A

Absolutely stable air: an atmospheric condition that exists when the environmental lapse rate is less than the moist adiabatic lapse rate.

Absolutely unstable air: an atmospheric condition that exists when the environmental lapse rate is greater than the dry adiabatic lapse rate.

Acid precipitation: precipitation, as rain, snow or sleet containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.

Acid rain: see acid precipitation.

Adiabatic lapse rate: the rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. For parcels that remain unsaturated during lifting, the (dry adiabatic) lapse rate is 9.8°C per kilometer.

Advection: the horizontal transport of atmospheric properties.Advection fog: a fog that forms when warm air flows over a cold surface and cools from below until dew point is reached.

Air mass: a body of air covering a relatively wide area and exhibiting horizontally uniform properties.

Air pollutant: harmful substance or product introduced into the atmosphere.

Albedo: reflectivity; the fraction of radiation striking a surface that is reflected by that surface.

Altocumulus: a cloud of a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. These clouds are of medium altitude, about 8000-20,000 ft (2400-6100 m).

Altostratus: a cloud of a class characterized by a generally uniform gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. These clouds are of medium altitude, about 8000 to 20,000 ft (2400-6100 m).

Ambient: of the surrounding area or environment.

Aneroid barometer: an instrument for measuring atmospheric pressure in which a needle, attached to the top of an evacuated box, is deflected as changes in atmospheric pressure cause the top of the box to bend in or out.

Anticyclone: a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere.

Atmospheric pressure: The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.

B

Barometric pressure: the pressure of the atmosphere as indicated by a barometer.

C

Cirrocumulus: a cirriform cloud characterized by thin, white patches, each of which is composed of very small granules or ripples. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Cirrostratus: a cloud of a class characterized by a composition of ice crystals and often by the production of halo phenomena and appearing as a whitish and usually somewhat fibrous veil, often covering the whole sky and sometimes so thin as to be hardly discernible. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Cirrus: a cloud of a class characterized by thin white filaments or narrow bands and a composition of ice crystals. These clouds are of high altitude (20,000-40,000 ft or 6000 -12,000 m).

Climate: the composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.

Climatology: the science that deals with the phenomena of climates or climatic conditions.

Cloud: a visible mass of water droplets or ice crystals suspended in the atmosphere.

Cloud amount: measured in OKTAS this is the portion of the sky which is covered in cloud

Cloud condensation nucleus: a particle, either liquid or solid, upon which water condenses to form cloud droplets.

Cold front: a zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.

Cold occlusion: A frontal zone formed when a cold front overtakes a warm front and, being colder than the air ahead of the warm front, slides under the warm front, lifting it aloft. Compare warm occlusion.

Conduction: flow of heat in response to a temperature gradient within an object or between objects that are in physical contact.

Convection: 1. vertical air circulation in which warm air rises and cool air sinks, resulting in vertical transport and mixing of atmospheric properties. 2. flow of heat by this circulation.

Convergence: a net flow of air into a given region. Compare divergence.

Coriolis force: a fictitious force used to account for the apparent deflection of a body in motion with respect to the earth, as seen by an observer on the earth. The deflection (to the right in the Northern Hemisphere) is caused by the rotation of the earth.

Corona: a white or colored circle or set of concentric circles of light of small radius seen around a luminous body, esp. around the sun or moon. The color varies from blue inside to red outside and the phenomenon is attributed to diffraction of light by thin clouds or mist (distinguished from halo).

Cumuliform: having the appearance or character of cumulus clouds.

Cumulonimbus: a cloud of a class indicative of thunderstorm conditions characterized by large, dense towers that often reach altitudes of 30,000 ft (9000 m) or more, cumuliform except for their tops, which appear fibrous because of the presence of ice crystals.

Cumulus: a cloud of a class characterized by dense individual elements in the form of puffs, mounds or towers, with flat bases and tops that often resemble cauliflower and found at a lower altitude than altocumulus, usually below 8000 ft (2400 m).

Cyclone: a large-scale circulation of winds around a central region of low atmospheric pressure, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

D

Day length: duration of the period from sunrise to sunset.

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.

Diurnal: daily, especially pertaining to actions which are completed in 24 hours and are repeated every 24 hours.

Divergence: the net flow of air from a given region. Compare convergence.

