GLOSSARY

All of the technical terms employed in this book arc defined or characterized on first occurrence in the text, and in many cases on later occurrences as well. Most of these terms arc also defined here, usually in more detail, and many arc listed in the Index. A few additional terms, not used in the text, arc included here to round out certain interrelated groups, and all technical terms employed within the definitions in the Glossary arc also defined in the Glossary. Time spent browsing and cross-referring in the Glossary should thus help to fix meanings and relationships in mind. The Glossary concludes with a list of some commonly used chemical symbols.

Absolute humidity is one of several measures of atmospheric moisture content. It is the mass of water vapor per unit volume of the air containing the water vapor, typically expressed as grams of water vapor per cubic meter of air. (Compare relative humidity, and see also humidity. )

Absorption is the conversion into heat of a fraction of the radiation that is incident on an object.

Acid deposition is the combination of dry deposition of acidic substances (coal ash, etc.) plus precipitation (rain, snow, etc.) made acidic typically by the addition of sulfur dioxide and nitrogen oxides into the atmosphere as a result of fossil-fuel burning. Automobile exhaust and the emissions from coal-fired powerplants arc two significant causes of acid deposition. In severe cases, acid precipitation kills fish and other aquatic life and damages and/or destroys trees, crops, and other vegetation. In some instances, it has rendered entire lakes and forests nearly lifeless.

Acidic (see pH ).

Acid precipitation is rain (or snow) with a pH less than about 5.6, usually because of the presence of nitric and/or sulfuric acid. Because natural rain is slightly acidic, rain with a pH value somewhat less than the strictly neutral criterion (a pH of 7) is not generally referred to as acid rain.

Adiabatic means occurring without the exchange of heat. In meteorology, an adiabatic exchange usually occurs between an air parcel under consideration and its surroundings.

Advection, in meteorology, is horizontal movement of air, or horizontal transport by air of any property, such as heat or humidity.

Aerosol is a gaseous suspension of fine liquid or solid particles in air.

Air mass is a large volume of air, typically thousands of kilometers in horizontal dimensions, that is relatively uniform horizontally in its properties, especially in temperature and moisture content.

Albedo is the total fraction of light (or, more generally, of electromagnetic radiation flux) striking a surface that is reflected by that surface, often expressed as a percentage. Light-colored, light-reflecting surfaces, such as snow and ice, have a high albedo, while dark, light-absorbing surfaces have a low albedo.

Alkaline (see pH ).

Antarctic ozone hole (see Ozone hole ).

Anthropogenic means caused or created by human beings. For example, an-thropogenic carbon-dioxide emissions arc those caused by such human activities as the burning of fossil fuels.

Anticyclone is a weather system characterized by relatively high atmospheric pressure compared with its surroundings, together with winds blowing clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (Compare cyclone .)

Atmosphere is the envelope of gases that surrounds a planet. Earth's atmosphere is one of the five basic interrelated components of the Earth system. (See the other four components: biosphere, cryosphere, hydrosphere, and pedosphere .)

Atmospheric pressure is the force exerted by the air on each unit of area of a surface, essentially equivalent to the weight of the overlying atmosphere. High atmospheric pressure generally leads to stable weather conditions, whereas low pressure can result in storms.

Atom is the smallest unit of a chemical element that can take part in a chemical reaction. An atom is composed of a nucleus, which contains protons and neutrons, surrounded by electrons.

Barometer is an instrument used to measure atmospheric pressure.

Biodiversity is a general term that describes several aspects of the variety inherent in the complex web of living things, and it has different specific meanings. Species diversity , the most common use of the term, is the total number of biological species occurring in a particular area. Habitat diversity is the variety, of places, or types of places, where life exists. Genetic diversity is primarily the variety of populations comprising each species, where population is seen as a group of members of a species that live together and so can mate with one another.

Biomass is the total dry weight of living material in a particular area.

Biome is a distinctive ecological system, characterized primarily by the nature and composition of its vegetation.

Biosphere, one of the five basic interrelated components of the Earth system, is the region on land, in the oceans, and in the atmosphere inhabited by living things. (See atmosphere. )

Biota is all living things, collectively, including animal and plant life.

Black body is an idealized or theoretical object that absorbs all of the radiation incident upon it and emits the maximum possible radiation for its temperature at every wavelength.

Blizzard is snow falling with winds faster than 35 miles per hour and visibility of a quarter-mile or less over an extended time period.

Business-as-usual, in the context of global-climate change, is a scenario for future world patterns of energy use and greenhouse-gas emission that assumes there will be no significant change in people's and governments' attitudes and priorities.

Carbon cycle is the exchange of carbon between land, atmosphere, and oceans. About one-fourth of the total quantity of atmospheric carbon (in the form of carbon dioxide) is cycled in and out of the atmosphere each year; approximately half of this amount is exchanged with the land biota, and the other half, through physical and chemical processes, passes across the ocean surface.

