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Atmosphere: the gaseous or air portion of the physical environment that encircles a planet. in the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. the divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere. cape: acronym for convective available potential energy. the amount of energy available to create convection, with higher values increasing the possibility for severe weather. ceilometer: an instrument that is used to measure the angular elevation of a projected light on the base of a cloud. it measures the angle of the cloud base included by the observer (or machine), the ceiling light and the illuminated spot on the cloud. crepuscular rays: contrasting, alternating bright and dark rays in the sky. sunlight is scattered by molecules and particles rendering these bright rays visible. contrast is enhanced by haze, dust, or mist. these rays are more likely to be seen in the late afternoon, as clouds come between the sun and the observer. a similar effect occurs when the sun shines though a break in a layer of clouds. eye: the center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. an eye will usually develop when the maximum sustained wind speeds exceed 78 mph. it can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. in general, when the eye begins to shrink in size, the storm is intensifying. firewhirl: a tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption. fujita-pearson scale: a scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. it is normally used to identify the most intense damage exhibited by a tornado. developed by t. theodore fujita and allen pearson. greenhouse effect: the overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space. hail: precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. individual lumps are called hailstones. it is reported as "gr" in an observation and on the metar. small hail and/or snow pellets is reported as "gs" in an observation and on the metar. high clouds: a term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. the main types of high clouds are cirrus,cirrocumulus, and cirrostratus. this altitude applies to the temperate zone. in the polar regions, these clouds may be found at lower altitudes. in the tropics, the defining altitudes for cloud types are generally higher. national hurricane center (nhc): a branch of the tropical prediction center, it is the office of the national weather service that is responsible for tracking and forecasting tropical cyclones over the north atlantic, caribbean, gulf of mexico, and the eastern pacific. : for further information, contact the nhc, located in miami, florida. partly cloudy: the state of the weather when the clouds are conspicuously present, but do not completely dull the sky or the day at any moment. the national weather service does not have an amount of sky cover for this condition. snow crust: the crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow. standing wave: an atmospheric wave that is stationary with respect to the medium in which it is embedded. steam fog: a type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. also called sea smoke when it occurs over the ocean. transparent: a condition where a material is clear enough not to block the passage of radiant energy, especially light. universal time coordinate: one of several names for the twenty-four hour time which is used throughout the scientific and military communities. visible light: the portion of the electromagnetic spectrum that can be detected by the human eye. it travels at the same speed as all other radiation, that is at 186,000 mile per second. it has a wave length longer than ultraviolet light and shorter than x-rays. visual flight rules (vfr): refers to the general weather conditions pilots can expect at the surface. vfr criteria means a ceiling greater than 3,000 feet and greater than 5 miles. wall cloud: an abrupt lowering of a cloud from its parent cloud base, a cumulonimbus or supercell, with no visible precipitation underneath. forming in the area of a thunderstorm updraft, or inflow area, it exhibits rapid upward movement and cyclonic rotation. it often develops before strong or violent tornadoes..

Crystallization: the process of a substance going directly from a vapor form (water vapor) to a solid (ice) at the same temperature, without going through the liquid phase (water). the opposite of sublimation. cumulus congestus: a strongly sprouting cumulus cloud with generally sharp outlines and often with great vertical development. it may occur as tower-like clouds with cauliflower tops. these clouds may produce abundant showers and may develop further into cumulonimbus. earthlight (earthshine): the faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere. heat lightning: lightning that appears as a glowing flash on the horizon. it is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard. hurricane: the name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the north atlantic ocean, caribbean sea, gulf of mexico, and in the eastern north pacific ocean. this same tropical cyclone is known as a typhoon in the western pacific and a cyclone in the indian ocean. katafront: a front where the warm air descends the frontal surface, except in the low layers of the atmosphere. lightning: a sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air. shear: it is the rate of change over a short duration. in wind shear, it can refer to the frequent change in wind speed within a short distance. it can occur vertically or horizontally. directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. when used in reference to doppler radar, it describes the change in radial velocity over short distances horizontally. swell: ocean waves that have traveled out of their generating area. swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch. thunder: the sound emitted by rapidly expanding gases along the channel of a lightning discharge. over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. temperatures can rise to over 10,000 °c in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. the rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away. tropical air mass: an air mass that forms in the tropics or subtropics over the low latitudes. maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry. upper air/upper level: the portion of the atmosphere which is above the lower troposphere. it is generally applied to the levels above 850 millibars. therefore, upper level lows and highs, troughs, winds, observations, and charts all apply to atmospheric phenomena above the surface..

