Absolute Humidity - The mass of water vapor per volome of air (usually expressed as grams of water vapor per cubic meter of air).

Absolute Instability - The condition of air that has an environmental lapse rate that is greater than the dry adiabatic rate (1°C per 100 meters).

Absolute Stability -The condition of air that has an environmental lapse rate that is less than the wet adiabatic rate.
Absolute Zero The zero point on the Kevin temperature scale, representing the temperature at which all molecular motion is presumed to cease.

Acid Precipitation - Rain or snow with a pH value that is less than the value for uncontaminated rain.
Adiabatic Temperature Change The cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted.

Advection - Horizontal convective motion (wind).

Advection Fog - Fog formed when warm moist air is blown over a cool surface and chilled below- the dew point.

Aerosols - Tiny solid and liquid particles suspended in the atmosphere.

Aerovane - A device that resembles a wind vane with a propeller at one end. Used to indicate wind speed and direction.

Air - A mixture of many discrete gases, of which nitrogen and oxygen are most abundant, in which vaning quantities of tiny solid and liquid particles are suspended.

Air Mass
- A large body of air, usually 1600 diameters or more across, that is characterized by homogeneous physical properties at any given altitude.

Air-Mass Thunderstorm - A localized thunderstorm that forms in a warm, moist, unstable air mass. Most frequent in the afternoon in spring and summer.

Air-Mass - Weather The conditions experienced in an area as an air mass passes over it. Because air masses are large and relatively homogeneous, air-mass weather will be fairly constant and may last for several days.

Air Pollutants - Airborne particles and gases occurring in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment

Air Pressure - The force exerted by the weight of a column of air above a given point.

Albedo - The reflection of a substance. usually expressed as a percentage of the incident radiation reflected. (in weather, it is the reflection of sunlight off the surface of the planet back in to space)

Aleutian Low - A large cell of low’ pressure centered over the Aleutian Islands of the North Pacific during the winter.

Altimeter - An aneroid barometer calibrated to indicate altitude instead of pressure.

Altitude (of the Sun) - The angle of the Sun above the horizon (sun angle).

Analog Method - A statistical approach to weather forecasting in which current conditions are matched with records of similar past weather events with the idea that the succession of events in the past will be paralleled by current conditions.

Anemometer - An instrument used to determine wind speed. Aneroid Barometer An instrument for measuring air pressure: it consists of evacuated metal chambers that are very sensitive to variations in air pressure.

Annual Mean Temperature - A average of the 12 monthly means.

Annual Temperature Range - The difference between the warmest and coldest monthly means.

Anticyclone - An area of high atmospheric pressure characterized by diverging and rotating winds and subsiding air aloft.

Anticyclonic Flow - Winds blow out and flow clockwise about an anticlonic high in the Northern Hemisphere, and the blow out and flow counterclockwise about an anticlone in the Southern Hemisphere.

Aphelion - The point in the orbit of a planet that is farthest from the Sun.

Apparent Temperature - The air temperature perceived by a person.

Arctic (A) Air Mass - A bitterly cold air mass that forms over the frozen Arctic Ocean.

Arctic Sea Smoke - A dense and often extensive steam fog occurring over high-latitude ocean areas in winter.

Arid - See Desert.

Atmosphere - The gaseous portion of a planet, the planet’s envelope of air; one of the traditional subdivisions of Earth’s physical environment.

Atmospheric Window - Refers to the fact that the troposphere is transparent (i.e. does not absorb) to terrestrial radiation between 8 and 12 micrometers in length.

Aurora - A bright and ever changing display of light caused by solar radiation interacting with the upper atmosphere in the region of the poles. It is called aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere.

Automated Surface Observing System (ASOS)
- A widely used, standardized set of automated weather instruments that provide routine surface observations. (less accurate than conventional measurements)

Azores High - Name given the subtropical anticyclone when it is situated over the eastern part of the N orth Atlantic Ocean.

Backing Wind Shift
- A wind shift in a counterclockwise direction, such as a shift from east to north.

Barograph - A recording barometer.

Beaufort Scale - A scale that can he used for estimating wind speed when an anemometer is not available.


Bergeron Process - A theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, arid the different saturation levels of ice and liquid water.

Bermuda High - Name given the subtropical high in the North Atlantic during the summer when it is centered near the island of Bermuda.

Bimetal Strip - A thermometer consisting of two thin strips of metal welded together and that have widely different coefficients of thermal expansion. When temperature changes, the two metals expand or contract unequally and cause changes in the curvature of the element. Commonly used in thermographs.