Carbon dioxide (CO₂ ) is a colorless, odorless gas that is a trace constituent of the Earth's atmosphere and one of the major greenhouse gases. Anthropogenic CO₂ results mainly from burning fossil fuels (coal, oil, natural gas) and from deforestation.

Celsius is a temperature scale, also called the Centigrade scale. Its fixed points are the freezing point of water (O°C) and the boiling point of water (100°C). To convert from Celsius to Fahrenheit, multiply the Celsius temperature by 1.8 and add 32°.

Chlorofluorocarbons (CFCs) arc synthetic compounds invented by Thomas Midgley, Jr., in 1928 as refrigerants. CFCs destroy stratospheric ozone and are also greenhouse gases. The primary use of CFCs today is as a coolant in refrigerators and air conditioners. CFCs are also used as solvents, foam-blowing agents, and aerosol propellants, though the use of CFCs in aerosol cans in the United States was outlawed in the late 1970s. Substitutes for CFCs are under development, and some are already available. The Montreal Protocol for the Protection of the Ozone Layer is an international agreement that requires that parties to the agreement in developed nations phase out the production of CFCs by 1996.

Climate is, collectively, the temperature, humidity, precipitation, winds, radiation, and other meteorological conditions characteristic of a locality or region over an extended period of time. Compared to weather, climate involves longer times and deals not only with the atmosphere but also with oceans, land, and biosphere. The essential characteristic of climate is that it is a statistical concept embracing the sum total of weather, thus including not only average conditions

but also probabilities of extreme events and other descriptions of thc variability of meteorological conditions.

Climate sensitivity is thc magnitude of thc climate change expected to result from a particular change in external influences. One description of climate sensitivity is the rise in global average temperature expected to result from a doubling of carbon dioxide in thc atmosphere.

Climatology is the scientific study of climate, particularly of its variability and its dependence on factors that influence its behavior.

Cloud is a visible aggregation of condensed water-vapor (liquid or ice) particles.

Cold front is a zone of transition marking thc boundary between advancing, relatively cold and dense air and retreating, relatively  warm and less dense air. (Compare warm front. )

Compound is any substance, for example carbon dioxide (CO₂ ), formed from two or more elements chemically combined in fixed proportions. (Compare element .)

Condensation is thc process of changing state from gas (or vapor) to liquid, as in the formation of water droplets in clouds.

Condensation nuclei arc small particles in thc air that attract water and en-courage condensation.

Convection is, generally, vertical motion in the atmosphere or ocean generated by temperature differences and resulting in the transfer of heat.

Coriolis effect is an apparent deflective force due to the Earth's rotation, affecting relatively large-scale, slow motions of thc atmosphere and ocean, causing a deflection to the right in thc Northern Hemisphere and to thc left in thc Southern Hemisphere.

Cryosphere, one of thc five basic interrelated components of thc Earth system, is that portion of thc Earth's surface with average temperatures below thc freezing point of water. The bulk of the cryosphere is at or near thc poles, but cryospheric regions also exist atop high mountain ranges on all continents. The cryosphere is composed of snow, permanently frozen ground (permafrost), floating ice, and glaciers. (See atmosphere. )

Cumulus clouds arc clouds exhibiting significant vertical development, often due to convective updrafts.

Cyclone is a weather system characterized by relatively low atmospheric pressure compared with its surroundings, together with winds blowing counterclockwise in thc Northern Hemisphere and clockwise in thc Southern Hemisphere. (Compare anticyclone. )

Deforestation is destruction of forests, usually by cutting or burning. Deforestation enhances thc greenhouse effect in two ways. First, when wood is burned or decomposes, it releases carbon dioxide. Second, trees that arc destroyed can no longer remove carbon dioxide from thc atmosphere in thc process of photosynthesis.

Demography is thc study of thc nature and structure of human populations, including their distribution, age structure, composition, lifestyles, and change.

Desertification is degradation of land characterized by reduced soil moisture and reduced vegetation, including crops, and by soil erosion. Like deforestation, desertification can affect climate in several ways, such as by altering the water cycle.

Developing countries, sometimes called less-developed countries (LDCs) or "Third World," are those characterized in general by low personal income, little industrialization, high rates of illiteracy, and poor public health. Most developing countries are in the Northern Hemisphere.

Dew point is a measure of humidity, given in terms of the air temperature at which dew begins to form, as water vapor condenses into liquid.

Dobson unit (DU) is a measure of ozone abundance. Ozone varies naturally, especially with latitude and season, ranging typically from about 250 to 460 Dobson units. (Gordon Dobson was a researcher at Oxford University, who, in the late 1920s, built the first instrument, now called the Dobson meter, for measuring total ozone from the ground.)

Drought is an extended period of abnormal dryness for a particular region.

