain or incline. In practice, it usually refers to the snow avalanche.
Avalanche AdvisoryA preliminary notification that conditions may be favorable for the development of avalanches in mountain regions.
AverageA stock market average is a mathematical way of reporting the composite change in prices of the stocks that the average includes.
Average Annual Daily Traffic (AADT)(AADT)A measurement representing the total number of vehicles passing a given location, based upon 24-hour counts taken over an entire year.
Average annual runoffFor a specified area, the average value of annual runoff amounts calculated for a selected period of record that represents average hydrologic conditions.
Average CostThe total cost of production divided by the total quantity produced.
Average degree of consolidationThe ratio of the total volume change in a soil mass at a given time to the total volume change anticipated in the soil mass due to primary consolidation.
Average DemandThe demand on, or the power output of, an electrical system or any of its parts over an interval of time, as determined by the total number of…
Average energyThe total power generation produced by a powerplant during all of the years of its actual or simulated operation divided by the number of years of actual…
Average megawatt (MWa, aMW)(MWa, aMW)One megawatt of capacity produced continuously over a period of one year. 1 aMW = 1 MW x 8760 hours/year = 8,760 MWh = 8,760,000 kWh.
Average valueAverage value Arithmetic mean (m) of a number (n) of values (x1, x2, ... xn), defined by the equation: m = Sxi/n.
Aviation gasolineAll special grades of gasoline for use in aviation reciprocating engines, as given in the American Society for Testing and Materials (ASTM) specification…
avidinA glycoprotein present in egg white, which has a strong affinity to biotin. Can lead to biotin deficiency if given in large quantities.
avidityA measure of the binding strength of an antibody to its antigen.
avirulence gene(Abbreviation: avr gene). Many plants contain R genes, which confer simply-inherited resistance to a specific pathogen race.
AVNThe Aviation model (120-hour numerical model of the atmosphere). The output from this model is now part of what is known as the GFS model.
Avoid HarmExecutive Order 13158 mandates that each federal agency whose actions affect an MPA shall, to the extent permitted by law and to the maximum extent…
Avoided CostThe incremental cost to an electric power producer to generate or purchase a unit of electricity or capacity or both.
Avoided costsAn investment guideline describing the value of a conservation or generation resource investment by the cost of more expensive resources that a utility…
Avoirdupois weightAn English and American system of weights based on a pound of 16 ounces.
AVPOn a buoy report, Average Wave Period is the average period (seconds) of the highest one-third of the wave observed during a 20 minute sampling period.
axenic cultureFree of external contaminants and internal symbionts; generally not possible with surface sterilization alone, sometimes used incorrectly to indicate.
Axial FansFans in which the direction of the flow of the air from inlet to outlet remains unchanged; includes propeller, tubaxial, and vaneaxial type fans.
Axial Flow CompressorA type of air compressor in which air is compressed in a series of stages as it flows axially through a decreasing tubular area.
Axial Flow TurbineA turbine in which the flow of a steam or gas is essentially parallel to the rotor axis.
axillary budA bud found at the axil of a leaf. Synonym: lateral bud.
axillary bud proliferationPropagation of plant tissue in vitro to promote axillary growth, to generate large numbers of plantlets in culture.
AxisA straight line passing through a spherical body between its two poles. The central line of the body or any of its parts. The vertebral column.
Axis of DamA vertical plane or curved surface, appearing as a line in plan or cross section, to which horizontal dimensions can be referred.
Axis of dam (concrete)A vertical reference surface coincident with the upstream face at the top of the dam.
AzimuthA direction in terms of a 360° compass. North is at 0°, east is at 90°, south is at 180°, and west is at 270°.
Azimuth (Solar)The angle between true south and the point on the horizon directly below the sun.
Azimuth AngleThe direction or bearing toward which a sloping surface faces (e.g., a north-facing slope has an azimuth angle of 360°; a northeast-facing slope, an…
Azores CurrentOne of the currents of the North Atlantic subtropical gyre.
Azores HighAlternate term for Bermuda High - a semi-permanent, subtropical area of high pressure in the North Atlantic Ocean off the East Coast of North America that…
BAbbreviation used on long-term climate outlooks issued by CPC to indicate areas that are likely to be below normal for a given parameter (temperature…
B cellA white blood cell that comes from bone marrow. As part of the immune system, B cells make antibodies and help fight infections. Also called B lymphocyte.
B cellsAlso known as B lymphocytes. Each B cell is capable of making one specific antibody.
B chromosomeA supernumerary chromosome present in some individuals (both plant and animal).
B lymphocyteA white blood cell that comes from bone marrow. As part of the immune system, B lymphocytes make antibodies and help fight infections. Also called B cell.
B-44 Form, Cooperative Station ReportA Weather Service form documenting station management, exposure, topography, driving instructions, payment information, hydrometeorologic equipment, and…
Bacillus thuringiensis(Abbreviation: Bt). A bacterium that produces a toxin against certain insects, particularly Coloeoptera and Lepidoptera; a major means of insecticide for…
Bacillus thuringiensis (Bt)A bacterium that kills insects; a major component of the microbial pesticide industry.
Back Door Cold FrontA cold front moving south or southwest along the Atlantic seaboard and Great Lakes; these are especially common during the spring months.
back mutationA second mutation at the same site in a gene as the original mutation. The second mutation restores the wild-type protein sequence.
Back pressureA pressure that can cause water to backflow into the water supply when a user's water system is at a higher pressure than the public water system.
Back-building ThunderstormA thunderstorm in which new development takes place on the upwind side (usually the west or southwest side), such that the storm seems to remain…
Back-sheared Anvil[Slang], a thunderstorm anvil which spreads upwind, against the flow aloft. A back-sheared anvil often implies a very strong updraft and a high severe…
BackcrossCrossing an organism with one of its parent organisms.
BackdraftingThe flow of air down a flue/chimney and into a house caused by low indoor air pressure that can occur when using several fans or fireplaces and/or if the…
BackfillMaterial used in refilling excavation, or the process of such refilling. Material used to fill an excavated trench.
Backfill concreteConcrete used in refilling excavation in lieu of earth material.
BackfireA fire started to stop an advancing fire by creating a burned area in its path.
BackflowA reverse flow condition, created by a difference in water pressures, which causes water to flow back into the distribution system.
BackfurrowThe first cut of a plow, from which the slice is laid on undisturbed soil.
Background levelThe average amount of a substance present in the environment. Originally referring to naturally occurring phenomena. Used in toxic substance monitoring.
Background level noiseNoise in the environment (other than noise emanating from the source of interest).
Background RadiationFilling all space, e.g. the radiation from the early, hot universe.
BackingAny type of line used to partially fill a reel before the main fishing line is added; commonly used in fly-fishing or by bass anglers who use many of the…
Backing WindsWinds which shift in a counterclockwise direction with time at a given location (e.g.
BackscatterThe portion of power scattered back in the incident direction.
backscattered radiationThe scattering of radiant energy into the hemisphere of space bounded by a plane normal to the direction of the incident radiation and lying on the same…
BacksightA rod reading taken on a point of known elevation, a benchmark or a turning point.
BacksiphonageA form of backflow caused by a negative or below atmospheric pressure within a water system.