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Cirrostratus: a cirriform cloud that develops from cirrus spreading out into a thin layer, creating a flat sheetlike appearance. it can give the sky a slightly milky or veiled look. when viewed from the surface of the earth, these ice crystals can create a halo effect around the sun or moon. this cloud is a good precursor of precipitation, indicating it may occur within 12 to 24 hours. condensation funnel: a funnel-shaped cloud consisting of condensed water drops that has possible rotation. convergence: wind movement that results in a horizontal net inflow of air into a particular region. convergent winds at lower levels are associated with upward motion. contrast with divergence. data buoys: buoys placed throughout the gulf of mexico and along the atlantic and pacific coasts of the united states that relay information on air and water temperature, wind speed, air pressure, and wave conditions via radio signals. 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. friction layer: the thin layer of atmosphere adjacent to the earth's surface. surface friction is effective in slowing down wind up to approximately 1,500 to 3,000 feet above the ground. above this level, air tends to flow parallel to the isobars. wind distribution within this layer is determined by vertical temperature gradient and the physical contours of the underlying surface features. front: the transition zone or interface between two air masses of different densities, which usually means different temperatures. for example, the area of convergence between warm, moist air and cool, dry air. ionosphere: a complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. it is located between the mesosphere and the exosphere and is included as part of the thermosphere. pressure tendency: the pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation. rainfall: the amount of precipitation of any type, primarily liquid. it is usually the amount that is measured by a rain gauge. sand: loose particles of hard, broken rock or minerals. in observing, sand is reported when particles of sand are raised to sufficient height that reduces visibility. it is reported as "sa" in an observation and on the metar. snow level: the elevation in mountainous terrain where the precipitation changes from rain to snow, depending on the temperature structure of the associated air mass. stratocumulus: a low cloud composed of layers or patches of cloud elements. it can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. it is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them. stratosphere: the layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. it extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. it is the location of the earth's ozone layer. triple point: the point at which any three atmospheric boundaries meet. it is most often used to refer to the point of occlusion of an extratropical cyclone where the cold, warm, and occluded fronts meet. cyclogenesis may occur at a triple point. it is also the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium..

Anticyclone: a relative pressure maximum. an area of pressure that has diverging winds and a rotation opposite to the earth's rotation. this is clockwise the northern hemisphere and counter-clockwise in the southern hemisphere. it is the opposite of an area of low pressure, or a cyclone. bathythermograph: a device used to obtain a record of temperature against depth (pressure) in the ocean. may be referred to as a b.t. broken: the amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer. cold front: the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the northern hemisphere). precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. convective condensation level (ccl): the height at which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated. cumulus: one of the three basic cloud forms (the others are cirrus and stratus). it is also one of the two low cloud types. a cloud that develops in a vertical direction from the base (bottom) up. they have flat bases and dome- or cauliflower-shaped upper surfaces. the base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. small, separate cumulus are associated with fair weather (cumulus humilis). with additional heating from the earth's surface, they can grow vertically throughout the day. the top of such a cloud can easily reach 20,000 or more into the troposphere. under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. further development may create a cumulonimbus. 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. frozen precipitation: precipitation that reaches the ground in a frozen state. examples include snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. geostationary satellite: an orbiting weather satellite that maintains the same position over the equator during the earth's rotation. also known as goes, an acronym for geostationary operational environmental satellite. icing: the forming or depositing of ice on an object. national weather association (nwa): an organization whose membership promotes excellence in operational meteorology and related activities, recognizing the professional as well as the volunteer. : for further information, contact the nwa. north pacific high: a semi-permanent, subtropical area of high pressure in the north pacific ocean. it is strongest in the northern hemispheric summer and is displaced towards the equator during the winter when the aleutian low becomes more dominate. observation: in meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. an observer is one who records the evaluations of the meteorological elements. overrunning: this occurs when a relatively warm air mass is forced above a cooler air mass of greater density. weather generally associated with this event includes cloudiness, cool temperatures, and steady precipitation. 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. 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. polar front: a semi-continuous, semi-permanent boundary between polar air masses and tropical air masses. an integral part of an early meteorological theory known as the polar front theory. 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. scattered: the amount of sky cover for a cloud layer between 3/8ths and 4/8ths, based on the summation layer amount for that layer. snowburn: a burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface. snow shower: frozen precipitation in the form of snow, characterized by its sudden beginning and ending. it is reported as "shsn" in an observation and on the metar. squall: a sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. the intensity and duration is longer than that of a gust. it is reported as "sq"s in an observation and on the metar. tropic of cancer: the most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees north latitude. troposphere: the lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. characterized by clouds and weather, temperature generally decreases with increasing altitude. updraft: a small scale current of air with vertical motion. if there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development. water vapor (h2o): water in gaseous form. it is one of the most import constituents of the atmosphere. due to its molecular content, air containing water vapor is lighter than dry air. this contributes to the reason why moist air has a tendency to rise. wedge: primarily refers to an elongated area of shallow high pressure at the earth's surface. it is generally associated with cold air east of the rockies or appalachians. it is another name for a ridge, ridge line, or ridge axis. contrast with a trough. wedge is also a slang term for a large, wide tornado with a wedge-like shape. 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..

