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Advection: the horizontal transfer of any property in the atmosphere by the movement of air (wind). examples include heat and moisture advection. 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. blowing sand: sand that is raised by the wind to heights of six feet or greater. it is reported as "blsa" in an observation and on the metar. centrifugal force: the apparent force in a rotating system that deflects masses radially outward from the axis of rotation. this force increases towards the equator and decreases towards the poles. chemosphere: a vaguely defined region of the upper atmosphere in which photochemical reactions take place. it includes the top of the stratosphere, all of the mesosphere, and sometimes the lower part of the thermosphere. dense fog advisory: advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions. divergence: wind movement that results in a horizontal net outflow of air from a particular region. divergence at lower levels is associated with a downward movement of air from aloft. contrast with convergence. doldrums: located between 30 degrees north and 30 degrees south latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. also a nautical term for the equatorial trough. dusk: the period of waning light from the time of sunset to dark. 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). 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. 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. hudson bay low: an area of low pressure over or near the hudson bay area of canada that often introduces cold air to the north central and northeast united states. newhall winds: the local name for winds blowing downward from desert uplands through the newhall pass southward into the san fernando valley, north of los angeles. noctilucent clouds: rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. they have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. they may appear bright against a dark night sky, with a blue-silver color or orange-red. palmer drought index: a long-term meteorological drought severity index produced by the noaa/usda (department of agriculture) joint agricultural weather facility. the index depicts prolonged times, as in months or years, of abnormal dryness or wetness. it responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration. sounding: a plot of the atmosphere, using data rom upper air or radiosonde observations. usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location. 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. tilt: the inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. for example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere. trajectory: the curve that a body, such as a celestial object, describes in space. this applies to air parcel movement also. tropics/tropical: the region of the earth located between the tropic of cancer, at 23.5 degrees north latitude, and the tropic of capricorn, at 23.5 degrees south latitude. it encompasses the equatorial region, an area of high temperatures and considerable precipitiation during part of the year. 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. typhoon: the name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western north pacific ocean. this same tropical cyclone is known as a hurricane in the eastern north pacific and north atlantic ocean, and as a cyclone in the indian ocean.: : undercast: in aviation, it is an opaque cloud layer viewed from an observation point above the layer. from the ground, it would be considered an overcast. united states weather bureau: the official name of the national weather service prior to 1970. whiteout: when visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista..

Adiabatic process: a thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. in this process, compression will result in warming and expansion will result in cooling. clear ice: a glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled in water droplets. the droplets spread out over an object, such as an aircraft wing's leading edge, prior to complete freezing and forms a sheet of clear ice. cold high: a high pressure system that has its coldest temperatures at or near the center of circulation, and horizontally, is thermally barotropic. it is shallow in nature, as circulation decreases with height. associated with cold arctic air, it is usually stationary. also known as a cold core high. contrast with a warm high. conduction: the transfer of heat through a substance by molecular action or from one substance by being in contact with another. 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. flood stage: the level of a river or stream where overflow onto surrounding areas can occur. national hurricane center (nhc): a branch of the tropical prediction center, it is the office of the national weather service that is responsible for tracking and forecasting tropical cyclones over the north atlantic, caribbean, gulf of mexico, and the eastern pacific. : for further information, contact the nhc, located in miami, florida. sea ice: ice that is formed by the freezing of sea water. it forms first as small crystals, thickens into sludge, and coagulates into sheet ice, pancake ice, or ice floes of various shapes and sizes. showalter stability index: a measure of the local static stability of the atmosphere. it is determined by lifting an air parcel to 500 millibars and then comparing its temperature to that of the environment. if the parcel is colder than its new environment, then the atmosphere is more stable. if the parcel is warmer than its new environment, then the atmosphere is unstable and the potential for thunderstorm development and severe weather increases. snow blindness: temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. the medical term is niphablepsia. 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..

Barometer: an instrument used to measure atmospheric pressure. two examples are the aneroid barometer and the mercurial barometer. 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. 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. conduction: the transfer of heat through a substance by molecular action or from one substance by being in contact with another. fresh water: water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity. funnel cloud: a violent, rotating column of air visibly extending from the base of a towering cumulus or cumulonimbus toward the ground, but not in contact with it. it is reported as "fc" in an observation and on the metar. isobar: the line drawn on a weather map connecting points of equal barometric pressure. mud slide: fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain. 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. oxygen (o2): a colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%. parhelion: the scientific name for sun dogs. either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. they are caused by the refraction of sunlight passing through ice crystals. they are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. they are also known as mock suns. prognostic chart: a chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. also known as a prog. salt water: the water of the ocean, distinguished from fresh water by its appreciable salinity. 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). thermograph: essentially, a self-recording thermometer. a thermometer that continuously records the temperature on a chart. universal time coordinate: one of several names for the twenty-four hour time which is used throughout the scientific and military communities. wave(s): in general, any pattern with some roughly identifiable periodicity in time and/or space. it is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. in meteorology, this applies to atmospheric waves, such as long waves and short waves. in oceanography, this applies to waves generated by mechanical means, such as currents, turbidity, and the wind. 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 speed: the rate of the motion of the air on a unit of time. it can be measured in a number of ways. in observing, it is measured in knots, or nautical miles per hour. the unit most often used in the united states is miles per hour. year: the interval required for the earth to complete one revolution around the sun. a sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. the calendar year begins at 12 o'clock midnight local time on the night of december 31st-january 1st. currently, the gregorian calendar of 365 days is used, with 366 days every four years, a leap year. the tropical year, also called the mean solar year, is dependent on the seasons. it is the interval between two consecutive returns of the sun to the vernal equinox. in 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century..