Downburst: a strong downdraft current of air from a cumulonimbus cloud, often associated with intense thunderstorms.

Drizzle: precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter.

Dry adiabatic lapse rate: the rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5°F per 1000 ft or 9.8°C per km.

E

Eddy: swirling currents of air at variance with the main current.

Environmental lapse rate: the rate of decrease of air temperature with height, usually measured with a radiosonde.

Equinox: the time when the sun crosses the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 (the spring or vernal equinox) and September 22 (autumnal equinox).

F

Flash flood: a sudden and destructive rush of water down a narrow gully or over a sloping surface, caused by heavy rainfall.

Flow: 1. wind; 2. volume of air carried by wind.

Foehn (fohn): a warm, dry wind on the lee side of a mountain range, the warmth and dryness of the air being due to adiabatic compression as the air descends the mountain slopes.

Fog: a cloud with its base at the earth's surface where visibility is reduced to less than 1000 metres.

Föhn: original yet alternate spelling of foehn.

Fractocumulus: a cumulus cloud presenting a ragged, shredded appearance, as if torn.Fractostratus: a stratus cloud presenting a ragged, shredded appearance, as if torn. It differs from a fractocumulus cloud in having a smaller vertical extent and darker color.

Freezing drizzle: a drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.

Freezing level: The altitude at which the air temperature first drops below freezing.Freezing rain: rain that falls as a liquid but freezes into glaze upon contact with the ground.

Front or frontal zone: an interface or zone of transition between two dissimilar air masses.Frontal inversion: a temperature inversion that develops aloft when warm air overruns the cold air behind a front.

Frostbite: human tissue damage caused by exposure to intense cold.

G

Geostrophic wind: a wind that is affected by coriolis force, blows parallel to isobars and whose strength is related to the pressure gradient (i.e., spacing of the isobars).

GMT: Greenwich Mean Time, same as UTC.

GPS: Global Positioning System, a navigation system which uses a constellation of artificial earth satellites to make precise determinations of the latitude and longitude of locations on the earth's surface or in the atmosphere.

Gradient: a rate of change with respect to distance of a variable quantity, as temperature or pressure, in the direction of maximum change.

Greenhouse effect: atmospheric heating caused by solar radiation being readily transmitted inward through the earth's atmosphere but longwave radiation less readily transmitted outward, due to absorption by certain gases in the atmosphere.

H

Hail: showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.

Halo: any of a variety of bright circles or arcs centered on the sun or moon, caused by the refraction or reflection of light by ice crystals suspended in the earth's atmosphere and exhibiting prismatic coloration ranging from red inside to blue outside. Often seen in Cirro-stratus clouds

Haze: an aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors.

Heat exhaustion: a mild form of heat stroke, characterized by faintness, dizziness, and heavy sweating.

Heatstroke: a condition resulting from excessive exposure to intense heat, characterized by high fever, collapse, and sometimes convulsions or coma.

Hypothermia: a rapid, progressive mental and physical collapse that accompanies the lowering of body temperature.

I

Ice crystal: precipitation consisting of small, slowly falling crystals of ice.

Ice pellets: precipitation consisting of particles of ice less than 5 mm in diameter, occurring either as frozen raindrops or as small hailstones.

Icing: a coating of ice on a solid object.Insolation: solar radiation received at the earth's surface.

Isobar: a line of equal or constant atmospheric pressure.

Isotach: a line on a weather map or chart connecting points where winds of equal speeds have been recorded.

Isotherm: a line of equal or constant temperature.

J

Jet: a fast-moving wind current surrounded by slower moving air.

Jet stream: strong, generally westerly winds concentrated in a relatively narrow and shallow stream in the upper troposphere.

K

L

Land breeze: a coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.

Lapse rate: the rate of decrease of air temperature with increase of elevation vertically above a given location.

Latent heat: heat absorbed or released during a change of phase at constant temperature and pressure.Lee: the side or part that is sheltered or turned away from the wind.

Leeward: the side away from the wind. Compare windward.

Lenticular cloud: a very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.

Lightning: A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground or between the ground and a cloud.

Longwave radiation: a term used to describe the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers. Compare shortwave radiation.

M

Mercury barometer: an instrument for measuring atmospheric pressure. The instrument contains an evacuated and graduated glass tube in which mercury rises or falls as the pressure of the atmosphere increases or decreases.