Advection fog: fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point. aviation weather center: as one of the national centers for environmental prediction, it is the national center for weather information that is used daily by the federal aviation administration, commercial airlines, and private pilots. it is entering a new phase of service, growing to accept global forecasting responsibilities. : for further information, contact the awc, located in kansas city, missouri. deepening: used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. the opposite of filling. dry line: the boundary between the dry desert air mass of the southwest u.s. and the moist air mass from the gulf of mexico. it usually lies north-south across the central and southern high plains states during spring and summer. the passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. its presence influences severe weather development in the great plains. instrument flight rules (ifr): refers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and landing. ifr conditions for fixed wing aircraft means the minimum cloud ceiling is greater than 500 feet and less than 1,000 feet and/or visibility is greater than 1 mile and less than 3 miles. ionosphere: a complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. it is located between the mesosphere and the exosphere and is included as part of the thermosphere. lapse rate: the change of an atmospheric variable, usually temperature, with height. a steep lapse rate implies a rapid decrease in temperature with height and is a sign of instability. mesocyclone: a area of rotation of storm size that may often be found on the southwest part of a supercell. its circulation can be larger than the tornado that may develop within it, but not necessarily. originally a radar term for a rotation signature that met certain criteria, it is best seen on doppler radar. microscale: the smallest scale of meteorological phenomena that range in size from a few centimeters to a few kilometers. larger phenomena are classified as mesoscale. it also refers to small scale meteorological phenomena with life spans of less than a few minutes that affect very small areas and are strongly influenced by local conditions of temperature and terrain. nor'easter: a cyclonic storm occurring off the east coast of north america. these winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto atlantic beaches, often causing beach erosion and structural damage. wind gusts associated with these storms can exceed hurricane force in intensity. a nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas. numerical forecasting: the use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. these models are run on high-speed computers at the national centers for environmental prediction. : ozone layer: an atmospheric layer that contains a high proportion of oxygen that exists as ozone. it acts as a filtering mechanism against incoming ultraviolet radiation. it is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface. pressure change: the net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation. reconnaissance (recco) code: an aircraft weather reconnaissance code that has come to refer primarily to in-flight tropical weather observations, but actually signifies any detailed weather observation or investigation from an aircraft in flight. sea spray: sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave. slush: snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures. snow garland: snow appearing as a beautiful long thick rope draped on trees, fences and other objects. formed by the surface tension of thin films of water bonding individual snow crystals. stable/stability: occurs when a rising air parcel becomes denser than the surrounding air. it will then return to its original position. when the density of the air parcel remains the same as the surrounding air after being lifted, it is also considered stable, since it does not have the tendency to rise or sink further. contrast with unstable air and instability. supercooling: the reduction of the temperature of any liquid below the melting point of that substance's solid phase. cooling a substance beyond its nominal freezing point. supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. the smaller and purer the water droplets, the more likely they can become supercooled. temperature: the measure of molecular motion or the degree of heat of a substance. it is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. it is also the degree of hotness or coldness. in surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth. 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. vorticity: the measurement of the rotation of a small air parcel. it has vorticity when the parcel spins as it moves along its path. although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. if it does not spin, it is said to have zero vorticity. in the northern hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. it is negative when the parcel has clockwise, or anticyclonic, rotation..