Aneroid barometer: an instrument for measuring the atmospheric pressure. it registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. the aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. the scales on the glass cover measure pressure in both inches and millibars. arctic air mass: an air mass that develops around the arctic, it is characterized by being cold from surface to great heights. the boundary of this air mass is often defined by the arctic front, a semi-permanent, semi-continuous feature. when this air mass moves from its source region, it may become more shallow in height as it spreads southward. atmosphere: the gaseous or air portion of the physical environment that encircles a planet. in the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. the divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere. biosphere: the transition zone between the earth and the atmosphere within which most terrestrial life forms are found. it is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere. central pressure: the atmospheric pressure at the center of a high or low. it is the highest pressure in a high and the lowest pressure in a low, referring to the sea level pressure of the system on a surface chart. cirrus: one of the three basic cloud forms (the others are cumulus and stratus). it is also one of the three high cloud types. cirrus are thin, wispy clouds composed of ice crystals and often appear as veil patches or strands. in the mid-latitudes, cloud bases are usually found between 20,000 to 30,000 feet, and it is the highest cloud that forms in the sky, except for the tops, or anvils, of cumulonimbus, which occasionally build to excessive heights. feeder bands: in tropical parlance, the lines or bands of thunderstorms that spiral into and around the center of a tropical system. also known as outer convective bands, a typical hurricane may have three or more of these bands. they occur in advance of the main rain shield and are usually 40 to 80 miles apart. in thunderstorm development, they are the lines or bands of low level clouds that move or feed into the updraft region of a thunderstorm. lightning: a sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air. low level jet (llj): strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. it is often amplified at night. the southerly wind over the us plains states during spring and summer is a notable example. marginal visual flight rules (mvfr): refers to the general weather conditions pilots can expect at the surface. mvfr means minimum or marginal visual flight rules. mvfr criteria means a ceiling between 1,000 and 3,000 feet and/or 3 to 5 miles visibility. nautical twilight: the time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. it ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars. nowcast: a short-term weather forecast for expected conditions in the next few hours. pascal: the unit of pressure produced when one newton acts on about one square meter. radiational cooling: the cooling of the earth's surface and the adjacent air. although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation. rain forest: a forest which grows in a region of heavy annual precipitation. there are two major types, tropical and temperate. shear line: a line of maximum horizontal wind shear. a narrow zone across which there is an abrupt change in the horizontal wind component parallel to it. sky cover: the amount of the celestial dome that is hidden by clouds and/or obscurations. storm winds: on the beaufort wind scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour). sun pillar: horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's surface, reflect sunlight into vertical sun pillars for a spectacular display. supercell: a severe thunderstorm characterized by a rotating, long-lived, intense updraft. although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes. twilight: often called dusk, it is the evening period of waning light from the time of sunset to dark. the time of increasing light in the morning is called dawn. twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. during dawn, the reverse order occurs until full daylight. visibility: a measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. the national weather service has various terms for visibility. surface visibility is the prevailing visibility determined from the usual point of observation. prevailing visibility is considered representative of visibility conditions at the station. sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. tower visibility is the prevailing visibility determined from the airport traffic control tower (atct) at stations that also report surface visibility. warm: to have or give out heat to a moderate or adequate degree. a subjective term for temperatures between cold and hot. in meteorology, an air parcel that is warm is only so in relation to another parcel. west virginia high: an area of stagnant high pressure located over west virginia during indian summer..

