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--> GLOSSARY A Algorithm. A component of a computer program (or set of programs) designed to solve a certain kind of problem. WSR-88D radars (NEXRAD) employ algorithms to analyze radar data and automatically determine storm motion, probability of hail, accumulated rainfall, and several other parameters. C Chemical weather. The state of the atmosphere as described by its chemical composition especially variable trace constituents such as ozone, oxides of nitrogen, carbon monoxide, etc. Climate. The qualitative and/or quantitative characterization of the weather at a particular place or region, or over an entire planet, for a specified period of years, by the statistics (e.g. averages, maxima and minima, probabilities of occurrence) of weather elements such as temperature, humidity, precipitation, and wind velocity. Changes in the statistics of weather elements over a series of specified climate-measuring periods provide a measure of the trends in climate. Corona. The pearly outer envelope of the Sun observed during solar eclipses or with the coronagraph. At sunspot minimum, the corona has large extensions along the Sun's equator, with short brush-like tufts near the poles. At sunspot maximum, the equatorial extensions are much smaller, and the corona is more regular in shape. Cyclone. An atmospheric cyclonic circulation, a closed circulation. A cyclone's direction of rotation (counterclockwise in the Northern Hemisphere) is opposite to the rotation of an anticyclone. Modern meteorology restricts the use of cyclone to the so-called synoptic-scale circulations (typically 1000-3000 km). Cyclonic. Having a sense of rotation about the local vertical the same as that of the Earth's rotation; that is, as viewed from above, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. D Data assimilation. Process of using observations to correct or adjust model predictions wherever and whenever the observations are available. Distributed modeling. In hydrology, the correlation of many observations and estimates of rainfall over space and time with information on topography, soil types, and soil saturation to determine how much rain is likely to be running off a region into a river and forecast where flooding will occur within a basin. Dropsonde (also called parachute radiosonde). A radiosonde dropped by parachute from an aircraft for the purpose of obtaining soundings of the atmosphere below. E Ensemble forecasting. A number of independent computer model integrations with initial conditions slightly perturbed to represent uncertainty in the analysis. Ensemble forecasting has three main objectives: (1) to improve a single forecast by averaging forecasts over the ensemble; (2) to estimate the reliability, or conversely the uncertainty, of a forecast; and (3) to provide a basis for probabilistic forecasts by assigning probability values to possible flow evolutions. F Flops. Floating point operations per second, a measure of the speed of a processor. Fuzzy logic. In meteorology, an objective decision-making
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--> process that quantitatively deals with uncertainty and imprecision in observation and forecasting algorithms. G Geostationary satellite. A satellite that orbits the Earth at the same rate that the Earth rotates, thus remaining over a fixed place above the equator. Gigaflops. A unit of computer speed equal to 109 floating point operations per second (flops). H Hybrid forecasting. The use of a mix of advanced forecasting techniques to forecast weather. Hydrograph. A graphical representation of stage or discharge at a point on a stream as a function of time. The most common type, the observed hydrograph, represents river gauge readings plotted at the time of observation. Other types of hydrographs are derived statistically from observed data and include distribution graphs and unit hydrographs. M Microscale. The smallest scale of atmospheric motions, typically from millimeters to a few meters. Mesoscale. Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles to several hundred miles. N Nowcast. A short-term weather forecast, generally for six hours or less. O Occultation. The gradual emergence or disappearance of the electromagnetic waves emitted from a celestial body or spacecraft behind another celestial body; for example, an eclipse of a star or planet by the moon as seen from the Earth, or the eclipse of a GPS satellite by Earth as observed from an orbiting GPS radio receiver. P Polarization. The state of electromagnetic radiation when transverse vibrations are regular, e.g., all in one plane, in a circle, an ellipse, or some other definite curve. Radiation may become polarized because of the nature of its emitting source, as is the case with many types of radar antennas, or because of processes to which it is subjected after leaving its source, like the scattering of solar radiation as it passes through the Earth's atmosphere. Predictability. The quality or state of being predictable or expected on the basis of observation, experience, or scientific reason. Probabilistic forecasting. Determining the probability of occurrence of different weather events. Probability. The chance that a prescribed event will occur, represented as a fraction between zero and one. The probability of an impossible event is zero, and of an inevitable event is one. Probability is estimated empirically by relative frequency, that is, the number of times a particular event occurs divided by the total count of all events in the class. R Radiation. 1. Electromagnetic energy being propagated through free space by virtue of joint variations in the electric and magnetic fields in space. 2. Energy propagated through any medium by virtue of the wave motion of that medium, as in the propagation of sound waves through the atmosphere or ocean waves along the water surface. Radiance. The flux density of radiant energy per unit solid angle and per unit projected area of radiating surface. Radio occultation. As applied in GPS meteorology, a technique whereby radio signals propagating from a GPS satellite to a LEO satellite are refracted by the Earth's atmosphere during about a 60 second period before the signal is occulted by the Earth, providing a refractivity profile of the atmosphere. From these measurements, temperature, pressure, and water vapor distributions can be determined. Radio occultation technique. The use of GPS transmitter radio path delay profiles through the Earth's atmosphere to infer the thermodynamic structure of the atmosphere. Radiosonde. A balloon-borne instrument for the simultaneous measurement and transmission of meteorological data. Rawinsonde. Method of upper-air observation consisting of an evaluation of the wind speed and direction, temperature, pressure, and relative humidity aloft by means of a balloon-borne radiosonde tracked by a radar or radio direction-finder. S Soil moisture accounting. A hydrologic model used by the NWS that produces a river forecast based on calibrated six-hour historical rain gauge measurements combined with six-hour rainfall measurements and estimation of precipitation and run-off over a basin. Sounding. 1. In geophysics, any penetration of the natural
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--> environment for scientific observation. 2. In meteorology, same as upper-air observation. However, a common connotation is that of a single complete rawinsonde observation. Space weather. Conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of spaceborne and ground-based technological systems and endanger human life or health. Adverse conditions in the space environment can disrupt satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses. Synoptic. In general, pertaining to or affording an overall view. In meteorology, this term has become somewhat specialized in referring to the use of meteorological data obtained simultaneously over a wide area for the purpose of presenting a comprehensive and nearly instantaneous picture of the state of the atmosphere. T Thermodynamics. In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.). U Upper air. In synoptic meteorology and in weather observing, that portion of the atmosphere above the shallow layer of air closest to the Earth. No distinct lower limit is set, but the term is usually applied to levels above 1 kilometer from the surface. W Water vapor (also called aqueous vapor, moisture). Water substance in vapor form; one of the most important constituents of the atmosphere. Weather. The state of the atmosphere near the ground as described by the temperature, pressure, humidity, wind speed and direction, cloud cover, visibility, cloud type, and the type and rate of precipitation.
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