Biosphere - The totality of life forms on Earth.

Blackbody - A material that is able to absorb 100 percent of the radiation that strikes it.

Blizzard - A violent and extremely cold wind laden with dry snow picked up from the ground.

Bora - In the region of the eastern shore of the Adriatic Sea, a cold, dry northeasterly wind that blows down from the mountains.

Buys Ballot’s Law - With your back to the wind in the Northern Hemisphere, low pressure will be to your left and high pressure to your right. The reverse is true in the Southern Hemisphere.

Calorie - The amount of heat required to raise the temperature of 1 gram of water 1°C. (represented as lower case c, the calorie on food labels represents raising 1000 grams of water 1°C, represented as a capital C)

Ceiling - The height ascribed to the lowest layer of clouds or obscuring phenomena when the sky is reported as broken, overcast, or obscured and the clouds are not classified “thin” or “partail” The ceiling is termed unlimited when the foregoing conditions are not present.

Celsius Scale - A temperature scale (at one time called the centigrade scale) devised by Anders Celsius in 1742 and used where the metric system is in use. For water at sea level, 0° is designated the ice point and 100° the steam point.
Chinook The name applied to a foehn wind in the Rocky Mountains.

Circle of Illumination - The line (great circle) separating day- light from darkness on Earth.

Cirrus - One of three basic cloud forms; also one of the three high cloud types. They are thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers.
Climate A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Climate Change
- A study dealing with variations in climate on many different time scales from decades to millions of years, and the possible causes of such variations.

Climate-Feedback Mechanisms - Because the atmosphere is a complex interactive physical system, several different possible outcomes may result when one of the system’s elements is altered. These various possibilities are called climate-feedback mechanisms.

Climate System - The exchanges of energy and moisture occurring among the atmosphere, hydrosphere, lithosphere, biosphere, and cryosphere.

Cloud - A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.

Cloud Seeding - The introduction into clouds of particles ( most commonly dry ice or silver iodide) for the purpose of altering the cloud’s natural development.

Clouds of Vertical Development - A cloud that has its base iii the low height range but extends upward into the middle or high altitudes.

Cold Front - The discontinuity at the forward edge of an advancing cold air mass that is displacing warmer air in its path.

Cold-Type Occluded Front
- A front that forms when the air behind the cold front is colder than the air underlying the warm front it is overtaking.

Cold Wave - A rapid and marked fall of temperature. The National Weather Service applies this term to a fall of tern- perature in 24 hours equaling or exceeding a specified number of degrees and reaching a specified minimum tern- perature or lower. These specifications vary for different parts of the country and for different periods of the year.

Coffision—Coalescence Process
- A theory of raindrop formation in warm clouds (above 0°C) in which large cloud droplets (“giants”) confide and join together with smaller droplets to form a raindrop. Opposite electrical charges may bind the cloud droplets together.

Condensation - The change of state from a gas to a liquid.

Condensation - Nuclei Microscopic particles that serve as surfaces on which water vapor condenses.

Conditional Instability - The condition of moist air with an environmental lapse rate between the dry and wet adiabatic rates.

Conduction - The transfer of heat through matter by molecular activity. Energy is transferred during collisions among molecules.

Constant-Pressure Surface - A surface along which the atmospheric pressure is everywhere equal at any given moment.

Continental (c) 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.

Contrail - A cloud-like streamer frequently observed behind aircraft flying in clear, cold, and humid air and caused by the addition to the atmosphere of water vapor from engine exhaust gases.

Controls of Temperature -
Those factors that cause variations in temperature from place to place, such as latitude and altitude.

Convection - The transfer of heat by the movement of a mass or substance. It can only take place in fluids.

Convection Cell - Circulation that results from the uneven heating of a fluid; the warmer parts of the fluid expand and rise because of their buoyancy and the cooler parts sink.

Convergence - The condition that exists when the wind distribution within a given region results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of all areas of convergent winds are regions favorable to cloud formation and precipitation.

Cooling Degree-Days - Each degree of temperature of the daily mean above 65°F is counted as one cooling degree-
day. The amount of energy required to maintain a certain temperature in a building is proportional to the cooling
degree-days total.

Coriolis Effect - The deflective effect of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Corona - A bright, whitish disk centered on the Moon or Sun that results from diffraction when the objects are veiled by a thin cloud layer.

