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Alaskan winds: the downslope air flow that blows through the alaskan valleys. it is usually given local names, such as knik, matanuska, pruga, stikine, taku, take, turnagain, or williwaw. alberta clipper: a fast moving, snow-producing weather system that originates in the lee of the canadian rockies. it moves quickly across the northern united states, often bring gusty winds and cold arctic air. altitude: in meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level. aphelion: the point on the earth's orbit that is farthest from the sun. although the position is part of a 21,000 year cycle, currently it occurs around july, when the earth is about 3 million miles farther from the sun than at perihelion. this term can be applied to any other celestial body in orbit around the sun. it is the opposite of perihelion. blowing dust: dust that is raised by the wind to heights of six feet or greater. it is reported as "bldu" in an observation and on the metar. closed low: a region of low pressure distinguished by a center of counterclockwise circulation (in the northern hemisphere), and is surrounded by one or more isobars or height contours. closed lows aloft (i.e., above the surface) may become disconnected from the primary westerly flow and thus progress eastward more slowly. it is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows. cyclogenesis: the process that creates a new low pressure system or cyclone, or intensifies a pre-existing one. it is also the first appearance of a trough. flood plain: level land that may be submerged by flood waters. geophysics: the study of the physics or nature of the earth and its environment. it deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. by extension, it often includes astronomy and the related astro-sciences. intermountain high: an area of high pressure that occurs during the winter between the rocky mountains and the sierra-cascade ranges. it blocks the eastward movement of pacific cyclones. also called plateau high or great basin high. k index: the measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer. macroburst: a large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds. omega block: a warm high aloft which has become displaced and is on the polarward side of the jet stream. it frequently occurs in the late winter and early spring in the northern hemisphere. the name comes from its resemblance to the greek letter, omega, when analyzed on upper air charts. 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. pollutant: particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. something unfavorable to health and life that has been added to the environment. radiation: the process by which energy is propagated through any medium by virtue of the wave motion of that medium. electromagnetic radiation, which emits heat and light, is one form. sound waves are another. 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. 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. straight-line winds: any surface wind that is not associated with rotation. an example is the first gust from a thunderstorm, as opposed to tornadic winds. 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. terminal doppler weather radar (tdwr): doppler radar installed at major airports throughout the united states to detect microbursts. terrestrial radiation: long wave radiation that is emitted by the earth back into the atmosphere. most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space. tule fog: ground fog in the central valley of california and the leading cause of weather-related casualties in that state. it forms at night and in the early morning when the ground cools, lowering the air temperature near the ground to or below its initial dew point. 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. variable ceiling: occurs when the height of a ceiling layer increases and decreases rapidly, the ascribed height is the average of all the varying values..

Aphelion: the point on the earth's orbit that is farthest from the sun. although the position is part of a 21,000 year cycle, currently it occurs around july, when the earth is about 3 million miles farther from the sun than at perihelion. this term can be applied to any other celestial body in orbit around the sun. it is the opposite of perihelion. asos: acronym for automated surface observing system. this system is a collection of automated weather instruments that collect data. it performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day. autumn: the season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. customarily, this refers to the months of september, october, and november in the north hemisphere and the months of march, april, and may in the southern hemisphere. astronomically, this is the period between the autumnal equinox and the winter solstice. easterlies: usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds. few: the amount of sky cover for a cloud layer between 1/8th and 2/8ths, based on the summation layer amount for that layer. firewhirl: a tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption. first gust: another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm. haboob: sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought. isotherm: the line of equal or constant air temperature. if something is isothermal, it is of equal or constant temperature with respect to either time or space. : : 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. national severe storms forecast center (nssfc): as of october 1995, the responsibilities of this center were divided into two branches, the storm prediction center and the aviation weather center. national weather service (nws): a primary branch of the national oceanic and atmospheric administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. : for further information, contact the nws. pressure altimeter: an aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. it is read accurately only in a standard atmosphere and when the correct altimeter setting is used..