Mesopause: the top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.Mesosphere: the atmospheric shell between about 20 km and about 70 to 80 km, extending from the top of the stratosphere (the stratopause) to the upper temperature minimum that defines the mesopause (the base of the thermosphere).

METAR: an international code (Aviation Routine Weather Report) used for reporting, recording and transmitting weather observations.

Meteorology: the science dealing with the atmosphere and its phenomena. A distinction can be drawn between meteorology and climatology, the latter being primarily concerned with average, not actual, weather conditions.

Microclimate: the climate of a small area such as a cave, house, city or valley that may be different from that in the general region.

Millibar: a unit of atmospheric pressure equal to 1/1000 bar, or 1000 dynes per square centimeter.

Monsoon: a thermally driven wind arising from differential heating between a land mass and the adjacent ocean that reverses its direction seasonally.

Mountain meteorology: meteorology of a mountainous or topographically complex area.MSL: above mean sea level.

N

Neutral stability (neutrally stable air): An atmospheric condition that exists in unsaturated (saturated) air when the environmental lapse rate equals the dry (moist) adiabatic rate.

Nimbostratus: a cloud of the class characterized by a formless layer that is almost uniformly dark gray; a rain cloud of the layer type, of low altitude, usually below 8000 ft (2400 m).

O

Occluded front: a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front. A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.

Okta: meaning eighth, is the form used to report coverage of the sky in cloud. Eight Oktas is full coverage with Zero Oktas being a clear sky.

Orographic waves: a wavelike airflow produced over and in the lee of a mountain barrier.

Ozone: A form of oxygen, O3. A powerful oxidizing agent that is considered a pollutant in the lower troposphere but an essential chemical in the stratosphere where it protects the earth from high-energy ultraviolet radiation from the sun.

P

Powder snow: dry, loose, unconsolidated snow.

Precipitation: Water in either liquid or solid form that comes from the atmosphere and falls to earth. It includes, drizzle, rain, freezing rain, hail, ice crystals, ice pellets and snow. It specifically excludes clouds, dew, fog and frost.

Pressure: the exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it.

Primary pollutant: substances that are pollutants immediately on entering the atmosphere. Compare secondary pollutant.

Q

R

Radiation: energy transport through electromagnetic waves. See shortwave radiation and longwave radiation.

Radiation fog: a fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.

Radiosonde: an instrument that is carried aloft by a balloon to send back information on atmospheric temperature, pressure and humidity by means of a small, expendable radio transmitter. Radiosondes can be tracked by radar, radio direction finding, or navigation systems (such as the satellite Global Positioning System) to obtain wind data. See also rawinsonde.

Rain: precipitation that falls to earth in drops more than 0.5 mm in diameter.

Relative humidity: the ratio of the actual water vapor pressure at a given time to the vapor pressure that would occur if the air were saturated at the same ambient temperature.

Return stroke: an electrical discharge that propagates upward along a lightning channel from the ground to the cloud.

Ridge: on a weather chart, a narrow elongated area of relatively high pressure.

Rime ice: an opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. See also clear ice.

RMS: Royal Meteorologial Society.

S

Sea breeze: a thermally produced wind blowing during the day from a cool ocean surface onto the adjoining warm land, caused by the difference in the rates of heating of the surfaces of the ocean and of the land.

Secondary pollutant: pollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.

Severe thunderstorm: a thunderstorm that produces heavy precipitation, frequent lightning, strong, gusty surface winds or hail. A severe thunderstorm can cause flash flooding and wind and hail damage and may spawn tornadoes.

Shortwave radiation: a term used to describe the radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths (between about 0.1 and 2 micrometers). Compare longwave radiation.

Sling psychrometer: an instrument used to measure the water vapor content of the atmosphere in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.

Small hail: same as snow pellets or graupel.Snow: precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing [deposition] of the water vapor in the air.

Snowflake: an agglomeration of snow crystals falling as a unit.

Snow grain: precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.

Snow pellets: precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.

Solstice: Either of the two times per year when the sun is at its greatest angular distance from the celestial equator: about June 21 (the Northern Hemisphere summer solstice), when the sun reaches its northernmost point on the celestial sphere, or about December 22 (the Northern Hemisphere winter solstice), when it reaches its southernmost point.