Albedo: the ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it. this varies according to the texture, color, and expanse of the object's surface and is reported in percentage. surfaces with high albedo include sand and snow, while low albedo rates include forests and freshly turned earth. altocumulus castellanus: a middle cloud with vertical development that forms from altocumulus clouds. it is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. its formation indicates instability and turbulence at the altitudes of occurrence. bright band: a narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest. climatology: the study of climate. it includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems. cyclonic flow: winds that blow in and around a cyclone, that is counterclockwise in the northern hemisphere, clockwise in the southern hemisphere. diurnal: pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind. dynamics: a branch of mechanics that deals with forces and their relations to patterns of motion. in metorology, this relates especially to wind and water patterns. electromagnetic spectrum: the band of electromagnetic radiation with components that are separated into their relative wave lengths. the portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. the various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays. ground fog: fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. primarily takes place at night or early morning. high clouds: a term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. the main types of high clouds are cirrus,cirrocumulus, and cirrostratus. this altitude applies to the temperate zone. in the polar regions, these clouds may be found at lower altitudes. in the tropics, the defining altitudes for cloud types are generally higher. hydrology: the study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface. lithosphere: the solid, outer portion of the earth's crust coupled to the rigid upper mantle. part of the geosphere. measured ceiling: a ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. mercurial barometer: an instrument used for measuring the change in atmospheric pressure. it uses a long glass tube, open at one end and closed at the other. after first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. a nearly perfect vacuum is established at the closed end after the mercury descends. the height of the column of mercury in the tube is a measurement of air pressure. as atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. measurement is taken in inches of mercury. although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. first used by evangelista torricelli (1608-1647), an italian physicist and mathematician, to explain the fundamental principles of hydromechanics. outflow: also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. it is the result of cold downdrafts and its passage includes a wind shift and temperature drop. perihelion: the point of the earth's orbit that is nearest to the sun. although the position is part of a 21,000 year cycle, currently it occurs around january, when the earth is about 3 million miles closer to the sun than at aphelion. this term can be applied to any other celestial body in orbit around the sun. it is the opposite of aphelion. pressure characteristic: the pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. this is recorded in three categories: falling, rising, or steady. st. elmo's fire: a luminous, and often audible, electric discharge that is sporadic in nature. it occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. it often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. also known as corposant or corona discharge. season: a division of the year according to some regularly recurring phenomena, usually astronomical or climatic. for example, in the northern hemisphere, winter is said to begin on the winter solstice and end on the vernal equinox when spring begins, covering the months of december, january, and february. in the tropics, there is the dry and the rainy season, depending on the amount of precipitation. unstable/ instability: occurs when a rising air parcel becomes less dense than the surrounding air. since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own. wind shift: the term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. it may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt..

Altocumulus castellanus: a middle cloud with vertical development that forms from altocumulus clouds. it is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. its formation indicates instability and turbulence at the altitudes of occurrence. barograph: an instrument that continuously records a barometer's reading of atmospheric pressure. barometric pressure: the pressure exerted by the atmosphere at a given point. its measurement can be expressed in several ways. one is in millibars. another is in inches or millimeters of mercury (hg). cold air funnel: funnel clouds, usually short-lived, that develop from relatively small showers or thunderstorms when the air aloft is very in cold. cold air funnels may touch down briefly, but in general are less violent than most other types of tornadoes. condensation: the process by which water vapor undergoes a change in state from a gas to a liquid. it is the opposite physical process of evaporation. confluence: a rate at which wind flow comes together along an axis oriented normal to the flow in question. the opposite of diffluence. depression: in meteorology, it is another name for an area of low pressure, a low, or trough. it also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features. earthquake: a sudden, transient motion or trembling of the earth's crust, resulting from the waves in the earth caused by faulting of the rocks or by volcanic activity. hurricane: the name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the north atlantic ocean, caribbean sea, gulf of mexico, and in the eastern north pacific ocean. this same tropical cyclone is known as a typhoon in the western pacific and a cyclone in the indian ocean. pressure tendency: the pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation. sandstorm: a strong wind carrying sand particles through the air. they are low level occurences, usually only ten feet in height to not more than fifty feet above the surface. due to the frequent winds created by surface heating, they are most predominate during the day and die out in the night. visibility is reduced to between 5/8ths and 6/16ths statute mile, and if less than 5/16ths, then the storm is considered a heavy sandstorm. it is reported as "ss" in an observation and on the metar. short wave: a progressive wave of smaller amplitude, wave length, and duration than a long wave. it moves in the same direction as the basic current in which it is embedded and may induce upward vertical motion ahead of it. they are more numerous than long waves and often disappear with height in the atmosphere. snow cover: the areal extent of ground covered by the snow. it is usually expressed as a percent of the total area of a given region. twilight: often called dusk, it is the evening period of waning light from the time of sunset to dark. the time of increasing light in the morning is called dawn. twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. during dawn, the reverse order occurs until full daylight. typhoon: the name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western north pacific ocean. this same tropical cyclone is known as a hurricane in the eastern north pacific and north atlantic ocean, and as a cyclone in the indian ocean.: : warm: to have or give out heat to a moderate or adequate degree. a subjective term for temperatures between cold and hot. in meteorology, an air parcel that is warm is only so in relation to another parcel. zulu time: one of several names for the twenty-four hour time which is used throughout the scientific and military communities..