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American meteorological society: an organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences. : for further information, contact the ams. boulder wind: a local name referring to an extremely strong downslope wind in the front range of the rocky mountains near boulder, colorado. carbon dioxide (co2): a heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of the total. collada: a strong, steady wind blowing from the north or northwest in the upper part of the gulf of california and from the northeast in the lower part. 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. eclipse: the obscuring of one celestial body by another. flanking line: a line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell. freezing drizzle: drizzle, falling as a liquid, but freezing on impact with the colder ground or other exposed surfaces. it is reported as "fzdz" in an observation and on the metar. gradient wind: a steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence. 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 warning: a formal advisory issued by forecasters at the national hurricane center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. a warning is used to inform the public and marine interests of the storm's location, intensity, and movement. ice: the solid form of water. it can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example. mackerel sky: the name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. the rippled effect gives the appearance of fish scales. national climatic data center (ncdc): the agency that archives climatic data from the national oceanic and atmospheric administration as well as other climatological organizations. : for further information, contact the ncdc, located in asheville, north carolina. 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. : 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. 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. 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. stationary front: a front which is nearly stationary or moves very little since the last synoptic position. may be known as a quasi-stationary front. warning: a forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc..

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. 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. bwer: acronym for bounded weak echo region. refers to radar echo signatures with low reflectivity in the center, surrounded by higher reflectivity. it is associated with strong updrafts and is found in the inflow region of a thunderstorm. calorie: in meteorology, it is the amount of heat required to raise the temperature of one (1) gram of water one (1) degree celsius. it is a unit of heat energy. ceiling: the lowest cloud layer that is reported as broken or overcast. if the sky is totally obscured, then it is the height of the vertical visibility. climate: the historical record and description of average daily and in seasonal weather events that help describe a region. statistics are generally drawn over several decades. the word is derived from the greek klima, meaning inclination, and reflects the importance early scholars attributed to the sun's influence. 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. cut-off low: a closed cold core low completely removed from the primary westerly flow. cutoff lows may remain detached from the westerlies for days while exhibiting very little forward (eastward) progress. in some instances, a cutoff low may move to the west, or retrograde, opposite to the prevailing flow. it is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows. 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. geostrophic wind: a steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. it is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration. instrument shelter: a boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation. mud slide: fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain. 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. siberian high: the semi-permanent high pressure area that forms over siberia during the winter. the average central pressure exceeds 1030 millibars from late november to early march. it is characterized by clear, dry weather. over southern asia, the predominate surface wind is northeasterly, just the opposite of the predominate summer winds which bring the monsoon. snowpack: the amount of annual accumulation of snow at higher elevations. standing cloud: any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain. winter storm: any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice..

Absolute humidity: a type of humidity that considers the mass of water vapor present per unit volume of space. also considered as the density of the water vapor. it is usually expressed in grams per cubic meter. abyssal plain: the flat, gently sloping or nearly level region of the sea floor. air mass thunderstorm: a thunderstorm that is produced by convection within an unstable air mass through an instability mechanism. such thunderstorms normally occur within a tropical or warm, moist air mass during the summer afternoon as the result of afternoon heating and dissipate soon after sunset. such thunderstorms are not generally associated with fronts and are less likely to become severe than other types of thunderstorms. however, that does not preclude them from having brief heavy downpours. anvil: the upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds of miles downstream from the parent cloud. it may look smooth or fibrous, but in shape, it resembles a blacksmith's anvil. it indicates the mature or decaying stage of a thunderstorm. 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. black ice: thin, new ice on fresh or salt water that appears dark in color because of its transparency. also refers to thin, transparent ice on road surfaces. 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. centripetal force: the force required to keep an object moving in a curved or circular path. it is directed inwards toward the center of the curved path. cheyenne fog: an upslope fog formed by the westward flow of air from the missouri river valley, producing fog on the eastern slopes of the rockies. cold core thunderstorms: thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlies warmer surface air. constant pressure surface: a surface along which the atmospheric pressure is equal everywhere. 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. eclipse: the obscuring of one celestial body by another. friction: in meteorology, it is the turbulent resistance of the earth on the atmosphere. considered as the resistance of fluids (air and water) to the relative motion of a solid body. the amount is dependent on the size and shape of the body. 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. nadir: the point on any given observer's celestial sphere diametrically opposite of one's zenith. 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. 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. oxygen (o2): a colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%. palouser: a strong, dangerous, katabatic wind that descends from the mountains into the palouse river valley in northern idaho and eastern washington. may be called a cow-killer. 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. 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. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. teleconnections: information used by forecasters to determine what the weather might be elsewhere when compared with past weather conditions at the same degree of longitude. trace: generally, an unmeasurable or insignificant quantity. a precipitation amount of less than 0.005 inch. 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. 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. 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. vorticity maximum: a center of vorticity, or the maximum of the vorticity field of a fluid. : : wind vane: an instrument that indicates the wind direction. the end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position..

