Glossary

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Chapter 11
absolute humidity Amount of water vapor found in a certain mass of air (usually expressed as grams of water per kilogram of air). Compare relative humidity.
acid deposition The falling of acids and acid-forming compounds from the atmosphere to the earth's surface. Acid deposition is commonly known as acid rain, a term that refers only to wet deposition of droplets of acids and acid-forming compounds.
acid rain See acid deposition.
air pollution One or more chemicals in high enough concentrations in the air to (1) harm humans, other animals, vegetation, or materials or (2) alter climate. Excess heat and noise are also considered forms of air pollution. Such chemicals or physical conditions are called air pollutants. See primary pollutant, secondary pollutant.
albedo Ability of a surface to reflect light.
atmosphere Whole mass of air surrounding the earth. See stratosphere, troposphere.
buffer Substance that can react with hydrogen ions in a solution and thus hold the acidity or pH of a solution fairly constant. See pH.
CFCs See chlorofluorocarbons.
chlorofluorocarbons (CFCs) Organic compounds made up of atoms of carbon, chlorine, and fluorine. An example is Freon-12 (CCl2F2), used as a refrigerant in refrigerators and air conditioners and in making plastics such as Styrofoam. Gaseous CFCs can deplete the ozone layer when they slowly rise into the stratosphere and their chlorine atoms react with ozone molecules.
climate Physical properties of the troposphere of an area based on analysis of its weather records over a long period (at least 30 years). The two main factors determining an area's climate are temperature, with its seasonal variations, and the amount and distribution of precipitation. Compare weather.
cold front Leading edge of an advancing mass of cold air. Compare warm front.
condensation nuclei Tiny particles on which droplets of water vapor can collect.
dew point Temperature at which condensation occurs for a given amount of water vapor.
dust dome Dome of heated air that surrounds an urban area and traps pollutants, especially suspended particulate matter. See also urban heat island.
dust plume Elongation of a dust dome by winds that can spread a city's pollutants hundreds of kilometers downwind.
evaporation Conversion of a liquid into a gas.
Freons See chlorofluorocarbons.
front The boundary between two air masses with different temperatures and densities. See cold front, warm front.
global warming Warming of the earth's atmosphere because of increases in the concentrations of one or more greenhouse gases primarily as a result of human activities. See greenhouse effect, greenhouse gases.
greenhouse effect Natural effect that releases heat in the atmosphere (troposphere) near the earth's surface. Water vapor, carbon dioxide, ozone, and several other gases in the lower atmosphere (troposphere) absorb some of the infrared radiation (heat) radiated by the earth's surface. This causes their molecules to vibrate and transform the absorbed energy into longer-wavelength infrared radiation (heat) in the troposphere. If the atmospheric concentrations of these greenhouse gases rise and they are not removed by other natural processes, the average temperature of the lower atmosphere will increase gradually. Compare global warming.
greenhouse gases Gases in the earth's lower atmosphere (troposphere) that cause the greenhouse effect. Examples are carbon dioxide, chlorofluorocarbons, ozone, methane, water vapor, and nitrous oxide.
high Air mass with a high pressure. Compare low.
industrial smog Type of air pollution consisting mostly of a mixture of sulfur dioxide, suspended droplets of sulfuric acid formed from some of the sulfur dioxide, and a variety of suspended solid particles. Compare photochemical smog.
inversion See temperature inversion.
low Air mass.
mesosphere Third layer of the atmosphere; found above the stratosphere. Compare stratosphere, troposphere.
microclimates Local climatic conditions that differ from the general climate of a region. Various topographic features of the earth's surface such as mountains and cities typically create them.
ozone depletion Decrease in concentration of ozone (O3) in the stratosphere. See ozone layer.
ozone layer Layer of gaseous ozone (O3) in the stratosphere that protects life on earth by filtering out most harmful ultraviolet radiation from the sun.
PANs Peroxyacyl nitrates. Group of chemicals found in photochemical smog.
photochemical smog Complex mixture of air pollutants produced in the lower atmosphere by the reaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Especially harmful components include ozone, peroxyacyl nitrates (PANs), and various aldehydes. Compare industrial smog.
precipitation Water in the form of rain, sleet, hail, and snow that falls from the atmosphere onto the land and bodies of water.
primary pollutant Chemical that has been added directly to the air by natural events or human activities and occurs in a harmful concentration. Compare secondary pollutant.
radiation temperature inversion Temperature inversion that typically occurs at night in which a layer of warm air lies atop a layer of cooler air nearer the ground as the air near the ground cools faster than the air above it. As the sun rises and warms the earth's surface, the inversion normally disappears by noon and disperses the pollutants built up during the night. See temperature inversion. Compare subsidence temperature inversion.
rain shadow effect Low precipitation on the far side (leeward side) of a mountain when prevailing winds flow up and over a high mountain or range of high mountains. This creates semiarid and arid conditions on the leeward side of a high mountain range.
relative humidity Amount of water vapor in a certain mass of air, expressed as a percentage of the maximum amount it could hold at that temperature. Compare absolute humidity.
secondary pollutant Harmful chemical formed in the atmosphere when a primary air pollutant reacts with normal air components or other air pollutants. Compare primary pollutant.
smog Originally a combination of smoke and fog but now used to describe other mixtures of pollutants in the atmosphere. See industrial smog, photochemical smog.
stratosphere Second layer of the atmosphere, extending about 17-48 kilometers (11-30 miles) above the earth's surface. It contains small amounts of gaseous ozone (O3), which filters out about 95% of the incoming harmful ultraviolet (UV) radiation emitted by the sun. Compare troposphere.
subsidence temperature inversion Inversion of normal air temperature layers when a large mass of warm air moves into a region at a high altitude and floats over a mass of colder air near the ground. This keeps the air over a city stagnant and prevents vertical mixing and dispersion of air pollutants. See temperature inversion. Compare radiation temperature inversion.
temperature inversion Layer of dense, cool air trapped under a layer of less dense, warm air. This prevents upward-flowing air currents from developing. In a prolonged inversion, air pollution in the trapped layer may build up to harmful levels. See radiation temperature inversion, subsidence temperature inversion.
thermal inversion See temperature inversion.
troposphere Innermost layer of the atmosphere. It contains about 75% of the mass of earth's air and extends about 17 kilometers (11 miles) above sea level. Compare stratosphere.
urban heat island Buildup of heat in the atmosphere above an urban area. The large concentration of cars, buildings, factories, and other heat-producing activities produces this heat.
warm front Boundary between an advancing warm air mass and the cooler one it is replacing. Because warm air is less dense than cool air, an advancing warm front rises over a mass of cool air. Compare cold front.
weather Short-term changes in the temperature, barometric pressure, humidity, precipitation, sunshine, cloud cover, wind direction and speed, and other conditions in the troposphere at a given place and time. Compare climate.