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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. 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. 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. 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. isohel: a line drawn through geographic points having equal duration of sunshine or another form of solar radiation during a specified time period. low clouds: a term used to signify clouds with bases below 6,000 feet and are of a stratiform or a cumuliform variety. stratiform clouds include stratus and stratocumulus. cumuliform clouds include cumulus and cumulonimbus. this altitude applies to the temperate zone. in the polar regions, these clouds may be found at lower altitudes. in the tropics, the defining altitudes for cloud types are generally higher. north pacific high: a semi-permanent, subtropical area of high pressure in the north pacific ocean. it is strongest in the northern hemispheric summer and is displaced towards the equator during the winter when the aleutian low becomes more dominate. salt water: the water of the ocean, distinguished from fresh water by its appreciable salinity. 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. university corporation for atmospheric research (ucar): a non-profit university membership consortium which carries out programs to benefit atmospheric, oceanic, and related sciences around the globe. among other activities, ucar operates the national center for atmospheric research (ncar) with national science foundation sponsorship. : for further information, contact ucar, located in boulder, colorado..

Abyssal plain: the flat, gently sloping or nearly level region of the sea floor. 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. biosphere: the transition zone between the earth and the atmosphere within which most terrestrial life forms are found. it is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere. 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. 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. eddy: a small disturbance of wind in a large wind flow, which can produce turbulent conditions. they can also be areas of warmer air north of the main westerlies or colder air south of the westerlies. in oceanic circulation, it is a circular movement of water usually formed where currents pass obstructions, between two adjacent currents flowing counter to each other, or along the edge of a permanent current. evapotranspiration: the total amount of water that is transferred from the earth's surface to the atmosphere. it is made up of the evaporation of liquid or solid water plus the transpiration from plants. 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. fog bank: a fairly well-defined mass of fog observed in the distance. most commonly seen at sea, over a lake, or along coastal areas. gully washer: a heavy rain shower that occurs suddenly, possibly creating a flash flood. pressure: the force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. snow advisory: a statement or advisory issued when snow is expected to create hazardous travel conditions. it warns of less severe weather conditions than a winter storm upwind: the direction from which the wind is blowing. also the windward side of an object. the opposite of the downwind or leeward side. : : water vapor (h2o): water in gaseous form. it is one of the most import constituents of the atmosphere. due to its molecular content, air containing water vapor is lighter than dry air. this contributes to the reason why moist air has a tendency to rise. westerlies: usually applied to the broad patterns of persistent winds with a westerly component. it is the dominant persistent atmospheric motion, centered over the midlatitudes of each hemisphere. near the earth's surface, the westerlies extend from approximately 35 to 65 degrees latitude, while in the upper levels they extend further polarward and equatorward. 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. wind shift: the term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. it may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt. 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. wind wave: an ocean or lake wave resulting from the action of wind on the water's surface. after it leaves its fetch area, it is considered a swell..

Advisory: statements that are issued by the national weather service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead to hazardous situations. some examples include snow advisories stating possible slick streets, or fog advisories for patchy fog condition causing temporary restrictions to visibility. ceilometer: an instrument that is used to measure the angular elevation of a projected light on the base of a cloud. it measures the angle of the cloud base included by the observer (or machine), the ceiling light and the illuminated spot on the cloud. chromosphere: a thin layer of relatively transparent gases above the photosphere of the sun. it is observed best during a total eclipse of the sun. 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. 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. mesoscale convective system (mcs): a large organized convective weather system comprised of a number of individual thunderstorms. it normally persists for several hours and may be rounded or linear in shape. this term is often used to describe a cluster of thunderstorms that does not meet the criteria of a mesoscale convective complex (mcc). nephelococcygia: a term applied when people find familiar objects within the shape of a cloud. 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. 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. steam fog: a type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. also called sea smoke when it occurs over the ocean. vertical visibility: the distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling..

