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Advection fog: fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point. bellot winds: refers to the winds in the canadian arctic that blow through the narrow bellot strait between somerset island and the boothia peninsula, connecting the gulf of boothia and franklin strait. 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. 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. 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. 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 low: a low pressure system that has its coldest temperatures at or near the center of circulation, and is thermally barotropic with respect to a horizontal plane. also known as a cold core low. a cut off low is an example, where an isolated pool of colder air is located south of the main westerlies. cyclogenesis: the process that creates a new low pressure system or cyclone, or intensifies a pre-existing one. it is also the first appearance of a trough. derecho: a line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. they are characterized by damaging straight-line winds over hundreds of miles. spanish for straight. fresh water: water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity. ground fog: fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. primarily takes place at night or early morning. gully washer: a heavy rain shower that occurs suddenly, possibly creating a flash flood. ice pellets: precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. they have a diameter of 0.2 inches (5 mm) or less. they are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets. it is reported as "pe" in an observation and on the metar. 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. long wave trough: a wave in the prevailing westerly flow aloft which is characterized by a large length and amplitude. a long wave moves slowly and is persistent. its position and intensity govern weather patterns over a period of days or weeks. measured ceiling: a ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. polar-orbiting satellite: a satellite whose orbit passes over both of the earth's between poles. scattering: the process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. this is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. when working with radars, this often refers to the more or less random changes in direction of radio energy. snow depth: the actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms. snow line: the lowest elevation area of a perennial snow field on high terrain, such as a mountain range. spring: the season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. customarily, this refers to the months of march, april, and may in the north hemisphere, and the months of september, october, and november in the southern hemisphere. astronomically, this is the period between the vernal equinox and the summer solstice. 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. trace: generally, an unmeasurable or insignificant quantity. a precipitation amount of less than 0.005 inch. 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. 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. wet bulb thermometer: a thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. the wet bulb temperature is used to compute dew point and relative humidity. one of the two thermometers that make up a psychrometer. 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. : :.

Baroclinity: the state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. also known as baroclinicity. an example is the tight temperature gradient along the east coast of the united states during the winter that gives rise to intense cyclogenesis. 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. cold core thunderstorms: thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlies warmer surface air. cold low: a low pressure system that has its coldest temperatures at or near the center of circulation, and is thermally barotropic with respect to a horizontal plane. also known as a cold core low. a cut off low is an example, where an isolated pool of colder air is located south of the main westerlies. 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. firewhirl: a tornado-like rotating column of fire and smoke created by intense heat from a forest fire or volcanic eruption. fresh water: water found rivers, lakes, and rain, that is distinguished from salt water by its appreciable lack of salinity. 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. geostrophic wind: a steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. it is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration. glaze: a smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. a storm that produces the accretion of glaze is called an ice storm. 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. instrument shelter: a boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ventilation. isohyet: the line drawn through geographic points recording equal amounts of rainfall during a given time or for a given of storm. latent heat: the energy released or absorbed during a change of state. 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. maximum: the greatest value attained by a function, for example, temperature, pressure, or wind speed. the opposite of minimum. mesolow: a small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles. nitrogen (n2): a colorless, tasteless, odorless gas that is the most abundant constituent of dry air. it comprises 78.09%. 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. 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. radiational cooling: the cooling of the earth's surface and the adjacent air. although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation. 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. season: a division of the year according to some regularly recurring phenomena, usually astronomical or climatic. for example, in the northern hemisphere, winter is said to begin on the winter solstice and end on the vernal equinox when spring begins, covering the months of december, january, and february. in the tropics, there is the dry and the rainy season, depending on the amount of precipitation. 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..