Drylands are areas of the world where precipitation is low and where rainfall typically consists of erratic, short, local, high-intensity storms.

Earth system (see atmosphere ).

Ecology is the science that deals with the interrelationships between living organisms and their environments.

Ecosystem is a distinct system of interdependent plants and animals, together with their physical environment. An ecosystem may be as large as the entire Earth, or as small as a pond.

Electromagnetic radiation is energy transfer by waves having both electrical and magnetic properties.

Electromagnetic spectrum is the range of types of radiation, ordered by wavelength or frequency, and includes all forms of electric, magnetic, and visible radiation.

Electron is a negatively charged component of an atom.

Element is any substance, for example iron or oxygen, that cannot be separated by chemical means into two or more simpler substances. (Compare compound.)

El Niño is a warming of the surface waters of the eastern tropical Pacific, occurring at irregular intervals of about 2 to 7 years, usually lasting from several months up to about 2 years, that has a significant influence on regional and global climate. El Niño has been linked to colder, wetter winters in parts of the United States, drier, hotter summers in South America and Europe, and drought in Africa, as well as reduced numbers of fish in South American Pacific coastal waters due to reduced upwelling of nutrient-rich waters. (See also El Niño-Southern Oscillation. )

El Niño-Southern Oscillation (ENSO) is the closely linked phenomena of El Niño (see above) and a global-scale shift in atmospheric pressure called the Southern Oscillation (SO). In the so-called warm phase of ENSO, El Niño warming extends over much of the tropical Pacific and becomes clearly linked

to the SO pattern. Many of the countries most affected by ENSO events arc developing countries with economics that are largely dependent on their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities for predicting the onset of ENSO events can thus have important human impacts. ENSO is fundamentally an aspect of the combined ocean-atmosphere system and cannot be understood as either entirely meteorological or entirely oceanographic in nature. Although ENSO is a natural part of Earth's climate variability, whether its intensity or frequency may change as a result of global warming is a concern. Recent progress toward predicting the occurrence and characteristics of ENSO events shows great promise.

Endemic means occurring naturally only in a certain region, as a species that is endemic to a particular place.

Environment is the complex of physical, chemical, and biological factors in which a living organism or community exists.

Environmentalrefugees arc people obliged to leave their traditional or established homelands because of environmental problems (deforestation, desertification, floods, drought, sea-level rise nuclear-plant accidents), on a permanent or semipermanent basis, with little or no hope of ever returning. Though no formal accounting has been taken, there may currently be about 25 million environmental refugees in the world, according to one estimate, and it is likely that the number will rise.

Equator is an imaginary circle around the Earth that is equally distant from the North and South poles and defines the latitude 0°.

Evaporation is the process of changing from a liquid state to a gaseous state, or vapor.

Evapotranspiration is the discharge of water from Earth's surface to the atmosphere by evaporation from bodies of water, or other surfaces, and by transpiration from plants.

Exotic means originating outside of an area, as for example an exotic species.

Fahrenheit is a temperature scale based on the freezing point of water (32°F) and the boiling point of water (212°F) under standard atmospheric pressure. To convert from Fahrenheit to Centigrade, subtract 32° from the Fahrenheit temperature and divide the resulting quantity by 1.8.

Feedback is a sequence of interactions in which the final interaction (or some set of interactions) influences the original one. In such a sequence, a cause produces a result, and the result then in turn influences its cause. As a system changes, it may generate processes that affect the original change. If one of these processes amplifies the change (global warming, for example, may cause a change in cloudiness, which then adds to the warming), it is called a positive feedback. If it dampens the change, it is called a negative feedback.

Fog is a cloud in contact with, or close to, the surface of the Earth, reducing visibility to less than one mile, or about 1.6 kilometers.

Food chain is a series of plants and/or animals that depend, one on another, as food sources (that is, a plant is catch by a small fish, which is eaten by a larger

fish, which is eaten by a bird, and so on, the series thus represented constituting the chain).

Fossil is the hardened remains or traces of particular plant or animal life from a previous geological period, preserved in the Earth's crust.

Fossil fuels, basically coal, oil, and natural gas, are fuels created by the decomposition of ancient animal and plant remains. They are finite (limited) resources, and they release carbon dioxide and other gases when burned.

Front is a narrow zone marking the boundary between two air masses of significantly different meteorological properties, usually the properties of temperature, humidity, wind speed, and wind direction.

General Circulation Models (GCMs) are computer models of Earth's climate that are used to improve our understanding of factors that influence climate and enhance our ability to forecast future climate patterns. One reason GCMs arc so useful is that they allow researchers to vary individual factors and observe the results, isolating processes in a way that is not possible in the physical world. GCMs are also sometimes called "Global Climate Models," a more accurate and more descriptive characterization. "General circulation" is an older technical term in meteorology that originally referred to the long-term average aspects of large-scale atmospheric behavior. In modern usage, a GCM can be an atmospheric model or an oceanic model or a model of the coupled climate system involving the atmosphere and ocean and other components as well.