Afos: acronym for automation of field operations and services. it is the computer system that links national weather service offices together for weather data transmission. air mass: an extensive body of air throughout which the horizontal temperature and moisture characteristics are similar. anabatic wind: a wind that is created by air flowing uphill. valley breezes, produced by local daytime heating, are an example of these winds. the opposite of a katabatic wind. arctic jet: the jet stream that is situated high in the stratosphere in and around the arctic or antarctic circles. it marks the boundary of polar and arctic air masses. blowing dust: dust that is raised by the wind to heights of six feet or greater. it is reported as "bldu" in an observation and on the metar. 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. 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. 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. cumulonimbus mammatus: a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. although they do not cause severe weather, they often accompany storms. they may slowly vary in size, since they are an area of negative buoyancy convection, and is associated with severe turbulence in the lower sections of the cloud. graupel: a form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together. greenhouse effect: the overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space. ground clutter: a pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. this may hide or confuse the proper return echo signifying actual precipitation. latitude: the location north or south in reference to the equator, which is designated at zero (0) degrees. parallel lines that circle the globe both north and south of the equator. the poles are at 90° north and south latitude. mammatocumulus: an obsolete term for cumulonimbus mammatus, it is a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. although they do not cause severe weather, they often accompany storms. mesolow: a small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles. millibar (mb): the standard unit of measurement for atmospheric pressure used by the national weather service. one millibar is equivalent to 100 newtons per square meter. standard surface pressure is 1,013.2 millibars. 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. 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. : 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. pre-frontal squall line: a line of thunderstorms that precedes an advancing cold front. 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. radiation fog: fog that is created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its dew point. formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. this primarily occurs during the night or early morning. 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. 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. sea fog: a type of advection fog which forms in warm moist air cooled to saturation as the air moves across cold water. 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. 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. stratocumulus: a low cloud composed of layers or patches of cloud elements. it can form from cumulus clouds becoming more stratiformed and often appears as regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. it is light or dark gray in color, depending on the size of the water droplets and the amount of sunlight that is passing through them..

Anomalous propagation: this refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. may be referred to as a.p. backscatter: a radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. also the scattering of radiant energy into space before it reaches the earth's surface. boulder wind: a local name referring to an extremely strong downslope wind in the front range of the rocky mountains near boulder, colorado. 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. circulation: the flow or motion of a fluid in or through a given area or volume. in meteorology, it is used to describe the flow of air as it moves around a pressure system in the atmosphere. it describes smaller patterns in semi-permanent pressure systems as well as the relatively permanent global currents of air. in oceanic terms, it is used to describe a water in current flow within a large area, usually a closed circular pattern such as in the north atlantic. 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. 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. freezing rain: rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces. it is reported as "fzra" in an observation and on the metar. front: the transition zone or interface between two air masses of different densities, which usually means different temperatures. for example, the area of convergence between warm, moist air and cool, dry air. heat balance: the equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. the balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation). hygrograph: an instrument that records the hygrometer's measure of water vapor. instability: the state of equilibrium in which a parcel of air when displaced has a tendency to move further away from its original position. it is the condition of the atmosphere when spontaneous convection and severe weather can occur. air parcels, when displaced vertically, will accelerate upward, often forming cumulus clouds and possibly thunderstorms. isobar: the line drawn on a weather map connecting points of equal barometric pressure. 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. 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. pilot balloon: a small balloon whose ascent is used to determine the direction and speed of low level atmospheric winds. also known as a pibal. 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. pressure jump: a sudden increase in the observed atmospheric pressure or station pressure. profiler: a type of doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere. 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. sea mile: a unit of length distinguished from a nautical mile. one sea mile is equivalent to 1,000 fathoms (6,000 feet). subsidence: a sinking or downward motion of air, often seen in anticyclones. it is most prevalent when there is colder, denser air aloft. it is often used to imply the opposite of atmospheric convection. texas norther: local name in the south-central great plains for strong winter winds blowing north or northwest following a sharp cold front with dropping temperatures. marked by a dark, blue-black sky. variable ceiling: occurs when the height of a ceiling layer increases and decreases rapidly, the ascribed height is the average of all the varying values. windward: the direction from which the wind is blowing. also the upwind side of an object. the opposite of the downwind or leeward side..