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Alberta clipper: a fast moving, snow-producing weather system that originates in the lee of the canadian rockies. it moves quickly across the northern united states, often bring gusty winds and cold arctic air. 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. 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. 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. cumulus congestus: a strongly sprouting cumulus cloud with generally sharp outlines and often with great vertical development. it may occur as tower-like clouds with cauliflower tops. these clouds may produce abundant showers and may develop further into cumulonimbus. cut-off high: a warm high which has become displaced and is on the polarward side of the jet stream. it occurs mostly during the spring and is most frequent over northeastern siberia, alaska, and greenland. it is an example of a blocking high. cyclonic flow: winds that blow in and around a cyclone, that is counterclockwise in the northern hemisphere, clockwise in the southern hemisphere. deepening: used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. the opposite of filling. eclipse: the obscuring of one celestial body by another. fathom: the common unit of depth in the ocean for countries using the english system of measurement. it is six feet or 1.83 meters. it can also be used in expressing horizontal distance, since 120 fathoms is equal to one cable or nearly on tenth of a nautical mile. hook echo: a radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. however, hook echoes and tornadoes do not always accompany each other. hydrology: the study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface. 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. land breeze: a diurnal coastal breeze that blows offshore, from the land to the sea. it is caused by the temperature difference when the sea surface is warmer than the adjacent land. predominate during the night, it reaches its maximum about dawn. it blows in the opposite direction of a sea breeze. 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. lifting condensation level (lcl): the height at which a parcel of moist air becomes saturated when it is lifted dry adiabatically. mesocyclone: a area of rotation of storm size that may often be found on the southwest part of a supercell. its circulation can be larger than the tornado that may develop within it, but not necessarily. originally a radar term for a rotation signature that met certain criteria, it is best seen on doppler radar. mountain breeze: a katabatic wind, it is formed at night by the radiational cooling along mountainsides. as the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. it may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. it blows in the opposite direction of a valley 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. sea mile: a unit of length distinguished from a nautical mile. one sea mile is equivalent to 1,000 fathoms (6,000 feet). 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. terminal doppler weather radar (tdwr): doppler radar installed at major airports throughout the united states to detect microbursts. towering cumulus: another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm. 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. tropical disturbance: an area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. it is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane. vernal equinox: taking place in the northern hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. days and nights are most nearly equal in duration. it falls on or about march 20 and is considered the beginning of spring in the northern hemisphere and autumn in the southern hemisphere. it is the astronomical opposite of the autumnal equinox..

Atmosphere: the gaseous or air portion of the physical environment that encircles a planet. in the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. the divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere. barometer: an instrument used to measure atmospheric pressure. two examples are the aneroid barometer and the mercurial barometer. 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. 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. charles' law: states that when the pressure is held constant, the volume of a gas varies directly with the temperature. therefore, if the pressure remains constant, the volume of a gas will increase with the increase of temperature. it was developed by jacques charles and is also known as the charles-guy-lussac law. climate analysis center (cac): the u.s. national weather service division that applies new technology and approaches to the analysis, diagnosis, and projection of short term climate fluctuations on a regional and global basis. : for further information, contact the cac, located in camp spring, maryland. cloud bank: a well-defined cloud mass that can be observed at a distance. it covers the horizon, but is not directly overhead. comma cloud: a feature seen on satellite images with a distinctive comma-shape. this is indicative of a synoptic cloud pattern associated with large, well-developed low pressure systems. confluence: a rate at which wind flow comes together along an axis oriented normal to the flow in question. the opposite of diffluence. cooling degree day: a cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees fahrenheit. it is used to estimate the energy requirements and is an indication of fuel consumption for air conditioning or refrigeration. 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. degree: a measure of temperature difference representing a single division on a temperature scale. 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. ice crystals: precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. they make up cirriform clouds, frost, and ice fog. also, they produce optical phenomena such as halos, coronas, and sun pillars. may be called "diamond dust." it is reported as "ic" in an observation and on the metar. 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. low pressure system: an area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. this is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. national meteorological center (nmc): now incorporated into the national centers for environmental prediction, it was the division of the national weather service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the northern hemisphere, specifically u.s. governmental organizations. polar-orbiting satellite: a satellite whose orbit passes over both of the earth's between poles. rain: precipitation in the form of liquid water droplets greater than 0.5 mm. if widely scattered, the drop size may be smaller. it is reported as "r" in an observation and on the metar. the intensity of rain is based on rate of fall. "very light" (r--) means that the scattered drops do not completely wet a surface. "light" (r-) means it is greater than a trace and up to 0.10 inch an hour. "moderate" (r) means the rate of fall is between 0.11 to 0.30 inch per hour. "heavy" (r+) means over 0.30 inch per hour. saturate: to treat or charge something to the point where no more can be absorbed, dissolved, or retained. in meteorology, it is used when discussing the amount of water vapor in a volume of air. siberian express: a fierce, cold flow of air that originates in siberia, then moves into alaska and northern canada before moving southward into the united states. snow creep: a continuous, extremely slow, downhill movement of a layer of snow. solstice: the point at which the sun is the furthest on the ecliptic from the celestial equator. the point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. the tropic of cancer and the tropic of capricorn are those parallels of latitude which lies directly beneath a solstice. in the northern hemisphere, the winter solstice falls on or about december 21 and the summer solstice on or about june 21. 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. 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. thermometer: an instrument used for measuring temperature. the different scales used in meteorology are celsius, fahrenheit, and kelvin or absolute. 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. upslope effect: the cooling of an air flow as it ascends a hill or mountain slope. if there is enough moisture and the air is stable, stratiform clouds and precipitation may form. if the air is unstable, there might be an increased chance of thunderstorm development. 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. 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..

