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Atmospheric 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). avhrr: acronym for advanced very high resolution radiometer. it is the main sensor on the u.s. polar orbiting satellites. boyle's law: states that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. therefore, if the pressure increases, the volume decreases and visa versa. for example, if the volume if halved, then the pressure is doubled. if the temperature is held constant, it becomes an isothermal process. discovered by robert boyle (1627-1691), an irish physicist and chemist and co-founder of the royal society. cold: a condition marked by low or decidedly subnormal temperature. the lack of heat. 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. heat index: the combination of air temperature and humidity that gives a description of how the temperature feels. this is not the actual air temperature. isohel: a line drawn through geographic points having equal duration of sunshine or another form of solar radiation during a specified time period. 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. metar: acronym for meteorological aerodrome report. it is the primary observation code used in the united states to satisfy requirements for reporting surface meteorological data. minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting. psychrometer: an instrument used to measure water vapor content of the atmosphere. it consists of two thermometers, a wet bulb and dry bulb. may also be referred to as a sling psychrometer. pulse: a very short duration of time. in regard to a radar, it is a brief burst of a electromagnetic radiation emitted by the radar. : saffir-simpson damage-potential scale: developed in the early 1970s by herbert saffir, a consulting engineer, and robert simpson, then director of the national hurricane center, it is a measure of hurricane intensity on a scale of 1 to 5. the scale categorizes potential damage based on barometric pressure, wind speeds, and surge. snowflakes: an ice crystal or an aggregate of ice crystals which fall from clouds. squall line: a narrow band or line of active thunderstorms that is not associated with a cold front. it may form from an outflow boundary or the leading edge of a mesohigh. 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. 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. wind: air that flows in relation to the earth's surface, generally horizontally. there are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports..

Altostratus: this middle cloud genus is composed of water droplets, and sometimes ice crystals, in the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. white to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. it is a good indicator of precipitation, as it often precedes a storm system. virga often falls from these clouds. 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. fetch: an area of the water surface over which waves are generated by a wind having a constant direction and speed. also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. one of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture. opaque: a condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. opaque sky cover refers to the amount of sky cover that completely hides all that might be above it. polar air mass: an air mass that forms over a high latitude region. continental polar air (cp) is formed over cold surface regions and is typically very stable with low moisture. maritime polar air (mp), produced over warmer waters, is less stable with high moisture. sea mile: a unit of length distinguished from a nautical mile. one sea mile is equivalent to 1,000 fathoms (6,000 feet). snow grains: frozen precipitation in the form of very small, white, opaque grains of ice. the solid equivalent of drizzle. it is reported as "sg" in an observation and on the metar. thickness: the thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. the layer most often used in meteorology is between 1000 and 500 millibars. there can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. for example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer. 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. vertical temperature profile: a series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram..

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. barrier winds: refers to the westerly flow of air along the northern slope of the brooks range in northern alaska that precedes the arrival of colder air from the north. 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. 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. cirrocumulus: a cirriform cloud with vertical development, appearing as a thin sheet of small white puffs which give it a rippled effect. it often creates a "mackerel sky", since the ripples may look like fish scales. sometimes it is confused with altocumulus, however, it has smaller individual masses and does not cast a shadow on other elements. it is also the least common cloud type, often forming from cirrus or cirrostratus, with which it is associated in the sky. clear air turbulence: name given to turbulence that may occur in perfectly clear air without any visual in warning in the form of clouds. it is often found in the vicinity of the jet stream where large shears in the horizontal and vertical are found, although this turbulence is not limited just to jet stream locale. other areas where it may occur include near mountains, in closed lows aloft, and in regions of wind shear. may be referred to as cat. cumulonimbus mammatus: a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. although they do not cause severe weather, they often accompany storms. they may slowly vary in size, since they are an area of negative buoyancy convection, and is associated with severe turbulence in the lower sections of the cloud. cyclonic flow: winds that blow in and around a cyclone, that is counterclockwise in the northern hemisphere, clockwise in the southern hemisphere. 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. gust: a sudden significant increase in or rapid fluctuations of wind speed. peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). the duration is usually less twenty seconds. inches of mercury (hg): the name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. one inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. first devised in 1644 by evangelista torricelli (1608-1647), an italian physicist and mathematician, to explain the fundamental principles of hydromechanics. 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. lithosphere: the solid, outer portion of the earth's crust coupled to the rigid upper mantle. part of the geosphere. microbarograph: a instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure. 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. quantitative precipitation forecast (qpf): a forecast of rainfall, snowfall or liquid equivalent of snowfall. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. subsidence: a sinking or downward motion of air, often seen in anticyclones. it is most prevalent when there is colder, denser air aloft. it is often used to imply the opposite of atmospheric convection. subtropical air: an air mass that forms over the subtropical region. the air is typically warm with a high moisture content due to the low evaporative process. 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. 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. x-rays: the portion of the electromagnetic spectrum that has a very short wave length. it has a wave length longer than gamma rays, yet shorter than visible light. x-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization. : :.