Stability: the degree of resistance of a layer of air to vertical motion.Stable air: see absolutely stable air.

Standard atmosphere: a hypothetical vertical distribution of temperature, pressure and density which, by international consent, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.

Station Model (Station Plot): a specified pattern for plotting, on a weather map, the meteorological symbols that represent the state of the weather at a particular observing station.

Stationary front: a front between warm and cold air masses that is moving very slowly or not at all.

Stepped leader: a faint, negatively charged channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.

Stratiform: (of a cloud) having predominantly horizontal development.

Stratocumulus: a cloud of a class characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus and the whole being at a lower altitude, usually below 8000 ft (2400 m).

Stratopause: the boundary between the stratosphere and mesosphere.

Stratosphere: the atmospheric shell above the troposphere and below the mesosphere. It extends from the tropopause, usually 10-25 km high, to the height where temperature begins to decrease in the 20 to 25 km region.

Stratus: a cloud of a class characterized by a gray, horizontal layer with a uniform base, found at a lower altitude than altostratus, usually below 8000 ft (2400 m).

Supercool: to cool a liquid below its freezing point without solidification or crystallization.

Surface weather chart: an analyzed synoptic chart of surface weather observations. A surface chart shows the distribution of sea-level pressure (therefore, the position of highs, lows, ridges and troughs) and the location and nature of fronts and air masses. Often added to this are symbols for occurring weather phenomena. Although the pressure is referred to mean sea level, all other elements on this chart are presented as they occur at the surface point of observation.Synoptic weather: weather occurring over a wide region on time scales exceeding 12 hours.

T

TAF: an international code (Aerodrome Forecast) used for issuing and transmitting weather forecasts.

Thermosphere: the atmospheric shell extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at 70 or 80 km.

Thunder: the sound caused by rapidly expanding gases in a lightning discharge.

Thunderstorm: a local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.

Time constant: the time required for a measuring instrument to respond to 63.2% of a stepwise change in a measured quantity.

Towering cumulus: a tall cumulus cloud, extending through low and middle cloud levels.

Tropopause: The boundary between the troposphere and stratosphere, characterized by an abrupt change in temperature lapse rate (temperatures decrease with height in the troposphere, but increase or remain constant with height in the stratosphere).

Troposphere: The portion of the earth's atmosphere from the surface to the tropopause; that is, the lowest 10-20 km of the atmosphere. The troposphere is characterized by decreasing temperature with height, and is the layer of the atmosphere containing most clouds and other common weather phenomena.

Trough: on a weather chart, a narrow, elongated area of relatively low pressure.Turbulence: irregular motion of the atmosphere, as indicated by gusts and lulls in the wind.

U

Unstable air: see absolutely unstable air.

Upper-air weather chart: weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.

Upslope fog: a fog that forms when moist, stable air is carried up a mountain slope.

UTC: Coordinated Universal Time; the international time standard kept at Greenwich, London.

V

Venturi effect: the speedup of air through a constriction due to the pressure rise on the upwind side of the constriction and the pressure drop on the downwind side as the air diverges to leave the constriction.

Visibility: the distance at which a given standard object can be seen and identified with the unaided eye.

Vortex: a whirling mass of air in the form of a column or spiral. It need not be oriented vertically but, for example, could be rotating around a horizontal axis.

W

Warm front: a transition zone between a mass of warm air and the colder air it is replacing.

Warm occlusion: A frontal zone formed when a cold front overtakes a warm front and, finding colder air ahead of the warm front, leaves the ground and rises up and over this denser air. Compare cold occlusion.

Weather: the state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc.

Westerlies: the prevailing winds that blow from the west in the mid-latitudes.

Wind chill equivalent temperature: the apparent temperature felt on the exposed human body owing to the combination of temperature and wind speed.

Wind chill: see wind chill equivalent temperature.Wind rose: a diagram, for a given locality or area, showing the frequency and strength of the wind from various directions.

Wind shear: the rate at which wind velocity changes from point to point in a given direction (as, vertically). The shear can be speed shear (where speed changes between the two points, but not direction), direction shear (where direction changes between the two points, but not speed) or a combination of the two.

Windward: the side toward the wind. Compare leeward.

X

Y
 
Z Zulu time, same as UTC.