Geoengineering is the intentional artificial modification of Earth systems to counteract anthropogenic effects such as global warming or stratospheric ozone depletion. An example of geoengineering aimed at reducing global warming is the "iron hypothesis," which suggests that adding iron to the oceans could stimulate the growth of small plants that would photosynthesize at greater levels, thus removing carbon dioxide from the atmosphere. In general, geoengineering proposals would be costly and logistically difficult, and before any were to be undertaken, it would be prudent to be certain that unanticipated adverse consequences would not occur.

Geosphere is, collectively, the physical elements of the Earth's surface, crust, mantle, and interior. (Compare pedosphere.)

Geostationary, or geosynchronous, describes an orbit in which a satellite is always in the same position with respect to the Earth. The satellite travels around the Earth, in the same direction and at the same speed as the Earth's rotation, completing one orbit in a 24-hour period, thus remaining above the same point on the Earth's surface. All geostationary satellites arc directly above Earth's Equator and are at the same altitude, namely about 36,000 kilometers, or 22,300 miles, above the Earth's surface.

Geothermal energy is energy obtained by the transfer of heat to Earth's surface from its depths. A natural hot spring is one such example, and significant amounts of electricity are generated by power plants that pump superheated fluids from the depths.

Glacier is a multi-year accumulation of snowfall in excess of snowmelt on land, resulting over time in a mass of ice covering at least a tenth of a square kilometer,

that shows some evidence of movement in response to gravity. Glacial ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers arc found on every continent except Australia.

Global change is change in the Earth system that is either a global phenomenon or occurs regionally but strongly enough and often enough to be of global significance. The leading current global-change issues include climate change due to an enhanced greenhonse effect, stratospheric ozone depletion, acid precipitation, urban air pollution, and loss of biodiversity.

Global warming is a predicted warming of Earth's climate due to increased concentrations of greenhouse gases in the atmosphere. The Intergovernmental Panel on Climate Change (IPCC), composed of many of the world's leading authorities on the subject, estimates that if atmospheric carbon dioxide were to double, global average temperature would eventually increase by l.5° to 4.5°C (about 3° to 8°F), with a "best guess" of 2.5°C (about 4°F). The IPCC also estimates that about half the magnitude of this warming will have occurred by the year 2030.

Greenhouse effect is the natural process whereby gases in Earth's atmosphere act like the glass in greenhouse, letting the Sun's energy in, but preventing some of the Earth's radiation from escaping to space. Were it not for this natural effect, Earth's climate would be about 33°C (60°F) colder, and life as we know it would not exist. The "enhanced greenhouse effect" refers to an increase in this natural heat-trapping phenomenon caused by anthropogenic emissions of greenhouse gases.

Greenhouse gases arc water vapor, carbon dioxide, methane, tropospheric ozone, nitrous oxide, CFCs, and other gases that absorb some of the long-wave thermal radiation emitted from Earth's surface, thereby contributing to the greenhouse effect and warming the atmosphere. With the exception of water vapor, these arc also called "trace gases," since they total less than 1% of the atmosphere.

Green revolution is a dramatic increase in global or regional food production, primarily as a result of the development of new strains of crops.

Ground truth is information collected at the Earth's surface at the same place and time as a remote sensor gathers analogous data. Ground-truth information is used to interpret and calibrate remotely sensed data from satellites.

Gulf Stream is the warm, swift ocean current that flows from the Gulf of Mexico, along the coast of the Eastern United States, across the Atlantic to the European coast, and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.

GWP, which stands for Global Warming Potential, is the ratio of how much a gas contributes, molecule for molecule, to enhancing the greenhouse effect, compared to the contribution of carbon dioxide.

Habitat is the environment in which an individual or population of humans, plants, or animals occurs.

Hadley cell, a feature of the tropical general circulation of the atmosphere, is characterized by rising air near the equator, poleward flow aloft, sinking air in the subtropics, and return flow toward the Equator near the surface.

Hail is precipitation composed of lumps of ice. Hail is produced when large frozen raindrops, or other particles in cumulonimbus clouds, grow by accumulating supercooled liquid droplets. Violent updrafts in the clouds carry the particles up through the freezing air, allowing the frozen core to accumulate more ice. When the piece of hail becomes too heavy to be carried by rising air currents, it falls to the ground.

Halocarbons are halons, chlorofluorocarbons, hydrochlorofluorocarbons, and other chemicals that deplete the stratospheric ozone layer. The term "halocarbons" is used in the Montreal Protocol on Substances that Deplete the Ozone Layer.

Halons are a class of ozone-depleting compounds containing bromine.

