Air Discharge - A lightning discharge between a cloud and a cloud-free (or clear air) region of the atmosphere.

Air Drainage - General term for gravity-induced, downslope flow of relatively cold air; a form of katabatic wind. Winds thus produced are called gravity winds.

Air Mass - A widespread body of air, the thermal, moisture, and stability properties of which can be identified as(a) having been established while that air was situated over a particular region of the earth's surface (climatology), and (b) undergoing specific modifications while in transit away from the source region. An air msss is often defined as a widespread body of air extending over an area of several million square killometers that is aproxamatly homogenous in it's horizontal extent, particularly with reference to tempature and water vapor consentration; in addition, the vertical tempature and humidity variations, together with the stability, are approxamately the same over it's horizontal extent. Depending upon it's type the depth of the air mass may range from one to several kilometers

Air parcel- An imaginary volume of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air; often used as a convenient tracer of air movement or atmospheric process.

Air pollution - Same as atmospheric pollution.

Air Pressure - same as atmospheric pressure.

Air Quality - a measure of the cleanliness of air described in terms of levels of contaniments in air especialy with regard to their potential effects on human health.

Air Quality Standards - A set of objectives established

Albedo - The percent of radiation returning from a surface compared to that which strikes it.

Altimeter - An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.

Altocumulus - A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.

Altocumulus Castellanus - An altocumulus showing vertical development, individual cloud elements have towerlike tops, often in the shape of tiny castles.

Altocumulus Lenticularis - A lens-shaped altocumulus cloud; a mountain-wave cloud generated by the disturbance of horizontal airflow caused by a prominent mountain range.

Altostratus - A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.

Ambient Air - The air surrounding a cloud, or the air surrounding rising or sinking air parcels.

Ambient Temperature - Temperature of the surrounding (ambient) air.

Anemometer - An instrument designed to measure wind speed.

Aneroid Barometer - An instrument designed to measure atmospheric pressure. It contains no liquid.

Annual Range of Temperature - The difference between the warmest and coldest months at any given location.

Anomalies - Departures of temperature, precipitation, or other weather elements from long-term averages.

Arctic air - A very cold and dry air mass that forms primarily in winter and the northern interior of North America.

Atmospheric Window - A region of the electromagnetic spectrum from 8 to 12 ?m where the atmosphere is transparent to radiation.

Autumnal Equinox - The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.

Barograph A recording instrument that provides a continuous trace of air pressure variation with time.

Barometer An instrument that measures atmospheric pressure. The two most common barometers are the mercury barometer and the aneroid barometer.

Beaufort scale A scale of wind strength based on visual assessment of the effects of wind on seas and vegetation.

Black body A hypothetical object that absorbs all of the radiation that strikes it. It also emits radiation at a maximum rate for its given temperature.

Blizzard A severe weather condition characterized by low temperatures and strong winds (greater than 32 mi/hr) bearing a great amount of snow. When these conditions continue after the falling snow has ended, it is termed a ground blizzard.

Bora A cold katabatic wind that originates in Yugoslavia and flows onto the coastal plain of the Adriatic Sea.

Bowen ratio The ratio of energy available for sensible heating to energy available for latent heating.

Boyle's law When the temperature is held constant, the pressure and density of an ideal gas are directly proportional.

Ceilometer An instrument that automatically records cloud height.

Centrifugal force A force directed outward, away from the center of a rotating object; equal in magnitude to the centripetal force but in the opposite direction.

Centripetal force An inward-directed force that confines an object to a circular path; equal in magnitude to the centrifugal force but in the opposite direction.

Charles's law With constant pressure, the temperature of an ideal gas is inversely proportional to the density of the gas.

Chinook A warm, dry wind on the eastern side of the Rocky Mountains. In the Alps, the wind is called a Foehn.

Cirrocumulus A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.

Cirrostratus A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.

Cirrus A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

Climate The accumulation of daily and seasonal weather events over a long period of time. A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Cloud base The lowest portion of a cloud.

Cloudburst Any sudden and heavy rain shower.

Cloud cover The amount of the sky obscured by clouds when observed at a particular location.

Cloud deck The top of a cloud layer, usually viewed from an aircraft.

Cloud seeding The introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation.

Coalescence The merging of cloud droplets into a single larger droplet.

Cold fog See Supercooled cloud.

Cold front The leading edge of a cold air mass.

Condensation Process by which water changes phase from a vapor to a liquid.

Condensation nuclei Small particles in the atmosphere that serve as the core of tiny condensing cloud droplets. These may be dust, salt, or other material.

Conduction The transfer of heat by molecular activity from one substance to another, or through a substance. Transfer is always from warmer to colder regions.