Aneroid barometer: an instrument for measuring the atmospheric pressure. it registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. the aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. the scales on the glass cover measure pressure in both inches and millibars. barometric 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). cold low: a low pressure system that has its coldest temperatures at or near the center of circulation, and is thermally barotropic with respect to a horizontal plane. also known as a cold core low. a cut off low is an example, where an isolated pool of colder air is located south of the main westerlies. continental air mass: an air mass with continental characteristics. it is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region. for example, cp is an air mass that is continental polar in nature. friction: in meteorology, it is the turbulent resistance of the earth on the atmosphere. considered as the resistance of fluids (air and water) to the relative motion of a solid body. the amount is dependent on the size and shape of the body. glaze: a smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. a storm that produces the accretion of glaze is called an ice storm. isotherm: the line of equal or constant air temperature. if something is isothermal, it is of equal or constant temperature with respect to either time or space. : : 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. maximum: the greatest value attained by a function, for example, temperature, pressure, or wind speed. the opposite of minimum. 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. sea surface temperature (sst): the temperature of the water's surface. it is measured using buoy and ship data, infrared satellite imagery, and coastal observations. snow: frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. it most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. it usually appears clustered into snowflakes. it is reported as "sn" in an observation and on the metar..

Antarctic ocean: although not officially recognized as a separate ocean body, it is commonly applied to those portions of the atlantic, pacific, and indian oceans that reach the antarctic continent on their southern extremes. 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. collada: a strong, steady wind blowing from the north or northwest in the upper part of the gulf of california and from the northeast in the lower part. directional shear: the shear created by a rapid change in wind direction with height. erosion: the movement of soil or rock from one area to another by the action of the sea, running water, moving ice, precipitation, or wind. 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. 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. 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. meridional flow: atmospheric circulation in which the north and south, or meridional, component of motion is unusually pronounced. this weakens the zonal flow. 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. 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. : 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. 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. radial velocity: a type of velocity that expresses motion toward or away from a given location. in doppler radar, it is the component of motion that is parallel to the radar beam. radiational cooling: the cooling of the earth's surface and the adjacent air. although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation. sea mile: a unit of length distinguished from a nautical mile. one sea mile is equivalent to 1,000 fathoms (6,000 feet). 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. twilight: often called dusk, it is the evening period of waning light from the time of sunset to dark. the time of increasing light in the morning is called dawn. twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. during dawn, the reverse order occurs until full daylight. 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. wet bulb thermometer: a thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. the wet bulb temperature is used to compute dew point and relative humidity. one of the two thermometers that make up a psychrometer..

Altocumulus: composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. in the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. a defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. it may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. virga may also fall from these clouds. apogee: the point farthest from the earth on the moon's orbit. this term can be applied to any other body orbiting the earth, such as satellites. it is the opposite of perigee. 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. 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. coalescence: the merging of two water drops into a single larger drop. condensation: the process by which water vapor undergoes a change in state from a gas to a liquid. it is the opposite physical process of evaporation. continental air mass: an air mass with continental characteristics. it is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region. for example, cp is an air mass that is continental polar in nature. directional shear: the shear created by a rapid change in wind direction with height. flanking line: a line of attached cumulus or towering cumulus clouds of descending height, appearing as stair steps (usually on the southwest side) of the most active part of a supercell. flood: high water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas. 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. isotherm: the line of equal or constant air temperature. if something is isothermal, it is of equal or constant temperature with respect to either time or space. : : melting level: the altitude at which ice crystals and snow flakes melt as they descend through the atmosphere. 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. palouser: a strong, dangerous, katabatic wind that descends from the mountains into the palouse river valley in northern idaho and eastern washington. may be called a cow-killer. quasi-stationary front: a front which is nearly stationary or moves very little since the last synoptic position. also known as a stationary front. rotation: the spinning of a body, such as the earth, about its axis. satellite: any object that orbits a celestial body, such as a moon. however, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. some of the information that is gathered by weather satellites, such as goes9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world. snow flurry/flurries: light showers of snow, generally very brief without any measurable accumulation. may be reported as "shsn--" in an observation and on the metar. thermohaline: in oceanography, it pertains to when both temperature and salinity act together. an example is thermohaline circulation which is vertical circulation induced by surface cooling, which causes convective overturning and consequent mixing. 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. upwind: the direction from which the wind is blowing. also the windward side of an object. the opposite of the downwind or leeward side. : : warm front: the leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the northern hemisphere), it is not as pronounced as with a cold frontal passage. precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. fog is common in the cold air ahead of the front. although clearing usually occurs after passage, some conditions may produced fog in the warm air..