HCFCs. (See hydrochlorofluorocarbons. )

Horse latitudes, the regions near latitude 30° in both hemispheres, are associated with high pressures and weak winds, because of subtropical anticyclones. The term originates from the days of sailing ships, from which, according to legend, horses were either thrown overboard or eaten as food ran out when the ships were becalmed.

Humidity is the amount of water vapor in the air. The higher the temperature of the air, the greater the number of water molecules it can bold. (See absolute humidity, relative humidity. )

Hurricane is an intense warm-core tropical storm with winds exceeding 74 miles per hour. Hurricanes originate over the tropical and subtropical North Atlantic, Indian, and Pacific oceans (in the latter they are known as typhoons ), because high sea-surface temperatures are essential to their formation.

Hydrocarbons are compounds containing only carbon and hydrogen.

Hydrochlorofluorocarbons (HCFCs) are replacements for chlorofluorocarbons (CFCs) that are less ozone-depleting than CFCs but not totally nondepleting. HCFCs are also greenhouse gases, and thus contribute to global warming.

Hydrologic cycle is the natural sequence through which water evaporates from the ocean, land surface, and plants into the atmosphere as water vapor, falls to Earth as precipitation, and largely returns to the ocean through pathways including rivers and ground water.

Hydrosphere, one of the five basic interrelated components of the Earth system, consists of all of Earth's waters, including the oceans, fresh waters, and water vapor in the atmosphere. (See atmosphere. )

Hydroxyl is a chemical group consisting of one atom of hydrogen and one atom of oxygen.

Ice Age is a geological time period during which sheets of ice cover extensive parts of the Earth.

Image resolution is the level of detail in an image, and is determined by the area represented by each pixel (picture clement). The smaller the area represented by a pixel, the more detailed the image and the higher the image resolution. For example, if a U.S. map and a world map arc printed on the same-sized paper, one square inch on the U.S. map will represent far less area and provide far more detail, relatively, than one square inch of the world map. The U.S. map would thus be said to have higher resolution.

Indigenous means occurring naturally in an area, as for example an indigenous species of animal or plant.

Infrared radiation is electromagnetic radiation at wavelengths longer than red visible light, but shorter than microwaves. Most radiation emitted by the Earth is infrared, and it is this radiation that is involved in the greenhouse effect.

In situ (Latin for "in original place"), usually refers to data collected at the actual location of the object or material measured, as opposed to remote sensing.

Insolation is the striking, by solar radiation, of a particular horizontal surface on or above Earth's surface.

Ion is an atom or molecule that has acquired an electric charge by the loss or gain of one or more electrons.

Ionosphere is an electrified region of the upper atmosphere.

Isothermal means of or indicating equality of temperature. In meteorology, isotherms arc lines on a weather map connecting points of equal temperature.

Kilometer is a metric unit of distance approximately equal to 3,280.8 feet, or 0.621 miles.

Lightning is a discharge of atmospheric electricity accompanied by a vivid flash of light. During thunderstorms, static electricity builds up in clouds. A positive charge builds in the upper part of the cloud, while a negative charge builds in the lower portion. When the difference between the charges becomes great, the charge jumps from one area to another, creating a lightning bolt. Most lightning bolts strike from one cloud to another, but they can also strike the ground. Such bolts can occur when positive charges build up on the ground. A negative charge or "leader" flows from the cloud toward the ground and then a positively charged stroke (called the return stroke) runs from the ground to the cloud. What appears as a lightning bolt is actually a series of downward and upward strokes, all taking place in less than a second. (Sec thunder. )

Mean is the scientific term for arithmetic average, as in "global mean temperature."

Mesosphere is an atmospheric layer above the stratosphere, at an average elevation between 50 and 80 kilometers above the Earth's surface.

Meteorology is the science of the atmosphere, in all its ramifications.

Microwave is a comparatively short electromagnetic wave, typically with a wavelength between about 1 millimeter and 1 meter.

Modeling is an investigative technique that uses a mathematical and/or physical representation of a system or theory to test for the effects that changes in

system components may have on the overall functioning of the system. Mathematical modeling using computers plays a major role in climate research, by simulating how Earth's climate will respond to changes in the atmospheric concentrations of greenhouse gases.

Molecule is two or more atoms of one or more elements chemically combined in fixed proportions. For example, atoms of the elements carbon and oxygen, chemically bonded in a 1:2 proportion, create molecules of the compound known as carbon dioxide (CO₂ ). Molecules can also be formed of a single element, as in ozone (O₃ ).

Monsoon is a particular seasonal weather pattern in subtropical regions, especially when characterized by periods of heavy winds and rainfall. Monsoons are caused by a pronounced seasonal change in wind direction. Winds usually blow from land to sea in winter, whereas, in the summer, this reverses, bringing precipitation. Monsoons are most typical in India and elsewhere in southern Asia. They also occur in Africa and Australia.