Blocking high: the development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. it prevents the movement of migratory cyclones across its latitudes. buys ballot's law: describes the relationship of the horizontal wind direction to the pressure distribution. in the northern hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. it is reversed in the southern hemisphere. this law was named after the dutch meteorologist, buys ballot, who developed the formula in 1857. 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. dust devil: a small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. it usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating. first gust: another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm. gravity: the force of attraction of the earth on an object. the direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface. lithosphere: the solid, outer portion of the earth's crust coupled to the rigid upper mantle. part of the geosphere. mixed layer: it is the upper portion of the boundary layer in which air is thoroughly mixed by convection. in oceanography, it is the layer of the water that is mixed through wave action or thermohaline convection. mountain breeze: a katabatic wind, it is formed at night by the radiational cooling along mountainsides. as the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. it may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. it blows in the opposite direction of a valley breeze. nephelococcygia: a term applied when people find familiar objects within the shape of a cloud. newton: the unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second. 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. 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. 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. rotor cloud: an altocumulus cloud formation that can be found in the lee of a mountain or similar barrier. the air rotates around a horizontal axis, creating turbulence. altocumulus lenticularis is an example. saturate: to treat or charge something to the point where no more can be absorbed, dissolved, or retained. in meteorology, it is used when discussing the amount of water vapor in a volume of air. 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. standing cloud: any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain. thermosphere: a thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. it is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere..

Altitude: in meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level. 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. clinometer: an instrument used to measure angles of inclination. used in conjunction with a ceiling light, it determines cloud height at night, based on the angle of a projected light on the clouds, the observer, and the ceiling light. cold front: the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the northern hemisphere). precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. convective condensation level (ccl): the height at which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated. 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. 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. dusk: the period of waning light from the time of sunset to dark. filling: used in describing the history of a low pressure system or an area of cyclonic circulation, it means an increase in the central pressure of the system. although it usually describes the action of a pressure system on a constant pressure chart, it also means a surface low is decreasing in cyclonic circulation and losing its characteristics. the opposite of deepening. fog: a visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. it is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. it is reported as "fg" in an observation and on the metar. 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. icicle: ice that forms in the shape of a narrow cone hanging point down. it usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows. maritime air mass: an air mass influenced by the sea. it is a secondary characteristic of an air mass classification, signified by the small "m" before the primary characteristic, which is based on source region. for example, mp is an air mass that is maritime polar in nature. also known as a marine air mass. melting point: the temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. contrast with freezing point. 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. nowcast: a short-term weather forecast for expected conditions in the next few hours. 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. photometer: any of a number of atmospheric phenomena which appear as luminous patterns in the sky. they do not directly cause adverse weather. they include halos, coronas. rainbows, and fogbows. pollutant: particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. something unfavorable to health and life that has been added to the environment. 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. roll cloud: a relatively rare, low-level, horizontal, tube-shaped cloud. although they are associated with a thunderstorm, they are completely detached from the base of the cumulonimbus cloud. salinity: a measure of the quantity of dissolved salts in sea water. the total amount of dissolved solids in sea water in parts per thousand by weight. saturate: to treat or charge something to the point where no more can be absorbed, dissolved, or retained. in meteorology, it is used when discussing the amount of water vapor in a volume of air. scud: low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. they are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. when observed from a distance, they are sometimes mistaken for tornadoes. severe weather: generally, any destructive weather event, but usually applies to localized storms, such as blizzards, intense thunderstorms, or tornadoes. stratosphere: the layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds. it extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. it is the location of the earth's ozone layer. transpiration: the process by which water in plants is transferred as water vapor to the atmosphere. 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. watch: a forecast issued well in advance of a severe weather event to alert the public of the possibility of a particular hazard, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, or heavy snows..

Air quality standards: the maximum level which will be permitted for a given pollutant. primary standards are to be sufficiently stringent to protect the public health. secondary standards must protect the public welfare, including property and aesthetics. cap: composed of a layer of warmer, dryer air aloft which may suppress or delay the development of thunderstorms. as an air parcel rises, it becomes cooler relative to the ambient, or surrounding, air in the cap and therefore, less buoyant and unable to rise further. also referred to as a lid. climatology: the study of climate. it includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems. cold front: the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the northern hemisphere). precipitation is generally at and/or behind the front, and with a fast-moving system, a squall line may develop ahead of the front. 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. daily mean: the average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures. depression: in meteorology, it is another name for an area of low pressure, a low, or trough. it also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features. diablo winds: dry winds in the diablo mountain range in central california that can exceed 60 miles per hour. similar to the santa ana winds, they develop as the wind flows from high pressure over nevada to lower pressure along the central california coast. dust: small particles of earth or other matter suspended in the air. it is reported as "du" in an observation and for wide spread dust on the metar. gully washer: a heavy rain shower that occurs suddenly, possibly creating a flash flood. 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. mountain wave: a wave in the atmosphere caused by a barrier, such as a mountain. sometimes it is marked by lenticular clouds to the lee side of mountain barriers. may be called a standing wave or a lee wave. 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. transparent: a condition where a material is clear enough not to block the passage of radiant energy, especially light. 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. wasatch winds: strong winds blowing easterly out of the wasatch mountains in utah, sometimes reaching speeds greater than 75 miles per hour..