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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. 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). 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. 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. 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. 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. 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. easterlies: usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds. 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. 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). 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. katafront: a front where the warm air descends the frontal surface, except in the low layers of the atmosphere. mesosphere: the layer of the atmosphere located between the stratosphere and the ionosphere, where temperatures drop rapidly with increasing height. it extends between 31 and 50 miles (17 to 80 kilometers) above the earth's surface. national center for atmospheric research (ncar): a division of the university corporation for atmospheric research, the center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. : for further information, contact ncar, located in boulder, colorado. national severe storms laboratory (nssl): a branch of the national oceanic and atmospheric administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms. : for further information, contact the nssl, headquartered in norman, oklahoma. partial obscuration: denotes that 1/8th or more of the sky, but not all of the sky, is hidden by any surface-based phenomena in the atmosphere, excluding precipitation. it often reduces horizontal visibility but not the vertical. it is reported as "x" in an observation and on the metar. radarsonde observation: an upper air observation used to determine winds and other meteorological data, by tracking the range, elevation, and azimuth of a radar target carried aloft. a type of rawinsonde. 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. stratus fractus: stratus clouds that appear in irregular fragments, as if they had been shred or torn. also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds. zonal index: the measure of the strength of the westerly winds of the middle latitudes. it is expressed as the horizontal pressure difference between 35 degrees and 55 degrees latitude, or as the corresponding geostrophic wind..

Air pollution: the soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors. 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. daily mean: the average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures. 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. 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. 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. 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. isodrosotherm: the line drawn on a weather map connecting points of equal dew point. 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. lifted index (li): a measure of atmospheric instability that is obtained by computing the temperature that the air near the ground would have if it were lifted to a higher level and comparing it to the actual temperature at that altitude. positive values indicate more stable air and negative values indicate instability. line echo wave pattern (lewp): a wave-shaped bulge in a line of thunderstorms. it may often be seen as a "s"-shaped radar echo signature and is often associated with severe weather. low latitudes: the latitude belt between 30 and 0 degrees north and south of the equator. also referred to as the tropical or torrid region. 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. 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. pressure characteristic: the pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. this is recorded in three categories: falling, rising, or steady. refraction: the bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. rocketsonde: a type of radiosonde that is shot into the atmosphere by a rocket, allowing it to collect data during its parachute descent from a higher position in the atmosphere than a balloon could reach. runway visual range (rvr): it is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. this value is normally determined by visibility sensors located alongside and higher than the center line of the runway. rvr is calculated from visibility, ambient light level, and runway light intensity. : : santa ana winds: the hot, dry winds, generally from the east, that funnel through the santa ana river valley south of the san gabriel and san bernadino mountains in southern california, including the los angeles basin. classified as katabatic, it occurs most often during the winter and it is an example of a foehn wind. 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. snow blindness: temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. the medical term is niphablepsia. storm winds: on the beaufort wind scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour). temperate climate: climates with distinct winter and summer seasons, typical of regions found between the tropics of cancer and capricorn and the arctic and antarctic circles. considered the climate of the middle latitudes. ultraviolet: electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone. universal time coordinate: one of several names for the twenty-four hour time which is used throughout the scientific and military communities. 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..