Country Breeze - A circulation pattern characterized by a light wind blowing into a city from the surrounding countryside. It is best developed on clear and otherwise calm nights when the urban heat island is most pronounced.

Cryosphere - Collective term for the ice and snow that exist on Earth. One of the spheres of the climate system.

Cumulus - One of three basic cloud forms; also the name given one of the clouds of vertical development. Cumulus are billowy, individual cloud masses that often have flat bases.

Cumulus Stage
- The initial stage in thunderstorm develop- ment in which the growing cumulonimbus is dominated by strong updrafts.

Cup Anemometer - See Anemometer.

Cyclogenesis - The process that creates or develops a new cyclone; also the process that produces an intensification of a preexisting cyclone.

- An area of low atmospheric pressure characterized by rotating and converging winds and ascending air.

Cyclonic Flow - Winds blow in and counterclockwise about a cyclone (low) in the Northern Hemisphere and in and clockwise about a cyclone in the Southern Hemisphere.

Daily Mean Temperature - The mean temperature for a day that is determined by averaging the hourly readings or
more commonly, by averaging the maximum and minimum temperatures for a day.

Daily Temperature Range - The difference between the max- mum and minimum temperatures for a day.

Dart Leader - See Leader.

Deposition - The process whereby water vapor changes direct- Ely to ice without going through the liquid state.

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

Dew - A form of condensation consisting of small water drops on grass or other objects near the ground that forms when the surface temperature drops below the dew point. Usually associated with radiation cooling on clear, calm nights.

Dew Point - The temperature to which air has to be cooled in order to reach saturation.

Diffraction -
The slight bending of light as it passes sharp edges.

Diffused - Light Solar energy is scattered and reflected in the atmosphere and reaches Earth’s surface in the form of diffuse blue light from the sky.

Discontinuity - A zone characterized by a comparatively rapid transition of meteorological elements.

Dispersion - The separation of colors by refraction.

Dissipating Stage - The final stage of a thunderstorm that is dominated by downdrafts and entrainment leading to the evaporation of the cloud structure.

Diurnal - Daily, especially pertaining to actions that are completed within 24 hours and that recur every 24 hours.

Divergence - The condition that exists when the distribution of winds within a given area results in a net horizontal
outflow of air from the region. In divergence at lower 1ev- ells, the resulting deficit is compensated for by a down- ward movement of air from aloft; hence, areas of diver- gent winds are unfavorable to cloud formation and precipitation.

Doldrums - The equatorial belt of cairns or light variable winds lying between the two trade-wind belts.

Doppler Radar - A type of radar that has the capacity of detecting motion direction.

Drizzle - Precipitation from stratus clouds consisting of tiny droplets.

Dry Adiabatic Rate - The rate of adiabatic cooling or warming in unsaturated air. The rate of temperature change is 1°C per 100 meters.

Dry Climate - A climate in which yearly precipitation is not as great as the potential loss ofwater by evaporation.
Dryline A narrow zone in the atmosphere along which there is an abrupt change in moisture as when dry continental
tropical air converges with humid maritime tropical air. The denser cT air acts to lift the less dense roT air producing
clouds and storms.

Dry-Summer Subtropical Climate - A climate located on the west sides of continents between latitudes 30° and 45°. It is the only humid climate with a strong winter precipitation maximum.

Dynamic Seeding -
A type of cloud seeding that uses massive seeding, a process resulting in an increase of the release of latent heat and causing the cloud to grow larger.

Easterly Wave - A large migratory wavelike disturbance in the trade winds that sometimes triggers the formation of a hur ricane.

Eccentricity - The variation of an elipse from a circle.

Electromagnetic Radiation
- See Radiation.

Elements (atmospheric)
- Those quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.

El Niño - The name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. A major El Niño episode can cause extreme weather in many parts of the world.

Entrainment - The infiltration of surrounding air into a vertically nonmoving air column. For example, the influx of cool, dry air into the downdraft of a cumulonimbus cloud; a process that acts to intensify the downdraft.

Environmental Lapse Rate - The rate of temperature decrease with height in the troposphere.

Equatorial Low - A quasi-continuous belt of low pressure lying near the equator and between the subtropical highs.

Equinox - The point in time when the vertical rays of the Sun are striking the equator. In the Northern Hemisphere, March 20 or 21 is the vernal or spring equinox and September 22 or 23 is the aiitunnal equinox. Lengths of daylight and darkness are equal at all latitudes at equinox.

Evaporation - The process by which a liquid is transformed into gas.