Montreal Protocol on Substances that Deplete the Ozone Layer is an international agreement that prescribes a timetable for ending the production of chlorofluorocarbons (CFCs) and related compounds. Begun in 1987, this unprecedented international treaty is a unique example of scientists and industry working with governments to seek a global solution to the human-caused environmental challenge of ozone depletion. After the original agreement was signed, new evidence arose proving that deeper and quicker cuts in CFC production were necessary to protect the ozone layer. The 1990 London amendments and the 1992 Copenhagen amendments sped up the halocarbon phaseout and controlled several other chemicals that were not in the original agreement: methyl chloroform, carbon tetrachloride, methyl bromide, and hydrochlorofluorocarbons. The revised agreement now calls for the phaseout of CFCs to be complete by 1996. The treaty also attempts to make the phaseouts fair to developing countries by setting up a fund, paid for by developed nations, to assist developing countries in making the switch to ozone-safe chemicals.

Negative feedback (see feedback ).

Neutron is a component of most atomic nuclei that is without electric charge, and is of approximately the same mass as the proton (the positively charged component).

NO_x is the common pollutant gases nitric oxide (NO) and nitrogen dioxide (NO₂ ), considered collectively.

Nuclear winter is the combination of climatological and related phenomena that are the hypothetical environmental consequences of nuclear war, consisting especially of a sudden and severe cooling due to a reduction in the amount of sunlight reaching Earth's surface, because of heavy smoke concentrations in the atmosphere arising from fires caused by nuclear explosions.

Orbit is the path of a body, such as a planet or satellite, in its periodic revolution around another body in space. For example, artificial satellites that orbit Earth near latitude 0º are said to have equatorial orbits, since they remain above the Equator. Satellites with inclinations near 90º are said to be in polar orbits,

because they cross over or near Earth's North and South poles as they revolve around the planet.

Ozone is a gaseous molecule consisting of three atoms of oxygen (O₃ ). Ozone in Earth's stratosphere forms a protective layer that shields Earth's inhabitants from damaging ultraviolet radiation from the Sun. Ozone occurring in the troposphere, near Earth's surface, on the other hand, though identical, is a harmful pollutant resulting from the interaction of anthropogenic emissions of nitrogen oxides, volatile organic compounds, and sunlight.

Ozone depletion is the thinning of the stratospheric ozone layer that protects life on Earth from excess ultraviolet radiation from the Sun. Anthropogenic halocarbons are primarily responsible for this reduction in the amount of ozone in the stratosphere.

Ozone hole is a region of the atmosphere over Antarctica where, during the Southern Hemisphere springtime, a substantial fraction of the stratospheric ozone disappears, owing primarily to catalytic destruction by halocarbons of anthropogenic origin.

Paleoclimatology is the reconstruction of ancient climates by using evidence such as tree rings and air trapped in ice cores. Researchers use such evidence to understand natural climatic shifts, which can help us in understanding and eventually predicting future climate trends.

Pedosphere, one of the five basic interrelated components of the Earth system, is the solid portion of the Earth's surface. The pedosphere rides on continental structures that evolve over millions of years as a consequence of the tectonic motions of Earth's land masses. (See atmosphere; compare geosphere. )

pH is a measure of the acidity or alkalinity of a solution. A value of 7 is neutral, values less than 7 are acid, and values over 7 are alkaline or basic. A change of one unit on the pH scale represents a factor of ten in acidity; for example, a solution with a pH of five is ten times as acid as one with a pH of six.

Phenology is the science dealing with the relationships between climate and periodic biological phenomena that arc related to or caused by climatic conditions, such as the seasonal budding of trees and the migration of birds.

Photochemical smog, present in many large cities, is formed by chemical reactions involving nitrogen oxides and hydrocarbons (from human activities, including automobile use) taking place in the presence of sunlight, typically in still, stagnant air. An important component of photochemical smog is tropospheric or ground-level ozone. The term "smog" is misapplied ill this sense; its original meaning was a combination of smoke and fog.

Photosynthesis is the series of chemical reactions by which plants use the Sun's energy, carbon dioxide, and water vapor to form materials for growth, and in the process release oxygen.

Photovoltaic (PV) means capable of converting solar radiation directly into electricity. A photovoltaic or solar cell, often made of silicon, is a device for that purpose.

Phytoplankton are minute forms of plant life in the oceans at the base of the marine food chain.

Pixel is the smallest element of an electronically coded image. "Pixel" is a contraction of the words "picture element." (See image resolution. )

Plate tectonics is the concept that Earth's crust is composed of rigid plates that move over a less rigid interior. The movements of these plates cause geological events such as earthquakes and continental drift and, over long periods of time, can cause significant shifts in the relative positions of Earth's land masses.

Positive feedback (see feedback ).

ppbv stands for parts per billion by volume, a measure of abundance.

ppmv stands for parts per million by volume, a measure of abundance.