Continental air mass An air mass that forms over land; it is normally relatively dry.

Continental Climate A climate lacking marine influence and characterized by more extreme temperatures than in marine climates: therefore, it has a relatively high annual temperature range for its latitude.

Continental polar air Relatively dry air mass that develops over the northern interior of North America; very cold in winter and mild in summer.

Continental tropical air Warm, dry air mass that forms over the subtropical deserts of the south-western United States.

Contrail (condensation trail) A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

Convection Motions in a fluid that result in the transport and mixing of the fluid's properties. In meteorology, convection usually refers to atmospheric motions that are predominantly vertical, such as rising air currents due to surface heating. The rising of heated surface air and the sinking of cooler air aloft is often called free convection. (Compare with forced convection.)

Convective condensation level (CCL) The level above the surface marking the base of a cumiliform cloud that is forming due to surface heating and rising thermals.

Convergence An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.

Cooling degree-day A form of degree-day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. A cooling degree-day is a day on which the average temperature is one degree above a desired base temperature.

Coriolis effect A deflective force arising from the rotation of the earth on its axis; affects principally synoptic-scale and global-scale winds. Winds are deflected to the right of the initial direction in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Crepuscular rays Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.

Cumulonimbus An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud.

Cumulus A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are called cumulus humilis. Those that exhibit much vertical growth are called cumulus congestur or towering cumulus.

Cumulus Congestus An upward building convective cloud with vertical development between that of a cumulus cloud and a cumulonimbus.

Cup anemometer An instrument used to monitor wind-speed. Wind rotation of cups generates and electric current calibrated in wind speed.

Cutoff high Anticyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Cutoff low Cyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Daily range of temperature The difference between the maximum and minimum temperatures for any given day.

Degree days Computed from each day's mean temperature (max+min/2). For each degree that a day's mean temperature is below or above a reference temperature is counted as one degree day.

Density The ratio of the mass of a substance to the volume occupied by it.

Deposition A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. (Also called sublimation in meteorology.)

Deposition nuclei Tiny particles in the atmosphere that serve as the core of tiny ice crystals as water vapor changes to the solid form. These are also called ice nuclei.

Desert One of two types of dry climate-the driest of the dry climates.

Dew Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Dew point (dew-point temperature) The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. When the dew point falls below freezing it is called the frost point.

Diffraction The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Diffuse insolation Solar radiation that is scattered or reflected by atmospheric components (clouds, for example) to the earth's surface.

Direct insolation Solar radiation that is transmitted directly through the atmosphere to the earth's surface without interacting with atmospheric components.

Divergence An atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region.

Downburts A severe localized downdraft that can be experienced beneath a severe thunderstorm. (Compare Microburst)

Downdraft Downward moving air, usually within a thunderstorm cell.

Drainage basin A fixed geographical region from which a river and its tributaries drain water.

Drizzle Small drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.

Drought A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry adiabatic rate The rate of change of temperature in a rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 10?C per 1000 m (5.5?F per 1000 ft).

Dry climate A climate in which yearly precipitation is not as great as the potential loss of water by evaporation.

Dust devil (or whirlwind) A small but rapidly rotating wind made visible by the dust, sand, and debris it picks up from the surface. It develops best on clear, dry, hot afternoons.

Eddy A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Effective emissivity A correction factor, dependent on the radiational characteristics of the earth -atmosphere system, that permits application of black body radiation laws to the earth-atmosphere system

Emissivity The fractional amount of radiation emitted by a given object or substance in comparison to the amount emitted by a perfect emitter.

Emittance The rate at which a black body radiates energy across all wave-lengths.

Entrainment The mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.

Environmental lapse rate The rate of decrease of temperature with elevation. It is most often measured with a radiosonde.

Equilibrium vapor pressure The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called saturation vapor pressure.

Equinox The time when the sun crosses the plane of the earth's equator occurring about March 21 and September 22.

Evaporation The process by which a liquid changes into a gas.

Evapotranspiration Vaporization of water through direct evaporation from wet surfaces and the release of water vapor by vegetation.

Evaporation fog Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

Exosphere The outermost portion of the atmosphere.

Fall Freeze date The date of occurrence in the fall of the first minimum at or below a temperature threshold.

Fall streaks Falling ice crystals that evaporate before reaching the ground.

Foehn See Chinook.

Fog A cloud with its base at the earth's surface. It reduces visibility to below 1 km.

Forced convection On a small scale, a form of mechanical stirring taking place when twisting eddies of air are able to mix.

Free convection Convection triggered by intense solar heating of the earth's surface.

Freeze A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost.