Absolute humidity: a type of humidity that considers the mass of water vapor present per unit volume of space. also considered as the density of the water vapor. it is usually expressed in grams per cubic meter. 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). 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. avhrr: acronym for advanced very high resolution radiometer. it is the main sensor on the u.s. polar orbiting satellites. backing: a counterclockwise shift in the wind direction in the northern hemisphere at a certain location. in the southern hemisphere, it is clockwise. this can either happen in the horizontal or the vertical (with height). for example, the wind shifts from the northeast to the north to the northwest. it is the opposite of veering. blue norther: refers to a swift-moving cold frontal passage in the southern great plains, marked by a dark, blue-black sky with strong wintery winds from the northwest or north and temperatures that may drop 20°f to 30°f in a few minutes. boiling point: the temperature at which a liquid changes to a vaporous state. the temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. the boiling point of pure water at standard pressure is 100°c or 212°f. bow echo: a radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. it is bent outward in a "bow" shape. 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. debris cloud: considered a rotating cloud of debris or dust that is on the ground or near the ground. the debris cloud appearing beneath a thunderstorm will most likely confirm the presence of a tornado. 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. 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." frontolysis: the destruction or dying of a front where the transition zone is losing its contrasting properties. the opposite of frontogenesis. latitude: the location north or south in reference to the equator, which is designated at zero (0) degrees. parallel lines that circle the globe both north and south of the equator. the poles are at 90° north and south latitude. moisture: refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air. oceanography: the study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology. 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. rainbow: a luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). it is created by refraction, total reflection, and the dispersion of light. it is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. the bow is always observed in the opposite side of the sky from the sun. 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. sea surface temperature (sst): the temperature of the water's surface. it is measured using buoy and ship data, infrared satellite imagery, and coastal observations. 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. surge: the increase in sea water height from the level that would normally occur were there no storm. although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. it is estimated by subtracting the normal astronomic tide from the observed storm tide. 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. 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. tornado: a violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. it is the most destructive of all storm-scale atmospheric phenomena. they can occur anywhere in the world given the right conditions, but are most frequent in the united states in an area bounded by the rockies on the west and the appalachians in the east. zodiac: the position of the sun during the course of the year as it appears to move though successive constellations. also, the band where the ecliptic runs centrally through the celestial sphere and contains the sun, the moon, and all the planets except venus and pluto..

Altocumulus: composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. in the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. a defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. it may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. virga may also fall from these clouds. clear: the state of the sky when no clouds or obscurations are observed or detected from the point of observation. cumulus humilis: cumulus clouds with little or no vertical development characterized by a generally flat appearance. their growth is usually limited by a temperature inversion, which is marked by the unusually uniform height of the clouds. also called fair-weather cumulus. dense fog advisory: advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions. 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. dog days: the name given to the very hot summer weather that may persists for four to six weeks between mid-july through early september in the united states. in western europe, this period may exist from the first week in july to mid-august and is often the period of the greatest frequency of thunder. named for sirius, the dog star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months. 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. glacier winds: air flow that descends from glaciers, occasionally at a high rate of speed. caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum intensity in the early afternoon. 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. greenwich mean time (gmt): the name of the twenty-four hour time scale which is used throughout the scientific and military communities. standard time begins at greenwich, england, home of the royal observatory which first utilized this method of world time. this is also the prime meridian of longitude. the globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. to the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain greenwich time (gmt). to the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain gmt. lunar eclipse: an eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. the moon does not have any light of its own, instead, it reflects the sun's light. during a lunar eclipse, the moon is in the earth's shadow. it will often look dim and sometimes copper or orange in color. : nimbostratus: this cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. they are generally associated with fall and winter conditions, but can occur during any season. 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. 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. polar jet: marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. it often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. it is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. its position tends to migrate south in the northern hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer. precipitation: any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. this includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. the amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period. pressure: the force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. 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. siberian high: the semi-permanent high pressure area that forms over siberia during the winter. the average central pressure exceeds 1030 millibars from late november to early march. it is characterized by clear, dry weather. over southern asia, the predominate surface wind is northeasterly, just the opposite of the predominate summer winds which bring the monsoon. synoptic scale: the size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more. tropic of cancer: the most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees north latitude. virga: streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. from a distance, the event sometimes may be mistaken for a funnel cloud or tornado. typically, it may fall from altocumulus, altostratus, or high based cumuonimbus. walker circulation: a deep east-west overturning in the atmosphere normally confined to within about 20 degrees latitude of the equator extending from low-levels to near the tropopause. wind shear: the rate of wind speed or direction change with distance. vertical wind shear is the rate of change of the wind with respect to altitude. horizontal wind shear is the rate of change on a horizontal plane..