Descriptionary

degrees below the horizon and lightening of the sky begins.

astronomical dusk the time when the Sun drops 18 degrees below the horizon and illumination of the sky fades out.

atmospheric layers the series of distinctive layers that make up the Earth's atmosphere; the lowest is the troposhere, followed by the stratosphere, the meso- sphere, the ionosphere, the thermosphere, and the exosphere.

chlorofluorocarbon any of several compounds used as refrigerants and in aerosol cans that, when released into the atmosphere, rise to the stratosphere and deplete or destroy the Earth's protective ozone layer.

circumzenithal arc seen from the ground as an upside-down rainbow, usually in the far north and high in the sky—actually the bottom quarter or half of a circle of light, caused by sunlight passing through ice crystals several miles above Earth, and not associ­ated with rain.

civil dawn the period when the Sun is six degrees below the horizon, when the Earth is partly illumi­nated.

civil dusk the period when the Sun drops six degrees below the horizon, when darkness begins to fall.

exosphere the uppermost stratum of the Earth's atmosphere, starting about 300 miles up and eventu­ally giving way to the radiation belts and magnetic fields of space.

fata morgana complex multiple mirage seen along the Atlantic and Pacific coasts, where cliffs and rock formations may appear suspended in the sky or take on the appearance of castles and towers, caused by refraction of light in atmospheric zones of varying densities; named for the legendary castles of Morgan le Fay.

global warming see greenhouse effect.

glory a bull's-eye-light rainbow or aura caused by diffraction of sunlight through a thin cloud of water vapor, most frequently seen surrounding an airplane's shadow. Also known as a corona.

green flash a light-refracting phenomenon in which the last portion or bead of the setting Sun and sur­rounding sky flash out in a brilliant green, usually lasting only an instant and rarely seen.

greenhouse effect the trapping of heat and solar radiation in the atmosphere by excess carbon dioxide released by the increased burning of wood and fossil fuels, with the projected long-range effect of warm­ing the Earth's overall climate and raising sea level by melting the polar ice caps.

hydrocarbon pollution pollution of the Earth's atmosphere by unburned or partially burned gasoline vapors.

hydrologic cycle the natural cycle in which water evaporates, forms clouds, and returns water back to Earth in the form of precipitation.

ionosphere the ion-rich layer of Earth's atmosphere found within the thermosphere between 50 and 180 miles up, used for reflecting radio beams in long­distance radio communication.

jet streams rivers of high-speed winds, usually trav­elling from west to east, from 30,000 to 45,000 feet up.

magnetosphere the huge magnetic envelope that protects the Earth from the Sun's constant blast of ions, found above the atmosphere as part of space.

mesosphere the strata of Earth's atmosphere from 30 to 50 miles up where extremely dry air drops to temperatures as low as -225°F.

mock sun a false image of the Sun, often watery in appearance, formed by the refraction or bending of light by hexagonal crystals of ice in the air. Also known as a sun dog.

ozone hole any one of various atmospheric "holes," which expand seasonally, such as has been measured over Antarctica, resulting from the chemical depletion of the protective layer of ozone from polluting chlo- rofluorocarbons.

ozone layer the layer of ozone gas (an explosive form of oxygen) that extends from 10 to 30 miles up in the stratosphere, and protects the Earth by absorb­ing hazardous ultraviolet radiation from the Sun.

Rayleigh scattering the scientific explanation for why the sky is blue. Blue wavelengths of sunlight are scattered by air molecules in all directions while yellow, red, and orange are not and pass to Earth unobstructed.

stratosphere the stratum of the Earth's atmosphere from 10 to 30 miles up that contains the ozone layer.

thermal a rising column of warm air.

thermosphere the stratum of Earth's atmosphere from 50 to 300 miles up that contains ionized bands, collectively known as the ionosphere.

troposphere the stratum of Earth's atmosphere from 0 to 10 miles up, where most weather systems occur.

ultraviolet radiation a form of radiation from the sun, most of which is absorbed or blocked by the Earth's ozone layer.

X-rays a form of the Sun's radiation, which is fil­tered or absorbed by the Earth's thermosphere.

beaches and shores

barchan a crescent-shaped sand dune.

barrier beach a large, low-lying sand bar sur­rounded by the sea.