Your Weather Definitions placeholder text:

Absorption: the process in which incident radiant energy is retained by a substance. the absorbed radiation is then transformed into molecular energy. air mass: an extensive body of air throughout which the horizontal temperature and moisture characteristics are similar. climate prediction center (cpc): a branch of the national centers for environmental prediction,the center maintains a continuous watch on short-term climate fluctuations and diagnoses and predicts them. : for further information, contact the cpc, located in washington, d.c. cloud bank: a well-defined cloud mass that can be observed at a distance. it covers the horizon, but is not directly overhead. coalescence: the merging of two water drops into a single larger drop. continent: a large land mass rising abruptly from the deep ocean floor, including marginal regions that are shallowly submerged. continents constitute about one-third of the earth's surface. downburst: a severe localized downdraft from a thunderstorm or shower. this outward burst of cool or colder air creates damaging winds at or near the surface. sometimes the damage resembles tornadic damage. downslope effect: the warming of an air flow as it descends a hill or mountain slope. easterlies: usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds. electromagnetic spectrum: the band of electromagnetic radiation with components that are separated into their relative wave lengths. the portion of the spectrum that the human eye can detect is called visible light, between the longer infrared waves and the shorter ultraviolet waves. the various types of energy comprising the spectrum are (from longest to shortest) radio, infrared, visible, ultraviolet, x-rays, gamma rays, and cosmic rays. flood: high water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas. frontogenesis: the birth or creation of a front. this occurs when two adjacent air masses exhibiting different densities and temperatures are brought together by prevailing winds, creating a front. it could happen when either air mass, or both, move over a surface which strengthens their original properties. however, it occurs most often along the eastern coasts of north america and asia, when the air mass moving out over the ocean has a weak or no distinct boundary. the opposite of frontolysis. geophysics: the study of the physics or nature of the earth and its environment. it deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. by extension, it often includes astronomy and the related astro-sciences. 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. 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. longitude: the location east or west in reference to the prime meridian, which is designated as zero (0) degrees longitude. the distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's north and south poles. time zones are correlated to longitude. mesolow: a small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles. multiple vortex tornado: a tornado which has two or more condensation funnels or debris clouds, often rotating around a common center. normal: the recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. normals are concerned with the distribution of data within limits of common occurrence. the parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc. 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 altimeter: an aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. it is read accurately only in a standard atmosphere and when the correct altimeter setting is used. 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. squall line: a narrow band or line of active thunderstorms that is not associated with a cold front. it may form from an outflow boundary or the leading edge of a mesohigh. thermodynamics: study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body. 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. valley breeze: an anabatic wind, it is formed during the day by the heating of the valley floor. as the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. it blows in the opposite direction of a mountain breeze. vorticity maximum: a center of vorticity, or the maximum of the vorticity field of a fluid. : : zulu time: one of several names for the twenty-four hour time which is used throughout the scientific and military communities..

Broken: the amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer. 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. 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. gulf stream: the warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the united states, beginning near cape hatteras. the term also applies to the oceanic system of currents that dominate the western and northern atlantic ocean: the florida current, which flows through the florida straits between the florida keys and cuba and northwards; the gulf stream, which begins around cape hatteras and flows northeasterly off the continental slope into the north atlantic; and the north atlantic current, which begins around the grand banks off newfoundland and continues east-northeastwards towards the british isles. melting point: the temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. contrast with freezing point. mesolow: a small scale low pressure center, ranging from the size of an individual thunderstorm to many tens of miles. mud slide: fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain. 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. 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. photosphere: the intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. reaching temperatures estimated at about 11,000°f, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation. prognostic chart: a chart of forecast predictions that may include pressure, fronts. precipitation, temperature, and other meteorological elements. also known as a prog. 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. saffir-simpson damage-potential scale: developed in the early 1970s by herbert saffir, a consulting engineer, and robert simpson, then director of the national hurricane center, it is a measure of hurricane intensity on a scale of 1 to 5. the scale categorizes potential damage based on barometric pressure, wind speeds, and surge. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. spring tide: a tide of increased range, which occurs about every two weeks when the moon is new or full. terminal doppler weather radar (tdwr): doppler radar installed at major airports throughout the united states to detect microbursts. 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. transpiration: the process by which water in plants is transferred as water vapor to the atmosphere. tropic of cancer: the most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees north latitude. 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. water cycle: the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas. 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. whirlwind: a small-scale, rapidly rotating column of wind, formed thermally and most likely to develop on clear, dry, hot afternoons. often called a dust devil when visible by the dust, dirt or debris it picks up. also slang for a landspout or a tornado..