Precipitation, in meteorology, is liquid or solid forms of moisture that fall from clouds, including rain, snow, hail, and sleet. Raindrops typically form around condensation nuclei, which are often particles of salt or dust. Under appropriate conditions, water or ice droplets forming on these particles can attract more water and continue to grow until they are large enough to fall out of the cloud.

PV (see photovoltaic ).

Radiation budget is an accounting of the radiation that enters and leaves a planet's atmosphere. On global average, the quantity of solar radiation entering the atmosphere from space should be balanced by the thermal radiation leaving the Earth's surface and atmosphere.

Radioactive decay is the spontaneous breakdown of certain atomic nuclei (for example uranium) into one or more nuclei of different elements (for example radium from uranium).

Rainforest is a basically evergreen woodland of the tropics distinguished by a continuous leaf canopy and an average rainfall of about 100 inches (250 centimeters) per year. Rainforests play an important role in the global environment for several reasons. Though encompassing just 6-7% of Earth's land, they are the most biologically diverse biome on the planet, thought to house nearly half of its species. Rainforests also take up carbon dioxide, helping to balance anthropogenic emissions. When rainforests or other forests are cut or burned, the opposite occurs: they release stored carbon dioxide, adding to the greenhouse effect.

Rain gauge is a calibrated container that measures the amount of rainfall occurring during a specific period of time.

Relative humidity is the amount of water actually in the air compared with how much the air can hold at its current temperature. For example, 50% relative humidity means the air holds half the water vapor it is capable of holding. (Compare absolute humidity and see also humidity.)

Remote sensing is the process of obtaining information from a distance, especially from aircraft or satellites. Modern remote-sensing technology has greatly expanded our ability to see and understand the Earth and its systems and to observe changes. It has become a critical tool in activities ranging from the verification of arms-control treaties to the provision of emergency aid to disaster-stricken regions. Through remote sensing we learn about problems such as droughts, famines, and floods; we obtain information about agricultural practices, weather conditions, transportation systems, river flows, and terrain

changes. We use remote sensing to locate Earth's natural resources, and can then use that information to exploit or protect them.

Renewable energy is energy from sources that are not depleted by use. Examples include using passive solar energy to heat buildings, solar thermal energy to heat water or turn turbines to produce electricity, photovoltaic cells to convert sunlight directly to electricity, wind power, and hydroelectric energy.

Sequestration is the natural removal of a substance from one regime and its storage in another (the sink), as when carbon dioxide is sequestered from the atmosphere by plants via photosynthesis.

Sink is the place of storage of substances removed naturally from another regime (see sequestration ). Plants, for example, through photosynthesis, transform carbon dioxide from the air into organic matter which is then "stored" in the plant or in the soil. Plants arc thus said to be "sinks" for carbon. One of the key uncertainties regarding climate is that the quantity of carbon held in the various sinks and the rates of exchange between them are not well known.

Smog (see photochemical smog ).

Solar constant is the average total radiation reaching the top of Earth's atmosphere from the Sun. The numerical value of this constant, about 1,370 watts per square meter, is not, in fact, truly constant; variations of about a tenth of a percent have been measured during the last two decades.

Solar radiation is energy emitted from the Sun. It is the main energy source for Earth's climate system, heating the surface of the Earth and driving currents in the oceans and winds in the atmosphere. Ordinary visible sunlight is the most obvious form of solar radiation, but other forms are significant, too; for example, see ultraviolet radiation.

Sonde is a device sent up into the atmosphere, typically borne on a balloon, to obtain information about atmospheric conditions above the Earth's surface. Radiosondes, for example, measure temperature, pressure, and humidity and then transmit these data to Earth by radio.

Southern Oscillation (see El Niño-Southern Oscillation ).

Spectral band is a segment of wavelengths within the electromagnetic spectrum.

Stratosphere is the region of the atmosphere between the troposphere and mesosphere, having a lower boundary approximately 8 kilometers above sea level at the poles and 15 km at the Equator, and an upper boundary at an altitude of approximately 50 km above sea level. This is the region that contains the ozone layer that protects Earth's surface from excess ultraviolet solar radiation.

Sustainable development is development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs. Some people believe that the concept of sustainable development should include preserving the environment for other species as well as for people.

Synoptic means simultaneous. For example, a synoptic weather map displays meteorological conditions observed in different places at a single time. "Syn-

optic" also refers to a large or general view of something. For example, an aerial photograph provides a "synoptic," or "bird's-eye" view of an area.

Tectonics (see plate tectonics ).

Terrestrial means pertaining to the Earth, as distinct from other planets (as in extraterrestrial life). It also means pertaining to the land, as distinct from the water or air (as in a terrestrial, as opposed to aquatic, ecosystem).