Eye - A roughly circular area of relatively light winds and fair weather at the center of a hurricane.

Eye Wall - The doughnut-shaped area ofintensive cumulonimbus development and very strong winds that surrounds the eye of a hurricane.

Fahrenheit Scale - A temperature scale devised by Gabriel Daniel Fahrenheit in 1714 and used in the English system. For water at sea level, 32° is designated the ice point and 212° the steam point.

Fall Wind - See Katabatic Wind.

Fata Morgana - A mirage most frequently observed in coastal areas in which extreme towering occurs.

Fixed Points - Reference points, such as the steam point and the ice point, used in the construction oftemperature scales.

Flash - The total discharge of lightning, which is usually per- ceived as a single flash oflight but which actually consists of several flashes (see Stroke).

Foehn - A warm, dry wind on the ice side of a mountain range that owes its relatively high temperature largely to adiabatic heating during descent down mountain slopes.

Fog - A cloud with its base at or very near Earth’s surface.

Forecasting Skill -
See Skill.

Freezing - The change of state from a liquid to a solid.

Freezing Nuclei - Solid pardcles that have a crystal form resembling that of ice; they serve as cores for the formation of ice crystals.

Freezing Rain - See Glaze.

Front - A boundary (discontinuity) separating air masses of different densities, one warmer and often higher in moisture content than the other.

Frontal Fog - Fog formed when rain evaporates as it falls through a layer of cool air.

Frontal Wedging - The lifting of air resulting when cool air acts as a barrier over which wanner, lighter air will rise.

Frontogenesis - The beginning or creation of a front.

Frontolysis - The destruction and dying of a front.

Frost - Occurs when the temperature falls to 0°C or below (See White Frost)

Fujita Intensity Scale (F-scale) - A scale developed by T. Theodore Fujita for classifring the severity of a tornado, based on the correlation of wind speed with the degree of destruction.

Geostationary Satellite - A satellite that remains over a fixed point because its rate of travel corresponds to Earth’s rate of rotation. Because the satellite must orbit at distances of about 35,000 kilometers, images from this type of satellite are not as detailed as those from polar satellites.

Geostrophic Wind - A wind, usually above a height of 600 meters, that blows parallel to the isobars.

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

Global Circulation - The general circulation of the atmosphere; the average flow of air over the entire globe.

Glory - A series of rings of colored light most commonly appearing around the shadow of an airplane that is projected on clouds below.

Gradient Wind - The curved airflow patern around a pressure center resulting from a balance among pressure-gradient force, Coriolis force, and centrifugal force.

Greenhouse Effect - The transmission of shortwave solar radiation by the atmosphere coupled with the selective absorption oflonger-wavelength terrestrial radiation, especially by water vapor and carbon dioxide, resulting in warming of the atmosphere.

Growing Degree-Days - A practical application of tempera- hire data for determining the approximate date when crops will be ready for harvest.

Gust Front - The boundary separating the cold downdraft from a thunderstorm and the relatively warm, moist surface air. Lifting along this boundary may initiate the development of thunderstorms.

Hadley Cell - The thermally driven circulation system of equatonal and tropical latitudes consisting of two convection cells, one in each hemisphere. The existence of this circulation system was first proposed by George Hadley in 1735 as an explanation for the trade winds.

Hail - Precipitation in the form of hard, round pellets or irregular lumps of ice that may have concentric shells formed by the successive freezing of layers of water.

- A narrow whitish ring of large diameter centered around the Sun. The commonly observed 22° halo subtends an angle of 22° from the observer.

Heat - The kinetic energy of random molecular motion.

Heat Budget - The balance of incoming and outgoing radiation.

Heating Degree -
Day Each degree of temperature of the daily mean below 65°F is counted as one heating degree- day. The amount of heat required to maintain a certain temperature in a building is proportional to the heating degree-days total.

Heterosphere - A zone of the atmosphere beyond about 80 kilometers where the gases are arranged into four roughly spherical shells, each with a distinctive composition.

High Cloud - A cloud that normally has its base above 6000 meters; the base may be lower in winter and at high-latitude locations.

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

Homosphere - A zone of atmosphere extending from Earth’s surface to about 80 kilometers that is uniform in terms of the proportions of its component gases.

Horse Latitude - A belt of calm or light variable winds and subsiding air located near the center of the subtropical high. (30 degrees North Lattitude)

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.

Humidity - A general term referring to water vapor in the air.

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 tropical cyclonic storm having minimum winds of 119 kilometers per hour; also known as typhoon (western Pacific) and cyclone (Indian Ocean).