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. 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. 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. 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. cold wave: a rapid fall in temperature within twenty-four hours to temperatures requiring substantially increased protection to agriculture, industry, commerce, and social activities. national weather service criteria includes the rate of temperature fall and the minimum to which it falls, depending on the region of the country and time of the in year. the weather channel uses the following criteria for a cold wave: a cold spell of two days or more with below normal temperatures in at least fifteen states, with at least five of them more than fifteen degrees below normal. echo: the energy return of a radar signal after it has hit the target. electromagnetic radiation: also called radiation, it is waves of energy propagated though space or through a material media. 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. fujita-pearson scale: a scale that classifies the severity of wind damage intensity based on the degree of destruction as it relates to the wind speed as well as path length and path width of the event. it is normally used to identify the most intense damage exhibited by a tornado. developed by t. theodore fujita and allen pearson. isodrosotherm: the line drawn on a weather map connecting points of equal dew point. lee/leeside/leeward: the side of an object or obstacle, such as a ship's sail, a mountain, or a hill, furthest away from the wind, and therefore, protected from the direct force of the wind. the opposite of windward. nephelococcygia: a term applied when people find familiar objects within the shape of a cloud. shear: it is the rate of change over a short duration. in wind shear, it can refer to the frequent change in wind speed within a short distance. it can occur vertically or horizontally. directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. when used in reference to doppler radar, it describes the change in radial velocity over short distances horizontally. 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. thermocline: a vertical negative temperature gradient in some layer of a body of water which is appreciably greater than the gradients above and below it. in the ocean, this may be seasonal, due to the heating of the surface water in the summer, or permanent. thunder snow: a wintertime thunderstorm from which falls snow instead of rain. violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. intense snowfall rates often occur during these situations. transparent: a condition where a material is clear enough not to block the passage of radiant energy, especially light. tropical prediction center (tpc): a division of the national centers for environmental prediction, the center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities. the national hurricane center is a branch. : for further information, contact the tpc, located in miami, florida. trough: an elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. the opposite of a ridge. 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. yellow snow: snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it. : :.

Aleutian low: a semi-permanent, subpolar area of low pressure located in the gulf of alaska near the aleutian islands. it is a generating area for storms and migratory lows often reach maximum intensity in this area. it is most active during the late fall to late spring. during the summer, it is weaker, retreating towards the north pole and becoming almost nonexistent. during this time, the north pacific high pressure system dominates. convection: motions in a fluid that transport and mix the properties of the fluid. these properties could be heat and/or moisture. when used to imply only upward vertical motion, it is then the opposite of subsidence. equinox: the point at which the ecliptic intersects the celestial equator. days and nights are most nearly equal in duration. in the northern hemisphere, the vernal equinox falls on or about march 20 and the autumnal equinox on or about september 22. 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. 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. infrared: the long wave, electromagnetic radiation of radiant heat emitted by all hot objects. on the electromagnetic spectrum, it can be found between microwave radiation and visible light. water vapor, ozone, and carbon dioxide are capable of absorbing or transmitting infrared radiation. may be referred to as ir. international date line: the line of longitude located at 180° east or west (with a few local deviations) where the date changes by a day. west of the line it is one day later than east of the line. jet stream: an area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the northern and southern hemispheres. flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. it is marked by a concentration of isotherms and strong vertical shear. low level jet (llj): strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. it is often amplified at night. the southerly wind over the us plains states during spring and summer is a notable example. orographic lifting: where the flow of air is forced up and over barriers such as highlands or mountains. moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. by the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, and the dissipation of clouds. may be called an orographic uplift. 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. 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. : 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. slush: snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures. standing cloud: any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain. station pressure: the atmospheric pressure with respect to the station elevation. 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. surface boundary layer: the lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it. turbulence: the irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. atmospheric turbulence is caused by random fluctuations in the wind flow. it can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure. vertical wind profile: a series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind 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..

Aleutian low: a semi-permanent, subpolar area of low pressure located in the gulf of alaska near the aleutian islands. it is a generating area for storms and migratory lows often reach maximum intensity in this area. it is most active during the late fall to late spring. during the summer, it is weaker, retreating towards the north pole and becoming almost nonexistent. during this time, the north pacific high pressure system dominates. calm: atmospheric conditions devoid of wind or any other air motion. in oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell. 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. 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. 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. occluded front: also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. it develops when three thermally different air masses conflict. the type of frontal boundary they create depends on the manner in which they meet. 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. shear line: a line of maximum horizontal wind shear. a narrow zone across which there is an abrupt change in the horizontal wind component parallel to it. stratus fractus: stratus clouds that appear in irregular fragments, as if they had been shred or torn. also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds. synoptic chart: any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time. : the rising and sinking branches are climatologically anchored to specific geographical locations, but these locations are seasonally dependent. in winter the the rising branch is centered near the "maritime continent" of greater indonesia while in summer it is centered near southern india. the sinking branches are generally found in the eastern equatorial pacific in winter and summer. sub-branches of rising motion are generally tied to equatorial areas of south america and africa in summer. yellow snow: snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it. : :.