Carbon dioxide (co2): a heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of the total. current: a horizontal movement of water, such as the gulf stream off the east coast of north america, or air, such as the jet stream. downpour: a heavy rain. geosphere: considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. at their conjunction is the biosphere. horizon: one of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth. the geographic horizon, also called the apparent horizon, is the distant line along which earth and sky appear to meet. this is the usual concept of horizon and is used in weather observing. the local horizon is the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions, such as mountains. 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. 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. 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. mesoscale convective system (mcs): a large organized convective weather system comprised of a number of individual thunderstorms. it normally persists for several hours and may be rounded or linear in shape. this term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (mcc). 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. 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. : obscuration: any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. according to the national weather service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic ash. it is reported as "x" in an observation and on the metar. 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. pilot balloon: a small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. also known as a pibal. pounds per square inch (psi): a unit for measuring pressure. one psi equals the pressure resulting from a force of one pound force acting over an area of one square inch. 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. sea level pressure: the atmospheric pressure at mean sea level, usually determined from the observed station pressure. snowfall: the rate at which snow falls, usually expressed in inches of snow depth over a six hour period. snow line: the lowest elevation area of a perennial snow field on high terrain, such as a mountain range. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. superrefraction: greater than normal bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. 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. 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. windward: the direction from which the wind is blowing. also the upwind side of an object. the opposite of the downwind or leeward side..

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. atmospheric 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). bubble high: a small high that may be created by precipitation and vertical instability associated with thunderstorm activity. a product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. convergence along the leading edge of a bubble high may help form additional thunderstorms. 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. equatorial trough: the quasi-continuous area of low pressure between the subtropical high pressure areas in both the northern and southern hemisphere. geosphere: considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. at their conjunction is the biosphere. 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. ground clutter: a pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. this may hide or confuse the proper return echo signifying actual precipitation. 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. mesoscale convective system (mcs): a large organized convective weather system comprised of a number of individual thunderstorms. it normally persists for several hours and may be rounded or linear in shape. this term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (mcc). 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. monsoon: the seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. the monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern india. the name is derived from the word mausim, arabic for season. this pattern is most evident on the southern and eastern sides of asia, although it does occur elsewhere, such as in the southwestern united states. multicell storm: a thunderstorm made up of two or more single-cell storms. pre-frontal trough: an elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction. rossby waves: the movement of ridges and troughs in the upper wind patterns, primarily the jet stream, circling the earth. named for carl-gustaf rossby, a u.s. weather bureau (nws) employee, who first theorized about the existence of the jet stream in 1939. sea spray: sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave. skew t-log p diagram: a thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. it is used to evaluate and forecast air parcel properties. some values that can be determined are the convective condensation level (ccl), the lifting condensation level (lcl), and the level of free convection (lfc). 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. tropical storm: a tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). at this point, the system is given a name to identify and track it. warm advection: the horizontal movement of warmer air into a location. wasatch winds: strong winds blowing easterly out of the wasatch mountains in utah, sometimes reaching speeds greater than 75 miles per hour..