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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • earth - The solid, liquid, and gaseous parts of the planet taken as a whole. Near-earth space (such as the magnetosphere) is often included.

  • easterlies - Any winds with components from the east, usually applied to broad currents or patterns of persistent easterly winds, the "easterly belts," such as the equatorial easterlies, the tropical easterlies, and the polar easterlies. See also trade winds.

  • easterly wave - A migratory wavelike disturbance of the tropical easterlies. It is a wave within the broad easterly current and moves from east to west, generally more slowly than the current in which it is embedded. Although best described in terms of its wavelike characteristics in the wind field, it also consists of a weak trough of low pressure. Easterly waves do not extend across the equatorial trough. To the west of the trough line in an easterly wave over the ocean, there is generally found divergence, a shallow moist layer, and exceptionally fine weather. The moist layer rises rapidly near the trough line; in and to the east of the trough line intense convergence, much cloudiness, and heavy rain showers prevail. This asymmetric weather pattern may be greatly distorted by orographic and diurnal influences if the wave passes over land areas. Easterly waves occasionally intensify into tropical cyclones. Compare equatorial wave.

  • element - 1. Smallest entity that is capable of possessing chemical characteristics and that cannot be changed into smaller neutral units by chemical reaction. Elements in their pure form may exist in atomic forms, for example, He, Ne, or may be associated into molecular units such as H2, O2, graphite, or diamond. 2. Any one of the properties or conditions of the atmosphere that together specify the physical state of weather or climate at a given place for any particular moment or period of time; the climatic elements, meteorological elements. 3. The smallest definable object of interest in a scene. It is a single item in a collection, population, or sample.

  • elevation - 1. A measure (or condition) of height, especially with respect to the height of a point on the earth's surface above a reference plane (usually mean sea level), as "station elevation." The term altitude (e.g., "high-altitude station") and the general term "height" are also used in this sense. 2. Same as elevation angle.

  • ELF - Abbreviation for extremely low frequency. See radio frequency band.

  • energy - A measurable physical quantity, with dimensions mass times velocity squared, that is conserved for an isolated system. Energy of motion is kinetic energy; energy of position is potential energy. See energy conversion, internal energy, enthalpy.

  • entrainment - 1. In meteorology, the mixing of environmental air into a preexisting organized air current so that the environmental air becomes part of the current; the opposite of detrainment. Entrainment of air into clouds, especially cumulus, is said to be inhomogeneous when the timescale for mixing of environmental air is very much greater than the timescale for drop evaporation. Under these conditions, which are often found when environmental air is first entrained into cumulus, regions of cloud and entrained air are intertwined, with evaporation occurring only on the edges of the interface between the cloudy and entrained environmental air. 2. The process by which turbulent fluid within a mixed layer incorporates adjacent fluid that is nonturbulent, or much less turbulent; thus entrainment always proceeds toward the nonturbulent layer. In the absence of advection effects, this tends to deepen the mixed layer.

  • entrainment rate - The rate at which entrainment proceeds. In the absence of advection effects, the entrainment rate is equal to the rate of deepening of the mixed layer.

  • entrance region - The region of confluence at the upwind extremity of a jet stream; the opposite of exit region.

  • environment - External conditions and surroundings, especially those that affect the quality of life of plants, animals, and human beings. In agriculture the environment includes the air, soil, and water conditions.

  • episode - In the context of air pollution, a time period during which pollution standards are exceeded or, more loosely, during which pollutant levels are significantly greater than normal or background levels.

  • equator - Geographically, on the earth's surface, the imaginary great circle of latitude 0°, which is equidistant from the poles, and which separates the Northern Hemisphere from the Southern Hemisphere. See meteorological equator, heat equator, thermal equator, celestial equator, aclinic line, geomagnetic equator.

  • equatorial convergence zone - Same as intertropical convergence zone.

  • erosion - The movement of soil or rock from one point to another by the action of the sea, running water, moving ice, precipitation, or wind. Erosion is distinct from weathering, for the latter does not necessarily imply transport of material. Where human agency has increased erosion beyond the normal geologic rate, it is termed accelerated erosion. Wind erosion is a very important factor in the continued redistribution of earth surface material. Two measures of this effect are used by geologists, capacity of the wind and competence of the wind. Compare corrasion, corrosion, eolation.

  • error - 1. See instrument error, observational error, random error, systematic error, standard error. 2. See forecast error.

  • ERS - Abbreviation for European Remote Sensing Satellites.

  • ESSA - The first series of operational meteorological satellites launched by the United States. Based on the earlier TIROS series of satellites, the nine satellites launched between 1966 and 1969 are often referred to as the TIROS Operational System.

  • ETA - Abbreviation for estimated time of arrival.

  • evaporation - (Also called vaporization.) The physical process by which a liquid or solid is transformed to the gaseous state; the opposite of condensation. Evaporation is usually restricted in use to the change of water from liquid to gas, while sublimation is used for the change from solid to gas. According to the kinetic theory of gases, evaporation occurs when liquid molecules escape into the vapor phase as a result of the chance acquisition of above-average, outward-directed, translational velocities at a time when they happen to lie within about one mean free path below the effective liquid surface. It is conventionally stated that evaporation into a gas ceases when the gas reaches saturation. In reality, net evaporation does cease, but only because the numbers of molecules escaping from and returning to the liquid are equal, that is, evaporation is counteracted by condensation. Because the molecules that escape the condensed phase have above-average energies, those left behind have below-average energies, which is manifested by a decrease in temperature of the condensed phase (unless compensated for by energy transfer from the surroundings). See also evapotranspiration.

  • excess - See kurtosis.

  • exit region - 1. The region of diffluence at the downwind extremity of a jet stream; the opposite of entrance region. 2. Same as delta region.

  • expectation - Same as expected value.

  • extended forecast - A forecast of weather conditions for a period extending beyond three or more days from the day of issuance. The U.S. National Weather Service issues extended forecasts for the three- to five-day period ahead. Compare long-range forecast, medium-range forecast.

  • extrapolation - The extension of a relationship between two or more variables beyond the range covered by knowledge, or the calculation of a value outside that range. In synoptic meteorology, extrapolation commonly refers to the forecasting of the position of a weather-pattern feature based solely upon recent past motion of that feature. Compare interpolation.

  • extreme - In climatology, the highest and, in some cases, the lowest value of a climatic element observed during a given time interval or during a given month or season of that period. If this value were the greatest extreme for the whole period of record for which observations are available, the value would be the absolute extreme.

  • eye - In meteorology, usually the "eye of the storm" (hurricane, typhoon), that is, the roughly circular area of comparatively light winds found at the center of a severe tropical cyclone and surrounded by the eyewall. The winds increase gradually outward from the center but can remain very light up to the inner edge of the eyewall. No rain occurs and in intense tropical cyclones the eye is clear with blue sky overhead. Most, but not all, tropical cyclones with maximum winds in excess of 40 m s-1(78 knots) have eyes visible on satellite imagery. Eye diameters vary from 10 to more than 100 km.

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