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Aerosols
Atmospheric aerosols are relatively small solid or liquid particles that are suspended (float) in the atmosphere. They can be produced naturally or by human activities. Their size typically ranges from a few hundredths of a micrometer to several micrometers. Aerosols have an influence on the radiative balance of the Earth. See sections 2.1.6, 4.1.2.2, 5.5.2.1 and 5.5.3.
Albedo
The albedo (α) is the ratio between reflected and incoming radiation. It varies between 0 for a perfect black body that absorbs all the incoming radiation to 1 for a surface that reflects it all. It depends on the wavelength, but the general term usually refers to some appropriate average across the spectrum of visible light, or across the whole spectrum of solar radiation. See sections 1.5, 2.1.4 and 4.2.3.
Alkalinity
The total alkalinity (Alk) is defined as the excess of bases over acid in sea water. See section 2.3.2.1 (eq. 2.45) and 4.3.1.
Anomaly
The anomaly of a variable (e.g., temperature) is the difference between the value under consideration and the long-term mean for the corresponding period or location.
Aphelion
The aphelion is the point in the Earth’s orbit that is furthest from the Sun. See section 2.1.3.1 and see also perihelion.
Aragonite
Aragonite is a form of calcium carbonate (CaCO3). It has a different crystal lattice and crystal shape than calcite. See section 2.3.2.2.
Ascendance
An ascendance is an upward movement of air in the atmosphere. See section 1.2.2.
Atmospheric boundary layer
The atmospheric boundary layer is the lowest part of the atmosphere which is in direct contact with the Earth’s surface. The properties of this layer are directly influenced by the presence of the surface, and in turn they influence the exchanges between the surface and the atmosphere. Vertical mixing is usually strong in this layer because of the relatively intense turbulent motions. See sections 2.1.6 and 3.3.7.
Austral
Austral is a synonym for 'southern'.