Anemometer: an instrument that measures the speed or force of the wind. anomalous propagation: this refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. may be referred to as a.p. arctic jet: the jet stream that is situated high in the stratosphere in and around the arctic or antarctic circles. it marks the boundary of polar and arctic air masses. asos: acronym for automated surface observing system. this system is a collection of automated weather instruments that collect data. it performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day. awips: acronym for advanced weather interactive processing system. it is the computerized system that processes nexrad and asos data received at national weather service forecast offices. beaufort wind scale: a system of estimating and reporting wind speeds. it is based on the beaufort force or number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. the scale was devised by sir francis beaufort (1777-1857), hydrographer to the british royal navy. blowing snow: snow that is raised by the wind to heights of six feet or greater. it is reported as "blsn" in an observation and on the metar. celestial sphere: the apparent sphere of infinite radius having the earth as its center. all heavenly bodies (planets, stars, etc.) appear on the "inner surface" of this sphere and the sun moves along the ecliptic. corposant: a luminous, sporadic, and often audible, electric discharge. 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. cyclone: an area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. the circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. also called a low pressure system and the term used for a tropical cyclone in the indian ocean. other phenomena with cyclonic flow may be referred to by this term, such as dust devils, tornadoes, and tropical and extratropical systems. the opposite of an anticyclone or a high pressure system. drainage wind: a katabatic wind, it is caused by the cooling of air along the slopes of a mountain. flash flood: a flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam. 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. icelandic low: a semi-permanent, subpolar area of low pressure in the north atlantic ocean. because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. it is strongest during a northern hemisphere winter and early spring, centered over iceland and southern greenland, and is the dominate weather feature in the area. during the summer, it is weaker, less intense, and might divide into two parts, one west of iceland, the other over the davis strait between greenland and baffin island. then the azores or bermuda high becomes the dominate weather feature in the north atlantic. 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. mixed precipitation: any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. rain may also be present. mountain wave: a wave in the atmosphere caused by a barrier, such as a mountain. sometimes it is marked by lenticular clouds to the lee side of mountain barriers. may be called a standing wave or a lee wave. mud slide: fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain. normal: the recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. normals are concerned with the distribution of data within limits of common occurrence. the parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc. overcast: the amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer. plow/plough wind: the spreading downdraft and strong straight-line winds preceding a thunderstorm. so named in the american midwest because of its ability to flatten tall grasses as it passes. radiation fog: fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. this primarily occurs during the night or early morning. ridge: an elongated area of high atmospheric pressure that is associated with an area of maximum anticyclonic circulation. the opposite of a trough. scattering: the process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. this is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. when working with radars, this often refers to the more or less random changes in direction of radio energy. shower: precipitation from a convective cloud that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. it occurs in the form of rain (shra), snow (shsn), or ice (shpe). it is reported as "sh" in an observation and on the metar. snow creep: a continuous, extremely slow, downhill movement of a layer of snow. snow depth: the actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms. transmissometer: an electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. by showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined. weather vane: originally used as a wind vane, it is an instrument that indicates the wind direction. the name developed based on observations on what kind of weather occurred with certain wind directions. creative designs often adorn the tops of barns and houses..

Alberta clipper: a fast moving, snow-producing weather system that originates in the lee of the canadian rockies. it moves quickly across the northern united states, often bring gusty winds and cold arctic air. bermuda high: a semi-permanent, subtropical area of high pressure in the north atlantic ocean that migrates east and west with varying central pressure. depending on the season, it has different names. when it is displaced westward, during the northern hemispheric summer and fall, the center is located in the western north atlantic, near bermuda. in the winter and early spring, it is primarily centered near the azores islands. blowing dust: dust that is raised by the wind to heights of six feet or greater. it is reported as "bldu" in an observation and on the metar. climate prediction center (cpc): a branch of the national centers for environmental prediction,the center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them. : for further information, contact the cpc, located in washington, d.c. cooling degree day: a cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees fahrenheit. it is used to estimate the energy requirements and is an indication of fuel consumption for air conditioning or refrigeration. 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. 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. cumulus mediocris: cumulus clouds characterized by moderate vertical development with upper protuberances not very marked in appearance. this cloud does not produce precipitation, but could develop into towering cumulus or cumulonimbus which do. 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. drizzle: slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches or 0.5 millimeters. it falls from stratus clouds and is often associated with low visibility and fog. it is reported as "dz" in an observation and on the metar. eclipse: the obscuring of one celestial body by another. eddy: a small disturbance of wind in a large wind flow, which can produce turbulent conditions. they can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. in oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current. 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." gale: on the beaufort wind scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). for marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). in 1964, the world meteorological organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots). graupel: a form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together. gravitation: the mutual attraction between two masses of matter. the rotation of the earth and the atmosphere modifies this attraction to produce the field of gravity. ice fog: fog that is composed of minute ice particles. it occurs in very low temperatures under clear, calm conditions in the polar latitudes and may produce a halo around the sun or moon. indian summer: a period of abnormally warm weather in mid to late autumn with clear skies and cool nights. a first frost normally precedes this warm spell. middle latitudes: the latitude belt roughly between 35 and 65 degrees north and south. may be referred to as the temperate region. sky: the vault-like apparent surface against which all aerial objects are seen from the earth. sky cover: the amount of the celestial dome that is hidden by clouds and/or obscurations. 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. tropical depression: a tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize. upslope fog: fog that forms when warm, moist surface air is forced up a slope by the wind. it is adiabatically cooled to below its initial dew point, which means the air cools by expansion as it rises. it forms best where there is a gradual slope, and it can become quite deep, requiring considerable time to dissipate. vapor trail: a cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. a vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. also called a contrail, for condensation trail. 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. weather surveillance radar (wsr-88d): the newest generation of doppler radars, the 1988 doppler weather radar. the radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm..

Altimeter: an instrument used to determine the altitude of an object with respect to a fixed level. the type normally used by meteorologists measures the altitude with respect to sea level pressure. bernoulli's theorem: a statement of the conservation of energy for a steady, nonviscous, incompressible level flow. it is an inverse relationship in which pressures are least where velocities are greatest. theorized by daniel bernoulli (1700-1782), a swiss mathematician and physicist. ecology: the study of the relationships between living organisms and their environment. 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. overcast: the amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer. polar-orbiting satellite: a satellite whose orbit passes over both of the earth's between poles. 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. snow: frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. it most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. it usually appears clustered into snowflakes. it is reported as "sn" in an observation and on the metar. transmissometer: an electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. by showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined..