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Atoll

An atoll is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands/cays on the coral rim.

Absolute Humidity

Measurement of atmospheric humidity. Absolute humidity is the mass of water vapor in a given volume of air (this measurement is not influenced by the mass of the air). Normally expressed in grams of water vapor per cubic meter of atmosphere at a specific temperature.

Absolute zero

The theoretical temperature at which molecular motion vanishes and a body would have no heat energy; the zero point of the Kelvin and Rankine temperature scales. Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold source that would render a Carnot cycle 100 percent efficient. The value of absolute zero is not estimated to be -273.15° Celsius, -459.67° Fahrenheit, 0° Kelvin, and 0° Rankine.

Absorption

The process by which radiant energy is absorbed and converted into other forms of energy. A substance that absorbs energy may also be a medium of refraction, diffraction, or scattering; these processes, however, involve no energy retention or transformation and are to be clearly differentiated from absorption.

Abyssal Fan

Fan shaped accumulation of sediment from rivers that is deposited at the base of a submarine canyon within an ocean basin.

Abyssal Plain

Another name for ocean floor.

Acid

Substance having a pH less than 7 / Substance that releases hydrogen ions (H+).

Acid Rain

Rain with a pH less than 5.6. Normal pH of precipitation is 5.6.

Acid Shock

A sudden acidification of runoff waters from the spring melting of accumulated snow in the middle latitudes because of the winter deposition of acidic precipitation.

Advection

1. The process of transport of an atmospheric property solely by the mass motion of the atmosphere; also, the rate of change of the value of the advected property at a given point. 2. Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere whereas convection describes the predominantly vertical, locally induced motions. 3. To transport or carry. In air quality, the rate at which particulate matter is transported.

Aerosols

Particles, other than water or ice, suspended in the atmosphere ranging in radius from one-hundredth to one-ten-millionth of a centimeter -- or 102 to 10-3 microns (µ). Aerosols are important as nuclei for the condensation of water droplets and ice crystals, and as participants in various atmospheric chemical reactions. Perhaps most significantly, they absorb solar radiation, then emit and scatter it, influencing the radiation budget of the Earth-atmosphere system, which in turn influences the climate on Earth's surface. Aerosols from volcanic eruptions can lead to a cooling at the surface, which may delay greenhouse warming for a few years following a major eruption.

Aggregate

This is the collective term for sand, gravel and crushed rock. They can be compacted to firmly fill a space and are often bound together with cement (to make concrete) or bitumen (for road surfacing).

Antarctic Circle

Latitude of 66.5° South. The northern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year.

Anticyclone

An atmospheric pressure system consisting of an area of high pressure and outward circular surface wind flow. In the Northern Hemisphere winds from an anticyclone blow clockwise, while Southern Hemisphere systems blow counterclockwise and undefined at the Equator.

Antilles current

An ocean current, the northern branch of the north equatorial current flowing along the northern side of the Great Antilles carrying water that is identical with that of the Sargasso Sea. The Antilles current eventually joins the Florida current (after the latter emerges from the Straits of Florida) to form the Gulf.

Arctic Circle

Latitude of 66.5° North. The southern limit of the area of the Earth that experiences 24 hours of darkness or 24 hours of day at least one day during the year.

Archipelago

An archipelagos or island group is a chain or cluster of islands that are formed tectonically.

Atmospheric window

The spectral region between 8.5 and 11.0 microns where the atmosphere is essentially transparent to longwave radiation.

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