Thunder is the sound that results from lightning. A lightning bolt produces an intense burst of heat that makes the air around it expand explosively, producing the sound we hear as thunder. Since light travels faster than sound, we see the lightning before we hear the thunder. The difference in time between the two can tell us how far away from us the clouds producing the lightning and thunder are. (See lightning.)

Thunderstorm is a local storm resulting from rising, warm, humid air, which produces lightning and therefore thunder, usually accompanied by rain or hail, gusty winds, and strong updrafts.

TOMS (see Total Ozone Mapping Spectrometer ).

Tornado is a strong, rotating column of air extending from the base of a cumulonimbus cloud to the ground. These twisting, spinning, often violent funnels of low-pressure air, created during powerful thunderstorms, are the most unpredictable weather event.

Total Ozone Mapping Spectrometer (TOMS) is a remote sensing instrument, first flown on the Nimbus-7 satellite, whose primary function is to monitor global ozone. TOMS is a major contribution from NASA to the monitoring of the Earth. The original TOMS instrument began delivering ozone data in November 1978, providing high-resolution mapping of total ozone, from the ground to the top of the atmosphere, on a daily basis. TOMS provided the first maps of the ozone hole. The TOMS instrument on Nimbus 7 failed in May 1993, after a remarkably long and productive lifetime. Subsequent TOMS and other instruments continue to monitor ozone depletion and variability.

Trace gases (see greenhouse gases ).

Trade winds are global-scale winds in the tropics that blow generally toward the west in both hemispheres (from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere). These relatively steady winds came to be called "trade winds" because they provided trading ships with a sailing route to the "New World," America.

Transpiration is the transfer of water from plants to the atmosphere; water is taken up by the roots of plants and released as water vapor by the leaves.

Tropical cyclone is a low-pressure weather system in the tropics in which the central core is warmer than the surrounding atmosphere. A tropical storm is designated a cyclone when its winds are from 39 to 74 miles per hour; when winds exceed 74 miles per hour, the cyclone is called a hurricane or typhoon.

Tropics are the region of Earth from latitude 23.5º north (the Tropic of Cancer) southward across the Equator to latitude 23.5º south (the Tropic of Capricorn). This region has relatively small daily and seasonal changes in temperature, but great seasonal changes in precipitation.

Tropopause is the boundary or transition zone separating the troposphere, the lowest major layer of the Earth's atmosphere, from the stratosphere, which lies above it. Temperature generally decreases from the Earth's surface up to the tropopause, and then is approximately constant or increases with height above the tropopause. The altitude of the tropopause is variable, typically ranging from somewhat less than 10 up to 15 or more kilometers above sea level. The tropopause height is usually higher over the tropics than in polar regions, and higher in summer than in winter.

Troposphere is the region of the atmosphere that extends from the Earth's surface to about 7 miles (12 kilometers) above sea level, on average. (See stratosphere. )

Troposphere ozone is ozone (O₃ ) in the troposphere. As opposed to stratospheric ozone (the "good ozone" that protects us from excess ultraviolet radiation from the Sun), troposphere ozone, or "bad ozone," results (from the interaction of nitrogen oxides (NO_x ), volatile organic compounds, and sunlight. Most of the pollutants that lead to the formation of troposphere ozone come from automobiles, powerplants, and other human activities. In many cities, ozone is a significant health problem. Ozone also causes 3 to 5 billion dollars a year in lost crop production and substantial losses in forest products. Troposphere ozone is also a significant greenhouse gas.

Typhoon is a tropical cyclone with winds 75 miles per hour or greater, occurring in the northwest Pacific Ocean. In other parts of the world, such storms have different names, such as hurricane.

Ultraviolet radiation (UV) is the energy range just beyond the violet end of the visible spectrum. Most UV is blocked by Earth's atmosphere (particularly the stratospheric ozone layer), but some solar UV penetrates and aids in plant photosynthesis and the production of vitamin D in humans. Too much UV can burn the skin, cause skin cancer and cataracts, and damage vegetation.

Volatile organic compounds (VOCs) are precursors of troposphere ozone and photochemical smog. They are produced by human activities, including the use of dry-cleaning solvents.

Volcano is a naturally occurring vent or fissure at Earth's surface through which erupt molten, solid, and/or gaseous materials. Major volcanic eruptions inject large quantities of dust, gas, and aerosols into the atmosphere and can thus cause temporary climatic cooling.

Warm front is a zone of transition marking the boundary between advancing, relatively warm and light air and retreating, relatively cold and dense air. (Compare cold front.)

Water vapor is the invisible, gaseous form of water.

Weather is the state of the atmosphere at some place and time, particularly as characterized by variables such as temperature, cloudiness, wind, humidity, and precipitation.

Wind is a natural motion of the air, especially a noticeable current of air moving in the atmosphere parallel to Earth's surface.

Some often-used chemical symbols