Subj : Wx Terms (A)(4) To : All From : Daryl Stout Date : Sun Sep 03 2017 10:39 am Angle of Reflection The angle at which a reflected ray of energy leaves a reflecting surface. It is measured between the outgoing ray and a perpendicular to the surface at the point of incidence (i.e., where the ray strikes). Angstrom A unit of length equal to 10-8 cm. Annual Flood In hydrologic terms, the maximum discharge peak during a given water year (October 1 - September 30). Anomaly The deviation of a measurable unit (e.g., temperature or precipitation) over a period in a given region from the long-term average, often the thirty-year mean, for that region. Antedecent Precipitation Index (Abbrev. API) - an index of moisture stored within a drainage basin before a storm. Anthelion A luminous white spot that appears on the parhelic circle at the same altitude as the sun and 180ø from it in azimuth. Anthropogenic Source A pollutant source caused or produced by humans. Anti-wind The upper or return branch of an along-valley wind system, as confined within a valley, and blowing in a direction opposite to the winds in the lower altitudes of the valley. Anticyclogenesis The formation or intensification of an anticyclone or high pressure center. Anticyclone A large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere Anticyclonic Rotation Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation. Antilles Current A current which originates in the vicinity of the Leeward Islands as part of the Atlantic North Equatorial Current. Anvil The flat, spreading top of a cumulonimbus cloud, often shaped like an anvil. Thunderstorm anvils may spread hundreds of miles downwind from the thunderstorm itself, and sometimes may spread upwind. Anvil Crawler [Slang], a lightning discharge occurring within the anvil of a thunderstorm, characterized by one or more channels that appear to crawl along the underside of the anvil. They typically appear during the weakening or dissipating stage of the parent thunderstorm, or during an active MCS. Anvil Dome A large overshooting top or penetrating top. Anvil Rollover Slang for a circular or semicircular lip of clouds along the underside of the upwind part of a back-sheared anvil, indicating rapid expansion of the anvil. Anvil Zits Slang for frequent (often continuous or nearly continuous), localized lightning discharges occurring from within a thunderstorm anvil. AO Arctic Oscillation - the Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45ø north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO. AOA At or above AOB At or below AOPA Aircraft Owners and Pilots Association AP Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions. Sometimes referred to as "ground clutter". AP Index In solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific stations. APD On a buoy report, the average wave period (seconds) of all waves during the 20-minute period. Aphelion The point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion. API Method In hydrologic terms, a statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the antecedent precipitation index, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity. Apogee The farthest distance between the moon and earth or the earth and sun. Apparent Temperature A measure of human discomfort due to combined heat and humidity (e.g., heat index). Apparent Wind The speed and true direction from which the wind appears to blow with reference to a moving point. Sometimes called RELATIVE WIND. APRCH Approach APRCHG approaching APRNT apparent APST Aviation Products and Services Team Aquiclude In hydrologic terms, a formation which contains water but cannot transmit it rapidly enough to furnish a significant supply to a well or spring. Aquifer In hydrologic terms, permeable layers of underground rock, or sand that hold or transmit groundwater below the water table that will yield water to a well in sufficient quantities to produce water for beneficial use. Aquifuge In hydrologic terms, a geologic formation which has no interconnected openings and cannot hold or transmit water. ARAM Aviation, Range, and Aerospace Meteorology Arch Dam A concrete arch dam is used in sites where the ratio of width between abutments to height is not great and where the foundation at the abutments is solid rock capable of resisting great forces. The arch provides resistance to movement. When combined with the weight of concrete (arch-gravity dam), both the weight and shape of the structure provide great resistance to the pressure of water. Arch Filament System (AFS) In solar-terrestrial terms, a bright, compact plage crossed by a system of small, arched filaments, which is often a sign of rapid or continued growth in an Active Region. Arctic The region within the Arctic Circle, or, loosely, northern regions in general, characterized by very low temperatures. Arctic front The boundary or front separating deep, cold arctic air from shallower, relatively less cold polar air. Arctic Oscillation (abbrev. AO)- The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45ø north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO. --- þ Synchronet þ The Thunderbolt BBS - wx1der.dyndns.org .