Arctic sea smoke: a type of advection fog that forms primarily over water when cold air passes across warmer waters. bathythermograph: a device used to obtain a record of temperature against depth (pressure) in the ocean. may be referred to as a b.t. 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. clear: the state of the sky when no clouds or obscurations are observed or detected from the point of observation. degree day: a measure of the departure of the mean daily temperature from a given standard. that is one degree day for each degree (fahrenheit or celsius) of departure above or below the standard during one day. dust bowl: the term given to the area of the great plains including texas, oklahoma, kansas, colorado, and new mexico that was most greatly affected during the great drought of the 1930's. 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. equator: the geographic circle at 0 degrees latitude on the earth's surface. it is equal distance from the north and south poles and divides the northern hemisphere from the southern. forecast: a statement of expected future occurrences. weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. freezing precipitation: precipitation that is liquid, but freezes upon impact with a solid surface, such as the ground or other exposed surfaces. frontolysis: the destruction or dying of a front where the transition zone is losing its contrasting properties. the opposite of frontogenesis. gale warning: a warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour). lithosphere: the solid, outer portion of the earth's crust coupled to the rigid upper mantle. part of the geosphere. middle clouds: a term used to signify clouds with bases between 6,000 and 18,000 feet. at the higher altitudes, they may also have some ice crystals, but they are composed mainly of water droplets. altocumulus, altostratus, and nimbostratus are the main types of middle clouds. 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. palmer drought index: a long-term meteorological drought severity index produced by the noaa/usda (department of agriculture) joint agricultural weather facility. the index depicts prolonged times, as in months or years, of abnormal dryness or wetness. it responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration. positive vorticity advection: the advection of higher values of vorticity into an area. it is also known as cyclonic vorticity. 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. sea mile: a unit of length distinguished from a nautical mile. one sea mile is equivalent to 1,000 fathoms (6,000 feet). specific humidity: the ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. it is expressed in grams per gram or in grams per kilograms. the specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel. 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. thermodynamics: study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body. thickness: the thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. the layer most often used in meteorology is between 1000 and 500 millibars. there can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. for example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer. trajectory: the curve that a body, such as a celestial object, describes in space. this applies to air parcel movement also. upwind: the direction from which the wind is blowing. also the windward side of an object. the opposite of the downwind or leeward side. : : vorticity: the measurement of the rotation of a small air parcel. it has vorticity when the parcel spins as it moves along its path. although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. if it does not spin, it is said to have zero vorticity. in the northern hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. it is negative when the parcel has clockwise, or anticyclonic, rotation. 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. weather surveillance radar (wsr-88d): the newest generation of doppler radars, the 1988 doppler weather radar. the radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm. west virginia high: an area of stagnant high pressure located over west virginia during indian summer..

Aphelion: the point on the earth's orbit that is farthest from the sun. although the position is part of a 21,000 year cycle, currently it occurs around july, when the earth is about 3 million miles farther from the sun than at perihelion. this term can be applied to any other celestial body in orbit around the sun. it is the opposite of perihelion. 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. 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. 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. contrail: acronym for condensation trail. a cloud-like 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. cumulus: one of the three basic cloud forms (the others are cirrus and stratus). it is also one of the two low cloud types. a cloud that develops in a vertical direction from the base (bottom) up. they have flat bases and dome- or cauliflower-shaped upper surfaces. the base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. small, separate cumulus are associated with fair weather (cumulus humilis). with additional heating from the earth's surface, they can grow vertically throughout the day. the top of such a cloud can easily reach 20,000 or more into the troposphere. under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. further development may create a cumulonimbus. earthlight (earthshine): the faint illumination of the dark part of the moon's disk produced by sunlight reflected onto the moon from the earth's surface and atmosphere. haze: a suspension of fine dust and/or smoke particles in the air. invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance. it is reported as "hz" in an observation and on the metar. 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. 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. multiple vortex tornado: a tornado which has two or more condensation funnels or debris clouds, often rotating around a common center. newhall winds: the local name for winds blowing downward from desert uplands through the newhall pass southward into the san fernando valley, north of los angeles. 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. 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. 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. 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. 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. 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. swell: ocean waves that have traveled out of their generating area. swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch. tide: the periodic rising and falling of the earth's oceans and atmosphere. it is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. this propagates a wave through the atmosphere and along the surface of the earth's waters. 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. 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..

Celestial sphere: the apparent sphere of infinite radius having the earth as its center. all heavenly bodies (planets, stars, etc.) appear on the "inner surface" of this sphere and the sun moves along the ecliptic. doppler radar: weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. using the doppler effect, it measures the velocity of particles. named for j. christian doppler, an austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away. drifts: normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind. environment: the sum total of all the external conditions that effect an organism, community, material, or energy. 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. 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. helicity: a property of a moving fluid, such as air, representing the potential for helical flow (flow that follows a corkscrew pattern). computed from the vertical wind profile of the lower atmosphere and measured relative to the motion as a storm, it is used to forecast the formation of mesocyclones. 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. isobar: the line drawn on a weather map connecting points of equal barometric pressure. latent heat: the energy released or absorbed during a change of state. mean temperature: the average of temperature readings taken over a specified amount of time. often the average of the maximum and minimum temperatures. melting point: the temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. contrast with freezing point. nowcast: a short-term weather forecast for expected conditions in the next few hours. ozone (o3): a nearly colorless gas and a form of oxygen (o2). it is composed of an oxygen molecule made up of three oxygen atoms instead of two. polar air mass: an air mass that forms over a high latitude region. continental polar air (cp) is formed over cold surface regions and is typically very stable with low moisture. maritime polar air (mp), produced over warmer waters, is less stable with high moisture. positive vorticity advection: the advection of higher values of vorticity into an area. it is also known as cyclonic vorticity. 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. sea spray: sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave. 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. snowfall: the rate at which snow falls, usually expressed in inches of snow depth over a six hour period. water: refers to the chemical compound, h2o, as well as its liquid form. at atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). it is a vital, life-sustaining part of life on earth. 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. zenith: the point which is elevated 90 degrees from all points on a given observer's astronomical horizon. the point on any given observer's celestial sphere that lies directly above him. the opposite of nadir..