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absolute humidity the mass of water vapor in a unit volume of air.

acid rain rain with an abnormally high acid content caused by burning of fossil fuels, which release sulfur and nitrogen oxides. In the presence of sunlight, these pollutants form sulfates and nitrates that create sulfuric and nitric acids when dissolved in water.

barometric pressure the weight of the atmosphere over a unit area of Earth's surface. Air pressure fluctuations are associated with changes in weather.

biome a major life zone containing interrelated plants and animals adapted to a particular climate and soil, which covers a large geographical area.

CFCs short for "chlorofluorocarbons," a family of industrial gases. CFCs interfere with the natural cycle of ozone formation in the upper atmosphere, which acts as a barrier to harmful ultraviolet radiation.

coalescence process by which tiny water droplets in a cloud merge to form raindrops that eventually precipitate.

condensation formation of liquid water from vapor, either around a small particle or on a solid surface. See dew point, evaporation.

convection transfer of heat by movement of air or water; specifically, rise of an air mass caused by warming over land or sea that often causes the formation of clouds.

cyclone a rotating mass of air with minimum pressure in its center. In the Northern Hemisphere, such winds move counterclockwise, and in the Southern Hemisphere, clockwise.

dew point temperature at which air at constant pressure must cool to become saturated with water vapor, followed by condensation.

down draft a rapidly descending column of cooling air that causes heavy rains and violent wind gusts.

evaporation process by which a liquid such as water becomes a gas.

high pressure system an area of high barometric pressure that rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

hurricane an extremely low-pressure, destructive weather cell of tropical origin with wind speeds in excess of 74 miles per hour (119 kilometers per hour). Hurricanes causes widespread flooding and wind damage in North America and the Caribbean.

jet stream fast-moving currents of air reaching 180 mph (290 kmph ) in the troposphere 30,000 to 35,000 feet up (9,144 to 10,668 meters) that can strengthen and shift low-pressure systems. In winter, currents may shift toward the equator; in summer, toward the poles.

low pressure a system in which air pressure decreases toward the center, associated with unsettled weather, usually formed by a mass of warm air being forced up by cold air.

ozone a gas composed of three oxygen atoms that forms a thin diffuse layer in the upper atmosphere. This layer absorbs harmful ultraviolet rays from the sun and also prevents heat loss from Earth.

sublimation the change of ice to vapor or vapor to ice without a liquid phase.

temperature inversion an atmospheric condition in which a layer of warm air, usually from a high pressure system, moves over a colder layer, preventing it from rising or dispersing. In urban areas, automobile and industrial air pollutants become trapped, resulting in a band of smog from the ground to a height of 500 to 1,000 feet (152 to 305 meters).

typhoon a term for a hurricane occurring in the North Pacific or the China Sea.

wind shear vertical eddies or waves of air resulting from one layer of air sliding over another moving at different speeds or directions.