Aurora: it is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. it is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. in the northern hemisphere, it is known as the aurora borealis or northern lights, and in the southern hemisphere, this phenomena is called the aurora australis. cumuliform: clouds composed of water droplets that exhibit vertical development. the density of the droplets often blocks sunlight, casting shadows on the earth's surface. with increasing vertical height, they are often associated with convection. bases of these clouds are generally no more than 3,000 feet above the ground, but they can develop past the troposphere in both temperate and tropical latitudes. they are classified as low clouds and include all varieties of cumulus and cumulonimbus. the opposite in type are the horizontal development of stratiform clouds. dew: condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours. diffluence: a rate at which wind flow spreads apart along an axis oriented normal to the flow in question. the opposite of confluence. dry slot: an area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. best seen on a satellite picture, such as a water vapor image. electromagnetic radiation: also called radiation, it is waves of energy propagated though space or through a material media. filling: used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. the opposite of deepening. fogbow: a whitish semicircular arc seen opposite the sun in fog. the outer margin has a reddish tinge, its inner margin has a bluish tinge, and the middle of the band is white. an additional bow with reversed colors sometimes appears inside the first. freezing precipitation: precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces. frontogenesis: the birth or creation of a front. this occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. it could happen when either air mass, or both, move over a surface which strengthens their original properties. however, it occurs most often along the eastern coasts of north america and asia, when the air mass moving out over the ocean has a weak or no distinct boundary. the opposite of frontolysis. frontolysis: the destruction or dying of a front where the transition zone is losing its contrasting properties. the opposite of frontogenesis. meteorology/meteorologist: the science and study of the atmosphere and atmospheric phenomena. various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. a scientist who studies the atmosphere and atmospheric phenomena. multicell storm: a thunderstorm made up of two or more single-cell storms. quantitative precipitation forecast (qpf): a forecast of rainfall, snowfall or liquid equivalent of snowfall. rip current: it is formed by a strong surface water movement, or current, of a short duration that flows seaward from the shore. the return flow is piled up onshore by the incoming waves and wind. it is localized, of narrow width, and its position relative to the beach can change as the wave condition changes. therefore, the higher the waves, the stronger the current. sargasso sea: an area of the north atlantic ocean between bermuda and the azores. it is in the middle of the north atlantic oceanic gyre, with converging surface waters. consequently, it has less biological features than any other region of the ocean because the lack of mixing with more nutrient-rich waters. sea fog: a type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water. severe thunderstorm: a thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4 inch hail or larger, or tornadoes. severe thunderstorms may also produce torrential rain and frequent lightning. snow eater: any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow. theodolite: an optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles. tide: the periodic rising and falling of the earth's oceans and atmosphere. it is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. this propagates a wave through the atmosphere and along the surface of the earth's waters. 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..

Absolute instability: when the lapse rate of a column of air is greater than the dry adiabatic lapse rate. the term absolute is used because this applies whether or not the air is dry or saturated. air: this is considered the mixture of gases that make up the earth's atmosphere. the principal gases that compose dry air are nitrogen (n2) at 78.09%, oxygen (o2) at 20.946%, argon (a) at 0.93%, and carbon dioxide (co2) at 0.033%. one of the most important constituents of air and most important gases in meteorology is water vapor (h2o). cape: acronym for convective available potential energy. the amount of energy available to create convection, with higher values increasing the possibility for severe weather. 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. dusk: the period of waning light from the time of sunset to dark. 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. electromagnetic radiation: also called radiation, it is waves of energy propagated though space or through a material media. exosphere: this region is considered the very outer limits of the earth's atmosphere. its lower boundary is often called the critical level of escape, where gas atoms are so widely spaced that they rarely collide with one another and have individual orbits. it is estimated to be some 400 plus miles (640 kilometers) above the surface. fair: this is a subjective description. considered as pleasant weather conditions with regard to the time of year and the physical location. fog bank: a fairly well-defined mass of fog observed in the distance. most commonly seen at sea, over a lake, or along coastal areas. frontal passage: it is the passage of a front over a specific point on the surface. it is reflected by the change in dew point and temperature, the shift in wind direction, and the change in atmospheric pressure. accompanying a passage may be precipitation and clouds. may be referred to as "fropa." frost: the covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing. green flash: a brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon. heat exhaustion: the effect of excessive heat, particularly when combined with high humidity, on a human being. signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps. low latitudes: the latitude belt between 30 and 0 degrees north and south of the equator. also referred to as the tropical or torrid region. observation: in meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. an observer is one who records the evaluations of the meteorological elements. photometer: any of a number of atmospheric phenomena which appear as luminous patterns in the sky. they do not directly cause adverse weather. they include halos, coronas. rainbows, and fogbows. prevailing visibility: it is considered representative of visibility conditions at the observation station. it is the greatest distance that can be seen throughout at least half the horizon circle, but not necessarily continuous. resolution: in relation to radar, it is the ability to read two distinct targets separately. the clearer the resolution, the nearer the two objects can be to each other and still be distinguishable. santa ana winds: the hot, dry winds, generally from the east, that funnel through the santa ana river valley south of the san gabriel and san bernadino mountains in southern california, including the los angeles basin. classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind. snow flurry/flurries: light showers of snow, generally very brief without any measurable accumulation. may be reported as "shsn--" in an observation and on the metar. snowpack: the amount of annual accumulation of snow at higher elevations. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. station elevation: the vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station. stratiform: clouds composed of water droplets that exhibit no or have very little vertical development. the density of the droplets often blocks sunlight, casting shadows on the earth's surface. bases of these clouds are generally no more than 6,000 feet above the ground. they are classified as low clouds, and include all varieties of stratus and stratocumulus. the opposite in type are the vertical development of cumuliform clouds. stratopause: the boundary zone or transition layer between the stratosphere and the mesosphere. characterized by a decrease in temperature with increasing altitude. sublimation: the process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). the opposite of crystallization. surface boundary layer: the lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it. syzygy: the points in the moon's orbit about the earth at which the moon is new or full. : :.