Air mass: an extensive body of air throughout which the horizontal temperature and moisture characteristics are similar. anemometer: an instrument that measures the speed or force of the wind. barotropy: the state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. it is considered zero baroclinity. blizzard: a severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. a severe blizzard is characterized by temperatures near or below 10°f, winds exceeding 45 mph, and visibility reduced by snow to near zero. 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. drifts: normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind. 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. eye wall: an organized band of convection surrounding the eye, or center, of a tropical cyclone. it contains cumulonimbus clouds, intense rainfall and very strong winds. 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. 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). gale warning: a warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour). growing season: considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. the frost-free growing season is between the first and last occurrence of 32°f temperatures in spring and autumn. ice jam: an accumulation of broken river ice caught in a narrow channel, frequently producing local flooding. primarily occurs during a thaw in the late winter or early spring. isobar: the line drawn on a weather map connecting points of equal barometric pressure. jet streak: a region of accelerated wind speed along the axis of a jet stream. latent heat: the energy released or absorbed during a change of state. nocturnal thunderstorms: thunderstorms which develop after sunset. they are often associated with the strengthening of the low level jet and are most common over the plains states. they also occur over warm water and may be associated with the seaward extent of the overnight land breeze. 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. rawinsonde: an upper air observation that evaluates the winds, temperature, relative humidity, and pressure aloft by means of a balloon-attached radiosonde that is tracked by a radar or radio direction-finder. it is a radiosonde observation combined with a winds-aloft observation, called a rawin. snow banner: a plume of snow blown off a mountain crest, resembling smoke blowing from a volcano. snowpack: the amount of annual accumulation of snow at higher elevations. thaw: a warm spell of weather when ice and snow melt. to free something from the binding action of ice by warming it to a temperature above the melting point of ice. thunderstorm: produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes. 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. veering: a clockwise shift in the wind direction in the northern hemisphere at a certain location. in the southern hemisphere, it is counterclockwise. this can either happen horizontally or vertically (with height). for example, the wind shifts from the north to the northeast to the east. it is the opposite of backing..

Arid: a term used for an extremely dry climate. the degree to which a climate lacks effective, life-promoting moisture. it is considered the opposite of humid when speaking of climates. 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. 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. drifting snow: snow particles blown from the ground by the wind to a height of less than six feet. eclipse: the obscuring of one celestial body by another. evaporation: the physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. it is the opposite physical process of condensation. eye: the center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. an eye will usually develop when the maximum sustained wind speeds exceed 78 mph. it can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. in general, when the eye begins to shrink in size, the storm is intensifying. firewhirl: a tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption. hygrometer: an instrument that measures the water vapor content of the atmosphere. 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. lenticular cloud: a cloud species which has elements resembling smooth lenses or almonds and more or less isolated. these clouds are caused by a wave wind pattern created by the mountains. they are also indicative of down-stream turbulence on the leeward side of a barrier. negative vorticity advection: the advection of lower values of vorticity into an area. nocturnal thunderstorms: thunderstorms which develop after sunset. they are often associated with the strengthening of the low level jet and are most common over the plains states. they also occur over warm water and may be associated with the seaward extent of the overnight land breeze. profiler: a type of doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere. reflectivity: a measure of the process by which a surface can turn back a portion of incident radiation into the medium through which the radiation approached. it also refers to the degree by which precipitation is able to reflect a radar beam. specific humidity: the ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. it is expressed in grams per gram or in grams per kilograms. the specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel. storm prediction center (spc): a branch of the national centers for environmental prediction, the center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the united states. formerly known as the severe local storms (sels) unit of the national severe storms forecast center. : for further information, contact the spc, located in norman, oklahoma. subrefraction: less than normal bending of light or a radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. 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. sunset: the daily disappearance of the sun below the western horizon as a result of the earth's rotation. in the united states, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. in great britain, the center of the sun's disk is used instead. time of sunset is calculated for mean sea level. 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. trade winds: two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. primarily lower level winds, they are characterized by their great consistency of direction. in the northern hemisphere, the trades blow from the northeast, and in the southern hemisphere, the trades blow from the southeast. warm: to have or give out heat to a moderate or adequate degree. a subjective term for temperatures between cold and hot. in meteorology, an air parcel that is warm is only so in relation to another parcel. world meteorological organization (wmo): from weather prediction to air pollution research, climate change related activities, ozone layer depletion studies and tropical storm forecasting, the world meteorological organization coordinates global scientific activity to allow increasingly prompt and accurate weather information and other services for public, private and commercial use, including international airline and shipping industries. established by the united nations in 1951, it is composed of 184 members. : for more information, contact the woe, located in geneva, switzerland. : : zenith: the point which is elevated 90 degrees from all points on a given observer's astronomical horizon. the point on any given observer's celestial sphere that lies directly above him. the opposite of nadir..