Hurricane Warning - A warning issued when sustained winds of 119 kilometers per hour or higher are expected within a specified coastal area in 24 hours or less.

Hurricane Watch -
An announcement aimed at specific coastal areas that a hurricane poses a possible threat, generally within 36 hours.

Hydrologic Cycle - The continuous movement of water from the oceans to the atmosphere (by evaporation), from the atmosphere to the land (by condensation and precipitation), and from the land back to the sea (via stream flow).

Hydrosphere -
The water portion of our planet; one of the traditional subdivisions of Earth’s physical environment.

Hydrostatic Equilibrium - The balance maintained between the force of gravity and the vertical pressure gradient that does not allow air to escape to space.

Hygrometer -
An instrument designed to measure relative humidity.

Hygroscopic Nuclei - Condensation nuclei having a high affinity for water, such as salt particles.

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 per- petual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Icelandic Low - A large cell of low pressure centered over Iceland and southern Greenland in the North Atlantic during the winter.

Ice Point - The temperature at which ice melts.

Ideal Gas Law - The pressure exerted by a gas is proportional to its density and absolute temperature.

Inclination of the Axis - The tilt of Earth’s axis from the per- pendicular to the plane of Earth’s orbit (plane of the ediptic). Currently, the inclination is about 23 1/2° away from the perpendicular.

Inferior Mirage - A mirage in which the image appears below the true location of the object.

Infrared Radiation - with a wavelength from 0.7 to 200 micrometers.

Interference - Occurs when light rays of different frequencies (i.e., colors) meet. Such interference results in the cancellation or subtraction of some frequencies, which is responsible for the colors associated with coronas.
Internal Reflection Occurs when light that is traveling through a transparent material, such as water reaches the opposite surface and is reflected back into the material. This is an important factor in the formation of such optical phenomena as rainbows.

Intertropical Convergence Zone (ITCZ) - The zone of general convergence between the Northern and Southern Hemisphere trade winds.

Ionosphere - A complex atmospheric zone of ionized gases extending between 80 and 400 kilometers, thus coinciding with the lower thermosphere and heterosphere.

Isobar - A line drawn on a map connecting points of equal barometric pressure, usually corrected to sea level.

Isohyet - A line connecting places having equal rainfall.

Isotherm - A line connecting points of equal air temperature. ITCZ See Intertropical Convergence Zone.

Jet Stream - Swift geostrophic airstreams in the upper troposphere that meander in relatively narrow belts.

Jungle - An almost impenetrable growth oftangle vines, shrubs, and short trees characterizing areas where the tropical rain forest has been cleared.

Katabatic Wind - The flow of cold, dense air downslope under the influence of gravity; the direction of flow is controlled largely by topography.

Kelvin Scale - A temperature scale (also called the absolute scale) used primarily for scientific purposes and having intervals equivalent to those on the Celsius scale but beginning at absolute zero.

Koppen Classification
- Devised by Wladimir Koppen, a system for classifying climates that is based on mean monthly and annual values of temperature and precipitation.

Lake-Effect Snow - Snow showers associated with a cP air mass to which moisture and heat are added from below as it traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

Land Breeze -
A local wind blowing from the land toward the sea during the night in coastal areas.

La Nifia - An episode of strong trade winds and unusually low sea—surface temperatures in the central and eastern Pacific. The opposite of El Niflo.

Lapse Rate - See Environmental Lapse Rate; Normal Lapse Rate.

Latent Heat - The energy absorbed or released during a change of state.

Law of Conservation of Angular Momentum - The product of the velocity of an object around a center of rotation (axis) and the distance of the object from the axis is constant.

Leader - The conductive path of ionized air that forms near a cloud base prior to a lightning stroke. The initial conductive path is referred to as a step leader because it extends itself earthward in short, nearly invisible bursts. A dart leader, which is continuous and less branched than a step leader, precedes each subsequent stroke along the same path.

Lifting Condensation Level - The height at which rising air that is cooling at the dry adiabatic rate becomes saturated and condensation begins.

Lightning - A sudden flash of light generated by the flow of electrons between oppositely charged parts of a cumulonimbus cloud or between the cloud and the ground.

Liquid-in-Glass Thermometer - A device for measuring temperature that consists of a tube with a liquid-filled bulb at one end. The expansion or contraction of the fluid indicates temperature.

Lithosphere - Rigid outer layer of Earth that includes the crust and uppermost part of the mantle.