Freeze free season The number of days between the last spring freeze date and the first fall freeze date.

Freezing rain and freezing drizzle Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze.

Front The transition zone between two distinct air masses.

Frontal fog See Evaporation fog.

Frost (also called hoarfrost) A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point is below freezing).

Frost point See Dew point.

Frozen dew The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.

Funnel cloud A rotating conelike cloud that extends down-ward from the base of a thunderstorm. When it reaches the surface it is called a tornado.

Geostrophic wind A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.

Glaciation The conversion of all the supercooled liquid water in a cloud into ice crystals, thus reducing the growth rate of ice crystals and hail.

Glaciated cloud A cloud or portion of a cloud where only ice crystals exist.

Glaze A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is called an icing storm.

Glory Colored rings that appear around the shadow of an object.

Graupel See Snow pellets

Green flash A small, green color that occasionally appears on the upper part of the sun as it rises or sets.

Ground fog See Radiation fog.

Growing degree-day A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates.

Growing season The number of days between the last spring freeze date and the first fall freeze date.

Haboob A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Hail Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Hailstones Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair hygrometer An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Halos Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Haze Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat A form of energy transferred between systems by virtue of their temperature differences.

Heat capacity The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat index (HI) An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat of fusion Heat released when water changes phase from liquid to solid; 80 calories per gram

Heat of melting Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating degree-day A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69 ?F (18 ?C); computed by subtracting the day's average temperature from 65 ?F.

Heat lightning Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

Heiligenschein A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

Heterosphere The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High inversion fog A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland climate Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Hoarfrost Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

Homosphere The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid continental climate A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40 and 50? north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Hurricane A severe tropical cyclone having winds in excess of 64 knots (74 mi/hr).

Hydrograph An instrument that provides a continuous trace of relative humidity with time.

Hygrometer An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Hypothermia The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.

Ice Cap Climate A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice fog A type of fog composed of tiny suspended ice particles that forms at very low temperatures.

Ice nuclei Particles that act as nuclei for the formation of ice crystals in the atmosphere.

Ice pellets See Sleet

Indian summer An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Infrared radiation Electromagnetic radiation with wavelengths between about 0.7 and 1000 ?m. This radiation is longer than visible radiation but shorter than microwave radiation.

Insolation The incoming solar radiation that reaches the earth and the atmosphere.

Intertropical convergence zone (ITCZ) The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.

Inversion An increase in air temperature with height.

Ion An electrically charged atom, molecule, or particle.

Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist.

Iridescence Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30? from the sun.

Isobar A line connecting points of equal pressure

Isotach A line connecting points of equal wind speed.

Isotherm A line connecting points of equal wind temperature.

January thaw A period of relatively mild weather around January 20 to 23 that occurs primarily in New England; an example of a singularity in the climatic record.

Jet stream Relatively strong winds concentrated within a narrow band in the atmosphere.

Katabatic wind Any wind blowing downslope. Usually cold.

Lake breeze A wind blowing onshore from the surface of a lake.

Lake-effect snows Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water.

Land breeze A coastal breeze that blows from land to sea, usually at night.

Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.)

Latent heat The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.

Lenticular cloud A cloud in the shape of a lens.

Lightning A visible electrical discharge produced by thunderstorms.

Longwave radiation A term most often used to describe the infrared energy emitted by the earth and the atmosphere.

Magnetosphere The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.

Mammatus clouds Clouds that look like pouches hanging from the underside of a cloud.

Marine climate A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime air mass An air mass that originates over the ocean. These air masses are relatively humid.

Maritime polar air Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Maritime tropical air Warm, humid air mass that forms over tropical and subtropical oceans.

Mean annual temperature The average temperature at any given location for the entire year.

Mesoscale The scale of meteorological phenomena that ranges in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.

Mesosphere The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.

Meteorology The study of the atmosphere and atmospheric phenomena as well as the atmosphere's interaction with the earth's surface, oceans, and life in general.

Microburst A strong localized downdraft less than 4 km wide that occurs beneath severe thunderstorms. A strong downdraft greater than 4 km across is called a downburst.

Microclimate The climate structure of the air space near the surface of the earth.

Microscale The smallest scale of atmospheric motions.

Millibar(mb) A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb.

Mirage A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is, it is called a superior image. When an object appears lower than it actually is, it is an inferior mirage.

Mist Very thin fog in which visibility is greater than 1.0 km (0.62 mi).

Mistral A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing ratio The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Moist adiabatic rate The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6?C per 1000 m (3.3?F per 1000 ft) is used.

Molecular viscosity The small-scale internal fluid friction that is due to the random motion of the molecules within a smooth-flowing fluid, such as air.

Mountain and valley breeze A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.